Southeast Asian Ovalocytosis

Southeast Asian ovalocytosis (SAO) is a rare hereditary red cell membrane defect characterized by the presence of oval-shaped erythrocytes and with most patients being asymptomatic or occasionally manifesting with mild symptoms such as pallor, jaundice, anemia and gallstones.

Epidemiology

SAO is common in Southeast Asian and Western Pacific countries (i.e. Thailand, Malaysia, Indonesia, Philippines and Papua New Guinea). SAO is very common in malaria-endemic areas (prevalence 1/20-1/4) but in Europe it is very rare.

Clinical description

SAO can occur at any age. Newborns with SAO might be symptomatic with hemolysis at birth that leads to anemia, pallor or jaundice. Anemia and hyperbilirubinemia in neonates is common together with mild splenomegaly and gallstones. Hemolysis usually disappears in the first three years of life. Adults are asymptomatic or have only minimal hemolytic anemia. Resistance to malaria (see this term) is also noted.

Etiology

SAO results from a 27 bp deletion in the SLC4A1 gene, localized on chromosome 17q21.31 (SLC4A1del27 mutation). This gene codes for a band 3 anion transport protein which is the bicarbonate/chloride exchanger in red blood cell membranes and defects in this protein cause membrane rigidity. Heterozygous mutations for the deletion are found in almost all cases, as homozygosity is thought to be lethal to the developing embryo. A homozygous mutation was recently reported in one case of SAO as the primary cause of extremely severe dyserythropoietic anemia associated with distal renal tubular acidosis (see this term), and that which would have been lethal if the fetus had not been transfused in utero. The association of mutation SLC4A1del27 with other mutations in the SLC4A1 gene may result in distal renal tubular acidosis associated with SAO in some cases.

Diagnostic methods

Diagnosis is based on the presence on a peripheral blood smear of macro-ovalocytes, some of them stomatocytic with more than one stoma, in the absence of hemolysis. Genetic assays can also be used to identify the mutation in the SLC4A1 gene.

Differential diagnosis

Differential diagnosis includes all forms of hereditary elliptocytosis and hereditary spherocytosis, and dehydrated hereditary stomatocytosis (see these terms).

Antenatal diagnosis

Antenatal diagnosis is possible but not undertaken because of the benign nature of the disease.

Genetic counseling

SAO follows an autosomal dominant pattern of inheritance. Genetic counseling is possible.

Management and treatment

Most cases of SAO are asymptomatic and treatment is not necessary.

Prognosis

SAO is not life threatening.