Clinical features and diagnosis of hemophagocytic lymphohistiocytosis

INTRODUCTION — 

Hemophagocytic lymphohistiocytosis (HLH) is an aggressive and life-threatening syndrome of excessive immune activation. HLH most often affects infants from birth to 18 months of age, but it also occurs in children and adults of all ages.

HLH can occur as a familial or sporadic disorder, and it can be triggered by a variety of events that disrupt immune homeostasis. Infections, malignancies, rheumatologic disorders, and immunodeficiencies are common triggers, both in patients with a predisposing genetic condition and in sporadic cases.

Prompt treatment of HLH is critical for successful outcomes, but the greatest barrier to a successful outcome is often a delayed diagnosis. Factors that may delay HLH diagnosis include its rarity, variable clinical presentation, and lack of specificity of the clinical and laboratory findings, which overlap with several other disease states.

The clinical features and diagnosis of HLH and a related disorder, macrophage activation syndrome (MAS), are discussed in this topic.

The management of patients with these disorders is discussed separately. (See "Treatment and prognosis of hemophagocytic lymphohistiocytosis".)

TERMINOLOGY — 

Our approach to describing HLH is consistent with the recommendations from the North American Consortium for Histiocytosis (NACHO) [1].

We do not use the terms "primary HLH" and "secondary HLH," which have been used in the past to distinguish between an underlying genetic cause versus HLH in association with an alternative source of pathologic immune activation. However, both primary and secondary HLH can be triggered by infections or other immune-activating events and, conversely, pathogenic gene variants (mutations) can be found in individuals of any age and with any family history. Identifying pathogenic gene variants is important for accurate diagnosis, prognosis, and individualized treatment.

We use the following terminology:

●HLH syndrome – A clinical condition of severe immune hyperactivation that may be associated with a gene defect of lymphocyte cytotoxicity or immune regulation, infection, cancer, rheumatologic disorder, and/or an immunodeficiency. The clinical presentation and laboratory abnormalities are described below. (See 'Clinical features' below.)

●HLH disease – An HLH syndrome in which the distinctive immune activation is the core problem. HLH disease may be associated with a specific genetic and/or environmental cause, as described below. (See 'Genetic abnormalities' below and 'HLH triggers' below.)

●Macrophage activation syndrome (MAS) – MAS refers to a form of HLH that occurs primarily in patients with juvenile idiopathic arthritis or other rheumatologic diseases. (See 'Rheumatologic disorders' below.)

●HLH disease mimics – Disorders that resemble HLH syndrome but are caused by other conditions. (See 'Differential diagnosis' below.)

PATHOPHYSIOLOGY

Immunologic abnormalities — HLH is characterized by excessive inflammation and tissue damage caused by uncontrolled immune activation. This hyperinflammatory state arises from a failure in the normal regulatory mechanisms of macrophages and lymphocytes, which are unable to properly suppress immune responses, leading to persistent and unchecked inflammation [2].

The hyperinflammation of HLH is often triggered by infection, cancer, or rheumatologic disorders, as discussed below. (See 'HLH triggers' below.)

HLH involves the following cell types, immune phenomena, pathologic features, and gene disorders.

●Cell types – The cell types involved in the pathogenesis of HLH include:

•Macrophages – Macrophages are professional antigen-presenting cells derived from circulating monocytes. Macrophages, along with dendritic cells, present foreign antigens to lymphocytes. In HLH, macrophages become activated and secrete excessive amounts of cytokines, ultimately causing severe tissue damage that can lead to organ failure. (See "An overview of the innate immune system", section on 'Monocytes and macrophages'.)

•Natural killer cells – Natural killer (NK) cells constitute 10 to 15 percent of lymphocytes. NK cells eliminate damaged, stressed, or infected host cells, such as macrophages, typically in response to viral infection or malignancy, in a major histocompatibility complex (MHC)-unrestricted manner. (See "An overview of the innate immune system", section on 'Natural killer cells'.)

•Cytotoxic lymphocytes – Cytotoxic T lymphocytes (CTLs) are activated T cells (primarily expressing CD8) that target and destroy infected or abnormal autologous cells (eg, macrophages), and present foreign antigens in conjunction with class I MHC molecules. (See "The adaptive cellular immune response: T cells and cytokines", section on 'CD8+ T cell activation'.)

In HLH, NK cells and/or CTLs fail to eliminate activated macrophages. This lack of normal feedback regulation results in excessive CD8-positive T cell and macrophage activation with highly elevated levels of interferon-gamma (IFNg) and other cytokines, which drive the pathology of HLH [2-8]. CD4 and CD8 lymphocyte subsets may also be disordered in HLH [9].

The normal elimination of activated macrophages by NK cells and CTLs occurs through perforin-dependent cytotoxicity. NK cells and CTLs lyse target cells in a series of steps that include the formation of an immunologic synapse; the creation of a pore in the macrophage membrane; and the delivery of cytolytic granules into the macrophage. The granules contain a variety of proteases (eg, granzyme B) that can initiate cell death, often through apoptosis. Most of the genetic defects in patients with familial HLH encode proteins involved in this process. (See "The adaptive cellular immune response: T cells and cytokines" and "NK cell deficiency syndromes: Clinical manifestations and diagnosis", section on 'Mechanisms of killing' and 'Genetic abnormalities' below.)

Toll-like receptor (TLR) activation of the immune system can also cause HLH [10]. TLRs are nonantigen-specific receptors on the surface of NK cells that are activated by components of bacteria, fungi, viruses, or mycoplasma. Normal mice with repeated TLR9 stimulation develop an illness similar to macrophage activation syndrome (MAS) [11]. Genes associated with TLR/interleukin 1 receptor (IL-1R) signaling are upregulated in patients with juvenile idiopathic arthritis who develop MAS [12].

●Hemophagocytosis – Hemophagocytosis refers to the engulfment (literally "eating") of host blood cells by macrophages.

Hemophagocytosis is characterized by the presence of red blood cells, platelets, or white blood cells (or fragments of these cells) within the cytoplasm of macrophages (picture 1 and picture 2), which can lead to cytopenias. Hemophagocytosis can be seen in immune tissues (lymph nodes, spleen, liver) or bone marrow. Although hemophagocytosis can be a marker of excessive macrophage activation and supports the diagnosis of HLH, hemophagocytosis alone is neither pathognomonic nor required for the diagnosis of HLH. (See 'Bone marrow evaluation' below and 'Diagnosis' below.)

●Cytokine storm – The persistent activation of macrophages, NK cells, and CTLs in patients with HLH leads to excessive cytokine production (cytokine storm) by macrophages and lymphocytes and is thought to be responsible for multiorgan failure and the high mortality of this syndrome [2,13,14].

Some of the cytokines that mediate HLH can be measured in serum and may be helpful in distinguishing HLH from other conditions. Cytokines found at extremely high levels in the plasma of patients with HLH include IFNg, the chemokine CXCL9 (which is regulated by IFNg) [15]; tumor necrosis factor-alpha (TNF-alpha); interleukins (IL) such as IL-6, IL-10, IL-12; and the soluble IL-2 receptor (CD25) [16-18]. Elevated IL-16 levels promote a TH1-type immune response, facilitating the recruitment of macrophages and other immune cells associated with HLH [19].

IL-18 overproduction is a hallmark of HLH associated with rheumatologic disorders, which is often called MAS [20,21]. Markedly elevated levels of IL-18 and its binding protein were found in adults with HLH [22]. Levels of IFNg and IFNg-induced proteins were higher in patients with HLH than with systemic juvenile idiopathic arthritis (sJIA), but the ratio of IL-18:IFNg was higher in sJIA [23].

Genetic abnormalities — Genetic abnormalities that affect regulators of the immune system play a major role in childhood HLH, and they are increasingly recognized in adult cases.

The initiation of the immune response in HLH is generally normal, but there is an inability to terminate the response [24,25]. The resultant overactivation of inflammatory cells and elevated levels of inflammatory cytokines (eg, IFNg, IL-1beta, IL-6, IL-10, IL-18, and TNF) can lead to multiorgan failure and death [3,24-26].

Gene defects play a major role in childhood HLH and in some adult cases [8,27-31]. Identification of gene abnormalities can be helpful in determining the likelihood of recurrence, the choice of treatment, and the risk of HLH in family members. (See 'Diagnosis' below and "Treatment and prognosis of hemophagocytic lymphohistiocytosis".)

Most of the originally implicated HLH-associated genes encode components of the machinery for perforin-dependent cytotoxicity (figure 1), while others are associated with various immune deficiencies or disorders of inflammasomes.

●Incidence of pathogenic gene variants – The likelihood of identifying a pathogenic gene variant (ie, mutation) is highest in the youngest patients.

