Clinical features and diagnosis of hemophagocytic lymphohistiocytosis

INTRODUCTION — Hemophagocytic lymphohistiocytosis (HLH) is an aggressive and life-threatening syndrome of excessive immune activation. It most frequently affects infants from birth to 18 months of age, but the disease is also observed 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. Infection is a common trigger both in those with a genetic predisposition and in sporadic cases.

Prompt treatment is critical, but the greatest barrier to a successful outcome is often a delay in diagnosis due to the rarity of this syndrome, variable clinical presentation, and lack of specificity of the clinical and laboratory findings.

The clinical features and diagnosis of HLH and a related disorder, macrophage activation syndrome (MAS), will be discussed here. 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 favor not using the terms "primary HLH" and "secondary HLH," which have been applied in an attempt to distinguish between an underlying genetic cause versus an alternative source of pathologic immune activation. These terms have caused a great deal of confusion because both primary and secondary HLH can be triggered by infections or other immune activating events, and gene mutations can be found in individuals of any age and with any family history.

We use the following terminology:

●HLH syndrome – A condition of pathologic immune activation that is often associated with genetic defects of lymphocyte cytotoxicity. The clinical presentation is described below. (See 'Clinical features' below and 'Laboratory and radiographic abnormalities' below.)

●HLH disease – HLH syndrome in which the distinctive immune activation is the core problem; HLH disease may be associated with a specific genetic and/or environmental causes, as described below. (See 'Genetics' below and 'Associated illnesses' below.)

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

Macrophage activation syndrome (MAS) refers to a form of HLH that occurs primarily in patients with juvenile idiopathic arthritis or other rheumatologic diseases. Some authors call this "reactive hemophagocytic syndrome." (See 'Rheumatologic disorders/MAS' below.)

PATHOPHYSIOLOGY

Immunologic abnormalities — HLH is a syndrome of excessive inflammation and tissue destruction due to abnormal immune activation. The hyperinflammatory/dysregulated immune state is thought to be caused by the absence of normal downregulation by activated macrophages and lymphocytes [2].

The cell types involved in the pathogenesis of HLH include the following:

●Macrophages – Macrophages are professional antigen presenting cells derived from circulating monocytes; they 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 and cytotoxic lymphocytes – 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 an MHC-unrestricted manner. (See "An overview of the innate immune system", section on 'Natural killer cells'.)

Cytotoxic lymphocytes (CTLs) are activated T lymphocytes that lyse autologous cells such as macrophages bearing foreign antigen in association with class I histocompatibility proteins. Most CTLs express CD8. (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+ T cell and macrophage activation with highly elevated levels of interferon gamma and other cytokines, which drive the pathology of HLH [2-8]. (See 'Immunologic profile' below and 'Cytokine storm' below.)

Other lymphocyte abnormalities include altered numbers of CD4 and CD8 lymphocyte subsets [9]. In a series of adult patients, those with increased CD8 numbers and decreased CD4/CD8 ratios had the best survival. Decreased total CD3 numbers portended a bad outcome. (See "Treatment and prognosis of hemophagocytic lymphohistiocytosis", section on 'Prognosis'.)

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 formation of an immunologic synapse; creation of a pore in the macrophage membrane; and delivery of cytolytic granules into the macrophage. The granules contain a variety of proteases such as 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 'Genetics' below.)

Toll-like receptor (TLR) activation of the immune system can be another cause of HLH [10]. TLRs are non-antigen-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 and MAS [12].

Hemophagocytosis — In addition to antigen presentation and cytokine production, macrophages can also phagocytize host cells. 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). Hemophagocytosis can be observed in biopsies of immune tissues (lymph nodes, spleen, liver) or bone marrow aspirates/biopsies. Although it can be a marker of excessive macrophage activation and supports the diagnosis of HLH, hemophagocytosis alone is neither pathognomonic of, 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 all of these cell types, and is thought to be responsible for multiorgan failure and the high mortality of this syndrome [2,13,14].

Cytokines found at extremely high levels in the plasma of patients with HLH include interferon gamma (IFN gamma), the chemokine CXCL9 (which is regulated by IFN gamma) [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 may be important for a TH1-type response that recruits macrophages and other cells implicated in HLH [19]. In a study of adults with secondary HLH, markedly elevated levels of IL-18 and its binding protein were found [20]. Some of these cytokines can be measured in serum and are useful in distinguishing HLH from other conditions. A study of IFN gamma, IL-6, and IL-18 in patients with systemic JIA (sJIA) versus HLH showed higher levels of IFN gamma and IFN gamma-induced proteins in HLH compared with sJIA, but the ratio of IL-18/IFN gamma was higher in sJIA [21]. (See 'Specialized testing' below.)

An extensive study on the role of IL-18 in MAS and other rheumatologic conditions has shed light on the differences in pathophysiology of HLH and MAS [22]. Unbound (free) IL-18 levels >24,000 pg/mL could distinguish MAS from familial HLH with an 83 percent sensitivity and 94 percent specificity. Many patients with MAS had IL-18 levels >100,000 pg/mL, which helped distinguish MAS from other autoinflammatory conditions. A mouse model of MAS revealed that IL-18 was primarily produced by intestinal epithelium, which provides an intriguing biologic model for a syndrome of infantile enterocolitis and MAS caused by NLRC4 inflammasome hyperactivity [23].

Triggers — Patients with HLH can have a single episode of the disease or relapsing episodes, with relapses occurring most often in patients with familial HLH. The instigating trigger for an acute episode is often an infection or an alteration in immune homeostasis. The two broad categories of triggers include those that cause immune activation and those that lead to immune deficiency.

