To describe a rare case of alternating sixteen and a half syndrome.

There have only been a handful of Sixteen-and-a-half syndrome cases described in the literature, which involves one-and-a-half syndrome with additional Cranial Nerves (CN) VII and VIII dysfunction. The one-and-a-half syndrome is manifested by ipsilateral conjugate horizontal gaze palsy and ipsilateral internuclear ophthalmoplegia. It occurs due to a lesion in the pontine tegmentum involving the medial longitudinal fasciculus (MLF) and the paramedian pontine reticular formation (PPRF) or abducens nucleus.

A 68-year-old woman with a history of atrial fibrillation, mitral valve replacement with a mechanical valve on Coumadin presented with the complaint of acute onset horizontal binocular double vision. She was last seen normal the previous night. 

The examination was notable for right-sided exotropia and skew deviation with intact convergence. Vertical eye movements were intact. There was no horizontal eye movement in the left eye. In the right eye, there was absent adduction but intact abduction with horizontal nystagmus. Neurologic exam was also notable for right nasolabial fold flattening with a mild facial droop and reduced hearing to finger rub in the right ear, compared to the left side. There was no limb paresis, tremors, or incoordination. 

Her routine blood studies were normal. CT head revealed no acute intracranial hemorrhage. No large vessel occlusion was noted on the CT angiogram of the head and neck. MRI brain showed an acute infarct within the left parasagittal dorsal pons along the floor of the fourth ventricle in the region of the left facial colliculus.  

This is an atypical presentation of the sixteen-and-a-half syndrome with CN VII and CN VIII dysfunction present on the opposite side of the pontine infarct, suggesting a possible contralateral extension of the lesion, not clearly visible in MRI scan.