Overhydrated hereditary stomatocytosis

Overhydrated hereditary stomatocytosis (OHSt), also known as hereditary hydrocytosis, is a rare inherited red blood cell membrane disorder characterized by increased permeability of the red blood cell membrane to sodium and potassium ions. This leads to an influx of water into the red blood cells, causing them to swell and assume a characteristic 'stomatocyte' appearance on blood smear — red blood cells with a mouth-shaped (slit-like) area of central pallor instead of the normal circular pallor. The condition belongs to the broader group of hereditary stomatocytosis syndromes. The disease primarily affects the hematologic system. Swollen, overhydrated red blood cells are prematurely destroyed, leading to chronic hemolytic anemia of variable severity. Key clinical features include anemia, jaundice (due to elevated bilirubin from red blood cell breakdown), splenomegaly (enlarged spleen), and reticulocytosis (increased immature red blood cells as the bone marrow compensates for red cell destruction). Gallstones may develop as a complication of chronic hemolysis. Laboratory findings typically show an elevated mean corpuscular volume (MCV), decreased mean corpuscular hemoglobin concentration (MCHC), and markedly reduced red cell osmotic fragility. Mutations in the RHAG gene (encoding the Rh-associated glycoprotein) have been identified in many cases, though genetic heterogeneity exists. Treatment is primarily supportive. Blood transfusions may be required during severe anemic episodes or aplastic crises. Folic acid supplementation is commonly recommended to support increased red blood cell production. Importantly, splenectomy is strongly contraindicated in overhydrated hereditary stomatocytosis, as it has been associated with a high risk of severe thromboembolic complications, including potentially life-threatening venous thrombosis. This distinguishes the management of OHSt from many other hereditary hemolytic anemias where splenectomy may be beneficial. Iron overload may develop over time and should be monitored.

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