Immune activation from an infection is a common trigger both in patients with a genetic predisposition and in sporadic cases with no underlying genetic cause identified. The most common infectious trigger is a viral infection, especially Epstein-Barr virus (EBV) [2]. Primary EBV infection can trigger HLH in individuals with a defect in perforin-dependent cytotoxicity, as well as in those without a known predisposition; patients with X-linked lymphoproliferative disease (XLP) are at particularly high risk [24]. Many other infectious organisms are also implicated. Kawasaki disease, a common vasculitis of childhood, can also trigger HLH and can often be misdiagnosed initially. The immune checkpoint inhibitors, nivolumab and ipilimumab, may be linked to development of HLH, but the incidence has not yet been defined [25]. (See 'Immunodeficiency syndromes' below and 'Infections' below and "Kawasaki disease: Clinical features and diagnosis".)

Excessive cytokine release in patients with chronic granulomatous disease (CGD) may also lead to HLH. In one institution, 3 of 17 patients with CGD developed HLH [26]. Common causes of immunodeficiency triggers include inherited syndromes, malignancy, rheumatologic disorders, or HIV infection. (See 'Genetics' below and 'Malignancy' below and 'Rheumatologic disorders/MAS' below and 'Immunodeficiency' below.)

The coexistence of immune dysregulation with unchecked inflammation distinguishes HLH from other syndromes of immune activation, immunodeficiencies, and inflammatory states [24].

GENETICS — Genetic defects play a major role in childhood HLH and are increasingly found in adult cases [8,27-31]. Genetic information can be helpful in determining the likelihood of recurrence, the need for hematopoietic cell transplant, 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) [32] (see 'Pathophysiology' above). These genes act in an autosomal recessive fashion (ie, inheritance of a mutation at both alleles of a gene is required to manifest the disease) and many cases are related to consanguinity; however, heterozygosity for an HLH mutation is occasionally found in an individual (typically an adult) with HLH associated with another condition [33]. (See 'Associated illnesses' below.)

In addition to homozygous mutation in a single HLH gene, individuals with HLH may be compound heterozygotes (ie, they may have a different mutation in each allele of the same gene) or they may show digenic inheritance (ie, they may have separate mutations in two different genes). A review of 2701 patients referred for genetic testing revealed that 225 (8 percent) were homozygous or compound heterozygous for mutations, and 28 (1 percent) showed digenic inheritance [31]. Another study reported similar findings, with monoallelic mutations of known familial HLH genes found in 43 of 281 patients classified as having "sporadic" disease, suggesting that this disorder is not a simple recessive one [34].

In a study that used whole exome sequencing, heterozygous variants in LYST, MUNC13-4, and STXBP2 were discovered in 5 of 14 patients with juvenile idiopathic arthritis (JIA) who had macrophage activation syndrome (MAS), but in only 4 of 29 patients with JIA who did not have MAS [35]. Several other recessive pairs and compound heterozygotes were found.

The likelihood of identifying a gene mutation is highest in the youngest patients. In a review of 476 North American children, a gene mutation was identified in 45 percent of those less than one month of age [24]. In those aged between two months to one year, one to two years, and greater than two years, the frequencies of identifying a gene mutation were 39, 20, and 6 percent, respectively. In another study of 175 adults (age range, 18 to 75 years), 14 percent had gene mutations; these tended to cause partial defects in protein function rather than complete loss of the protein; this partial loss of function may explain the later age of HLH onset in some adults [36]. (See 'Features in adults' below.)

In a study of 101 patients who met the HLH-2004 criteria for diagnosis of HLH, only 19 percent had biallelic mutations in the six primary genes associated with HLH [37]. Heterozygous variants in patients with potentially two HLH-associated gene mutations were not statistically different from the general population, suggesting these "digenic" cases were not disease causing. Of 47 patients with none of the expected HLH-associated gene mutations, 28 (58 percent) had potential disease-causing genetic defects. These defects were in genes associated with primary immunodeficiency disease or dysregulated immune activation or proliferation associated genes such as NLRC4 and NLRP12 as well as biallelic variants in NLRP4, NLRC3, and NLRP13.

Mutations at FHL loci — Several HLH gene mutations map to loci that code for elements of the cytotoxic granule formation and release pathway, and have been labeled familial hemophagocytic lymphohistiocytosis (FHL) loci. (See 'Terminology' above.)

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

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

●STX11/Syntaxin 11 – 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].

●STXBP2/Munc18-2 – FHL5 results from mutations of STXBP2, which encodes Munc18-2 (also called syntaxin binding protein 2) [33,43]. This protein binds to syntaxin 11 and promotes the release of cytotoxic granules.

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

●CDC42/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].

The gene defect responsible for FHL1 remains uncharacterized.

Immunodeficiency syndromes — Several mutations that cause congenital immunodeficiency syndromes are also associated with an increased incidence of HLH. These include the following:

●Griscelli syndrome (GS) – GS type 2 is caused by mutations of RAB27A, which encodes a GTP-binding protein [47]. GS2 is characterized by hypopigmentation, immune deficiency, thrombocytopenia, and/or neurologic defects. (See "Syndromic immunodeficiencies", section on 'Griscelli syndrome'.)

●Chediak-Higashi syndrome (CHS) – CHS is caused by mutations of CHS1/LYST, which encodes a lysosomal trafficking regulatory protein [48]. CHS is characterized by partial oculocutaneous albinism, neutrophil defects, neutropenia, and neurologic abnormalities. (See "Chediak-Higashi syndrome".)

●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 [49]. XLP2 is caused by mutations in X-linked inhibitor of apoptosis (XIAP), also called baculoviral IAP-repeat-containing protein 4 (BIRC4); the encoded protein protects cells from apoptosis [50]. 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 – X-linked immunodeficiency with magnesium defect, EBV infection, and neoplasia (XMEN) disease is another immunodeficiency syndrome with EBV-associated malignancies and rarely HLH [51]. 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'.)