•Children – In a review of 476 North American children, a pathogenic gene variant was found in 45 percent of children <1 month old; the frequency was lower in older children: 2 to 12 months (39 percent), 1 to 2 years (20 percent), and >2 years (6 percent) [32].

•Adults – A study of 175 patients 18 to 75 years old reported that 14 percent had pathogenic gene variants [33]. The genetic lesions in adults tended to cause partial defects in protein function rather than complete loss of the protein; the partial loss of function may account for the later age of HLH onset in some adults. (See 'HLH manifestations in adults' below.)

●Inheritance patterns – HLH-associated pathogenic gene variants act in an autosomal recessive fashion; thus, inheritance of a pathogenic variant at both alleles of a gene is required to manifest the disease. Many cases of familial HLH are related to consanguinity.

While most individuals have inherited the same pathogenic variant at both alleles, other inheritance patterns can be seen. Some individuals are compound heterozygotes (ie, a different gene variant in each allele of the same gene) or they show digenic inheritance (ie, separate mutations in two different genes). Heterozygosity for an HLH mutation is occasionally found in an individual (usually an adult) who has HLH in association with another condition [34]. (See 'HLH triggers' below.)

•A review of 2701 patients referred for genetic testing reported that 8 percent were homozygous or compound heterozygous for gene variants, while 1 percent showed digenic inheritance [31]. Another study reported similar findings, with monoallelic variants of known HLH-associated genes in 15 percent of 281 patients classified as having "sporadic" disease, suggesting that this disorder is not a simple recessive one [35].

•Another study reported biallelic gene variants of the six primary genes associated with HLH in 19 percent of 101 patients with HLH [36]. The frequency of heterozygous variants of two potential HLH-associated gene variants did not differ significantly from the general population, suggesting that these "digenic" cases are unlikely to be disease-causing. Among 47 patients without the expected HLH-associated gene variants, 58 percent had potential disease-causing genetic variants, including genes linked to primary immunodeficiency or dysregulated immune activation and proliferation, such as NLRC4 or NLRP12, or biallelic variants in NLRP4, NLRC3, or NLRP13.

•In a study that used whole-exome sequencing, heterozygous variants in LYST, UNC13D, and STXBP2 were found in 5 of 14 children with juvenile idiopathic arthritis (JIA) who had MAS but only in 4 of 29 patients with JIA who did not have MAS [37]. Other recessive pairs and compound heterozygotes were found.

●Pathogenic gene variants – Several HLH-associated pathogenic gene variants map to loci that encode elements of the cytotoxic granule formation and release pathway; these are referred to as familial HLH loci. Others cause various immunodeficiencies or disorders of inflammasomes.

•Familial HLH (FHLH) genetic loci – The following are identified FHLH genetic loci; note that the gene defect responsible for FHL1 remains uncharacterized.

-FHL2 (PRF1) – FHL2 results from mutations of PRF1, which encodes perforin. Perforin is delivered in cytolytic granules and forms pores in the membrane of target cells. Variants in other genes that affect perforin expression have also been reported [28,38-40].

-FHL3 (UNC13D) – FHL3 results from mutations of UNC13D, which encodes Munc13-4 [29,41]. Proteins of the Unc (uncoordinated) family regulate cytolytic granule maturation.

-FHL4 (STX11) – FHL4 results from mutations of STX11, which encodes syntaxin 11. Syntaxins control granule exocytosis. Several syntaxin mutations were reported in a group of Kurdish families with HLH [30,42].

-FHL5 (STXBP2) – FHL5 results from mutations of STXBP2, which encodes Munc18-2 (also called syntaxin binding protein 2) [34,43]. This protein binds to syntaxin 11 and promotes the release of cytotoxic granules.

•Other lymphocyte disorders – Other HLH-associated pathogenic gene variants that affect lymphocyte function include:

-RHOG – Biallelic mutations of RHOG, which encodes RhoG, a protein that physically interacts with Munc13-4, cause defects of lymphocyte cytotoxicity by impairing cytoskeletal morphology and formation of the immune synapse [44].

-CDC42 – Heterozygous mutations of CDC42, which encodes Cdc42 (a Rho GTPase), lead to defects in lymphocyte proliferation, migration, and formation of actin-based structures [45,46].

-NBAS – Biallelic variants of NBAS impair Golgi body to endoplasmic reticulum transport prior to degranulation [47].

-Griscelli syndrome – Griscelli syndrome (GS) type 2 is caused by mutations of RAB27A, which encodes a GTP-binding protein [48]. GS2 is characterized by hypopigmentation, immune deficiency, thrombocytopenia, and/or neurologic defects. (See "Syndromic immunodeficiencies", section on 'Griscelli syndrome'.)

-Chediak-Higashi syndrome – Chediak-Higashi syndrome (CHS) is caused by pathogenic variants of CHS1/LYST, which encodes a lysosomal trafficking regulatory protein [49]. It is important for intracellular transport and processing of lysosomal enzymes and other proteins, particularly in lymphocytes and macrophages. CHS is characterized by partial oculocutaneous albinism, neutrophil defects, neutropenia, and neurologic abnormalities. (See "Chediak-Higashi syndrome".)

-IPEX (Immune dysregulation, polyendocrinopathy, enteropathy, X-linked) – Several mutations of the FOXP3 gene have been found in HLH-affected families.

•Inflammasomopathies – The inflammasome is an essential component of the innate immune system that identifies pathogens and cellular stress. Inflammasomopathies refer to disorders caused by the dysregulation of the inflammasome.

Dysregulated inflammasome activity is often caused by disorders of genes that are associated with excessive or inappropriate inflammatory reactions. This dysregulation may lead to excessive production of IL-18, which contributes to the hyperinflammatory state and can exacerbate symptoms and inflammation in various autoinflammatory diseases [50]. Examples include cryopyrin-associated periodic syndromes (CAPS), familial Mediterranean fever (FMF), and other autoinflammatory syndromes.

-NLRC4 – The gene product of NLRC4 (NLR family CARD domain containing gene) gene product is a component of the inflammasome. Missense mutations in the nucleotide-binding domain can lead to MAS. NLRC4 is involved in the activation of inflammatory responses, particularly by triggering the production of proinflammatory cytokines like IL-1beta and IL-18 when it detects certain pathogens [51].

-NLRP3 – The gene product of NLRP3 (NOD-like receptor family, pyrin domain containing gene) assembles into an inflammasome complex that activates caspase-1. Mutations in NLRP3 tend to have gain-of-function effects, resulting in its constitutive activation and leading to the uncontrolled production of IL-1beta and IL-18 [50,52].

-AIM2 – The gene product of AIM2 (Absent in melanoma 2) detects double-stranded deoxyribonucleic acid (DNA) and forms an inflammasome complex, leading to the activation of caspase-1. Pathogenic gene variants of AIM2 can impair its function, resulting in inadequate responses to infections and excessive inflammation.

-HMOX1 – The gene product of HMOX1 (Heme oxygenase 1) causes defective heme oxygenase activity, which can lead to interstitial lung inflammation and fibrosis. In HLH and other conditions of excessive immune activation and inflammation, defective HMOX1 activity may exacerbate the inflammatory response, leading to recurrent flares of HLH symptoms [53].

•Impaired control of viruses

-X-linked lymphoproliferative disease – X-linked lymphoproliferative disease type 1 (XLP1) is caused by mutations in SH2 domain protein 1A (SH2D1A), also called signaling lymphocyte activation molecule (SLAM)-associated protein (SAP), which encodes an activator of NK and T cells [54].

XLP2 is caused by mutations in the X-linked inhibitor of apoptosis (XIAP), also called baculoviral IAP-repeat-containing protein 4 (BIRC4); the encoded protein protects cells from apoptosis [55]. XLP (also called Duncan disease) is characterized by an abnormal response to Epstein-Barr virus (EBV) infection. (See "X-linked lymphoproliferative disease", section on 'Genetics'.)

-XMEN disease – XMEN (X-linked immunodeficiency with magnesium defect, EBV infection, and neoplasia) is an immunodeficiency syndrome associated with EBV-related malignancies and rare cases of HLH [56]. A loss-of-function mutation in a gene encoding magnesium transporter 1 (MAGT1) leads to CD4 lymphopenia, chronic, high-level EBV infection, normal levels of NK-T cells, and dysregulated immune responses to EBV. (See "Malignancy in inborn errors of immunity", section on 'XMEN disease'.)

-ITK deficiency – Interleukin-2-inducible T cell kinase (ITK) deficiency, like XLP and XMEN deficiencies, is unable to control EBV infections. Affected patients have various lymphoproliferative diseases, lymphomatoid granulomatosis, HLH, and dysgammaglobulinemia.

-CD27 deficiency – Missense pathogenic variants of TNFRSF7 that reduce expression of CD27 have been associated with a syndrome of severe EBV infections associated with HLH, Hodgkin lymphoma, uveitis, and recurrent infections [57].

