Southeast Asian ovalocytosis (SAO)

INTRODUCTION — Southeast Asian ovalocytosis (SAO; also called stomatocytic elliptocytosis) is an autosomal dominant hereditary hemolytic anemia caused by a specific deletion in the gene for band 3 and characterized by mild hemolytic anemia that typically resolves in childhood. Band 3 encodes an integral membrane protein of red blood cells (RBCs) that also functions as an ion channel; the deletion interferes with channel function, causing RBCs to become dehydrated, stiff, and oval shaped.

This topic discusses the pathophysiology, diagnosis, and management of SAO.

Separate topics discuss general approaches to the evaluation and other disorders of RBC membrane/cytoskeleton and RBC hydration:

●Hemolytic anemias in children – (See "Overview of hemolytic anemias in children".)

●Review of the blood smear – (See "Evaluation of the peripheral blood smear".)

●Hereditary spherocytosis – (See "Hereditary spherocytosis".)

●Hereditary elliptocytosis – (See "Hereditary elliptocytosis and related disorders".)

●Hereditary stomatocytosis and xerocytosis – (See "Hereditary stomatocytosis (HSt) and hereditary xerocytosis (HX)".)

●Acquired RBC membrane disorders – (See "Burr cells, acanthocytes, and target cells: Disorders of red blood cell membrane".)

PATHOPHYSIOLOGY

Specific SLC4A1 deletion — The SLC4A1 gene encodes band 3 (also known as anion exchanger 1 [AE1], band 3 anion transport protein, and solute carrier family 4 member 1), an integral membrane protein that functions as a chloride-bicarbonate exchanger and provides structural cohesion between the red blood cell (RBC) membrane and the underlying spectrin-based cytoskeleton. (See "Red blood cell membrane: Structure and dynamics", section on 'Band 3'.)

The connection between SAO and an abnormality of band 3 was discovered in 1990 [1]. It was subsequently determined that individuals with SAO are heterozygous for a specific 27 base pair deletion in the SLC4A1 gene, referred to as deletion 400-408 (Δ400-408). Homozygosity for the deletion was initially thought to be incompatible with life; however, some individuals with homozygosity for the deletion have been identified [2,3].

This deletion removes nine amino acids corresponding to codons 400 to 408 of the band 3 protein that span the region between the cytoplasmic and membrane domain. Removal of these nine amino acids causes abnormal folding of band 3 at the junction of the N-terminal cytoplasmic domain with the transmembrane domain and inactivates its anion exchanger function [4,5]. It also leads to a tighter attachment between band 3 and ankyrin, with reduced lateral mobility that likely underlies the marked rigidity (stiffness) of SAO RBCs [6]. (See 'Characteristics of RBCs' below.)

Other SLC4A1 variants cause distinct disorders that are discussed separately.

●Other hereditary hemolytic anemias:

•Hereditary spherocytosis – (See "Hereditary spherocytosis", section on 'Band 3 deficiency due to SLC4A1 variants'.)

•Disorders of RBC hydration – (See "Hereditary stomatocytosis (HSt) and hereditary xerocytosis (HX)", section on 'SCL4A1 (band 3 gene)'.)

●Renal tubular acidosis – (See "Etiology and diagnosis of distal (type 1) and proximal (type 2) renal tubular acidosis", section on 'Distal (type 1) RTA'.)

The altered band 3 channel leads to dehydration and a remarkable "stiffness" of the RBC cytoskeleton.

Characteristics of RBCs — SAO RBCs have distinct characteristics:

●Oval shape – The distinct oval shape (picture 1) originally led to SAO being classified as a form of hereditary elliptocytosis [1]. Many SAO cells have one or two mouth-shaped regions, leading to descriptions of stomatocytosis. (See 'Blood smear' below.)

●Alteration of intracellular cation content – The SAO deletion affects band 3 at the interface between the cytoplasmic N-terminal domain and the transport-active transmembrane domain [7]. This genetic alteration completely negates the anion exchange function of band 3 and results in significant structural alterations. Surprisingly, the mutant protein is successfully expressed at the plasma membrane but is misfolded and cannot transport anions [7].