●Interleukin-2-inducible T cell kinase (ITK) deficiency – Patients with ITK deficiency, like those with XLP and XMEN deficiencies, are unable to control EBV infections. They have a variety of lymphoproliferative diseases, lymphomatoid granulomatosis, HLH, and dysgammaglobulinemia.

●CD27 (TNFRSF7) deficiency – Missense mutations that reduce expression of CD27 have been associated with a syndrome of severe EBV infections associated with HLH, Hodgkin lymphoma, uveitis, and recurrent infections [52].

●Hermansky-Pudlak syndrome (HPS) – HPS is a rare disorder characterized by oculocutaneous albinism and platelet storage pool deficiency [53,54]. Several responsible gene mutations have been identified: HPS1, AP3B1 (HPS2), HPS3, HPS4, HPS5, HPS6, DTNBP1 (HPS7), BLOC1S3 (HPS8), and BLOC1S6 (PLDN). (See "Hermansky-Pudlak syndrome".)

●Lysinuric protein intolerance – Lysinuric protein intolerance (LPI; MIM 222700) 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 [55].

●Chronic granulomatous disease (CGD) – CGD is a genetically heterogeneous condition associated with recurrent, life-threatening bacterial and fungal infections. (See "Chronic granulomatous disease: Pathogenesis, clinical manifestations, and diagnosis".)

Genotype-phenotype correlations — Patients with HLH gene mutations tend to present at a younger age than those without mutations. The affected gene and specific type and site of mutation may affect the age of presentation and clinical features, but there is controversy regarding the contribution of hypomorphic mutations to development of HLH [36,37]. Informative studies that evaluated genotype-phenotype correlations with HLH include:

●Patients with PRF1 null mutations typically present within the first year of life, whereas those with missense mutations and variable degrees of perforin expression have a more variable age of presentation, even into adulthood [56-63].

●In a series of 76 patients with HLH, those with PRF1 mutations had a significantly higher risk of early disease onset (ie, <6 months) than those with STX11 mutations (adjusted odds ratio 8.2; 95% CI 1.2-56) [64].

●In another study, the most common PRF1 mutation in Black Africans (50delT-PRF1) was found to be associated with an earlier age of disease onset compared with that reported for other PRF1 mutations (median age at diagnosis, three months for 50delT-PRF1 versus 36 months for others) [59,65].

●In a series of patients with digenic inheritance (inheritance of mutations at two separate FHL loci), PRF1 mutation in combination with a mutation affecting degranulation (eg, UNC13D, STX11, STXBP2) predicted disease onset at age two years or greater, whereas two mutations affecting degranulation predicted disease onset at <2 years of age [31].

●Adult patients with hypomorphic mutations of PRF1, MUNC13-4, and STXBP2 often have a more indolent course than younger patients [36].

●Individuals with STXBP2/Munc18-2 mutations (FHL-5) have defective erythropoiesis with aberrant cell morphology and decreased CD235a expression resulting in hemolysis [66].

EPIDEMIOLOGY — HLH is primarily a pediatric syndrome. Infants are most commonly affected, with the highest incidence in those <3 months [67]. The male-to-female ratio is close to 1:1 [67]. In adults, there may be a slight male predisposition [68].

It is estimated that approximately 1 child in 3000 admitted to a tertiary care pediatric hospital will have HLH, which corresponds to several cases per center per year [24]. Earlier reviews reported much lower rates of incidence, likely reflecting underdiagnosis of the condition. 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 [67]. A review of HLH cases from the largest pediatric hospitals in Texas revealed an incidence of 1 in 100,000 children [69].

Although HLH is primarily a pediatric disease, it is diagnosed in patients of all ages, including adults as old as 70 years of age [56,57,70]. A review of 2197 adult cases worldwide found that approximately half of reported patients were from Japan [68]. A nationwide survey in Japan from 2001 to 2005 identified 799 patients with HLH; of the 470 with sufficient data for analysis, 192 (41 percent) were older than 14 years [71]. In addition, there seems to be an ethnic predisposition for 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 [72].

Up to one-quarter of HLH cases are thought to be familial. The frequency of specific HLH mutations was evaluated in a multi-ethnic cohort of 76 patients with familial HLH originating from 65 unrelated families [64]. In this cohort, mutations in STX11, PRF1, and UNC13D were found in 20, 18, and 10 percent of affected individuals, respectively.

A review of 224 North American patients with HLH mutations found the following distribution of specific mutations according to ethnicity [24]:

●White Americans were most likely to have mutations in UNC13D (47 percent), STXBP2 (22 percent), and PRF1 (20 percent)

●Hispanic Americans were most likely to have mutations in PRF1 (71 percent) and UNC13D (17 percent)

●Black Americans were most likely to have mutations in PRF1 (98 percent)

●Arabs were most likely to have mutations in PRF1 (36 percent), UNC13D (27 percent), and STXBP2 (18 percent)

Other studies of specific ethnic groups have found the following distributions:

●Individuals of Turkish origin had a high incidence of mutations in PRF1, UNC13D, or STX11 [73]

●Individuals from Saudi Arabia, the United Arab Emirates, and Turkey had a high incidence of STXBP2 mutations [58,74,75]

●Japanese individuals had a high incidence of PRF1 mutations [76]

Genetic causes of HLH are discussed above. (See 'Genetics' 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 [77]. (See 'Genotype-phenotype correlations' above.)

The HLH-2004 study, which included 369 patients, reported the following clinical findings [78]:

●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 NK cell activity – 71 percent

●Soluble CD25 elevation – 97 percent

In addition to the typical presenting signs and symptoms, some HLH gene mutations 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 [58]. Defective granule mobilization by neutrophils has also been identified in these patients [79]. This leads to inadequate bacterial killing, especially of gram negative bacteria, and is hypothesized to lead to the association of chronic diarrhea in this subset of HLH patients.

