Pearls & Oy-sters: SCA21 Due to TMEM240 Variation Presenting as Myoclonus Dystonia Syndrome

Spinocerebellar ataxia 21 due to TMEM240 disease-associated variation characteristically presents insidiously with a delay in language, motor, and social skill acquisition. The condition typically progresses to severe cognitive impairment. We report a patient with SCA21 who presented with myoclonus dystonia (M-D) syndrome and whose dystonia showed a modest response to levodopa. Affected family members (mother and sibling of the proband) also had a similar phenotype. Neuropsychology evaluation of the proband and afflicted family members revealed moderate impairments in attention, executive function, short-term and episodic memory, and marked impairments in planning, abstract reasoning, language, and visuospatial functions. Normal EEG, α-fetoprotein levels, and somatosensory evoked potentials helped to delineate SCA21 from other differential diagnoses. Motor impairment, pyramidal signs, and sensory impairment are usually absent in SCA21. This case highlights the importance of genetic testing in patients with M-D syndrome and supports a trial of levodopa for patients with dystonia from SCA21 due to TMEM240 variation.