Convergence Deficits in Myoclonus-Dystonia Point to Cerebellar Impairment

Background: Myoclonus-dystonia (M-D) is a monogenic movement disorder, with proposed cerebellar dysfunction. Vergence eye movement deficits, characteristics of degenerative cerebellar disease, have not been studied in M-D. Cerebellar transcranial alternating current stimulation (tACS) is considered a potential therapeutic approach. Objectives: To assess vergence and prosaccade performance as markers of cerebellar dysfunction in M-D and to evaluate the effects of cerebellar 50 Hz tACS on these eye movements. Methods: Vergence and prosaccade performance were examined in 14 M-D patients carrying pathogenic SGCE variants and 14 healthy controls. A subgroup (n = 7) received real and sham 50 Hz cerebellar tACS in a randomized, double-blind design. Results: M-D patients showed prolonged latency and reduced gain of convergence compared to controls. Divergence did not differ between groups. Prosaccade peak velocity was reduced in M-D patients. 50 Hz cerebellar tACS showed no effect on eye movements. Conclusion: Impaired convergence supports cerebellar involvement in M-D. Further studies should identify affected pathways.