Some clinical findings are observed less frequently in affected patients from different ethnic groups. This was illustrated in a case series of 20 neonates from Japan, in which 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) [80].

Laboratory and radiographic abnormalities

Cytopenias — Cytopenias, especially anemia and thrombocytopenia, are seen in >80 percent of patients on presentation [69,78,81,82]. 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 [69].

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 those with juvenile idiopathic arthritis (JIA), because patients with JIA often have elevated blood counts prior to developing MAS.

Serum ferritin levels — A very high serum ferritin level is common in HLH and, especially in children, has high sensitivity and specificity. In the HLH-94 study, ferritin levels greater than 500, 5000, and 10,000 ng/mL were seen in 93, 42, and 25 percent, respectively; the median ferritin level was 2950 ng/mL [81]. 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 iron overload syndromes (eg, in multiply transfused patients). 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 [83]. When the control group was re-analyzed 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 [84]. There was no difference when primary and secondary HLH cases were analyzed separately.

In adults and neonates, other potential causes of extremely high ferritin levels should also be evaluated. As an example, ferritin levels over 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 [85]. (See 'Other diagnostic considerations' below and 'Differential diagnosis' below.)

While a very high ferritin level is helpful in suggesting the possibility of HLH, a low ferritin (eg, ferritin <500 ng/mL) does not exclude the possibility of HLH. 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.

Macrophages are a primary source of ferritin, which may account for the association between HLH and very high ferritin levels [86]. 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 [87].

Liver function and coagulation abnormalities — Nearly all patients with HLH will have hepatitis, manifested by elevated liver function tests (LFTs), including liver enzymes (AST, ALT, GGT), 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 frequently seen. The degree of abnormality ranges from mild to hepatic failure; hydrops fetalis has been reported in neonates [88].

●Liver enzyme levels greater than three times the upper limit have been reported in 50 to 90 percent of patients with HLH [69,82,88]; LDH is elevated in 85 percent [82]. Bilirubin levels between 3 and 25 mg/dL are seen in greater than 80 percent. The GGT level is an especially sensitive number to follow because of biliary tract infiltration by lymphocytes and macrophages [24].

●Hypertriglyceridemia can be due to severe liver involvement, but may not be elevated until the liver has been affected for some time. In a review of patients with HLH associated with a variety of triggers, 68 percent had elevated triglycerides at diagnosis or during the course of the disease [89].

●Coagulation abnormalities due to impaired hepatic synthetic function and/or disseminated intravascular coagulation are common [90].

●Liver biopsy, if done, is likely to show lymphocytic infiltrates in patients with HLH. On autopsy, the livers of patients who have died from HLH show chronic persistent hepatitis with periportal lymphocytic infiltration [91].

Neurologic findings — Neurologic abnormalities have been observed in one-third of patients with HLH, are highly variable, and may include seizures, mental status changes (including severe changes consistent with encephalitis), and ataxia [2,24,92]. These findings may dominate the clinical picture or develop prior to the appearance of other signs and symptoms [93,94]. 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 [95,96].

Patients with HLH are at risk of developing posterior reversible encephalopathy syndrome (PRES), which presents with headache, altered consciousness, visual disturbances, and/or seizures. On examination, patients may have retinal hemorrhages and optic nerve edema. PRES is associated with characteristic findings on brain magnetic resonance imaging (MRI), including vasogenic cerebral edema predominantly in the posterior cerebral hemispheres. (See "Reversible posterior leukoencephalopathy syndrome".)

MRI of the brain in patients with HLH also may show hypodense or necrotic areas [97]. Approximately 50 percent of patients have abnormalities of the cerebrospinal fluid, which may carry an increased risk for mortality and neurologic sequelae [98]. In a series of 10 adults with HLH, seven had neurological impairment, which included encephalopathy and seizures. Basal ganglia abnormalities were found in four patients [99]. (See 'Initial evaluation' below and "Treatment and prognosis of hemophagocytic lymphohistiocytosis", section on 'Prognosis'.)

Other findings — 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. Deteriorating respiratory function may be due to worsening of the HLH (causing an acute respiratory distress syndrome [ARDS]-like syndrome), or due to an infection. Pulmonary involvement was reported in 42 percent of a series of 775 adults with HLH [68].

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

●Renal dysfunction occurs in many patients and may present with hyponatremia, perhaps caused by a syndrome of inappropriate ADH (SIADH) mechanism. Many patients develop renal failure and require dialysis. Renal involvement was reported in 16 percent of a series of 775 adults with HLH [68].

●Skin manifestations can be quite varied. These include generalized rashes, erythroderma, edema, petechiae, and purpura. Skin rash was reported in one-quarter of a series of 775 adults with HLH [68].

●Bleeding is also a common manifestation of HLH. It 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 'Genetics' above.)

●Patients with underlying immunodeficiency syndromes may also have syndrome-specific findings (eg, albinism). (See 'Immunodeficiency syndromes' above.)

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

Associated illnesses — Infection, malignancy, rheumatologic, and immunodeficiency syndromes are common in patients with HLH, especially adults. (See 'Features in adults' below.)

It is important to identify these conditions because effective treatment may lead to clinical improvement of the HLH and allow the patient to avoid more toxic therapy (eg, hematopoietic cell transplant). However, evaluation for these associated syndromes should not delay diagnostic testing or initiation of HLH-specific treatment in those who are acutely ill.

