Keyhole Aqueduct Syndrome.

Keyhole aqueduct syndrome is a rare progressive neurodegenerative disorder describing a unique set of neuro-ophthalmologic, neuroimaging, and histopathological findings on autopsy. A midline mesencephalic cleft communicating with the cerebral aqueduct resembling syrinx is seen on imaging and histopathology. There are 9 cases published in the literature. We encountered a patient with vertical nystagmus, internuclear ophthalmoplegia, and progressive ataxia who has a midline cleft connecting the cerebral aqueduct with the interpeduncular cistern highlighting a distinguishing feature of this syndrome.