-Hermansky-Pudlak syndrome – Hermansky-Pudlak syndrome (HPS) is a rare disorder characterized by oculocutaneous albinism and platelet storage pool deficiency [58,59]. Pathogenic variants of HPS1, AP3B1 (HPS2), HPS3, HPS4, HPS5, HPS6, DTNBP1 (HPS7), BLOC1S3 (HPS8), and BLOC1S6 (PLDN) have been reported. (See "Hermansky-Pudlak syndrome".)

•Metabolic disorders

-Wolman disease – Mutations in LIPA cause a lack of functional liposomal acid lipase that can lead to HLH [60].

-Lysinuric protein intolerance – Lysinuric protein intolerance (LPI) is a recessive aminoaciduria caused by defective cationic amino acid transport in epithelial cells of the intestine and kidney. SLC7A7 (also called y+LAT1), the gene mutated in LPI, encodes the light subunit of a cationic amino acid transporter. Patients with LPI frequently display severe complications, such as pulmonary disease, hematologic abnormalities, and disorders of the immune response [61].

•Impaired autophagy – Chronic granulomatous disease (CGD) is a genetically heterogeneous condition associated with recurrent, life-threatening bacterial and fungal infections. CGD can lead to HLH through interconnected mechanisms.

The impaired ability to produce reactive oxygen species in CGD hinders clearance of infections, resulting in chronic or recurrent infections that trigger excessive immune responses, and a cytokine storm with elevated levels of proinflammatory cytokines like IL-1beta, IL-6, and IL-18 [62]. Dysregulation of macrophages and T cells further drives uncontrolled immune activation and hemophagocytosis. (See "Chronic granulomatous disease: Pathogenesis, clinical manifestations, and diagnosis".)

●Genotype-phenotype correlations – The affected gene and the specific type and site of mutation may affect the age of presentation and clinical features.

Patients with HLH-related pathogenic gene variants tend to present at a younger age than those without mutations. There is controversy regarding the contribution of hypomorphic mutations to the development of HLH [33,36].

Informative studies that evaluated genotype-phenotype correlations with HLH include:

•Patients with PRF1 null mutations typically present in the first year of life, whereas those with missense mutations and variable degrees of perforin expression have more variable ages of presentation, including some cases in adults [63-70].

•In a series of 76 patients with HLH, those with PRF1 gene variants had earlier disease onset (ie, <6 months) than those with STX11 gene variants (adjusted odds ratio 8.2 [95% CI 1.2-56]) [71].

•The most common PRF1 gene variant in Black Africans (50delT-PRF1) was associated with an earlier age of disease onset compared with other PRF1 gene variants (median age at diagnosis 3 versus 36 months) [66,72].

•In patients with digenic inheritance (ie, gene variants at two separate FHL loci), PRF1 abnormalities in combination with a gene variant that affects degranulation (eg, UNC13D, STX11, STXBP2) were associated with onset at ≥2 years, whereas two gene variants affecting degranulation were associated with disease onset at <2 years [31].

•Adults with hypomorphic mutations of PRF1, MUNC13-4, and STXBP2 often have a more indolent course than younger patients [33].

•Individuals with FHL5 (STXBP2 gene variants) have defective erythropoiesis with aberrant cell morphology and decreased CD235a expression resulting in hemolysis [73].

EPIDEMIOLOGY — 

HLH is primarily a pediatric syndrome, but it can occur in patients of any age.

Infants are most commonly affected, with the highest incidence in those <3 months [74]. The male-to-female ratio is close to 1:1 [74]. In adults, there may be a slight male predisposition [75].

Approximately 1 child in 3000 admitted to a tertiary care pediatric hospital had HLH; this corresponds to several cases of HLH per center per year [32]. Earlier studies reported lower rates of incidence, which probably reflected underdiagnosis. As an example, in a series from the 1970s that reported an incidence of 1.2 children per million per year, the diagnosis of HLH was made antemortem in only 11 of 32 patients [74]. A review of HLH cases in large American pediatric hospitals reported an incidence of 1 in 100,000 children [76].

Although HLH is predominantly seen in young children, it can be seen in patients of any age, including adults as old as 70 years [63,64,77]. There may be an ethnic predisposition for the development of malignancy-associated HLH, with one large study demonstrating a much higher risk in Japanese and Eastern Asian patients with malignancy compared with Western patients [78]. A review of 2197 adult cases worldwide found that approximately one-half of reported patients were from Japan [75]. A nationwide survey in Japan from 2001 to 2005 identified 799 patients with HLH; of the 470 with sufficient data for analysis, 41 percent were >14 years [79].

Up to one-quarter of cases of HLH were originally thought to be caused by inherited gene variants. One study reported classic HLH-associated mutations in only 19 percent of patients, but 58 percent of the other patients had pathogenic variants in other immune-deficiency genes [36]. In a study of 65 unrelated families with HLH, gene variants affecting STX11, PRF1, and UNC13D were found in 20, 18, and 10 percent of affected individuals, respectively [71].

A review of 224 North American patients with HLH-associated gene variants reported the following distribution of specific gene variants according to race or ethnicity [32]:

●White Americans – Most likely to be UNC13D (47 percent), STXBP2 (22 percent), and PRF1 (20 percent)

●Hispanic Americans – Most likely to be PRF1 (71 percent) and UNC13D (17 percent)

●Black Americans – Most likely to be PRF1 (98 percent)

●Arabs – Most likely to be PRF1 (36 percent), UNC13D (27 percent), and STXBP2 (18 percent)

Other associations of ethnic groups with specific gene variants include a high incidence of variants of PRF1, UNC13D, or STX11 in individuals of Turkish origin [80]; a high incidence of STXBP2 variants in individuals from Saudi Arabia, the United Arab Emirates, and Turkey [65,81,82]; and a high incidence of PRF1 mutations in Japanese individuals [83].

Genetic causes of HLH are discussed above. (See 'Genetic abnormalities' above.)

CLINICAL FEATURES

Initial presentation — HLH usually presents as an acute or subacute febrile illness associated with multiple organ involvement.

Initial signs and symptoms of HLH can mimic common infections, fever of unknown origin, hepatitis, or encephalitis. With few exceptions, the clinical features are similar regardless of whether an underlying genetic defect has been identified. Some patients have chronic "stuttering" presentations, with recurrent fevers of unknown origin and manifest only a subset of the classic diagnostic criteria [84].

The HLH-2004 study, which included 369 patients <18 years, reported the following clinical findings [85]:

●Fever – 95 percent

●Splenomegaly – 89 percent

●Bicytopenia – 92 percent

●Hypertriglyceridemia or hypofibrinogenemia – 90 percent

●Hemophagocytosis – 82 percent

●Ferritin >500 mcg/L – 94 percent

●Low/absent natural killer (NK) cell activity – 71 percent

●Soluble CD25 elevation – 97 percent

An international panel of HLH experts reported that the HLH-2024 criteria were 99 percent accurate in distinguishing HLH from infections or systemic juvenile idiopathic arthritis [86].

Features of HLH in adults are discussed below. (See 'HLH manifestations in adults' below.)

In addition to the typical findings of HLH, some HLH-associated pathogenic gene variants are associated with distinct clinical features. As an example, a review of 37 patients with STXBP2 mutations reported hypogammaglobulinemia, severe diarrhea, bleeding, and sensorineural hearing loss in 59, 38, 22, and 16 percent, respectively [65]. Defective granule mobilization by neutrophils has also been identified in these patients [87]. These abnormalities lead to inadequate bacterial killing (especially of gram-negative bacteria) that may lead to the association of chronic diarrhea with this genetic abnormality.

Some clinical findings vary among different ethnic groups. A case series of 20 neonates from Japan reported that the incidence of fever was extremely low in the eight preterm infants (12 percent); hypertriglyceridemia and neutropenia were uncommon; and familial mutations were undetectable in most patients (65 percent) [88].

Laboratory and imaging

●Cytopenias – Cytopenias are seen in most patients with HLH.

Anemia and/or thrombocytopenia are seen in >80 percent of patients at presentation [76,85,89,90]. Platelet counts range from 3000 to 292,000 (median 69,000)/microL, and hemoglobin levels of 3.0 to 13.6 (median 7.2) g/dL are typical [76]. White blood cell counts can be elevated or decreased.

Cytopenias may occur later in the disease course in patients with macrophage activation syndrome (MAS; ie, HLH in the setting of a rheumatologic disorder), especially in those with juvenile idiopathic arthritis (JIA) because patients with JIA often have elevated blood counts prior to developing MAS.

●Serum ferritin – A very high serum ferritin level is common in HLH, and it has especially high sensitivity and specificity in children.

While a very high ferritin level is helpful in suggesting the possibility of HLH, lower levels of ferritin (eg, <500 ng/mL) do not exclude the diagnosis. A relatively normal ferritin can occasionally be seen in HLH genetic syndromes, even during a disease flare, and disease activity in some patients may correlate more closely with elevated sIL-2R or sCD25 than with ferritin.

