KCTD17-related myoclonus-dystonia syndrome: clinical and electrophysiological findings of a patient with atypical late onset

Introduction: Myoclonus-dystonia is a rare syndrome typically occurring during childhood or adolescence, mainly due to SGCE pathogenic variants. Early-onset, atypical presentations of myoclonus-dystonia have recently been associated with KCTD17 variants. In these cases, laryngeal involvement was reported in the advanced stages. Methods: We evaluated a 52-year-old man with myoclonus-dystonia and positive family history. He underwent an electromyographic investigation of vocal cord and forearm muscles. Whole-exome sequencing was also performed. Results: Onset of symptoms was at 51 years with dysphonia and vocal tremor. Electromyography disclosed abductor spasmodic dysphonia and laryngeal myoclonus. The patient later developed writer's cramp, upper limb myoclonus, and blepharospasm. Botulinum toxin injection led to improvement of the writer's cramp and to a lesser extent of the spasmodic dysphonia. Genetic analysis identified a heterozygous missense variant in exon 2 of KCTD17: c.229 C > A (p.Leu77Ile), consistently predicted as damaging. Conclusions: We suggest that the KCTD17-associated phenotypic spectrum may include late onset (even in late adulthood) as well as early and prominent laryngeal involvement.