An 82-year-old man with ataxia and dysarthria

Given this patient's medication exposure and MRI findings, the leading diagnosis is (c) toxic encephalopathy secondary to metronidazole, also known as metronidazole-induced encephalopathy. The most common imaging finding of metronidazole-induced encephalopathy is symmetric hyperintensity on T2-weighted images in the dentate nuclei.2,3 The splenium of the corpus callosum, periaqueductal grey matter and dorsal brainstem are often also involved.3,4 Less commonly, there is abnormality in the cerebral hemispheric white matter or basal ganglia.4 This patient had received metronidazole for 11 weeks for his foot ulcer, with a total cumulative dose of more than 80 g. Metronidazole-induced encephalopathy is a rare adverse drug reaction. To date, a manual literature review identified only 91 reported cases. Clinical features may include cerebellar dysfunction, altered mental status and seizures.4,6 Our patient presented with symptoms of cerebellar dysfunction. A systematic review of case reports and case series involving 64 patients showed that 75% of affected patients had cerebellar dysfunction, 33% had altered mental status and 13% had seizures; 17% of patients had both cerebellar dysfunction and altered mental status; and one patient presented with all three manifestations.6 Cases of metronidazole-induced encephalopathy have been reported with total cumulative doses of as low as 0.25 g to as high as 182 g, occurring over durations from one day to six months.3,6 In addition to discontinuation of the drug, treatment is primarily supportive. This patient's metronidazole was discontinued, resulting in rapid clinical improvement. Most reported cases of metronidazole-induced encephalopathy have had good outcomes with resolution of signs, symptoms and MRI findings after drug cessation. Although the prognosis appears to be favourable overall, there are two case reports in which metronidazole- induced encephalopathy contributed to death,10,11 emphasizing the importance of timely recognition of a condition that is largely reversible.