The precise mechanism of RBC dehydration remains unclear. RBCs from patients with SAO were shown to have abnormal intracellular sodium and potassium content that became more markedly abnormal after storage of blood at 0°C. Increased cation "leak" fluxes at 37°C and increased sodium-potassium (Na=K ATPase) pump activity have also been also reported. (See "Hereditary stomatocytosis (HSt) and hereditary xerocytosis (HX)", section on 'Control of RBC solute and water content'.)

●Rigidity – In vitro studies have revealed that the abnormal SAO band 3 protein can interact with and modify the normal band 3 protein expressed in the same cell [1,8]. In normal RBCs, band 3 typically exists as dimers and tetramers within the RBC membrane. In contrast, SAO band 3 is thought to form higher-order oligomers that associate with the underlying RBC cytoskeleton more readily than the wild-type band 3. This phenomenon may account for the observed increase in membrane rigidity in these cells.

Cellular dehydration and membrane rigidity can cause hemolysis and hemolytic anemia; approximately one-half of neonates with SAO have transient neonatal hemolysis [9,10]. These findings are usually absent after three years of age. (See 'Typical presentation' below.)

Intriguingly, the SAO band 3 deletion leads to a cytokinesis malfunction that results in dyserythropoiesis [11]. The more plausible rationale for dyserythropoiesis is that the substantial production of misfolded band 3 proteins overburdens the RBC's internal trafficking system, alters membrane protein composition, allows oxidative damage to accumulate, and impairs reticulocyte maturation. It is possible that adults with SAO could experience hemolysis when presenting with other conditions that stress the bone marrow and RBC production.

It is not known why hemolysis resolves and morphologic abnormalities persist.

PREVALENCE — SAO is a rare disorder worldwide, but the prevalence is very high in certain regions of the world:

●Papua New Guinea – In the Melanesian population of Papua New Guinea, as many as 12 to 30 percent of individuals may be affected [12]

●Malay Peninsula and Borneo

●Thailand – In one study of 297 newborns in south Thailand, 5.1 percent had SAO [13]

●Indonesia

●Philippines

●Brunei

●Cambodia

●Torres Strait Islands of Australia

●Taiwan

●Madagascar and regions of South Africa [14,15]

With migration, SAO is expected to become more common in other countries [16].

Some epidemiologic studies suggested SAO is protective against several malaria species and may be specifically protective against cerebral malaria, suggesting the likely possibility of malaria-driven selection pressure [17]. However, a 2023 systematic review and meta-analysis did not identify lower rates of malaria in individuals with SAO [18]. (See "Protection against malaria by variants in red blood cell (RBC) genes", section on 'Southeast Asian ovalocytosis (SAO)'.)

CLINICAL FEATURES

Typical presentation — There are three main settings in which SAO comes to medical attention:

●Positive family history – Because SAO is an autosomal dominant trait, some individuals may be evaluated based on a known diagnosis in one of their parents, siblings, or other relatives.

●Neonatal hemolysis – Infants with SAO may present with hyperbilirubinemia. In one study involving 54 infants with documented SAO band 3 deletion, hyperbilirubinemia at birth was present in 28 (52 percent; severe in three [0.6 percent]) and neonatal jaundice was seen in 51 (94 percent) [9]. During the first week of life, the SAO cohort had significantly lower mean hemoglobin concentration, RBC count, and MCV (mean corpuscular volume) than controls; they had significantly higher mean absolute reticulocyte count, MCH (mean corpuscular hemoglobin), and RDW (red cell distribution width).

●Incidental finding – In older children or adults, typically without anemia, SAO may present as an incidental finding on the blood smear, often after careful attention to the RBC indices or a flag for abnormal RBC morphology [16,19,20].

Typical course — In a prospective natural history study of 31 neonates with SAO, the following findings were observed [10]:

●Pallor – 78 percent

●Jaundice – 68 percent

Hemolytic anemia was typical, with reticulocytosis; the nadir hemoglobin occurred at approximately two months of age and increased to a stable value by approximately 24 months [10]. During three years of follow-up, there were no episodes of severe anemia or hemolysis. (See 'Typical presentation' above.)

