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A female child with a maternally inherited MT-TK mutation (m.8363G>A, 70% heteroplasmy in lymphocytes) associated with several years of intractable generalised and myoclonic epilepsy and prior normal neuroimaging presented with progressive weakness, imbalance and drooling over 2 days without fever. Three-generation pedigree analysis (supplemental figure) revealed the mother as the symptomatic carrier of the MT-TK mutation associated with a MERRF phenotype and ragged red fibres on prior muscle biopsy. The patient’s older brother died of complications from the MT-TK mutation as a late teenager. At presentation, the patient was tachypnoeic with an oxygen saturation of 92%. Neurological examination demonstrated slurred speech, palatal weakness with drooling, quadriparesis with diffuse hyperreflexia and ataxic gait. Venous blood gas demonstrated a respiratory acidosis (pH 7.28, pCO2 74, p02 58, HC03 34.2) and elevated lactic acid 3.74 mmol/L (normal 0.5–2.2 mmol/L) necessitating intubation. MRI revealed T2-weighted signal abnormalities of bilateral cerebral peduncles and dorsal midbrain ( figure 1) that were not identified on prior neuroimaging 4 and 6 years prior to current symptomatic presentation. Diffusion-weighted, apparent diffusion coefficient, and susceptibility-weighted MRI sequences were not interpretable due to artefact from braces. The patient underwent a sleep study given the brainstem abnormalities that revealed central hypoventilation with an apnoea-hypopnoea index of 10 (normal 1–2) (figure 2), necessitating bilevel positive airway pressure (BiPAP) at night following extubation. The patient was discharged 1 month after presentation with residual weakness and ataxia and without a hospital admission more than 2 years after presentation and remains on BiPAP at night.