Sialidosis is less common than the other causes of progressive myoclonic epilepsy. There are at least two variants. All cases are inherited in an autosomal recessive manner. Type I sialidosis (cherry-red spot myoclonus syndrome) is due to N-acetyl neuraminidase deficiency, which results in defective cleavage, and thus accumulation, of oligosaccha-rides, typically with inclusion bodies with vacuolation. It has a juvenile or adult onset and is characterized by action myoclonus and an intention tremor, gradual visual failure, and later tonic-clonic seizures. There is little in the way of mental deterioration. A gene has been mapped to chromosome 6p21.3. In type II sialidosis there are defects in P-galactosidosis activity in addition to those in N-acetyl neuraminidase. Timing of the onset is variable from infancy to the second decade, and clinical features include severe myoclonus, tonic-clonic seizures, dysmorphic features, coarse facies, corneal clouding, skeletal dysostosis, cardiac involvement, and organomegaly and dementia. One gene has been mapped to chromosome 20. The genetic basis of this disorder is complex and not completed elucidated, and involves both the gene for Neu1 and that for cathepsin A, which forms a complex with Neu1. Diagnosis is confirmed by finding elevated urinary sialyliloligosaccharides and by assaying enzyme activity in leucocytes and cultured skin fibroblasts.

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