Growth and development in the cohort were normal, and no other abnormalities arose [10].

Distal RTA — Individuals with SAO may have a distal renal tubular acidosis (distal RTA). It appears that these individuals are compound heterozygous for the SAO deletion and another variant in the SLC4A1 gene; in a series of 32 patients with SAO from 27 families, the most common variant on the other allele was a G701D, deletion 850 (Δ850), or A858D [21]. Several of the individuals were teenagers, suggesting that the RTA does not resolve with age as the hemolysis does.

When these 32 patients were initially assessed, the most striking characteristic was their inability to thrive, typically resulting in body weights falling below the third percentile [21]. Rickets afflicted 74 percent of these individuals, frequently causing significant skeletal distortions. Medullary nephrocalcinosis was identified in 80 percent of the cases where it was investigated, although it may have been underreported due to challenges in detection. Patients had typical blood and urine chemistries associated with distal RTA, with hyperchloremic acidosis (bicarbonate, 12 mEq/L [reference range, 23 to 28 mEq/L]; chloride 115 mmol/L [reference range, 98 to 106 mmol/L]) and increased urinary pH (>5.5, often >6.5, despite systemic acidosis). Most were hypokalemic (average serum potassium, 2.8 mmol/L).

Details of pathophysiology, evaluation, and management are discussed separately.

●Pathophysiology and evaluation – (See "Overview and pathophysiology of renal tubular acidosis and the effect on potassium balance", section on 'Distal (type 1) RTA' and "Etiology and clinical manifestations of renal tubular acidosis in infants and children", section on 'Distal (type 1) renal tubular acidosis' and "Etiology and diagnosis of distal (type 1) and proximal (type 2) renal tubular acidosis", section on 'Distal (type 1) RTA'.)

●Management – (See "Treatment of distal (type 1) and proximal (type 2) renal tubular acidosis", section on 'Distal (type 1) renal tubular acidosis'.)

EVALUATION

●Neonate or infant – The evaluation typically starts with a finding of hemolysis or hemolytic anemia in a neonate or infant. Testing for hemolysis will reveal non-immune hemolysis (negative Coombs test). Examination of the peripheral blood smear will reveal ovalocytes (≥25 percent), stomatocytes, and/or theta cells (RBCs with two slits). (See 'CBC findings and hemolysis testing' below.)

These findings are followed by ektacytometry and genetic testing. (See 'Ektacytometry and EMA binding' below and 'Genetic testing' below.)

●Older child or adult – Often when ovalocytes are seen on the blood smear in an older child or adult, the first test is ektacytometry, followed by genetic testing for the SLC4A1 gene deletion. (See 'Ektacytometry and EMA binding' below and 'Genetic testing' below.)

Family history — A family history can be helpful if positive.

SAO is an autosomal dominant trait. First-degree relatives (parents, full siblings, and children) have a 50 percent chance of carrying the SAO-specific SLC4A1 gene variant. (See "Inheritance patterns of monogenic disorders (Mendelian and non-Mendelian)", section on 'Autosomal dominant'.)

However, since anemia and hemolysis are often absent, especially after three years of age, many relatives may be unaware of their diagnosis, or they may have a history of anemia at birth without knowing the cause.

CBC findings and hemolysis testing — Hemolysis and anemia are seen in a subset of infants; adults usually have normal Hb values with slightly abnormal RBC indices [9,10]. Although the prevalence of hemolysis is challenging to estimate due to ascertainment bias, studies of newborns with SAO report neonatal hyperbilirubinemia in approximately one-half of newborns [9,10].

●Anemia – Anemia may occur if hemolysis is severe. Data are insufficient to estimate a range for hemoglobin values. (See 'Typical presentation' above.)

●RBC indices – Ovalocytes and other abnormal RBC morphologies persist throughout life. These are associated with abnormal RBC indices including increased MCHC [22,23].

Increased RDW (red cell distribution width), decreased MCV, and decreased RBC count have been reported in some patients.

●Findings of hemolysis – Hemolysis may be present in neonates and young infants but would be unusual in an older child or adult. Typical findings of hemolysis include high lactate dehydrogenase (LDH) and bilirubin and low haptoglobin (table 1). The hemolysis is non-immune (Coombs-negative). One report described significant hemolysis in an individual who had both SAO and glucose-6-phosphate dehydrogenase (G6PD) deficiency [24].

