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Hereditary elliptocytosis, also known as ovalocytosis, is an inherited blood disorder in which an abnormally large number of the sufferer’s erythrocytes (i.e. red blood cells) are elliptical rather than the typical biconcave disc shape. It is one of many red-cell membrane defects. In its severe forms, this disorder predisposes to haemolytic anaemia.
Elliptocytosis was first described in 1904,[1] and was first recognised as a hereditary condition in 1932.[2] More recently it has become clear that the severity of the condition is highly variable,[3] and there is much genetic variability amongst those affected.[4]
The incidence of hereditary elliptocytosis is hard to determine, as many sufferers of the milder forms of the disorder are asymptomatic and their condition never comes to medical attention. Around 90% of those with this disorder are thought to fall into the asymptomatic population. It is estimated that its incidence is between 3 and 5 per 10,000 in the USA,[5] and that those of African and Mediterranean descent are of higher risk. Because it can confer resistance to malaria, some subtypes of hereditary elliptocytosis are significantly more prevalent in regions where malaria is endemic. For example, in equatorial Africa its incidence is estimated at 60-160 per 10,000,[6] and in Malayan natives its incidence is 1500-2000 per 10,000.[7] Almost all forms of hereditary elliptocytosis are autosomal dominant disorder, and both sexes are therefore at equal risk of having the condition. The most important exception to this rule of autosomal dominance is for a subtype of hereditary elliptocytosis called hereditary pyropoikilocytosis (HPP), which is autosomal recessive.
There are three major forms of hereditary elliptocytosis: common hereditary elliptocytosis, spherocytic elliptocytosis and southeast asian ovalocytosis.
Common hereditary elliptocytosis is the most common form of elliptocytosis, and the form which has been most extensively researched. Even when looking only at this form of elliptocytosis, there is a large variation in its severity. A clinically significant haemolytic anaemia occurs only in 5-10% of sufferers, with a strong bias towards those with more severe subtypes of the disorder.
Spherocytic elliptocytosis and southeast asian ovalocytosis are less common subtypes predominantly affecting those of south-east asian and european ethnic groups, respectively.
The following categorisation of the disorder demonstrates its heterogeneity (in approximate order from least severe to most severe)[8]:
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