In the HLH-94 study, ferritin levels >500, >5000, and >10,000 ng/mL were seen in 93, 42, and 25 percent, respectively; the median ferritin level was 2950 ng/mL [89]. Serum ferritin in this range is seen in very few other inflammatory disorders in children, and when it does occur in other syndromes, it is often in the setting of multiply transfused patients with iron overload. This was illustrated in a series of 330 children with high serum ferritin levels (320 controls and 10 HLH patients), in which a ferritin level >10,000 ng/mL was 90 percent sensitive and 96 percent specific for HLH, with very minimal overlap with sepsis, infections, and liver failure [91]. When the control group was reanalyzed with a comparison cohort of 120 patients with HLH, a ferritin level ≥2000 mcg/L had a 70 percent sensitivity and 68 percent specificity for diagnosing HLH [92]. There was no difference when primary and secondary HLH cases were analyzed separately.

In both adults and neonates, other causes of extremely high ferritin levels should also be evaluated. As an example, ferritin levels >10,000 ng/mL can be seen in neonatal hemochromatosis or fulminant liver failure; however, the presence of cytopenias and fevers, as well as elevated soluble IL-2 receptor alpha (sIL-2R) and sCD163 in patients with HLH may help to exclude these other possible diagnoses [93]. (See 'Differential diagnosis' below.)

Macrophages are a primary source of ferritin, which may account for the association between HLH and very high ferritin levels [94]. A protein responsible for modulation of iron homeostasis, growth differentiation factor 15, is dramatically upregulated in patients with HLH and is responsible for increased serum ferritin by enhancing the ferroportin-mediated iron efflux [95].

●Other disease markers

•sCD25 – sCD25 (Interleukin 2 receptor protein) is a valuable indicator of HLH.

sCD25 >17,000 international units at diagnosis and a decrease by <25 percent after a week of therapy were bad prognostic findings [96].

•CXCL9 – This interferon-gamma (IFNg)-regulated chemokine is markedly elevated in HLH and decreases more rapidly with effective therapy than ferritin. The levels of CXCL9 and IL-6 and their ratio may help to differentiate HLH from sepsis and systemic inflammatory response syndrome (SIRS) [97].

•IL-18 – IL-18 is elevated in HLH and MAS, with the latter often having levels >100,000 international units [98].

●Liver function and coagulation tests – Nearly all patients with HLH have hepatitis, manifest by elevated liver function tests (LFTs), including elevated transaminases, lactate dehydrogenase (LDH), and bilirubin. Increased triglycerides and abnormal coagulation parameters (especially elevated D-dimer) caused by hepatic dysfunction and disseminated intravascular coagulopathy are also common. The degree of abnormality ranges from mild liver dysfunction to hepatic failure; hydrops fetalis has been reported in neonates [99].

•Liver enzyme levels >3 times the upper limit of normal (ULN) have been reported in 50 to 90 percent of patients with HLH [76,90,99]; LDH is elevated in 85 percent [90]. Bilirubin levels between 3 and 25 mg/dL are seen in >80 percent of patients. The GGT (gamma-glutamyl transferase) level, associated with infiltration of the biliary tract by lymphocytes and macrophages, can be a useful parameter to monitor the disease course [32].

•Hypertriglyceridemia can be due to severe liver involvement, but it may not be elevated until the liver has been affected for some time. In a review of patients with HLH associated with various triggers, 68 percent had elevated triglycerides at diagnosis or in the course of the disease [100].

•Coagulation abnormalities due to impaired hepatic synthetic function and/or disseminated intravascular coagulation are common [101].

•Liver biopsy, if done, is likely to show lymphocytic infiltrates in patients with HLH. On autopsy, the livers of patients who died from HLH showed chronic persistent hepatitis with periportal lymphocytic infiltration [102].

Neurologic abnormalities — Neurologic abnormalities are seen in up to one-third of patients with HLH.

Neurologic findings are highly variable and may include seizures, mental status changes (including encephalitis-like conditions), and ataxia [2,32,103]. Neurologic findings may dominate the clinical picture or they may develop prior to the appearance of other signs and symptoms [104,105].

●Neurologic syndromes in HLH

•Isolated central nervous system HLH – Isolated central nervous system (CNS) HLH refers to patients in whom the symptoms and pathological features of HLH primarily affect the CNS, without significant systemic involvement [106-110]. Isolated CNS HLH can occur in various contexts, including infectious diseases, autoimmune disorders, and malignant triggers.

Diagnosing isolated CNS HLH can be difficult since the classic systemic symptoms of HLH (eg, fever, splenomegaly, cytopenias) may be absent or minimal. The underlying mechanisms may involve hyperactivation of immune cells within the CNS, leading to inflammation and potential damage to brain tissue.

Diagnosis of isolated CNS HLH typically involves a combination of clinical evaluation, magnetic resonance imaging (MRI), and sometimes cerebrospinal fluid (CSF) analysis to detect inflammatory markers or evidence of hemophagocytosis [106-110].

•Posterior reversible encephalopathy syndrome (PRES) – Patients with HLH are at risk of developing PRES, which presents with headaches, altered consciousness, visual disturbances, and/or seizures. Patients may have retinal hemorrhages and optic nerve edema. PRES is associated with vasogenic cerebral edema predominantly in the posterior cerebral hemispheres by MRI. (See "Reversible posterior leukoencephalopathy syndrome".)

●Neurologic findings

•MRI – Brain MRI may show edema and/or hypodense or necrotic areas [111].

•CSF – Findings in the CSF typically include elevated protein levels, lymphocytic pleocytosis, presence of macrophages with or without hemophagocytosis, elevated cytokine levels (particularly neopterin), and normal glucose levels.

Neopterin is produced by macrophages in response to cytokines (particularly IFNg) that indicate immune activation and may be found in serum and CSF of patients with HLH [112]. Neopterin levels may be elevated with HLH triggered by infections, autoimmune diseases, or malignancies. The marked elevation of neopterin in CNS can help differentiate CNS HLH from other conditions affecting the CNS.

Approximately 50 percent of patients have abnormalities of the CSF, which may carry an increased risk for mortality and neurologic sequelae [113]. In a series of 10 adults with HLH, seven had neurologic impairment, which included encephalopathy and seizures. Basal ganglia abnormalities were found in four patients [114].

Neurologic manifestations of HLH can vary widely, even with the same underlying genetic abnormality. As an example, two patients with familial HLH due to a PRF1 mutation presented with isolated HLH of the central nervous system; one patient presented with severe encephalitis, while the other presented with a demyelinating peripheral neuropathy caused by diffuse macrophage infiltration of the nerve sheath [115,116].

Other organ systems — HLH can affect other organ systems, including the respiratory system, heart, and skin.

●Respiratory abnormalities may lead to an urgent need for ventilatory support and death from acute respiratory distress syndrome (ARDS). Deteriorating respiratory function may be due to worsening of the HLH (causing an ARDS-like syndrome) or due to an infection. Pulmonary involvement was reported in 42 percent of a series of 775 adults with HLH [75].

●Severe hypotension may require administration of one or more vasopressors.

●Kidney dysfunction occurs in many patients with HLH and may present with hyponatremia, often related to the excessive antidiuretic hormone. Many patients develop kidney failure and require dialysis. Kidney involvement was reported in 16 percent of a series of 775 adults with HLH [75].

●Skin manifestations can be quite varied, including generalized rash, erythroderma, edema, petechiae, and purpura. Skin rash was reported in one-quarter of a series of 775 adults with HLH [75].

●Bleeding is a common manifestation of HLH. Bleeding may be due to altered coagulation from liver failure, thrombocytopenia from bone marrow failure, or platelet function defects associated with an underlying genetic defect in platelet granule processing. (See 'Genetic abnormalities' above.)

●Patients with underlying immunodeficiency syndromes may manifest syndrome-specific findings (eg, albinism). (See 'Immunologic abnormalities' above.)

●Some patients have clinical features of Kawasaki disease, such as conjunctivitis, red lips, and cervical lymphadenopathy. (See "Kawasaki disease: Clinical features and diagnosis", section on 'Clinical manifestations'.)

HLH manifestations in adults — HLH presenting is increasingly recognized in adults [75,117-121].

