To investigate the clinical and neurophysiological findings in a cohort of patients with silaidosis type 1.

Sialidosis is a rare, autosomal recessive disorder. It is caused by a mutation in the gene neuraminidase (NEU1), located on chromosome 6 leading to abnormal intracellular accumulation of sialyloligosaccharides. Sialidosis type I is the normomorphic (or mild) form of the disorder and the main motor symptoms are myoclonus and ataxia. Myoclonus is thought to be of cortical origin.

This study involved 3 patients with the diagnosis of sialidosis type 1 who underwent clinical and electrophysiological evaluation. Patients were examined by a movement disorder trained neurologist. Unified Myoclonus Rating Scale (UMRS) and Scale for the assessment and rating of ataxia (SARA) were performed with the  UMRS being videotaped. Scalp electroencephalography (EEG) was recorded using a 64-channel EEG system. SEP, C-reflex, and startle reflex data were obtained and cortico-muscular and musculo-muscular coherences were analyzed.

3 cases with sialidosis type 1 who were in 3 different decades of their lives were evaluated. The age of onset was in the first or second decade of the life (4, 11 and 16-year-old). The main presenting symptoms were imbalance (case 1), seizure (case 2) and visual difficulty (case 3). Giant SEPs and C-reflexes during rest were observed in the 3 subjects. Cortico-muscular coherence was detected confirming the presence of cortical myoclonus. Strong musculo-muscular coherence was present between antagonist muscles.

Cortical hyperexcitability demonstrated in our cases is aligned with previously reported physiological findings in another cohort of sialidosis patients. The musculo-muscular coherence between the antagonistic muscle groups is supportive of the idea that cortical drive synchronizes muscle discharges leading to co-contraction of agonist-antagonist muscle groups during a cortical myoclonic jerk.