Objective: To describe the clinical and genetic features of an adolescent patient with MT-ND6-associated Leigh syndrome presenting with a prominent dystonia and myoclonus.

Background: Leigh syndrome is a mitochondrial neurodegenerative disorder characterized by progressive neurological deterioration and bilateral necrotizing lesions of the basal ganglia and brainstem. The disease typically presents in infancy and is associated with a poor prognosis. Mutations in the mitochondrial gene MT-ND6, encoding a subunit of respiratory chain complex I, are rare causes of Leigh syndrome and may present with atypical clinical phenotypes and prolonged survival.

Method: A comprehensive clinical evaluation, brain MRI, metabolic screening, and whole-genome sequencing were performed in a 14-year-old Uzbeki female presenting with progressive generalized dystonia, myoclonus and intelectual deterioration.

Results: Neurological symptoms began at age 12 with hand tremor and progressive gait disturbance, followed by generalized tonic-clonic seizures. Over two years, the patient developed a progressive movement disorder characterized by myoclonus and dystinia predominantly affecting the right upper limb and trunk and exacerbated by tactile stimulation. Neurological examination revealed psychomotor slowing and impaired ambulation requiring assistance, while cranial nerve function and tendon reflexes remained preserved. Brain MRI demonstrated progressive bilateral basal ganglia lesions with cortical involvement. Whole-genome sequencing identified a heteroplasmic pathogenic MT-ND6 variant (m.14487T>C; p.Met63Val) with a heteroplasmy level of 39%, confirming the diagnosis of Leigh syndrome.

Conclusion: This report describes genetically confirmed case of MT-ND6-associated Leigh syndrome highlighting an atypical presentation with myoclonus dystonia. The case expands the phenotypic spectrum of Leigh syndrome and emphasizes the importance of considering mitochondrial disorders in adolescents with myoclonus dystonia.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/adolescent-onset-myoclonus-dystonia-in-mt-nd6-m-14487tc-associated-leigh-syndrome/