These disorders involve paralysis of one of the cranial nerves that control eye movement (the 3rd, 4th, or 6th nerve), impairing the ability to move the eyes. How eye movement is affected depends on which nerve is affected.
The eye is moved by three pairs of muscles, controlled by the 3rd, 4th, and 6th cranial nerves. These muscles move the eye up and down, right and left, and diagonally. People with one of these palsies may have double vision when they look in certain directions.
Third Cranial Nerve (Oculomotor Nerve) Palsy
A palsy of the 3rd cranial nerve can impair eye movements, the response of pupils to light, or both. This palsy can be caused by brain disorders—such as a head injury, a bulge (aneurysm) in an artery supplying the brain, a hemorrhage, or a tumor—or by diabetes or high blood pressure.
The affected eye turns outward when the unaffected eye looks straight ahead, causing double vision. The affected eye can move only to the middle when looking inward and cannot move up and down. Because the 3rd cranial nerve also raises the eyelids and controls the pupils, the eyelid droops, and the pupil may be widened (dilated). It may not narrow (constrict) in response to light.
The disorder causing the palsy may worsen, resulting in a serious, life-threatening condition. For example, a severe headache may occur suddenly, or a person may become increasingly drowsy or less responsive. In such cases, the cause may be a ruptured aneurysm, which then bleeds. People may go into a coma. In such people, dilation of both pupils and lack of response to light (fixation) by both pupils indicates deep coma and possibly brain death (see Brain Death).
Diagnosis and Treatment
The diagnosis is based on results of a neurologic examination and computed tomography (CT) or magnetic resonance imaging (MRI). If the pupil is affected or if symptoms suggest a serious underlying disorder, CT is done immediately. If a ruptured aneurysm is suspected and CT does not detect blood, a spinal tap (lumbar puncture—Fig. 2: How a Spinal Tap Is Done), magnetic resonance angiography, CT angiography, or cerebral angiography is done (see Angiography).
Treatment depends on the cause. Emergency treatment is required if a life-threatening disorder is the cause.
Fourth Cranial Nerve (Trochlear Nerve) Palsy
A palsy of the 4th cranial nerve affects vertical eye movements. Often, the cause cannot be identified. The most common identified cause is a head injury, often due to a motorcycle accident. Occasionally, diabetes causes this palsy. Rarely, the cause is a tumor, an aneurysm, or multiple sclerosis.
One or both eyes may be affected. The affected eye cannot turn inward and down. As a result, people see double images, one above and slightly to the side of the other. Thus, going down stairs, which requires looking inward and down, is difficult. However, tilting the head to the side opposite the affected eye muscle can compensate and eliminate the double images. This position can eliminate the double images because people use eye muscles that are unaffected by the palsy to focus both eyes on an object.
Usually, the diagnosis is suspected if a person has characteristic limited eye movement. CT or MRI may be done.
The disorder causing the palsy, if identified, is treated. Eye exercises may help. Sometimes surgery is necessary to eliminate double vision.
Sixth Cranial Nerve (Abducens Nerve) Palsy
A palsy of the 6th cranial nerve affects the ability to turn the eye outward. Many disorders can cause this palsy:
Some of these disorders put pressure on the nerve by causing nearby swelling or by increasing pressure within the skull. Others interfere with blood flow to the nerve.
If this palsy occurs alone (without other cranial nerve palsies), its cause is often never identified.
The affected eye cannot turn fully outward and may turn inward when people look straight ahead. Double vision occurs when people look toward the side of the affected eye. Other symptoms depend on the cause. They may include severe headache, accumulation of fluid (edema) in the conjunctiva (the membrane covering the white of the eye), numbness in the face and mouth, loss of vision, and inability to move the eye in other directions.
Diagnosis and Treatment
Usually, doctors can easily identify a 6th cranial nerve palsy, but the cause is less obvious. An ophthalmoscope is used to look into the eye and check for evidence of tumors, increased pressure, and abnormalities in blood vessels. CT or, preferably, MRI is done to exclude tumors and other abnormalities. If the results are unclear, a spinal tap (lumbar puncture) may be done to determine whether pressure within the skull is increased and whether an infection is present. If symptoms suggest vasculitis, blood is withdrawn to check for evidence of inflammation, such as certain abnormal antibodies (antinuclear antibodies and rheumatoid factor) and an abnormal erythrocyte sedimentation rate (ESR—how quickly red blood cells settle to the bottom of a test tube containing blood). After all tests are done, the cause may remain unknown.
Treatment depends on the cause. When the cause is treated, the palsy usually resolves. Palsies with no identifiable cause usually resolve without treatment within 2 months, as do those due to a blocked blood vessel.
Last full review/revision September 2012 by Michael Rubin, MDCM