This is a paralysis of the sixth cranial nerve. This nerve controls a single eye muscle, the abducens muscle, which is responsible for making the eye look to the side. If this muscle stops working the eye turns inward (cross-eyed) and has a hard time moving outwards. Most people complain of a side-by-side horizontal double vision. Like all cranial nerve palsies, there are many potential causes for an abducens palsy. The most common cause is microvascular injury from diabetes or hypertension. You can think of this like a miniature “stroke” to the nerve. Other potential causes are mass lesions (tumors or aneurysms) and high intracranial pressure such as from pseudotumor cerebri. Most sixth nerve palsies are self-limited and improve after a few months. Ophthalmologists used to watch these palsies for spontaneous improvement, but in this day and age of MRI imaging, we tend to image everyone with a new palsy to rule out the dangerous stuff.