Progressive myoclonic epilepsy due to rare mitochondrial ND6 mutation, m.14487T>C

We report the case of a 43-year old man with adult-onset drug-resistant epilepsy, cortical myoclonus and bilateral optic neuropathies. Serial MRI scans over a two year period demonstrated the interval devel- opment of bihemispheric stroke-like lesions. Giant somatosensory evoked potentials and short-duration myoclonic jerks with craniocaudal spread on surface-EMG were consistent with cortical myoclonus. Optical coherence tomography showed bilateral symmetric thinning of the nerve fibre layer in the pap- illomacular bundles. Muscle biopsy was non-diagnostic, however next-generation whole mitochondrial gene sequencing identified a pathogenic m.14487T>C mitochondrial gene mutation. This gene is most commonly implicated in cases of infantile-onset Leigh syndrome, although a broader phenotypic spectrum including migraine with aura and progressive myoclonic epilepsy have recently been described. Bilateral optic neuropathies consistent with Leber hereditary optic neuropathy are commonly seen with other ND6 mitochondrial gene mutations and were in our case a striking feature. Debilitating right-sided facial myoclonus affecting speech and swallowing remained highly drug-resistant however symptomatic benefit was derived from targeted onabotulinum toxin A injections (80 units orbicularis oculi, 10 units nasalis, 10 units risorius, 20 units levator anguli oris, 60 units platysma) administered every three months.