Adults with HLH can have similar clinical as children. A series of 775 adults with HLH reported similar predominance of fever (96 percent), splenomegaly (69 percent), and hepatomegaly (67 percent) [75]. However, adults with HLH also have certain distinctive features. A report from an expert panel determined the following clinical features to be important in adults [122]:

●Underlying predisposing disease

●Fever

●Organomegaly

●Cytopenias

●Elevated ferritin

●Elevated LDH

●Hemophagocytosis on the bone marrow aspirate

HLH in adults is more likely to be associated with a hematologic malignancy, and an elevated ferritin level is less specific in adults due to the higher incidence of other inflammatory conditions [117]. The reduced specificity of extremely high ferritin in adults was illustrated in a series of 113 adults with a serum ferritin level >50,000 ng/mL (median age, 58 years) [123]. Of these, only 17 percent were ultimately diagnosed with HLH; among those with HLH, one-half had secondary HLH due to a malignancy and one-third had secondary HLH due to infection. Non-HLH diagnoses in these patients with hyperferritinemia included kidney failure (65 percent), hepatocellular injury (54 percent), infection (46 percent), and hematologic malignancy (32 percent). (See 'Differential diagnosis' below.)

The later age of onset in some adults may be explained by the presence of a mutation with partial residual protein function, which may be able to compensate in the setting of some immune triggers. (See 'Genetic abnormalities' above.)

Single-institution studies of HLH in adults reported infections in 23 to 41 percent of patients and rheumatologic/autoimmune disease in 8 to 20 percent [124-126].

HLH TRIGGERS — 

An infection, cancer, rheumatologic disorder, or an alteration in immune homeostasis is often the instigating trigger for an acute episode of HLH. Identification of a trigger is important because treating the trigger may lessen the severity of HLH and, in some cases, enable avoidance of more toxic HLH-specific therapy. However, evaluation for HLH triggers should not delay the diagnosis of HLH or initiation of HLH-specific treatment in acutely ill patients.

HLH triggers can be broadly classified as:

●Immune activation – Immune activation from an infection, a malignancy (or its treatment), or a rheumatologic disorder are common triggers, both in patients with a genetic predisposition and in sporadic cases with no identifiable underlying genetic cause.

●Immunodeficiencies – Immunodeficiency related to inherited syndromes, malignancy, rheumatologic disorders, or HIV (human immunodeficiency virus) infection can trigger HLH. As an example, in one study, 3 of 17 patients with chronic granulomatous disease (CGD) developed HLH in association with excessive cytokine release [127].

The coexistence of immune dysregulation with unchecked inflammation distinguishes HLH from other syndromes of immune activation, immunodeficiencies, and inflammatory states [32].

Infections — HLH can be triggered by various types of infections, but viral infections are most common.

●Viral – HLH is often associated with viral infections, including Epstein-Barr virus (EBV), cytomegalovirus (CMV), parvovirus, herpes simplex virus, varicella-zoster virus, measles virus, human herpes virus 8, H1N1 influenza virus, parechovirus, and HIV, alone or in combination [75,128-136].

Some notable viral infections include:

•EBV – Primary EBV infection can trigger HLH in individuals with a defect in perforin-dependent cytotoxicity and in cases of sporadic HLH. Children with X-linked lymphoproliferative diseases (XLPs) are at particularly high risk [32].

•HIV – The development of HLH shortly after the initiation of antiretroviral therapy for the treatment of HIV infection has been reported [137].

•COVID-19 – SARS-CoV-2, the coronavirus that causes COVID-19, has been reported to cause HLH [138,139]. Patients who are treated with anti-tumor necrosis factor (TNF) agents (eg, for rheumatologic diseases) and develop HLH may be infected with mycobacterium tuberculosis, CMV, EBV, Histoplasma capsulatum, and other bacteria [140].

●Other infections – Although less common, HLH may also occur in the setting of infections due to bacteria (eg, Brucella, gram-negative bacteria, tuberculosis), parasites (eg, Leishmaniasis, malaria), and fungi [75,136,141,142].

Cancers — HLH can be triggered by various types of cancer (especially lymphomas) and some cancer treatments.

●Malignancies – Lymphomas are the most common malignant triggers of HLH, but numerous other cancers have also been associated. When it is associated with a malignancy, HLH is often more immediately life-threatening than the cancer itself.

HLH has been most commonly associated with lymphoid cancers (including B, T, and natural killer [NK] cell lymphomas) and leukemias but also with solid tumors [32,136,143-155]. Many patients with malignancy-associated HLH also have an acute infectious trigger.

•Among 458 cases of malignancy-associated HLH in a Swedish registry study, HLH was associated with lymphoma in 52 percent, leukemias in 29 percent, other hematologic malignancies in 8 percent, and solid tumors in 11 percent [156].

•A review of lymphoma-associated HLH in 542 patients (mean age 60.2 years) reported B cell non-Hodgkin lymphoma [NHL] in 45 percent, T cell NHL in 45 percent, and Hodgkin lymphoma in 9 percent [157]. In a single-institution study, HLH was reported in 18 percent of patients with acute myeloid leukemia and in 4 percent of patients with acute lymphocytic leukemia [158].

•In a series of 162 adults with HLH, hematologic malignancies (especially NHL) were the most common trigger, seen in 57 percent [159]. An additional 25 percent had infections (caused by bacterial, viral, parasitic, or fungal organisms), while 4 percent had both hematologic malignancy and an infection. Further analysis of these patients reported immunosuppression in 45 percent (eg, HIV infection or an immunosuppressive medication) [160]. A review of 22 patients with hematologic malignancies and HLH reported BK virus in 54 percent, HHV-6 in 33 percent, EBV in 28 percent, CMV in 24 percent, adenovirus in 17 percent, and parvovirus-type B in 17 percent [158].

In rare cases, HLH may be diagnosed before a malignancy is identified, highlighting the importance of ruling out any underlying cancer in new presentations of HLH [161]. The overall prognosis is quite poor for any malignancy-associated HLH, regardless of the patient's age at presentation, as discussed separately. (See "Treatment and prognosis of hemophagocytic lymphohistiocytosis", section on 'Prognosis'.)

●Cancer treatments – Certain cancer treatments can also trigger HLH.

•Immune checkpoint inhibitors (ICIs), including pembrolizumab, nivolumab, and ipilimumab, can trigger HLH, but the incidence is not well defined [162]. In clinical settings, it is important to recognize HLH symptoms (eg, fever, cytopenias, liver dysfunction), particularly in cancer patients receiving ICI therapy [163].

•Immunotherapies, such as chimeric antigen receptor (CAR)-T cell therapy, blinatumomab, and brentuximab vedotin, can cause cytokine release syndrome (CRS), which manifests immune hyperactivity like that of HLH [164-166]. (See "Cytokine release syndrome (CRS)".)

Rheumatologic disorders — HLH can be triggered by various rheumatologic disorders.

The term macrophage activation syndrome (MAS) is often used when a hemophagocytic syndrome develops in children with rheumatologic conditions; we consider MAS to be HLH in the setting of a rheumatologic disorder, rather than a separate syndrome.

HLH can arise at the initial presentation of the rheumatologic disorder, during treatment, or in association with a concurrent infection. Some patients with MAS are found to have heterozygosity for HLH-associated gene variants (eg, PRF1, UNC13D), while others have disorders associated with dysregulated inflammasomes [37]. (See 'Genetic abnormalities' above.)

The most common rheumatologic association is in children with systemic juvenile idiopathic arthritis (sJIA; formerly called Still's disease). When MAS is a presenting manifestation of sJIA, systemic juvenile or adult rheumatoid arthritis, or systemic lupus erythematosus, the diagnosis of both conditions can be challenging. Kawasaki disease, a common vasculitis of childhood, can trigger HLH and may be misdiagnosed initially. Other autoimmune diseases associated with HLH include dermatomyositis, systemic sclerosis, mixed connective tissue disease, antiphospholipid syndrome, Sjögren's disease, ankylosing spondylitis, vasculitis, and sarcoidosis [101,136,167-173]. (See "Systemic juvenile idiopathic arthritis: Complications", section on 'Macrophage activation syndrome' and "Kawasaki disease: Clinical features and diagnosis".)

Excessive proinflammatory cytokine production causes chronic inflammation in conditions like rheumatoid arthritis and systemic lupus erythematosus [174]. Some rheumatologic disorders exhibit dysregulated inflammasomes, which are crucial for the innate immune response; gene variants in inflammasome-related genes (eg, NLRP3, AIM2) may increase susceptibility to both systemic inflammation and rheumatologic disorders. The intensified inflammatory response in association with increased IL-1beta and IL-18 can perpetuate the cycle of inflammation by causing tissue damage. Patients with significantly elevated IL-18 levels (eg, >50,000 pg/mL) should be evaluated for possible underlying rheumatologic conditions if not previously diagnosed.

Treatment of the rheumatologic disorder can also trigger HLH. In a study of 394 children with sJIA who were treated with tocilizumab, 23 developed confirmed or probable MAS [175].

Immunodeficiencies — Inherited and acquired immunodeficiencies can trigger HLH.

Some inherited immunodeficiency disorders are associated with pathogenic gene variants that are also associated with HLH [48,49,54,127,176-179]. (See 'Immunologic abnormalities' above.)

