The Ear and Auditory System

Excerpt

The ears—pinna, external auditory canal, and eardrum— are the sound-collection system for the body. Abnormalities involving the skin, cartilage, bone, and eardrum may interfere with hearing.

The Rinne and Weber tuning-fork tests can be used in the office to evaluate hearing. With the Weber test, a tuning fork (usually 256 Hz) is activated and applied to the skull, the forehead, the chin, or the upper incisor teeth. A normal response is no lateralization of the sound energy generated from the fork to either ear. (It is perceived by the patient as being in the middle of the head or on top of the head.) In an abnormal patient, the vibrating fork will be perceived in the ear with conductive hearing loss (drum perforation, impacted wax, middle ear fluid, stapes fixation, or otosclerosis), provided the other ear is normal. If some sensorineural dysfunction exists in both ears, caused by aging deafness (presbycusis), ototoxicity from drugs, acoustic trauma from excess noise exposure, or following a central nervous system infection, a vibrating fork will be perceived in the ear with the best sensorineural function.

In the Rinne test, when an activated tuning fork is held 2.5 cm from the ear and then placed on the mastoid process, a normal subject will hear it better (louder or more distinctly) in front of the ear than behind the ear. The proper notation is AC > BC; this indicates that air conduction is better than bone conduction. If the patient has abnormal hearing, a tuning fork activated and held 2.5 cm from the ear and then placed on the mastoid process will be heard better behind the ear. This result is written BC > AC (i.e., bone conduction is better than air conduction).

Air conduction testing measures the integrity of the entire hearing apparatus from external ear to auditory cortex. Bone conduction testing measures the integrity of the sensorineural structures (cochlea, eighth nerve, brainstem nuclei, and relays to the auditory cortex). The combination of these two tests permits the examiner to use fundamental physiologic information in categorizing the patient's hearing as being within normal limits.