Blood, brain and binocular vision.

A man aged 51 years presented with sudden onset, horizontal, binocular, double vision and right facial weakness. Ocular motility examination demonstrated a right horizontal gaze palsy pattern in keeping with a one-and-a-half syndrome. Since this was associated with a concomitant, ipsilateral, lower motor neuron (LMN) facial (VIIth) cranial nerve palsy, he had acquired an eight-and-a-half syndrome. Diffusion-weighted MRI confirmed a small infarcted area in the pons of the brainstem which correlated with anatomical location of the horizontal gaze centre and VIIth cranial nerve fasciculus. As a result of this presentation, further investigations uncovered a hitherto undiagnosed blood dyscrasia-namely polycythaemia vera. Regular venesection was started which resulted in complete resolution of his ocular motility dysfunction and an improvement of his LMN facial nerve palsy.