One and a Half Syndrome Following Resection of a Posterior Fossa Epidermoid Cyst

This is a case of One and a Half Syndrome following resection of a posterior fossa epidermoid cyst. A 31-year-old male initially presented with left facial droop and bilateral ptosis, and a "down and out" gaze of the left eye. He underwent imaging and was diagnosed with an epidermoid cyst located in the posterior cranial fossa. Suboccipital craniotomy was performed with resection of the cyst and expansile duraplasty. Post-op month 2 after the resection, he presented to neuro-ophthalmology clinic with binocular horizontal double vision. On exam, there was resolution of the prior ptosis, left down and out gaze, and left facial droop. On testing of extraocular movements, nystagmus was present in up and down gaze. The patient was unable to look to the right. When attempting to look left, there was left nystagmus and a deficit of right adduction. With the left eye being covered, there was some improvement in right adduction. The findings were consistent with One and a Half Syndrome, suggesting involvement of the right 6th nerve nucleus which was responsible for right gaze palsy, and involvement of the right medial longitudinal fasciculus leading to left intranuclear ophthalmoplegia. Although intracranial masses can cause One and a Half Syndrome such as in this case, other causes include vascular or demyelinating disease.