Sialidosis type1 with cardiac malformation: A case report

Sialidosis is a rare autosomal recessive lysosomal storage disease caused by a variant in the neuraminidase 1 (NEU1) gene encoding lysosomal neuraminidase, and is a rare cause of progressive myoclonus epilepsies (PME). Sialidosis is classified into two types. Sialidosis type 1 is a relatively mild late-onset form with ataxia, myoclonus, macular cherry-red spot, seizures, and non-dysmorphic features. Sialidosis type 2 has congenital, infantile and juvenile-onset forms, and has more severe feature, including ascites, coarse facies, dysostosis multiplex, macular cherry-red spot, hepatosplenomegaly, and developmental delay than type 1. Our case, a 29-year-old male, had shown normal development up to the age of 12 years and 5 months. Subsequently, he developed ataxia and myoclonus. At age 12 years and 11 months, generalized tonic-clonic seizures occurred with ataxia, myoclonus, intentional tremor, borderline intelligence, and cherry-red macular spot. Skin fibroblast enzymological analysis of neuraminidase yielded a value below 1 nmol/h/mg protein. NEU1 gene findings were consistent with compound heterozygous missense variant c.1034C>T(p.Thr345Ile) and c.239C>T(p.Pro80Leu), while electron microscopy of skinfibroblasts showed vacuoles and dense body deposition in both neuroblasts and Schwann cells. In addition, this is the first reported case of sialidosis type 1 associated with a quadricuspid aortic valve malformation. Collectively, the above findings indicated a diagnosis of sialidosis type 1 with cardiac malformation. The involuntary movement improved temporarily in response to clonazepam but then gradually worsened. During 17 years of follow-up, his seizures were controlled with anticonvulsants, but the ataxia, myoclonus and intentional tremor gradually worsened.