Blood smear — SAO RBCs have a characteristic morphology often described as stomatocytic elliptocytosis.

On the blood smear, these cells can appear similar to stomatocytes (RBCs containing either a longitudinal slit or one or two transverse ridges), ovalocytes, or macro-ovalocytes with one or more transverse slits (picture 1) [14,23]. RBCs with two slits are sometimes referred to as theta cells or knizocytes [25].

Ektacytometry and EMA binding — Ektacytometry (when available) is the preferred biochemical test for SAO because it shows a specific pattern of deformability, as illustrated in panel C of the figure (figure 1). In SAO, the deformability curve is flattened and undeformable with DImax close to zero [23].

●Ektacytometry – Ektacytometry is an in vitro test in which a laser diffractometer is used to measure deformability of a population of RBCs exposed to an osmotic gradient, creating a graph with specific parameters that can be compared with a reference population of RBCs [26]. Deformability is based on light scatter when the cells change from a discoid to an elliptical shape as they are subjected to a shear force.

The graph shows a tracing with osmolality on the X-axis (from 0 to 500 milliosms) and RBC deformability on the Y-axis. As osmolality decreases, the RBCs gain intracellular water and eventually lyse; as it increases, the RBCs lose intracellular water, becoming dehydrated and less flexible. The tracing on the graph yields the following parameters:

•Omin – The point at which the RBCs will lyse due to increased intracellular water.

•DImax – The point at which the RBCs have maximum flexibility (blood osmolality).

•Ohyper – The point halfway between the Omin and the maximal deformability.

Each RBC disorder has a unique and characteristic tracing. For SAO, the tracing is very flat due to limited flexibility of the cells. Ektacytometry patterns have been determined in numerous other hereditary hemolytic anemias [27].

●EMA binding – SAO has decreased binding to eosin-5-maleimide (EMA) (figure 1); however, this finding is not specific to SAO and is also seen in hereditary spherocytosis and other disorders. (See 'Differential diagnosis' below and "Hereditary spherocytosis", section on 'Confirmatory tests'.)

Data on findings of osmotic fragility testing in SAO are limited.

Genetic testing — Genetic testing for SAO (evaluation for the specific 27 nucleotide deletion in the SLC4A1 gene, Δ400-408) can be done by polymerase chain reaction (PCR) using primers specific for the region surrounding the deletion, followed by Sanger sequencing [13]. (See "Polymerase chain reaction (PCR)".)

PCR should be considered for confirmation, especially when ektacytometry is not available.

A more contemporary approach involves next-generation DNA sequencing (NGS), either as part of a gene panel or whole exome sequencing. (See "Next-generation DNA sequencing (NGS): Principles and clinical applications", section on 'Whole genome, exome, or gene panel'.)

NGS has several advantages over traditional Sanger sequencing:

●Speed – NGS is considerably faster than Sanger sequencing and can process a large amount of data in a relatively short period of time. (See "Next-generation DNA sequencing (NGS): Principles and clinical applications", section on 'Practical issues'.)

●Cost – NGS is often more cost-effective than Sanger sequencing, especially when analyzing a large number of samples and querying a large number of genes. Cost considerations may vary by health system, and clinicians should familiarize themselves with cost considerations specific to their practice.

●Comprehensiveness – A gene panel that includes gene variants that cause other hereditary hemolytic anemias such as hereditary spherocytosis and cryohydrocytosis can be useful to evaluate SAO and other disorders in the differential diagnosis.

Diagnosis — The diagnosis of SAO requires a biochemical test result consistent with the diagnosis (such as from ektacytometry) as well as confirmatory genetic testing that reveals the 27 nucleotide deletion in the SLC4A1 gene. (See 'Ektacytometry and EMA binding' above and 'Genetic testing' above.)

This generally follows the classical three lines of investigation, starting with family history and blood smear review. (See 'Family history' above and 'CBC findings and hemolysis testing' above and 'Blood smear' above.)