Infections — 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 [68,100-108]. An HLH-like syndrome has been reported in association with SARS-CoV-2 (the novel coronavirus that causes COVID-19) [109,110]. The development of HLH shortly after the initiation of antiretroviral therapy (ART) for the treatment of HIV infection has also been reported [111]. Patients with rheumatologic diseases who are treated with anti-TNF agents and develop HLH may be infected with mycobacterium tuberculosis, CMV, EBV, Histoplasma capsulatum, and other bacteria [112].

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 [68,108,113,114].

Malignancy — HLH has been reported in association with malignancies, most commonly lymphoid cancers (including B, T, and NK cell) and leukemias, but also solid tumors [24,108,115-127]. Rarely, the diagnosis of HLH may precede the identification of the malignancy [128]. A review of malignancy-associated HLH from a single institution revealed an 18 percent incidence of HLH in patients with acute myeloid leukemia and a 4 percent incidence in patients with acute lymphocytic leukemia [129].

Many patients with a malignancy who develop secondary HLH seem to have an acute infectious trigger. A retrospective review of 22 patients with hematologic malignancies and secondary HLH at one center 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 [129]. When it is associated with a malignancy, HLH is often more immediately life-threatening than the malignancy itself. (See "Causes of anemia in patients with cancer" and "Clinical manifestations, pathologic features, and diagnosis of subcutaneous panniculitis-like T cell lymphoma" and "Clinical manifestations, pathologic features, and diagnosis of extranodal NK/T cell lymphoma, nasal type".)

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'.)

Rheumatologic disorders/MAS — HLH can occur in the setting of rheumatologic disorders. The most common association is in children with systemic juvenile idiopathic arthritis (sJIA, formerly called Still's disease, systemic onset JIA, or systemic onset juvenile rheumatoid arthritis). The term macrophage activation syndrome (MAS) is used when a hemophagocytic syndrome develops in children with JIA and other rheumatologic conditions. MAS should be thought of as HLH in the setting of a rheumatologic disorder rather than as a separate syndrome. Performance guidelines for the diagnosis of MAS have been published [130]. (See "Systemic juvenile idiopathic arthritis: Course, prognosis, and complications", section on 'Macrophage activation syndrome' and "Kawasaki disease: Complications".)

HLH may develop any time during the course of a rheumatologic disorder (eg, on presentation, during therapy, in association with a concurrent infection). In patients with sJIA treated with tocilizumab, 23 of 394 developed confirmed or probable MAS [131]. When MAS occurs as a presenting manifestation of lupus and systemic juvenile or adult rheumatoid arthritis, the diagnosis of both conditions may be challenging. Other autoimmune diseases associated with HLH include dermatomyositis, systemic sclerosis, mixed connective tissue disease, antiphospholipid syndrome, Sjögren's disease, ankylosing spondylitis, vasculitis, and sarcoidosis [90]. Some patients with MAS have also been found to have heterozygosity for mutations in HLH genes (eg, PRF1, UNC13D) [35]. (See 'Genetics' above.)

Immunodeficiency — HLH has been found in patients with inherited immunodeficiency disorders, including those due to mutations that are associated with HLH as well as others [26,47-49,132-135]. (See 'Immunodeficiency syndromes' above.)

Acquired immunodeficiencies have also been associated with HLH, including HIV/AIDS, hematopoietic cell transplantation, or kidney or liver transplant [108,136,137]. Sometimes HLH occurs in the setting of a concurrent infection or a lymphoproliferative syndrome [138-140]. In one small series, the development of HLH in kidney transplant patients appeared to be associated with the combination of splenectomy and the administration of anti-thymocyte globulin [141].

Features in adults — HLH presenting in adulthood is increasingly recognized [68,142-146]. Adults can have similar clinical features of HLH as children. As an example, a series of 775 adults with HLH reported similar predominance of fever (96 percent), splenomegaly (69 percent), and hepatomegaly (67 percent) [68].

However, emerging diagnostic criteria for adults with HLH indicate several differences from those used in pediatric patients. A Delphi analysis (a method for finding consensus using iterative anonymous questionnaires) from an expert panel determined the following clinical features to be important in adults [147]:

●Underlying predisposing disease

●Fever

●Organomegaly

●Cytopenias

●Elevated ferritin

●Elevated LDH

●Hemophagocytosis on the bone marrow aspirate

A 2015 review noted that 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 [142].

The frequency of underlying conditions such as hematologic malignancies, infections, and rheumatologic conditions in adults with HLH have been illustrated in various case series:

●In a series of 162 adults with HLH, hematologic malignancies (especially non-Hodgkin lymphoma) were the most common trigger, seen in 92 (57 percent) [148]. An additional 40 patients (25 percent) had infections, which were caused by bacterial, viral, parasitic, or fungal organisms; six (4 percent) had both hematologic malignancy and infection. Additional analysis of this cohort identified a source of immunosuppression in 73 (45 percent) [149]. For 61, the source of immunosuppression was HIV infection; for the remainder, it was an immunosuppressive medication.

●Several single-institution studies focusing on HLH in adults have documented infection in 23 to 41 percent of patients and rheumatologic/autoimmune disease in 8 to 20 percent [150-152]. In a series of 30 patients who had testing for EBV DNA, 10 were found to be positive [153]. Adults with EBV-associated HLH have higher ferritin, LDH, AST, and ALT levels than those without EBV infection [154].

●Reports of coexisting autoimmune and rheumatologic diseases in adults include systemic lupus erythematosus, rheumatoid arthritis, Still's disease, polyarteritis nodosa, mixed connective tissue disease, pulmonary sarcoidosis, systemic sclerosis, and Sjögren's disease [90,108,141,155-160].

The reduced specificity of an 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) [161]. Of these, only 19 (17 percent) were ultimately diagnosed with HLH; 9 of the 19 had secondary HLH due to a malignancy and 6 of the 19 had secondary HLH due to infection. More common diagnoses than HLH included renal failure (65 percent), hepatocellular injury (54 percent), infection (46 percent), and hematologic malignancy (32 percent). Other diagnoses associated with extremely high ferritin levels in adults are discussed separately. (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 'Genetics' above.)

