A 57 year old man presented with a 2 month history of occipital headache radiating frontally, a 4 week history of unsteadiness, right sided paresthesia, deafness, nausea and anorexia. Shortly after admission he developed a left Horner’s syndrome. On examination there was a left Horner’s syndrome, a mild spastic tetraparesis with right sided predominance but normal jaw jerk, joint position and pin prick sensation. He had a markedly ataxic gait.

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Brain MRI showed a mass lesion in the left side of the pons and midbrain. On axial T2 weighted images (figure A) there was high signal centrally and low signal peripherally whereas the signal characteristics were reversed on T1 enhanced axial images (figure B). Haematological and biochemical screens were normal except for a raised prostate specific antigen of 73 μg/l (normal<4 μg/l). Bone scan, abdominal ultrasound, and chest radiography were normal. Ultrasound guided transrectal prostate biopsy confirmed prostatic adenocarcinoma. The histology of a transcerebellar stereotactic biopsy of the brain stem lesion showed metastatic adenocarcinoma which was positive for prostate specific antigen.

The primary presentation of prostatic carcinoma as brain stem dysfunction and the rarity of solitary brain stem metastasis from prostatic carcinoma1-3 make this a most unusual case.