Isolated trochlear nerve palsy secondary to a juvenile pilocytic astrocytoma

A 9-year-old female developed a fourth nerve palsy which was the first sign of a posterior fossa astrocytoma. The fourth nerve palsy was believed to have resulted from direct invasion of the trochlear nerve rather than compression by hydrocephalus. Sagittal reconstruction of a computed tomogram revealed extension of the tumor into the dorsal midbrain where the trochlear nerve exits.