Superficial siderosis and episodic fourth nerve paresis. Report of a case with clinical and magnetic resonance imaging findings.

We describe a patient with superficial siderosis who had an episodic unilateral fourth nerve paresis. The superficial siderosis was caused by small repeated intraventricular hemorrhages from a periventricular cavernous angioma. T2-weighted magnetic resonance images demonstrated a rim of low signal intensity at the brain surface, characteristic of hemosiderin deposition. These low-signal-intensity deposits included the dorsal brain stem around the anterior medullary velum. We suggest that the hemosiderin deposits affected the proximal portion of the fourth nerve where it contains central myelin and that this in some way caused unstable conduction of nerve impulses through the nerve.