Acquired immunodeficiencies have also been associated with HLH. Examples include HIV infection, hematopoietic cell transplantation, and kidney or liver transplant [136,180,181]. Some cases of HLH in association with an immunodeficiency are triggered by a concurrent infection or a lymphoproliferative syndrome [182-184].

EVALUATION — 

Clinical and laboratory evaluation is used to identify the HLH syndrome and confirm the underlying cause.

Clinical and laboratory — Most patients with HLH are acutely ill with multiorgan involvement, cytopenias, liver function abnormalities, and neurologic symptoms.

Patients may have already experienced a prolonged hospitalization or clinical deterioration without a clear diagnosis before HLH was suspected. A priority should be placed on rapid evaluation for organ involvement (eg, bone marrow insufficiency, liver abnormalities, neurologic involvement, immune activation), with the goal of starting treatment as rapidly as possible once the diagnosis (or a high likelihood) of HLH is established. The diagnostic approach is similar in infants, children, and adults [32].

●History – Patients with suspected HLH (or relatives) should be asked about parental consanguinity, familial disorders, antecedent infections, recurrent fevers, and pre-existing immunologic defects (eg, HIV infection, rheumatologic disorders, immunosuppressive medications). (See 'Genetic abnormalities' above and 'HLH triggers' above.)

●Examination – Physical examination should focus on identifying rashes, bleeding, lymphadenopathy, hepatosplenomegaly, neurologic abnormalities, and other organ dysfunction (eg, heart, lungs).

●Laboratory – Many tests will have already been done (eg, during an evaluation of an unexplained febrile illness that involves multiple organs). Others (eg, ferritin, triglycerides, screening immunologic studies) should be done immediately. We suggest the following for all patients:

•Hematology

-Complete blood count with differential

-Coagulation studies, including prothrombin time (PT), partial thromboplastin time (aPTT), fibrinogen, D-dimer

•Serum chemistries

-Liver function tests, including transaminases, GGT (gamma-glutamyl transferase), total bilirubin, albumin, and lactate dehydrogenase (LDH).

-Triglycerides (fasting).

-Ferritin.

-Soluble IL-2 receptor alpha (sCD25 or sIL-2R), IL-18, CD163, and CXCL9.

•Infections

-Cultures of blood, bone marrow, urine, cerebrospinal fluid (CSF), and other potentially infected body fluids.

-Viral titers and quantitative polymerase chain reaction (PCR) testing for Epstein-Barr virus (EBV), cytomegalovirus (CMV), adenovirus, and other suspected viruses. It is critical to follow the levels of any identified virus during treatment with the appropriate antiviral therapy.

●Other evaluation – The patient is evaluated for possible organ injury. Testing may include:

•Heart and lungs – Electrocardiogram and echocardiogram

•Imaging

-Chest radiography.

-Computed tomography (CT) scans of the neck, chest, abdomen, and pelvis or a positron emission tomography (PET) scan to evaluate for occult malignancy.

-Abdominal ultrasound, if the physical examination for splenomegaly is inconclusive.

Bone marrow evaluation — All patients with suspected HLH should have a bone marrow aspirate and biopsy to evaluate the cause of cytopenias and/or detect hemophagocytosis. Bone marrow specimens should be evaluated for possible malignancy and infections (including cultures).

Bone marrow cellularity can be high, low, or normal in HLH [32].

Infiltration of the bone marrow by activated macrophages is consistent with HLH. The macrophages in HLH do not have the cellular atypia associated with malignant histiocytes, and they are clearly different from the CD1a-staining Langerhans cells of Langerhans cell histiocytosis (formerly called histiocytosis-X). It is helpful to stain the bone marrow for the hemoglobin-haptoglobin scavenger receptor CD163 to highlight the macrophages whether there is evidence of hemophagocytosis. (See 'Diagnosis' below.)

Hemophagocytosis on bone marrow examination is reported in 25 to 100 percent of cases of HLH [159]. However, hemophagocytosis is not pathognomonic for HLH. A review of 78 bone marrow aspirates that showed hemophagocytosis included 40 that were associated with a diagnosis of HLH and 38 without an associated diagnosis of HLH; however, phagocytosis of nucleated cells or multiple nucleated cells was strongly correlated with a diagnosis of HLH [185]. Some patients may only show hemophagocytosis later in the disease course, even as they are clinically improving [32].

Neurologic — All patients with suspected HLH should have a lumbar puncture (LP). Imaging is performed for patients with unexplained neurologic abnormalities.

●LP – LP with analysis of the CSF is done for all patients with HLH.

Analysis of CSF should include cell counts, chemistries, neopterin (a biomarker produced by immune cells), microbiologic cultures, and testing for viruses, as indicated by clinical findings and epidemiology.

CSF is abnormal in one-half of patients with HLH, with elevated protein, cellular pleocytosis, and occasional hemophagocytosis.

●Brain imaging – Brain MRI, with and without contrast, should be performed if there are unexplained neurologic findings. Brain MRI may show parameningeal infiltrations, subdural effusions, necrosis, and other abnormalities.

Specialized testing

●Immunologic profile – Immunologic and cytokine studies are appropriate for patients suspected of having HLH based on the results of the initial evaluation.

We typically perform the following immunologic testing:

•Soluble IL-2 receptor alpha (sCD25 or sIL-2R)

•Flow cytometry for:

-Lymphocyte subsets

-Natural killer (NK) cell function/degranulation using surface expression of CD107alpha (also called LAMP-1 [lysosomal-associated membrane protein 1])

-Perforin and granzyme B

-Cell surface expression of SAP and XIAP (in males)

•Soluble hemoglobin-haptoglobin scavenger receptor (sCD163)

•Immunoassay for serum CXCL9 and IL-18

•Serum IL-18 levels

•Quantitative immunoglobulins (eg, IgG, IgA, IgM)

Not all laboratories can provide all the above studies.

Levels of sIL-2R are generally available in one to two days, while the other tests may take longer. Nevertheless, therapy should not be delayed while awaiting the results of this immunologic testing. Levels of sIL-2R most closely correlate with disease activity [32]. The ratio of sIL-2R to serum ferritin may be useful in patients with lymphoma. A review of patients with lymphoma-associated HLH versus nonlymphoma-associated cases found that the former had a much higher ratio of sIL-2R to ferritin than the latter (ratio 8.56 versus 0.66) [186].

Elevated sIL-2R, CXCL-9, and IL-18; reduced NK function or cell surface expression of CD107alpha; elevated sCD163 (especially in macrophage activation syndrome [MAS]); and reduced perforin, SAP, or XIAP are also consistent with a diagnosis of HLH, but none is entirely specific [5,187-192]. Immunoglobulin levels are variable [5], while peripheral blood lymphocyte subsets generally show normal T cell numbers and normal helper/suppressor ratio, and may show decreased numbers of B cells or NK cells [5,193]. Elevated granzyme B may be thought to as part of the immune signature of lymphocyte activation [194].

The NK cytotoxicity assay is not widely available, is labor-intensive, and has limited utility in cases of low-circulating NK cells. Flow cytometry for reduced/absent NK cell perforin and CD107alpha is more sensitive and has equivalent specificity in screening patients for HLH, and may be an acceptable surrogate [195].

●Gene testing – Genetic testing (ie, identification of an HLH gene mutation) is indicated in all patients who meet the diagnostic criteria for HLH, and in those with a high likelihood of HLH based on the initial evaluation.

Testing for an HLH-associated pathologic gene variant/mutation is indicated in all patients who meet the diagnostic criteria for HLH, and in those patients with a high likelihood of HLH based on the initial evaluation.

Selective genetic testing may be used to confirm the genetic disorder in patients whose relatives have a known familial syndrome.

For patients without an identified familial syndrome, we favor genetic testing with either a next-generation sequencing panel of HLH-associated genes or whole-exome sequencing; for some cases, it may be necessary to request intronic sequencing to find rare variants [36]. These approaches offer the efficiency of testing, the possibility of identifying biallelic or hypomorphic mutations, and/or the possibility of identifying novel HLH-associated gene defects [36]. Moreover, if allogeneic hematopoietic cell transplantation (HCT) is being considered, the patient and any potentially matched related donor should have whole-exome sequencing performed to avoid the risk of transplantation with a similar genetic defect.

An acceptable alternative for evaluating patients without a known familial syndrome is selective testing guided by the immunophenotype. As examples:

•If cellular perforin protein levels are low, we do PRF mutation analysis.

•If CD107alpha mobilization is low, we do UNC13D, STX11, STXBP2, and RAB27A mutation analysis.

•If SAP expression is low, we do SH2D1A mutation analysis.

•If XIAP expression is low, we do BIRC4 mutation analysis.

Laboratories that can perform genetic testing can be found in the Genetic Testing Registry.

