(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 Overview of the Cranial Nerves Twelve pairs of nerves—the cranial nerves—lead directly from the brain to various parts of the head, neck, and trunk. Some of the cranial nerves are involved in the special senses (such as seeing... read more .)
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 younger people: Multiple sclerosis Multiple Sclerosis (MS) In multiple sclerosis, patches of myelin (the substance that covers most nerve fibers) and underlying nerve fibers in the brain, optic nerves, and spinal cord are damaged or destroyed. The cause... read more (both eyes are often affected)
Less common causes of internuclear ophthalmoplegia include Lyme disease Lyme Disease Lyme disease is a tick-transmitted infection caused by Borrelia species, primarily by Borrelia burgdorferi and sometimes by Borrelia mayonii in the United States. These... read more 
, neurosyphilis Tertiary (third, or late) syphilis 
, tumors, head injuries Overview of Head Injuries Head injuries that involve the brain are particularly concerning. Common causes of head injuries include falls, motor vehicle crashes, assaults, and mishaps during sports and recreational activities... read more , nutritional deficiencies such as Wernicke encephalopathy Wernicke Encephalopathy Wernicke encephalopathy is a brain disorder that causes confusion, eye problems, and loss of balance and results from thiamin deficiency. Wernicke encephalopathy is caused by a severe deficiency... read more , and certain drugs, such as phenothiazines (which are antipsychotic drugs Antipsychotic Drugs Psychosis refers to symptoms such as delusions, hallucinations, disorganized thinking and speech, and bizarre and inappropriate motor behavior that indicate loss of contact with reality. A number... read more ), opioids, and tricyclic antidepressants Heterocyclic (including tricyclic) antidepressants Agomelatine, a new type of antidepressant, is a possible treatment for major depressive episodes. Several types of drugs can be used to treat depression: Selective serotonin reuptake inhibitors... read more .
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 Multiple Sclerosis (MS) In multiple sclerosis, patches of myelin (the substance that covers most nerve fibers) and underlying nerve fibers in the brain, optic nerves, and spinal cord are damaged or destroyed. The cause... read more , 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.
