This is a paralysis or stroke to the fourth cranial nerve. This nerve controls a single eye muscle called the superior oblique muscle. This muscle is located behind the eyeball and it helps the eye look downward and assists with eye rotation when you tilt your head sideways. When the nerve is blocked, this muscle stops working, and people complain of vertical double vision where objects look stacked on top of each other. The double vision may get worse when looking to the side or trying to read a book. Fourth nerve palsies can be subtle. In fact, this is the hardest cranial nerve palsy to detect as the eyes appear normal to casual inspection. There are many causes for a 4th nerve palsy: congenital, trauma, vascular insults (hypertension/diabetes), and lesions in the brain. If there isn’t an obvious cause for a nerve palsy, then further imaging such as MRI should be obtained. If the double vision persists, prism glasses are an option as is strabismus surgery.