A 38-year-old man gave a 5-day history of progressively slurred speech, confusion and unsteadiness. He had taken metronidazole for 7 days following a dental procedure. He had a history of Leber's hereditary optic atrophy (LHON), causing reduced visual acuity, but no movement disorder, tremor or cardiac arrhythmia to suggest a ‘LHON plus’ syndrome. On examination, he was encephalopathic with marked dysarthria and ataxia of the trunk, limbs and gait. There was no peripheral neuropathy or palatal myoclonus. Contrast-enhanced CT scan of brain and cerebrospinal fluid (CSF) analysis, including oligoclonal band testing, were normal. T2-weighted MR scan of brain showed bilateral hyperintensity of the dentato-rubro-olivary pathway (Guillain–Mollaret triangle), a functional circuit linking the cerebellar dentate nucleus …