Arachnoid cyst causing third cranial nerve palsy manifesting as isolated internal ophthalmoplegia and iris cholinergic supersensitivity.

An 8-month-old boy presented with anisocoria, a sluggishly reactive right pupil, and cholinergic supersensitivity as the only signs of what proved months later to be compressive third cranial nerve palsy due to an arachnoid cyst. Tonic constriction and dilation, segmental iris sphincter palsy, aberrant regeneration phenomena, ductional deficits, and ptosis were absent. The initial diagnosis was postganglionic internal ophthalmoplegia attributed to a viral ciliary ganglionopathy. Nineteen months later, he had developed an incomitant exodeviation and a supraduction deficit. Brain MRI revealed a mass consistent with an arachnoid cyst compressing the third cranial nerve in the right interpeduncular cistern. Resection of the cyst led to a persistent complete third cranial nerve palsy. This is the second reported case of prolonged internal ophthalmoplegia in a young child as a manifestation of a compressive third cranial nerve palsy. Our patient serves as a reminder that isolated internal ophthalmoplegia with cholinergic supersensitivity is compatible with a preganglionic compressive third nerve lesion, particularly in a young child.