EVALUATION AND DIAGNOSTIC TESTING

Initial evaluation — 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 the possibility of HLH is raised. A priority should be placed on rapid evaluation for organ involvement including testing for signs of bone marrow insufficiency, liver abnormalities, neurologic involvement, and 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 [24].

Patients with suspected HLH (or their families) 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 'Genetics' above and 'Associated illnesses' above.)

The physical examination should focus on identifying rashes, bleeding, lymphadenopathy, hepatosplenomegaly, and neurologic abnormalities. A thorough examination for signs of other organ involvement (eg, cardiac, respiratory) is also necessary.

Many of the initial tests that are helpful in evaluating HLH will have already been done as part of the evaluation of an unexplained febrile illness that involves multiple organs. Others, including serum ferritin, triglycerides, and screening immunologic studies, should be done immediately.

We suggest the following tests in all patients:

●Complete blood count with differential

●Coagulation studies, including PT, aPTT, fibrinogen, D-dimer

●Liver function tests, including ALT, AST, GGT, total bilirubin, albumin, and lactate dehydrogenase (LDH)

●Serum triglycerides (fasting)

●Serum ferritin

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

Identifying signs of infection and specific organ injury is helpful in making the diagnosis of HLH, as well as for management of organ-specific complications. Based on the symptoms and signs of specific organ involvement and/or the degree of suspicion for the presence of HLH, we perform the following studies in all patients:

●Cultures of blood, bone marrow, urine, cerebrospinal fluid, and other potentially infected body fluids; and 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 anti-viral therapy.

●Bone marrow evaluation. (See 'Bone marrow evaluation' below.)

●Electrocardiograph, chest radiography, and an echocardiogram.

●Lumbar puncture with cerebrospinal fluid (CSF) analysis should be performed for all patients, including cultures and testing for viruses (eg, by PCR), as indicated by clinical findings and epidemiology. CSF is abnormal in over half of patients with HLH, with findings of cellular pleocytosis, rarely hemophagocytosis, and elevated protein. (See "Viral encephalitis in adults" and "Acute viral encephalitis in children: Clinical manifestations and diagnosis".)

●Brain magnetic resonance imaging (MRI) scan, with and without contrast (unless contrast is contraindicated). Imaging of the central nervous system may show parameningeal infiltrations, subdural effusions, necrosis, and other abnormalities.

●Computed tomography (CT) scans of 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.

We do a rapid immunologic evaluation in those with a high clinical suspicion of HLH. (See 'Immunologic profile' below.)

Bone marrow evaluation — All patients should have a bone marrow aspirate and biopsy to evaluate the cause of cytopenias and/or detect hemophagocytosis. Bone marrow specimens should also be cultured, and examined for infectious organisms and evidence of malignancy. Bone marrow cellularity can be high, low, or normal in HLH [24]. Hemophagocytosis on bone marrow examination is reported in 25 to 100 percent of cases of HLH [148].

Hemophagocytosis is not pathognomonic for HLH. A review of 78 bone marrow aspirates that showed hemophagocytosis included 40 that were associated with 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 [162]. Some patients may only show hemophagocytosis later in the disease course, even as they are clinically improving [24]. A review of adult patients exhibiting hemophagocytosis in bone marrow aspirates revealed that 170 (64 percent) had lymphoma, especially T/NK and B cell lymphoma. Of 182 patients with sufficient clinical data to judge HLH-2004 diagnostic criteria for HLH, only 77 (29 percent) fulfilled 5 of 8 criteria (see 'Diagnostic criteria' below). Of those who had a malignancy, survival was a median of 9 months, versus 71.8 months in those with non-malignant disorders [163].

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 (both hemophagocytosing and not). (See 'Diagnosis' below.)

Specialized testing

Immunologic profile — Immunologic and cytokine studies are appropriate for those suspected of having HLH based on the results of the initial evaluation. (See 'Initial evaluation' above.)

We typically perform the following immunologic testing:

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

●Tests of NK cell function/degranulation (eg, by flow cytometry for surface expression of CD107alpha, also called LAMP-1 [lysosomal-associated membrane protein 1])

●Flow cytometry for cell surface expression of perforin and granzyme B proteins

●Flow cytometry for cell surface expression of SAP and XIAP proteins in males

●Soluble levels of the hemoglobin-haptoglobin scavenger receptor (sCD163)

●Immunoassay for serum CXCL9

●Immunoglobulin levels (eg, IgG, IgA, IgM)

●Lymphocyte subsets (underlying immune deficiency diseases are sometimes found)

The first six are only available in specialized centers [164].

Findings consistent with HLH include elevated sIL-2R; reduced NK function or cell surface expression of CD107alpha; elevated sCD163; and reduced perforin, SAP, or XIAP [5,165-170]. Immunoglobulin levels are variable [5]. Peripheral blood lymphocyte subsets generally show normal T cell numbers and helper/suppressor ratio, and may show decreased numbers of B cells or NK cells [5,171]. Elevation of granzyme B has been found and is thought to be part of the immune signature of lymphocyte activation [172].

Of all the immunologic studies, we find sIL-2R to correlate most closely with disease activity [24]. The ratio of sIL-2R to serum ferritin may be useful in patients with lymphoma. A review of patients with lymphoma-associated HLH versus non-lymphoma-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) [173].

Levels of the sIL-2R will be available in one to two days, while the other tests take longer. Thus, therapy should not be delayed while awaiting results of this immunologic testing.

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 [174].