●HLA typing – HLA (human leukocyte antigen) typing is indicated during the initial evaluation in preparation for identifying a donor for allogeneic HCT. Performing this testing at the time of initial presentation avoids delays in identifying donors should they be needed. (See "Treatment and prognosis of hemophagocytic lymphohistiocytosis".)

DIAGNOSIS — 

HLH should be suspected in an infant, child, or adult with unexplained cytopenias, hepatitis, or inflammatory central nervous system findings who has unexplained fever, hepatosplenomegaly, prior HLH-like episodes, a family history of HLH, or a known genetic disorder associated with HLH.

The diagnosis of HLH is based on a compatible clinical presentation in the setting of elevated inflammatory markers (eg, ferritin, sCD25, and/or CXCL9). HLH can be diagnosed using any of the following:

●HLH-2024 [196] (see 'HLH-2024 criteria' below)

●HLH-2004 [85] (see 'HLH-2004 criteria' below)

●Familial HLH [86] (see 'Familial HLH' below)

A high degree of suspicion is required to diagnose HLH. The natural history of untreated HLH syndrome is almost uniformly fatal, and diagnosis is challenging because of the clinical complexity, rarity, and diversity of causes. Prompt recognition and diagnosis of the underlying cause of HLH are critical to enable urgent and appropriate treatment.

For critically ill patients with a strong suspicion of HLH, we make a presumptive diagnosis of HLH and urgently initiate treatment, whether all diagnostic criteria are fulfilled. (See "Treatment and prognosis of hemophagocytic lymphohistiocytosis", section on 'Acutely ill or deteriorating patients'.)

HLH-2024 criteria — Diagnosis is based on fulfilling ≥5 of the following [196]:

●Fever – ≥38.5° C

●Splenomegaly – ≥2 cm below the costal margin

●Cytopenias – ≥2 of the following

•Hemoglobin <90 g/L (<100 g/L in neonates)

•Platelets <100 x 109/L

•Neutrophils <109/L

●Hypofibrinogenemia or hypertriglyceridemia – ≥1 of the following:

•Fibrinogen ≤1.5 g/L

•Triglycerides ≥3.0 mmol/L

●Hyperferritinemia – ≥500 microg/L

●Hemophagocytosis – In bone marrow or other tissues

●Elevated soluble CD25 (also called soluble interleukin 2 receptor alpha) – ≥2400 units/mL

Other features that are not included as criteria in HLH-2024 but may be helpful for recognizing HLH and distinguishing it from other conditions include [196]:

●Reduced or absent natural killer (NK) cell activity by genetic and lymphocyte cytotoxicity assays

●Hepatomegaly

●Elevated aminotransferases

●Elevated bilirubin

●Elevated lactate dehydrogenase (LDH)

●Elevated d-dimers

●Elevated cells or protein in cerebrospinal fluid (CSF)

●Known underlying immunosuppression

Some of the findings described above are used in the H score, which is a weighted scoring system based on nine clinical/laboratory variables [197], as described below.

HLH-2004 criteria — The HLH-2004 criteria [85] are an earlier version of the HLH-2024 criteria described above. (See 'HLH-2024 criteria' above.)

The HLH-2004 criteria differ from the HLH-2024 criteria, in that low/absent NK cell activity was included as a criterion; diagnosis was based on fulfilling ≥5 of the criteria [85].

Familial HLH — The variable presentation can be difficult to diagnose using the HLH-2024 or HLH-2004 criteria.

An alternative approach can be used to diagnose familial HLH (FHLH).

FHLH manifests characteristic features in many children, but it can also present in adolescents and adults [33]. Not all patients with FHLH will fulfill the diagnostic criteria of HLH-2024 or HLH-2004 and, in some cases, HLH-directed therapy must be initiated based on a strong clinical suspicion before there is irreversible damage to the brain or other organs.

Diagnosis of FHLH requires the identification of a pathogenic gene variant/mutation [86]. FHLH can be caused by autosomal recessive gene variants in PRF1, UNC13D, STX11, or STXBP2, any of which can impair lymphocyte cytotoxicity.

Because the turnaround time for genetic testing may delay the diagnosis and result in irreversible organ damage, a presumptive diagnosis of FHLH can be made using flow cytometry or lymphocyte cytotoxicity assays, which are often faster.

●Flow cytometry to quantify intracellular perforin expression in cytotoxic cells can suggest PRF1 gene variants [198].

●Reduced or absent cell membrane CD107a fluorescence, which indicates decreased or defective perforin exocytosis, can suggest gene variants in UNC13D, STXBP2, STX11, and RAB27A [192].

●In males, flow cytometry can identify abnormalities of XLP1 and XLP2 [199].

Unaffected siblings of patients with FHLH can be evaluated by genetic testing or lymphocyte cytotoxicity studies.

Predictive models — Various clinical and laboratory features have been incorporated into models to estimate the probability of HLH.

●H score – Some clinicians use the "H score" to estimate the probability of HLH [197].

This model incorporates points for immunosuppression; fever; organomegaly; levels of triglycerides, ferritin, alanine aminotransferase, and fibrinogen; degree of cytopenias; and presence of hemophagocytosis on the bone marrow aspirate. An H score ≥250 confers a 99 percent probability of HLH, whereas a score of ≤90 is associated with a <1 percent probability of HLH.

●OHI index – The OHI (Optimized HLH inflammatory) index was reported to be more sensitive and specific than the H score [200]. sCD25 >3900 units/microL and ferritin >1000 ng/microL identified patients with the highest mortality and defined hematologic malignancy-associated HLH with a diagnostic prediction of 94 percent sensitivity and 72 percent specificity.

DIFFERENTIAL DIAGNOSIS — 

HLH can resemble various common conditions that cause fever, pancytopenia, hepatic abnormalities, or neurologic findings, but clinical presentations vary. Diagnosis is based on fulfilling multiple clinical and laboratory findings, but no single feature distinguishes HLH from other disorders in the differential diagnosis. Diagnostic criteria are presented above. (See 'Diagnosis' above.)

The presence of cytopenias, a very high ferritin level, and liver function abnormalities can be especially helpful in distinguishing HLH from these other conditions. The frequency of liver function test (LFT) abnormalities is so high in HLH that we believe the absence of LFT abnormalities should prompt a thorough search for an alternative diagnosis. HLH is also unlikely if neither sCD25 nor CXCL9 is elevated.

It is also important to remember that HLH can develop in association with many of the conditions in its differential diagnosis.

Among conditions that should be considered in the differential diagnosis are:

●Macrophage activation syndrome (MAS) – MAS should be thought of as a form of HLH associated with a rheumatologic disease, rather than as a separate clinical entity. (See 'Rheumatologic disorders' above.)

●Infection/sepsis – Systemic infections and/or sepsis share many features with HLH, including fever, cytopenias, and hepatic involvement. Both sepsis and HLH can have findings of disseminated intravascular coagulation and widespread inflammation with cytokine abnormalities. Unlike HLH, which is often triggered by a viral infection, sepsis is typically caused by a bacterial or fungal micro-organism and is typically not characterized by ongoing lymphocyte activation. While there is no ideal test to distinguish between sepsis and HLH, extremely high ferritin and elevated lactate dehydrogenase levels were highly predictive of a subsequent diagnosis of HLH in a series of 19 children with an initial diagnosis of fever of unknown origin [90]. Ferritin levels tend to be static in patients with infections but are prone to dramatic increases in those with HLH. (See "Sepsis syndromes in adults: Epidemiology, definitions, clinical presentation, diagnosis, and prognosis".)

●Liver disease/liver failure – Primary liver disease and HLH can both present with hepatomegaly and elevated LFTs. Both can cause coagulopathy with prolonged prothrombin time (PT) and partial thromboplastin time (aPTT), low fibrinogen, and elevated D-dimer, and both can cause encephalopathy. Unlike liver disease, HLH is a multisystem disorder. Those with HLH typically have more extensive organ involvement, cytopenias, extremely high ferritin, and neurologic findings. Cytokine profiles seen in HLH are not typically seen in primary liver disease. (See "Acute liver failure in adults: Etiology, clinical manifestations, and diagnosis".)

●Multiple organ dysfunction syndrome – Multiple organ dysfunction syndrome (MODS) refers to progressive organ dysfunction in an acutely ill patient. Like HLH, MODS can affect any organ system, and there may be some overlap between these diagnoses [201]. It is possible that a subset of patients who have been diagnosed with MODS have in fact had HLH. An extremely high ferritin or dramatically increasing ferritin is more consistent with HLH than with MODS. (See "Sepsis syndromes in adults: Epidemiology, definitions, clinical presentation, diagnosis, and prognosis", section on 'Multiple organ dysfunction syndrome' and 'Evaluation' above.)

