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Autosomal recessive inherited metabolic disorder characterized by hyperexcretion of free sialic acid in the urine and by its storage in the lysosomes of different tissues. Clinical features include coarse facial abnormalities, clear cornea, albinoid fungi, ptosis, nystagmus, anteverted nose, high-arched palate, cardiomegaly, heart failure, hepatosplenomegaly, nephrotic syndrome, hypotonia, and developmental delay. Neonatal ascites, hydrops fetalis, and early death can occur.

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Sialuria: Rarest form of sialic acid storage disease.

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Moderate Form (Salla Disease; Free Sialic Storage Disease; Sialuria Finnish Type): Adult form of sialuria mostly observed in the northeastern part of Finland. Clinical features include progressive mental and psychomotor retardation, clumsiness, onset at age 12 to 18 months with deterioration in the second decade, 4 to 15% vacuolated lymphocytes, enlarged storage lysosomes, and increased sialic acid in the urine. Ataxia, athetosis, rigidity, spasticity, impaired speech, growth retardation, thick calvaria, and exotropia are present in more than 50% of patients. Life expectancy is reduced to the seventh decade in most patients.

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Severe Form: Infantile Sialic Acid Storage Disease (ISSD; Sialuria, Infantile Form; N-Acetyl-Neuraminic Acid Storage Disease; NANA Storage Disease): Usually diagnosed in the newborn period. Unlike Salla disease, there is no ethnic prevalence. Clinically, it presents with severe visceral involvement, dysostosis multiplex, psychomotor retardation, and early death.

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Autosomal recessive.

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Sialuria differs from the sialidosis in the accumulation and excretion of free sialic acid in the presence of normal or increased levels of neuraminidase activity. Sialuria occurs because of defective feedback inhibition on the enzyme UDP-GlcNAc 2-epimerase in the process of NANA synthesis. Both Salla disease and infantile sialic acid storage disease are caused by impairment of an active transport system of free sialic acid across lysosomal membrane and probably are allelic to each other, despite different clinical features. Genetic mapping of both forms is assigned to 6q14-q15 on the long arm of chromosome 6.

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Clinical features. Peripheral blood count may show anemia and vacuolated lymphocytes, also seen in bone marrow aspiration. Urinary free sialic acid level is increased. Cultured fibroblast presents increased free sialic acid level. Electron microscopy shows accumulation of NANA in the cytosolic fraction only in sialuria and in the lysosomal fraction only in Salla disease and infantile sialic acid storage disease, resulting in vacuole appearance. Prenatal diagnosis can be obtained by measuring free sialic acid in amniotic fluid. Normal lysosomal enzymes exclude sialidosis.

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Sialuria: Rarest form. Clinical features include coarse facies, hepatosplenomegaly, and near-normal growth and development.

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Salla Disease: Most common form. High prevalence in northern part of Finland. Presents in adulthood with growth and mental retardation, ataxia, athetosis, rigidity, spasticity, and speech impairment.

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Infantile Sialic Acid Storage Disease: Most severe form. Commonly presents in infancy with early-onset psychomotor retardation, coarse facial “gargoyle-like” appearance with wispy hair, hypertelorism, prominent epicanthic folds, fluffy eyebrows, long philtrum, high-arched palate, and hepatosplenomegaly. Other presentations may include nephrotic syndrome (one case), fetal and ...

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