Internuclear ophthalmoplegia is impairment of horizontal eye movements caused by damage to certain connections between nerve centers in the brain stem (the lower part of the brain).
In internuclear ophthalmoplegia, horizontal eye movements are impaired (for example, the eye on the damaged side can turn outward but not inward), but vertical eye movements are not affected.
In one-and-a-half syndrome, when the person tries to look to either side, the eye on the damaged side looks straight ahead and the other eye can turn outward but not inward, but movements up and down are not affected.
People with internuclear ophthalmoplegia or one-and-a-half syndrome may have double vision when they look in certain directions.
Treatment and prognosis (whether the disorder abates or eventually resolves) depends on the cause of impaired horizontal eye movements.
(See also Overview of the Cranial Nerves.)
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) of the 3rd cranial nerve (oculomotor nerve), the 4th cranial nerve (trochlear nerve), and the 6th cranial nerve (abducens nerve).
Internuclear ophthalmoplegia usually results from
In older people: A stroke (typically only one eye is affected)
In younger people: Multiple sclerosis (both eyes are often affected)
Less common causes of internuclear ophthalmoplegia include Lyme disease, neurosyphilis, tumors, head injuries, nutritional deficiencies such as Wernicke encephalopathy, and certain drugs, such as phenothiazines (which are antipsychotic medications), opioids, and tricyclic antidepressants.
Horizontal (side-to-side) eye movements are impaired, but vertical (up-and-down) eye movements are not. The affected eye cannot turn inward, but it can turn outward. When only one eye is affected and 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 unaffected 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 when they look to the side but may not have double vision when they look straight ahead.
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 to either side, the eye on the damaged side remains motionless in the middle. The other eye can turn outward but not inward. As in internuclear ophthalmoplegia, vertical eye movements are not affected. Causes of one-and-a-half syndrome include multiple sclerosis, stroke, and tumors.
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 prognosis (whether the disorder abates or eventually resolves) depends on the disorder that caused it.
