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Tectal fourth nerve palsy due to infarction.

51-YEAR-OLD WOMAN with a history of a heart murmur and a small stroke several years earlier suddenly began walking unsteadily and noticed vertical double vision. Examination findings revealed limitation of the right eye on looking down and in (Figure )a nd mild ataxia of gait and of each arm. She had a slight spontaneous left head tilt, and additional left tilt decreased, whereas right head tilt maximized image separation, supporting the diagnosis of a right trochlear nerve palsy. Magnetic resonance image findings showed bilateral cerebellar and left pontomesencephalic tectal infarctions in the territory of the superior cerebellar arteries (SCAs) (Figure). Echocardiography revealed a large atrial septal defect as the avenue of a probable paradoxical embolus. During the next 2 weeks, diplopia disappeared and ataxia improved. COMMENT Intramedullary involvement of the fourth nerve from any cause is uncommon, 1 though probably underdiagnosed and confused with skew deviation. Brainstem infarction of the fourth nerve usually arises from midbrain tegmental involvement of the trochlear nucleus or proximal fascicles in the territory of the paramedian or short circumferential vessels. 2 Guillain and colleagues 3 described the clinicopathologic findings of infarction of the SCA territory, including tectal trochlear palsy, in 1928. However, SCA infarcts were considered rare until the introduction of computed scanning showed that infarcts of the cerebellar SCA territory are common, and it is symptomatic involvement of the SCA territory of the brainstem, as in the present