●Encephalitis – Encephalitis can result from infection, autoimmunity, and a number of viral infections; and the clinical manifestations can range from subtle neurologic deficits to complete unresponsiveness. The neurologic presentation of those with encephalitis can thus be identical to those with HLH. However, those with HLH typically have more extensive organ involvement, cytopenias, liver abnormalities, and high ferritin, whereas findings in encephalitis are typically confined to the central nervous system. (See "Acute viral encephalitis in children: Clinical manifestations and diagnosis" and "Viral encephalitis in adults".)

●Autoimmune lymphoproliferative syndrome (ALPS) – ALPS is an immune dysregulation syndrome caused by genetic defects in the machinery for Fas (CD95)-mediated apoptosis, which leads to the expansion of some autoreactive lymphocyte populations. Patients present with hepatosplenomegaly, rash, and autoimmune cytopenias, along with other autoimmune manifestations that could mimic findings of HLH (eg, autoimmune hepatitis, Guillain Barré syndrome). Unlike those with HLH, patients with ALPS typically do not manifest multiorgan failure and signs of excessive inflammation, such as extremely high ferritin levels and severe liver failure. (See "Autoimmune lymphoproliferative syndrome (ALPS): Clinical features and diagnosis".)

●Drug reaction with eosinophilia and systemic symptoms (DRESS) – DRESS is a severe drug-induced hypersensitivity reaction possibly initiated by viral reactivation. Like HLH, DRESS is characterized by fever and LFT abnormalities. DRESS can also be associated with hemophagocytosis, although this is rare [202]. Unlike HLH, DRESS is characterized by a temporal relationship to a drug, eosinophilia, and skin rash. DRESS is unlikely to cause extremely high ferritin or cytopenias, which are found in most patients with HLH. (See "Drug eruptions", section on 'Drug reaction with eosinophilia and systemic symptoms'.)

●Kawasaki disease – Kawasaki disease (KD), a vasculitis that predominantly affects children, is characterized by widespread inflammation that includes fever, rash, lymphadenopathy, elevated triglycerides, and abnormal cerebrospinal fluid. KD typically causes bilateral conjunctivitis and mucositis, as well as cardiac findings (eg, coronary artery aneurysms), which are much less common in HLH; conversely, HLH is more likely to be associated with cytopenias and liver abnormalities. KD can act as a trigger for HLH, so its diagnosis does not eliminate the possibility of HLH. Compared with patients with KD, those with HLH associated with KD have a longer history of fever and higher levels of the N-terminal pro-brain natriuretic peptide (NT-proBNP; 889 pg/mL versus 233 pg/mL, respectively) [203]. A patient with the diagnosis of KD, especially if "atypical," whose symptoms do not respond to intravenous immune globulin (IVIG) therapy, should be evaluated for HLH. (See "Kawasaki disease: Clinical features and diagnosis" and "Kawasaki disease: Complications", section on 'Cardiac complications'.)

●Cytophagic histiocytic panniculitis – Cytophagic histiocytic panniculitis is a rare systemic disorder consisting of lobular panniculitis (ie, inflammation of the subcutaneous fat), fever, hepatosplenomegaly, and liver failure. This panniculitis can be associated with a form of T cell lymphoma [204,205]. A subset of patients diagnosed with this condition may have had HLH. These patients with panniculitis, who primarily present with subcutaneous nodules, are less likely to have the severe multiorgan involvement seen in HLH. (See "Clinical manifestations, pathologic features, and diagnosis of subcutaneous panniculitis-like T cell lymphoma" and "Panniculitis: Recognition and diagnosis", section on 'Malignancy'.)

●Thrombotic thrombocytopenic purpura (TTP), hemolytic uremic syndrome (HUS), or drug-induced thrombotic microangiopathy (DITMA) – TTP, HUS, and DITMA (also called drug-induced TTP) are characterized by endothelial damage, microvascular thrombosis, and anemia; fever, neurologic findings, or renal failure may be present. Unlike the anemia of HLH, the anemia in these syndromes is microangiopathic (ie, Coombs negative, characterized by schistocytes). Patients with TTP, HUS, or DITMA generally do not have rising ferritin or liver function abnormalities, although a DITMA syndrome associated with quinine may have multiorgan failure. (See "Diagnostic approach to suspected TTP, HUS, or other thrombotic microangiopathy (TMA)" and "Drug-induced thrombotic microangiopathy (DITMA)".)

●Transfusion-associated graft-versus-host disease (ta-GVHD) – Ta-GVHD is a rare complication of transfusion of any nonirradiated blood component. It occurs when viable donor lymphocytes in the transfusion attack the recipient's tissues (skin, bone marrow, gastrointestinal tract), often fatally. It is most common after hematopoietic cell transplantation but can also occur in immunocompetent individuals who have a partial human leukocyte antigen (HLA) match with the donor (eg, in ethnically homogenous populations or after directed donation). The typical presentation includes fever, rash, pancytopenia, and elevated liver enzymes, 4 to 30 days after transfusion. High ferritin levels and hemophagocytosis in the bone marrow can also be seen. Unlike HLH, skin biopsy in ta-GVHD shows vacuolization of the basal layer and a histiocytic infiltrate, and sometimes the pathognomonic finding of satellite dyskeratosis. In cases where the two diagnoses cannot be distinguished, a trial of dexamethasone, and, if not sufficient, etoposide, can be used. (See "Transfusion-associated graft-versus-host disease".)

SUMMARY

●Description – Hemophagocytic lymphohistiocytosis (HLH) is a potentially life-threatening syndrome of excessive immune activation. HLH is most common in infants and young children, but it can affect patients of any age. (See 'Epidemiology' above.)

●Pathophysiology – HLH is associated with an inability to restrict immune responses of activated macrophages and lymphocytes. Many patients, especially infants and children, have an inherited abnormality of perforin-dependent cytotoxicity (figure 1), while others have an underlying immunologic trigger, such as infection, malignancy, or rheumatologic disorder. (See 'Genetic abnormalities' above and 'HLH triggers' above.)

●Presentation – Most patients are acutely ill with multiorgan involvement, but presentations vary. Common clinical and laboratory findings include fever, hepatosplenomegaly, rash, lymphadenopathy, neurologic symptoms, cytopenias, high serum ferritin, and liver function test abnormalities. (See 'Initial presentation' above.)

●Evaluation – Initial evaluation includes (see 'Evaluation' above):

•Clinical, laboratory, and imaging – History should include parental consanguinity, familial disorders, antecedent infections, recurrent fevers, and pre-existing immunologic defects. Examine the patient for rashes, bleeding, lymphadenopathy, hepatosplenomegaly, neurologic abnormalities, and organ dysfunction. Hematology, chemistries, markers of inflammation (eg, sCD25 or sIL-2R, IL-18, CD163, CXCL9), infectious disease testing, imaging, and other clinical testing are described. (See 'Clinical and laboratory' above.)

•Bone marrow – Bone marrow aspirate and biopsy specimens are evaluated for causes of cytopenias and to detect hemophagocytosis, malignancy, and infections. (See 'Bone marrow evaluation' above.)

•Neurologic – All patients should have a lumbar puncture (LP); brain magnetic resonance imaging is performed as clinically indicated. (See 'Neurologic' above.)

•Specialized testing – Immunologic and gene testing are important components of the diagnosis and classification of HLH. (see 'Specialized testing' above):

●Diagnosis – HLH should be suspected in an infant, child, or adult with unexplained cytopenias, hepatitis, or inflammatory central nervous system findings who has unexplained fever, hepatosplenomegaly, prior HLH-like episodes, a family history of HLH, or a known genetic disorder associated with HLH.

HLH can be diagnosed using any of the following methods:

•HLH-2024 criteria – ≥5 of the following (details are discussed above). (See 'HLH-2024 criteria' above.)

-Fever

-Splenomegaly

-Bicytopenia

-Hypofibrinogenemia or hypertriglyceridemia

-Hyperferritinemia

-Hemophagocytosis

-Elevated soluble CD25

•HLH 2004 criteria – Differences between HLH-2004 and HLH-2024 are discussed above. (See 'HLH-2004 criteria' above.)

•Familial HLH – Diagnosis requires identification of a pathogenic gene variant/mutation. Mutation of an HLH-associated gene (eg, affecting cytolytic function of T cells and/or natural killer cells or defects in inflammasome regulation). (See 'Familial HLH' above.)

●Predictive models – Various clinical and laboratory features can be used to estimate the probability of HLH, including the H score and the OHI (Optimized HLH inflammatory) index. (See 'Predictive models' above.)

●Differential diagnosis – HLH should be distinguished from other multisystem illnesses associated with fever, liver failure, and/or neurologic symptoms. Notably, many of the conditions in the differential diagnosis can also be triggers for HLH. (See 'Differential diagnosis' above.)

ACKNOWLEDGMENT — 

The editorial staff at UpToDate acknowledge the late Laurence A Boxer, MD, for his previous role as a section editor for this topic.