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, hearing, and taste), and others control muscles in the face or regulate glands. The nerves are named and numbered (according to their location, from the front of the brain to the back).
Viewing the Cranial Nerves
A cranial nerve disorder may result when the following are damaged or malfunction:
Disorders that directly damage cranial nerves include injuries, tumors, inflammation, infections (such as shingles), an inadequate blood supply (as occurs in diabetes), drugs, and toxins.
Some cranial nerve disorders interfere with eye movement. Eye movement is controlled by 3 pairs of muscles. These muscles move the eye up and down, right and left, and diagonally. The muscles are controlled by the following cranial nerves:
If one of one of these nerves is damaged, the muscles it controls may become paralyzed to varying degrees (called a palsy), and people may not be able to move their eyes normally. How eye movement is affected depends on which nerve is affected. People with one of these palsies may have double vision when they look in certain directions.
Symptoms of cranial nerve disorders depend on which nerves are damaged and how they were damaged. Cranial nerve disorders can affect smell, taste, vision, sensation in the face, facial expression, hearing, balance, speech, swallowing, and muscles of the neck. For example, vision may be affected in various ways:
If one of the 2nd cranial nerves (optic nerve) is damaged, the affected eye is blind.
If any of the three cranial nerves that control eye movement (3rd, 4th, or 6th cranial nerve) is damaged, people cannot move their eyes normally. Symptoms include double vision when looking in certain directions.
If the 3rd cranial nerve (oculomotor nerve) is paralyzed, the upper eyelid is paralyzed. It droops down over the eye and interferes with vision.
If the 8th cranial nerve (auditory or vestibulocochlear nerve) is damaged or malfunctions, people may have problems hearing and/or vertigo—a feeling that they, their environment, or both are spinning.
Cranial nerve disorders can also cause various kinds of facial or head pain.
When doctors suspect a cranial nerve disorder, they ask the person detailed questions about the symptoms and test the function of the cranial nerve by asking the person to do simple tasks, such as to follow a moving target with the eyes.
Imaging of the brain with magnetic resonance imaging (MRI) is often needed.