Differential diagnosis — The differential diagnosis of SAO includes other hereditary hemolytic anemias with elongated RBC shape, dehydration, or abnormal biochemical testing. The table summarizes differences in biochemical test results (figure 1) [26]. Like SAO, these disorders cause non-immune hemolytic anemia. Unlike SAO, in each of these other disorders, hemolysis persists throughout life. Each disorder has a distinct pattern of RBC deformability on ektacytometry. Although the RBC morphologies may be similar in some cases, different genes and different pathophysiologies are involved, as discussed in the linked topic reviews.

●Hereditary spherocytosis (HS) – HS is a hereditary RBC membrane disorder characterized by chronic hemolytic anemia and spherocytes on the blood smear. Different genetic variants in SLC4A1 and other genes are involved, but not the SAO-specific SLC4A1 deletion. (See "Hereditary spherocytosis".)

●Hereditary stomatocytosis (HSt) – HSt refers to a group of disorders with alterations in RBC hydration, including dehydrated stomatocytosis (DHSt; also called hereditary xerocytosis [HX]) and cryohydrocytosis (CHC) [28,29]. Different genetic variants in SLC4A1 and other genes are involved, but not the SAO-specific SLC4A1 deletion. (See "Hereditary stomatocytosis (HSt) and hereditary xerocytosis (HX)".)

●Hereditary elliptocytosis (HE) – HE is a hereditary RBC membrane disorder characterized by elliptocytes on the blood smear. Variants in genes other than SLC4A1 are involved. (See "Hereditary elliptocytosis and related disorders".)

MANAGEMENT

Supportive care — Birth should occur in a hospital with a neonatal intensive care unit (NICU) due to the potential severity of the disease.

Supportive care in the neonatal period is the same as other neonatal hemolytic anemias. Transfusions and treatments for hyperbilirubinemia may be required [9]. (See "Unconjugated hyperbilirubinemia in term and late preterm newborns: Initial management".)

Folic acid is administered to infants with hemolysis, as with other chronic hemolytic anemias.

Testing for RTA — Clinicians should be aware of the association of SAO with distal renal tubular acidosis (distal RTA). (See 'Distal RTA' above.)

Distal RTA is suggested by normal anion gap (hyperchloremic) metabolic acidosis. Nephrolithiasis or nephrocalcinosis may occur. Details of the evaluation, laboratory testing, interpretation of urinary pH, and management are presented separately. (See "Etiology and diagnosis of distal (type 1) and proximal (type 2) renal tubular acidosis", section on 'Diagnosis' and "Treatment of distal (type 1) and proximal (type 2) renal tubular acidosis", section on 'Management of distal RTA'.)

Splenectomy (not used) — Splenectomy is not used since hemolysis generally resolves by three years of age and adults are generally asymptomatic.

Reproductive and genetic counseling — First-degree relatives can be informed of the possibility of carrying the SAO variant (27 base pair deletion) in the SLC4A1 gene.

●Reproductive counseling – Since hemolytic anemia predominantly affects infants and young children, this information is likely to be most useful for reproductive counseling. Knowledge of the likelihood of transmitting SAO can help in managing the newborn by directing diagnostic testing and allowing the patient to avoid more extensive testing and interventions directed at other disorders.

•One parent affected – If one parent carries the SAO pathogenic variant, their children have a 50 percent chance of inheriting the variant.

•Both parents affected – If both parents carry the SAO pathogenic variant, their offspring have a 25 percent chance of inheriting the unaffected gene from both parents, a 50 percent chance of inheriting the variant from one of their parents, and a 25 percent chance of inheriting the variant from both parents. However, since homozygosity for the variant may be incompatible with life in many cases, the proportions may be closer to one-third unaffected and two-thirds affected.

●Pregnancy care – When a parent is known to have SAO, it is important to take precautions and inform the health care team involved in the delivery and care of the newborn.

Considerations include:

•Information – All relevant medical personnel (obstetrician, pediatrician) should be informed about the parent's SAO diagnosis well before the due date, to enable the team to prepare for care that may be needed, such as planning for the birth hospitalization and neonatal care.

