Unilateral abducens nerve palsy in a child with rapidly increasing head circumference.

A 12-month-old healthy, normally developing boy presented with rapidly increasing head circumference over 2 months and 1 week of strabismus. Head circumference was 50 cm (.99th percentile; mother 5 59.5 cm; father 5 56.5 cm). Examination showed limited abduction OD, overshoot of adduction OD, and no papilledema. MRI (figure) showed the right anterior inferior cerebellar artery traversing the root entry zone of the right abducens nerve, a cause of unilateral cranial nerve VI palsy recognized in adults but not in children. Neuroimaging was otherwise normal. The patient was diagnosed with benign familial macrocephaly and discharged with ophthalmology follow-up for conservative management.