Teaching Case

A 51-year-old female patient presented with 3 days history of dysarthria, difficulty in walking, tremors and proximal weakness. There was a history of on and off diarrhoea for the past year, for which she was treated with an antiprotozoal drug (ornidazole). She was also on treatment for cervical spondylosis.

Bilateral symmetrical T2W and FLAIR hyperintense signal was seen in dentate nucleus of cerebellum and splenium of corpus callosum. Subtle T2W hyperintense signal was also noted in dorsal pons. No abnormal areas of restricted diffusion were seen. These findings were consistent with ornidazole toxicity.

Ornidazole is a nitroimidazole derivative with actions similar to metronidazole and is used in the treatment of protozoal infections and anaerobic bacterial infections. Many case reports of metronidazole-induced encephalopathy have been described in the literature but only two case reports of ornidazole toxicity so far. One of them is a case of reversible cerebellar toxicity and the other is peripheral neuropathy due to treatment with ornidazole [1, 2].
The various neurologic side effects of metronidazole toxicity include peripheral neuropathy, ataxic gait, dysarthria, convulsive seizures, and encephalopathy. Similar but fewer side effects are seen with ornidazole. While the precise mechanism for neurotoxicity is not entirely clear, one hypothesis is that it occurs via axonal swelling secondary to metronidazole-induced vasogenic oedema or a reversible ischaemia due to vasospasm [3].
The typical MRI findings described are bilateral and symmetrical increased T2 signal intensity in the cerebellar dentate nuclei, dorsal medulla, dorsal pons, midbrain and splenium of the corpus callosum, and uncommonly in the inferior olivary nucleus of the medulla and white matter of the cerebral hemispheres. These findings are almost similar to those seen in metronidazole toxicity. Restricted diffusion in these regions have also been described [4]. Our case did not have increased signal on diffusion weighted images; probably diffusion restriction occurs only in the acute setting of lesion development.
These changes are reversible on discontinuation of the drug, though the exact time period is variable ranging from days to many weeks.
Our patient was prescribed ornidazole for diarrhoea initially and then later the patient had been taking the drug without proper medical advice for nearly a year. The patient’s symptoms improved dramatically after discontinuation of the drug.
MRI plays an important role in diagnosing this condition and is also helpful in the follow-up of these patients. Our patient was not clinically suspected to have drug toxicity at the outset. Following imaging, a detailed drug history was obtained and excessive use of ornidazole was confirmed. Follow-up MRI is awaited.