•Hospital birth – Birth in a hospital setting is advised, preferably one with a neonatal intensive care unit (NICU) or access to a high-risk pediatric team. This is a precautionary measure in case the infant experiences any complications related to SAO, particularly hemolytic anemia and hyperbilirubinemia. (See 'Typical presentation' above.)

•Pediatric evaluation – A pediatrician should evaluate the newborn as soon as possible after birth. They can conduct a thorough physical examination and order any necessary testing. They should be provided with a detailed family medical history, especially concerning SAO and any other inherited or relevant conditions (such as glucose-6-phosphate dehydrogenase [G6PD] deficiency) and maternal iron deficiency, to allow them to best evaluate neonatal anemia. (See "Unconjugated hyperbilirubinemia in neonates: Risk factors, clinical manifestations, and neurologic complications" and "Unconjugated hyperbilirubinemia in term and late preterm newborns: Initial management", section on 'Pretreatment laboratory evaluation'.)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Anemia in adults".)

SUMMARY AND RECOMMENDATIONS

●Genetic basis – Southeast Asian ovalocytosis (SAO) is an autosomal dominant hemolytic anemia caused by a specific 27 base pair deletion in the SLC4A1 gene, which encodes band 3 of the red blood cell (RBC) cytoskeleton. Affected individuals are heterozygous for the deletion. This causes their RBCs to become rigid, dehydrated, and oval in shape. (See 'Pathophysiology' above.)

●Epidemiology – SAO is rare overall but very common in regions of Papua New Guinea, with prevalence up to 30 percent of individuals. Other regions with higher prevalence are listed above. The high prevalence in these regions is likely due to significant protection against several malaria species and against cerebral malaria, with relatively minimal clinical complications. (See 'Prevalence' above.)

●Presentation and clinical course – Neonates with SAO can have hemolysis or hemolytic anemia. Typically, this resolves by three years of age. Ovalocytic or stomatocytic RBCs persist on the blood smear throughout life and may lead to an incidental diagnosis in older children and adults. Some individuals have a distal renal tubular acidosis (distal RTA) that causes failure to thrive and persists through adulthood. (See 'Clinical features' above.)

●Diagnostic evaluation – The evaluation starts with a positive family history, hemolysis or hemolytic anemia in an infant, or finding of ovalocytes on the blood smear. It follows three lines of investigation, starting with a detailed family history and review of RBC morphology, followed by a biochemical test such as ektacytometry that demonstrates a characteristically flattened tracing (figure 1), and confirmed by genetic testing for the SAO-specific SLC4A1 deletion. Genetic testing can be done by polymerase chain reaction (PCR) or a next-generation sequencing (NGS) gene panel. (See 'Evaluation' above.)

●Differential – The differential diagnosis of SAO includes other hereditary hemolytic anemias and other causes of oval-shaped RBCs, such as hereditary spherocytosis, hereditary elliptocytosis, and hereditary stomatocytosis. These other disorders can cause hemolysis, but hemolysis can persist throughout life; the pattern on ektacytometry is different; and genetic testing shows different genetic variants. (See 'Differential diagnosis' above.)

●Management – Individuals with SAO may require no interventions, although neonates and infants with hemolysis or hemolytic anemia may require intensive monitoring and interventions including the following (see 'Management' above):

•Neonates may require treatment for hyperbilirubinemia and hemolytic anemia. Splenectomy is not used, as hemolysis resolves by three years of age and is absent in older children and adults. (See 'Supportive care' above and 'Splenectomy (not used)' above and "Unconjugated hyperbilirubinemia in term and late preterm newborns: Initial management".)

•The possibility of distal RTA should be considered. (See 'Testing for RTA' above and "Etiology and diagnosis of distal (type 1) and proximal (type 2) renal tubular acidosis".)

•If a parent carries the SAO variant, their children have a 50 percent chance of inheriting the variant. The health care team involved in the delivery and care of the newborn should be aware of the diagnosis well before the due date, and birth in a hospital setting is advised, preferably one with a neonatal intensive care unit (NICU) or access to a high-risk pediatric team. A pediatrician should evaluate the newborn as soon as possible after birth. (See 'Reproductive and genetic counseling' above.)