The most common cause of sudden binocular double vision, meaning double vision that goes away when you close one eye, is a cranial nerve palsy. Three cranial nerves, numbered III, IV, and VI, control the muscles that move each eye. When one of these nerves is affected, the corresponding eye muscle weakens and the two eyes no longer aim at the same point.
In adults over 50, the most frequent cause of an isolated cranial nerve palsy is reduced blood flow to the nerve, often related to diabetes or high blood pressure. When no other neurological signs are present, these palsies often resolve on their own within three months with proper management of the underlying condition.