HLH associated with lymphoma can be challenging to differentiate from clinical presentations of sepsis. A study in 15 adults with lymphoma-associated HLH showed potential for the use of assays of the cytokines CXCL9 and CXCL10; elevated levels had a high sensitivity and specificity for lymphoma-associated HLH compared to sepsis [175].

Genetic 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.

For patients whose relatives have a known familial syndrome, selective genetic testing may be used to confirm the genetic disorder.

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 [37]. These approaches offer efficiency of testing, the possibility of identifying biallelic or hypomorphic mutations, and/or the possibility of identifying novel HLH-associated gene defects [37]. Moreover, if allogeneic hematopoietic cell transplant (HCT) is being considered, the patient and any matched related donor should have expedited whole exome sequencing performed to mitigate the risk of an unsuccessful transplant due to transplantation with a similar genetic defect.

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

●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 (http://www.ncbi.nlm.nih.gov/gtr/tests/). (See "Next-generation DNA sequencing (NGS): Principles and clinical applications", section on 'Whole genome, exome, or gene panel'.)

HLA testing — Human leukocyte antigen (HLA) 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 natural history of untreated HLH syndrome is almost uniformly fatal. A high degree of suspicion is required because of the clinical complexity, rarity, and diversity of causes. Prompt recognition of the HLH syndrome and diagnosis of the underlying cause of HLH disease is critical to enable urgent and appropriate treatment.

Diagnostic criteria — The diagnosis of HLH syndrome is based on a compatible clinical presentation in the setting of elevated inflammatory markers (eg, ferritin, sCD25, and/or CXCL9). (See 'Clinical features' above.)

If neither sCD25 nor CXCL9 is elevated, the diagnosis of HLH is unlikely and other disorders in the differential diagnosis should be strongly considered. (See 'Differential diagnosis' below.)

Ideally, the diagnosis of HLH is based on fulfilling the published diagnostic criteria used in the HLH-2004 trial [24]. Not infrequently, however, a diagnosis of HLH is made in the patient who only partly meets the most stringent criteria because definitive HLH therapy must be initiated due to an inadequate response to general supportive care. A presumptive diagnosis depends on a careful consideration of the presence or absence of the specific elements of the diagnostic criteria, the results of additional laboratory tests (eg, D-dimer and liver function tests), and a nuanced view of the overall clinical status.

We recommend that the diagnosis of HLH be based on the following criteria, which were used in the HLH-2004 trial [24,78]:

In children, homozygosity or compound heterozygosity for verified HLH-associated mutations (eg, PRF1, UNC13D, STX11, STXBP2, Rab27A, SH2D1A, BIRC4, LYST, ITK, SLC7A7, XMEN, HPS) or gene defects of other immune regulatory genes (identified by whole exome sequencing [WES]), described above. (See 'Terminology' above.)

In adults, heterozygosity of one of the above genes together with clinical findings associated with HLH [36,37].

OR

Five of the following nine findings:

●Fever ≥38.5°C

●Splenomegaly

●Peripheral blood cytopenia, with at least two of the following: hemoglobin <9 g/dL (for infants <4 weeks, hemoglobin <10 g/dL); platelets <100,000/microL; absolute neutrophil count <1000/microL

●Hypertriglyceridemia (fasting triglycerides >265 mg/dL) and/or hypofibrinogenemia (fibrinogen <150 mg/dL)

●Hemophagocytosis in bone marrow, spleen, lymph node, or liver

●Low or absent NK cell activity

●Ferritin >500 ng/mL (the authors prefer to consider a ferritin >3000 ng/mL as more indicative of HLH [83])

●Elevated soluble CD25 (soluble IL-2 receptor alpha [sIL-2R]) two standard deviations above age-adjusted laboratory-specific norms

●Elevated CXCL9 [176]

It should be noted that these diagnostic criteria were devised for use in clinical trials and are therefore unlikely to capture every case of HLH. Because of the high mortality of HLH in the absence of appropriate treatment, we do not always require these diagnostic criteria to be met in order to initiate treatment. Patients may be too critically ill to undergo biopsy. Specifically, we do not delay treatment while awaiting the results of genetic or specialized immunologic testing.

Diagnostic criteria are essentially the same in adults, with the caveat that adults are more likely to have a secondary form of HLH than children, and adults with secondary HLH are more likely to have an underlying malignancy as the cause.

We consider flow cytometry for reduced/absent NK cell perforin and/or CD107alpha a satisfactory alternative to the NK cytotoxicity assay.

We consider the following modified criteria sufficient to diagnose HLH: three of four clinical findings (fever, splenomegaly, cytopenias, hepatitis) plus abnormality of one of four immune markers (hemophagocytosis, increased ferritin, hypofibrinogenemia, absent or very decreased NK cell function) [24,177]. These criteria are useful because it is common for a patient with HLH to exhibit only three or four of the eight diagnostic criteria, but also have central nervous system (CNS) symptoms, hypotension, and renal or respiratory failure.

Examples of others we would be likely to treat include the following:

●A patient with CNS symptoms, cytopenias, fever, and

ferritin over 3000 ng/mL or rapidly rising ferritin or elevated sCD25

●A patient with CNS symptoms, hepatitis, coagulopathy, and

ferritin over 3000 ng/mL or rapidly rising ferritin or elevated sCD25

●A patient with hypotension, fever, no response to broad spectrum antibiotics, and

ferritin over 3000 ng/mL or rapidly rising ferritin or elevated sCD25

In contrast, we would not give HLH-specific therapy to a patient with fever, hepatitis, hypofibrinogenemia, and cytopenias, with a ferritin <3000 ng/mL and sCD25 only slightly above the age-related norm, because of the possibility that this could represent bacterial or viral infections. HLH patients rarely have prominent lymphadenopathy; patients with prominent adenopathy and HLH-like findings should be evaluated for lymphoma, Castleman disease, and various infections. Leukocytosis also suggests that there should be a more thorough evaluation for a diagnosis other than HLH. Infants with a low or slightly elevated sCD25 and high ferritin may have an underlying immune deficiency with an infection [1].

