The two peripheral forms of deafness—conductive and sensorineural deafness—must be distinguished from each other, because important remedial measures are available, particularly for the former. In differentiating them, the tuning-fork tests are often of value. When a vibrating fork, preferably of 512-Hz frequency, is held about 2.5 cm from the ear (test for air conduction), sound waves can be appreciated only as they are transmitted through the middle ear; they will be reduced with disease in this location. When the vibrating fork is applied to the skull (test for bone conduction), the sound waves are conveyed directly to the cochlea, without intervention of the sound-transmission apparatus of the middle ear, and will therefore not be reduced or lost in outer or middle ear disease. Normally air conduction is better than bone conduction, and the sound transmitted though the air is appreciated for about twice as long as that passing through the bone.
These principles form the basis for several simple tests of auditory function. In the Weber test, the vibrating fork is applied to the forehead in the midline (or to a central incisor). A normal person hears the bone-conducted sound equally in both ears. In nerve deafness, the sound is localized to the normal ear for the reasons noted previously; in conductive deafness, the sound is perceived as louder in the affected ear because interference from ambient sounds is muted on the affected side. In the Rinne test, the fork is applied to the mastoid process. At the moment the patient indicates that the sound ceases, the fork is held at the auditory meatus. In middle ear deafness, the sound cannot be heard by air conduction after bone conduction has ceased (abnormal Rinne test). In nerve deafness, the reverse may be true (normal Rinne test), but more saliently, both air and bone conduction are quantitatively decreased. The Schwabach test consists of comparing the patient’s bone conduction with that of the normal examiner.
In general, early sensorineural deafness is characterized by a partial loss of perception of high-pitched sounds and conductive deafness by a partial loss of low-pitched sounds. This can be ascertained by the use of tuning forks of different frequencies but most accurately by the use of an audiometer and the construction of an audiogram, which reveals the entire range of hearing at a glance. The audiogram is an essential test in the evaluation of hearing loss and the point of departure for subsequent diagnostic evaluation. A ticking watch (infrequently found on doctor’s wrists or pockets any longer) or rubbing the patient’s hair together near the ear can be used as a surrogate test of gross hearing for the bedside, but these maneuvers emit mostly high-frequency sound and will not detect low-frequency conductive loss.
A cochlear type of hearing loss can be recognized by the presence of the symptoms of recruitment and diplacusis. Recruitment refers to a heightened perception of loudness once the threshold for hearing has been exceeded; thus the patient’s retort “You don’t have to shout” when the examiner raises his voice (see the following text).
Diplacusis refers to a defect in frequency discrimination that is manifest by a lack of clarity of spoken syllables or by the perception that music is out of tune and unpleasant (described by patients as a “mushiness” of sounds).
Because each cochlear nucleus is connected with the cortex of both temporal lobes, hearing is unaffected by unilateral cerebral lesions as already mentioned. Deafness caused by brainstem lesions is observed only rarely, as a massive lesion is required to interrupt both the crossed and uncrossed projections from the cochlear nuclei—so massive, as a rule, that other neurologic abnormalities usually make the testing of hearing impossible.
A number of special tests prove to be helpful in distinguishing cochlear from retrocochlear (nerve) lesions. Although an absolute distinction cannot be made on the basis of any one test, the results taken together (particularly loudness recruitment, speech discrimination and tone decay) make it possible to predict the site of the lesion with considerable accuracy. These tests, usually carried out by an otologist or audiologist, include the following: 1.