A palsy of the 3rd cranial nerve can impair eye movements, the response of pupils to light, or both.
- These palsies can occur when pressure is put on the nerve or the nerve does not get enough blood.
- People have double vision when they look in a certain direction, the eyelid droops, and the pupil may be widened (dilated).
- Doctors do a neurologic examination and magnetic resonance imaging (MRI) or computed tomography (CT) of the brain.
- Treatment depends on the cause.
(See also Overview of the Cranial Nerves.)
Palsy refers to paralysis, which can range from partial to complete.
The most common causes of 3rd cranial nerve palsy are
- Pressure on (compression of) the nerve
- Inadequate blood flow to the nerve
Compression tends to result from serious disorders, such as
- A bulge (aneurysm) in an artery supplying the brain
- A disorder that causes herniation of the brain
Herniation occurs when the brain is forced downward through a small natural opening in the sheets of tissue that separate the brain into compartments. Herniation may result from bleeding (sometimes caused by a head injury), a tumor, or another mass in the brain.
Inadequate blood flow is more common and usually less serious. It typically results from
- High blood pressure (hypertension)
- Other disorders that affect blood vessels, such as a bulge in an artery (aneurysm) that carries blood to the brain
The affected eye turns slightly outward and downward when the unaffected eye looks straight ahead, causing double vision. The affected eye may turn inward very slowly and may move only to the middle when looking inward. It cannot move up and down. Because the 3rd cranial nerve also raises the eyelids and controls the pupils, the eyelid droops. The pupil may be normal or be widened (dilated) and may not narrow (constrict) in response to light. The pupil is often affected when the cause is compression of the nerve.
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 by both pupils indicates deep coma and possibly brain death.
- Neurologic examination
- Magnetic resonance imaging or computed tomography
- Sometimes other tests
The diagnosis of 3rd cranial nerve palsy is based on results of a neurologic examination.
Magnetic resonance imaging (MRI) or computed tomography (CT) of the brain is done to identify the cause.
If the pupil is affected or if symptoms suggest a serious underlying disorder, brain MRI or CT is done immediately.
If a ruptured aneurysm is suspected, CT (or MRI) is done immediately. If CT (or MRI) does not detect blood, a spinal tap (lumbar puncture), magnetic resonance angiography, CT angiography, or cerebral angiography is done.
- Treatment of the cause
Treatment of 3rd cranial nerve palsy depends on the cause. Emergency treatment is required if a life-threatening disorder is the cause.