Cochlear Nerve

Lesions of this special sensory nerve cause deafness. Nerve cells are in the spiral ganglion of the cochlea. Their peripheral connections are to the auditory cells of the organ of Corti, and their central connections are to the cochlear nucleus. The next-order neuron is the lateral lemniscus. When impulses concerned with sound have reached the cochlear nucleus, their further passage is bilateral (ie, a unilateral lesion above the level of the nucleus cannot cause deafness).

Deafness Does the patient have some hearing loss, and if so, is it nerve deafness (or sensorineural deafness) or conductive deafness? Conductive deafness is caused by a disease of the external canal, middle ear, or ossicles. The key features of nerve deafness are

• Loss of perception of high pitched sounds

• Loss of hearing of bone conducted sound

How to Examine Hearing You can learn a lot from the patient's history. If the patient can hear on the telephone using either ear, his hearing is reasonably good.

A formal test is awareness of whispered speech at 6 ft. It is often difficult to find a quiet room in which to test the patient. Follow this method:

• Have the patient stand, facing at right angles to you, at a distance of 6 ft.

• Ask the patient to put his finger in the ear farther from you. Whisper a series of numbers, asking him to repeat each number after you.

A person with normal hearing can hear and repeat nine out of 10 whispered numbers at this distance. Then turn the patient so that he faces the opposite way, ask him to plug the far ear, and repeat the procedure. Your whispered voice can vary from very faint to almost conversational speech. After you have done this a few times, you will get the sense of a constant-volume whisper and the test becomes quite reliable. If his hearing is normal, record it as "Whispered voice @ 6', Rt and Lt." If you had to move up for the patient to hear the numbers when testing the left ear, record it as, for example, "Whispered voice @ 6' Rt and 4' Lt."

If the patient is deaf in one or both ears, do the following to establish whether the deafness is caused by a defect in the conducting system of the ear or in the nerve leading from the ear.

• Place the handle of a vibrating 256 tuning fork on the patient's mastoid.

• Ask him to tell you when the vibrations can no longer be heard.

• At that point, put the vibrating tines of the tuning fork close to the patient's ear.

If the sound reappears, one can say that air conduction is better than bone conduction. These are the findings in most people and in early nerve deafness. If your hearing is normal, put the handle of the 256 tuning fork on the patient's mastoid and when he says he can no longer hear it, put the handle on your own mastoid. If you hear it through your mastoid longer than the patient does, he has nerve deafness. If nerve deafness is complete, there is no appreciation of sound by either bone conduction or air conduction. This is the Rinne test. It is reliable only with bilateral hearing loss. The opposite situation—when bone conduction is longer than air conduction—is found in conductive or middle ear deafness.

There is another way to tell nerve deafness from conductive deafness:

Put the handle of a vibrating 256 tuning fork on the center of the patient's head, over the vertex. Ask him if he hears it better in one ear or the other or in the center of his head.

If the patient has a conductive hearing loss in one ear, he will hear the tuning fork in this ear. You can simulate this for yourself, provided that you have normal hearing. When the vibrating tuning fork is on the vertex of your head, put your finger in one ear. The sound will immediately become loudest in this ear. You have thus produced a degree of conductive deafness in yourself. This is the Weber test. On the other hand, if the patient has nerve deafness in one ear, he will hear the tuning fork in the opposite normal ear. Weber's test is no help in bilateral deafness and it will not work if there is both nerve and conductive deafness in the same ear.

Remember, nerve deafness results in decreased bone conduction and loss of high-tone appreciation; conduction deafness results in decreased air conduction and loss of low-tone appreciation. Total hearing loss always means nerve deafness.

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