Nerves in the brain are called cranial nerves. There are 12 pairs of them, each having specific sensory or motor functions. One of these nerves is the vestivulocochlear nerve, also known as the eighth cranial nerve. It is composed of two sensory branches, the vestibular nerve and the cochlear or auditory nerve. Hearing is the function of the cochlear nerve, while body position sense comes from the vestibular nerve.
The eighth cranial nerve arises from the brain stem, which is connected with the spinal cord and is located at the base of the skull. It travels from the inner part of the ear towards the brain. In the inner ear are receptor cells that receive vibrations of sound and noise from the outside, which then stimulate the cochlear nerve to send these sounds to the brain for interpretation. Head movements also affect the fluid inside the ear and stimulate the vestibular nerve to send information to the brain regarding the body's sense of balance or position.
Injury or damage to either branch of the eighth cranial nerve often results in several symptoms. When the cochlear nerve is affected, manifestations include tinnitus or ear ringing, poor hearing ability, and deafness. Hearing loss is usually due to noise exposure, otitis media, or the inflammation of the middle part of the ear, ear wax impaction or blockage, and presbycusis. Presbycusis is a term referring to the hearing loss brought about by old age. Some infants may also be born with defects in the cochlear nerve and become deaf for life.
When the vestibular branch of the eighth cranial nerve is affected by injury or disease, symptoms usually include dizziness, nausea, ataxia, vomiting, and nystagmus. Ataxia means instability or unsteadiness when doing voluntary movements. Nystagmus denotes abnormal and involuntary rapid eye movements. Affected patients also present with vertigo, or the false sensation that the environment is spinning. Causes frequently include tumors, degenerative diseases, vestibular neuritis, and intoxication from medications or alcohol.
Diagnosis of injuries to this nerve is often done by a neurologist, a doctor who treats diseases of the nervous system. He usually takes the patient's medical and family history, and then performs special tests to evaluate the functions of the eighth cranial nerve. Other diagnostic tools that may also be helpful include the magnetic resonance imaging (MRI) and computed tomography (CT) scan. Treatment often depends on the cause of the patient's symptoms.