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Sialic acid storage disease is a genetic condition affecting mainly the nervous system. The severity of its symptoms varies significantly among affected individuals. It is typically categorized into three subtypes: infantile free sialic acid storage disease, Salla disease, and intermediate severe Salla disease.
Infantile free sialic acid storage disease (ISSD) represents the most severe form. Infants with ISSD experience significant developmental delays, low muscle tone (hypotonia), and struggle to gain weight and grow appropriately (failure to thrive). They may also exhibit distinctive "coarse" facial features, seizures, bone abnormalities, enlargement of the liver and spleen (hepatosplenomegaly), and an enlarged heart (cardiomegaly). Swelling of the abdomen can occur due to enlarged organs and fluid accumulation (ascites). Some affected infants develop hydrops fetalis, characterized by excessive fluid buildup in the body before birth. Life expectancy for children with ISSD is typically limited to early childhood.
Salla disease is a milder form of sialic acid storage disease. Infants with Salla disease usually display hypotonia within the first year and subsequently develop progressive neurological issues. Symptoms of Salla disease include intellectual disability and developmental delay, seizures, impaired movement and balance (ataxia), muscle stiffness (spasticity), and involuntary, slow, writhing movements of the limbs (athetosis). Individuals with Salla disease generally live into adulthood.
Intermediate severe Salla disease presents with symptoms that are intermediate in severity between those seen in ISSD and Salla disease.
This disorder follows an autosomal recessive inheritance pattern. This means that both copies of the responsible gene in each cell must carry mutations for the condition to manifest. Individuals with only one copy of the mutated gene are carriers and usually do not exhibit any signs or symptoms of the condition. Both parents of an affected individual are carriers.
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