Other diagnostic considerations — Although hemophagocytosis and a very high serum ferritin are quite helpful in the diagnosis of HLH (see 'Serum ferritin levels' above), the following caveats are important to keep in mind:

●Hemophagocytosis is neither pathognomonic of, nor required for, the diagnosis of HLH. For patients with multiorgan failure and an immunologic profile typical of HLH who are acutely ill, serial bone marrow evaluations for hemophagocytosis can be conducted concurrently with initiation of treatment.

●Results from the HLH-94 study indicated that a ferritin level >500 ng/mL was only 80 percent specific for the diagnosis of HLH.

•Based on our experience, in children we consider serum ferritin levels >2000 to 3000 ng/mL in the proper clinical setting as concerning for HLH, and ferritin >10,000 ng/mL as highly suggestive of the disease. Support for our approach comes from a retrospective review of all patients admitted to Texas Children's Hospital, in Houston, Texas, with ferritin levels >500 mcg/L over a two-year period [83]. In this cohort, a ferritin level >500 mcg/L was 100 percent sensitive for HLH, but less specific. A ferritin level >10,000 mcg/L in children was 90 percent sensitive and 96 percent specific for HLH, with very minimal overlap with sepsis, infections, and liver failure. (See 'Serum ferritin levels' above.)

•In adults, we rely less heavily on an isolated serum ferritin elevation, because serum ferritin is less specific for HLH in adults. (See 'Features in adults' above.)

A scoring system has been developed to generate a diagnostic score referred to as an "Hscore" that estimates the probability of HLH [178]; this 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 Hscore ≥250 confers a 99 percent probability of HLH, whereas a score of ≤90 confers a <1 percent probability of HLH.

DIFFERENTIAL DIAGNOSIS — HLH may simulate a number of common conditions that cause fever, pancytopenia, hepatic abnormalities, or neurologic findings. We find cytopenias, a very high ferritin level, and liver function abnormalities to 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. (See 'Cytopenias' above and 'Serum ferritin levels' above and 'Liver function and coagulation abnormalities' above.)

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

●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/MAS' 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 sepsis is typically not characterized by ongoing lymphocyte activation. While there is no ideal test to distinguish between sepsis and HLH, an extremely high ferritin and elevated lactate dehydrogenase level were highly predictive of a subsequent diagnosis of HLH in a series of 19 children with an initial diagnosis of fever of unknown origin [82]. 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 a coagulopathy with prolonged PT and 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 [179]. 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 and diagnostic testing' 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-mediated apoptosis, which leads to 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 liver function test abnormalities. DRESS can also be associated with hemophagocytosis, although this is rare [180]. Unlike HLH, DRESS is characterized by temporal relationship to a drug, eosinophilia and skin rash. DRESS is unlikely to cause an 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'.)

●Child abuse – Child abuse and HLH may present with similar features involving the central nervous system [181,182]. The majority of child abuse victims with brain injury also have some laboratory abnormalities such as a prolonged aPTT [183]. However, cytopenias, abnormal LFTs, and high serum ferritin typical of HLH are not features of child abuse. (See "Differential diagnosis of suspected child physical abuse".)

●Kawasaki disease – Kawasaki disease (KD), a vasculitis that predominantly affects children, is characterized by widespread inflammation that include 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) [184]. 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 [185,186]. 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 non-irradiated 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

●Definition – Hemophagocytic lymphohistiocytosis (HLH) is a potentially life-threatening syndrome of excessive immune activation that 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 adequately 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 'Genetics' above and 'Associated illnesses' above.)

●Presentation – Most patients are acutely ill with multiorgan involvement; 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 should include (see 'Evaluation and diagnostic testing' above):

•Complete blood count

•Coagulation studies

•Liver function tests, ferritin, triglycerides

•Blood cultures and viral testing, as appropriate

•Bone marrow examination

•Lumbar puncture

•Magnetic resonance imaging (MRI) of the brain

•Other imaging (eg, positron emission tomography [PET]/computed tomography [CT]), if an underlying malignancy is suspected

●Specialized testing – Additional testing, including immunologic parameters and genetic testing should be performed in selected cases (see 'Specialized testing' above):

•Soluble IL-2 receptor alpha (sCD25) and soluble CD163

•IgG, IgA, IgM levels

•Lymphocyte subsets

•Flow cytometry:

-NK cell function/degranulation (eg, surface expression of CD107alpha/LAMP-1)

-Expression of perforin, granzyme B, and SAP and XIAP in males

•Serum CXCL9 (by immunoassay)

●Diagnosis – The diagnosis of HLH is based upon a compatible clinical presentation in the setting of inflammatory/immunologic markers (see 'Diagnosis' above), as follows:

•Molecular diagnosis – Mutation of an HLH-associated gene (eg, affecting cytolytic function of T cells and/or NK cells or defects in inflammasome regulation) (See 'Mutations at FHL loci' above.)

or

•HLH-2004 diagnostic criteria – ≥5 of the following, in conjunction with clinical judgment and the patient’s history (see 'Diagnostic criteria' above):

-Fever ≥38.5°C

-Splenomegaly

-Cytopenias in ≥2 blood lineages

-Hypertriglyceridemia and/or hypofibrinogenemia

-Hemophagocytosis in bone marrow, spleen, lymph node, or liver

-Ferritin >500 ng/mL (typically considerably higher)

-Low or absent NK cell activity

-Elevated sCD25

-Elevated CXCL9  

●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.)

●Treatment – Management of HLH is discussed separately. (See "Treatment and prognosis of hemophagocytic lymphohistiocytosis".)

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