Internuclear ophthalmoplegia is impairment of horizontal eye movements caused by damage to certain connections between nerve centers in the brain stem.

In internuclear ophthalmoplegia, the nerve fibers that coordinate both eyes in horizontal movements—looking from side to side—are damaged. These fibers connect collections of nerve cells (centers or nuclei) that the 3rd cranial nerve (oculomotor nerve) and the 6th cranial nerve (abducens nerve) originate from. In older people, the disorder usually results from a stroke, and only one eye is affected. In younger people, it usually results from multiple sclerosis, and both eyes are often affected. Less common causes include Lyme disease, tumors, and toxicity due to a drug (such as tricyclic antidepressants).

Horizontal eye movements are impaired, but vertical ones are not. The affected eye cannot turn inward, but it can turn outward. When a person looks to the side opposite the affected eye, the following happens:

• The affected eye, which should turn inward, cannot move past the midline. That is, the affected eye looks straight ahead.
• As the other eye turns outward, it often makes involuntary, repetitive fluttering movements called nystagmus That is, the eye rapidly moves in one direction, then slowly drifts in the other direction.

People with internuclear ophthalmoplegia may have double vision.

One-and-a-half syndrome results when the disorder that causes internuclear ophthalmoplegia also damages the center that coordinates and controls horizontal eye movements (horizontal gaze center). When the person tries to look the either side, the affected eye remains motionless in the middle. The other eye can turn outward but not inward. As in internuclear ophthalmoplegia, vertical eye movements are not affected.

In internuclear ophthalmoplegia and one-and-a-half syndrome, the eyes can turn inward when the person looks inward (as when focusing on a nearby object) even though the eyes cannot turn inward when the person looks to the side.

For internuclear ophthalmoplegia or one-and-a-half syndrome, treatment and outlook (whether the disorder abates or eventually resolves) depends on the disorder that caused it.

Last full review/revision July 2007 by Michael Rubin, MD