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. 2017 BMJ Publishing Group Ltd.