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Hearing loss can result from a lesion anywhere within the auditory
system (Nadol, 1993). Usually hearing loss is slowly progressive and can
come from a variety of conditions as discussed below. Sudden hearing
loss is much less common and is usually unilateral. Sudden hearing loss
is estimated to occur at an incidence of 5-10 per 100,000 per year in
the current U.S. population (Shih, 1994) and may be caused by
perilymphatic fistula, leukemia, macroglobulinemia polycythemia, viral
and metabolic disorders. Autoimmune and other immune mediate diseases,
multiple sclerosis, barro trauma, head injury, metastatic cancer, and
ototoxicity are the potential etiologies, many which can be determined
by history and appropriate diagnostic testing. Most commonly, sudden
hearing loss is idiopathic. Conductive hearing loss is due to an abnormality within the outer or middle ear resulting in inefficient transmission of sound to the inner ear system. When the loss of hearing is due to pathology in the cochlea or along the VII cranial nerve from the inner ear to the brainstem, the loss is referred to as a sensorineural hearing loss. Patients may exhibit both conductive and sensorineural loss which is referred to as a mixed hearing loss. Central hearing loss (or central auditory dysfunction) is present when a lesion exists in the central auditory pathway beyond the VIII cranial nerve, for instance in the cochlear nucleus in the pons or in the primary or association auditory cortex of the temporal lobe. In addition to organic types of hearing loss, there is also the possibility of functional hearing loss. Functional hearing loss is diagnosed when an individual claims to have a hearing loss, but discrepancies in objective test measures suggest that the loss does not exist, or is exaggerated. Conductive Hearing Loss Conductive hearing loss occurs with pathology in the outer or middle ear. The bone conduction thresholds are normal, but air conduction results suggest a decrease in hearing sensitivity. The patient with a conductive hearing loss typically demonstrates decreased sensitivity across all frequencies. Sometimes hearing is better for the higher frequencies than it is for the lower ones. (Figure 3) Another finding of conductive loss is that speech discrimination is relatively unimpaired. A patient with a conductive loss has good discrimination ability if the sound or speech signal is loud enough to hear. Patients with a conductive hearing loss complain of tinnitus frequently. The tinnitus may be localized in one ear, perceived in both ears, or unlocalized within the head. In the case of a conductive impairment, the tinnitus tends to be of relatively low pitch. Sensorineural Hearing LossSensorineural hearing loss occurs with pathology in the inner ear or along the nerve pathway from the inner ear to the brainstem. Hearing loss specifically from cochlear disorders alone is termed sensory loss. There is ambiguity among audiologists, neurologists, and otologists concerning what is a retrocochlear and what is a central problem. Most neurologists think of central disorders as those involving just the central nervous system and not the cranial nerves. Most audiologists and otolaryngologists think of central disorders as anything proximal to the ear and cochlea. For the purposes of this discussion, it is better to be more specific and we will define retrocochlear as an abnormality between the cochlea and the brainstem. The term sensorineural includes both cochlear and retrocochlear disorders. A pure sensorineural impairment exists when the sound-conducting mechanism (outer and middle ear) is normal in every respect, but a disorder is present in the cochlea or auditory nerve. Sensorineural impairment can be congenital or acquired. Congenital sensorineural hearing loss may result from hereditary factors, malformation of the cochlea, intrauterine viral infections, or birth trauma. The etiology of most sensorineural hearing loss is unknown. Acquired sensorineural hearing loss may be caused by noise exposure, acoustic tumor, head injury, infection, toxic drug effects, vascular disease, or presbycusis. The configuration of the audiogram demonstrating a sensorineural hearing loss may vary significantly and in some instances may suggest the etiology of the loss. Many people with sensorineural losses experience a loss only in the high frequency region. These individuals have no difficulty understanding speech at normal intensities in a quiet environment since low-frequency hearing is unimpaired. However, they do experience difficulty in understanding speech in a noisy environment. Generally, the low frequencies are defined as the range from 250 Hz to 750 Hz, the middle frequencies as 1,000 Hz to 3,000 Hz, and the high frequencies as 4,000 Hz to 8,000 Hz on the standard audiogram. Loudness recruitment is usually associated with sensory loss of cochlear origin, which constitutes the majority of sensorineural losses. Recruitment is an abnormally rapid growth of loudness with an increase in intensity (Sanders, 1984). The recruiting patient with sensory loss will not hear low-intensity sounds at all, and may just barely hear sounds of moderate intensity, but the recruitment of loudness may cause moderately loud sounds to be perceived as uncomfortably loud. This disruption of normal loudness function may be painful to the individual and require the utilization of variable compression circuitry should the patient pursue hearing aid use. The patient with sensorineural hearing loss is usually subject to tinnitus of a somewhat different sort from that associated with conductive hearing loss. Generally, the patient with sensorineural loss reports a constant ringing or buzzing noise, which may be localized in either ear or may not be localized. In general, the pitch of tinnitus tends to be higher in sensorineural impairment than in conductive impairment. In addition, the patient may report that tinnitus is only present at night or when background noise is minimal, when in fact it is always present but the patient=s perception is only in quiet environments. In sensorineural losses, the audiometric Weber test is expected to lateralize to the better hearing ear. Audiometrically, sensorineural loss is characterized by overlapping air and bone conduction thresholds. The tympanogram is typically normal, and acoustic reflexes may be present, elevated, or absent. The audiometric findings for a typical sensorineural hearing loss are displayed in Figure 4. Contrary to a commonly held misconception, sensorineural hearing loss may be helped by the use of hearing aids. Current technology utilizes full dynamic range compression to significantly increase the effectiveness of amplification. Mixed Hearing LossMixed hearing loss consists of a conductive and a sensorineural component in the same ear. The patient's behavior will reflect attributes of both a conductive and a sensorineural disorder. Causes of mixed hearing loss may be any combination of the conditions described previously for conductive and sensorineural hearing loss. The conductive component of the mixed hearing loss may be corrected by successful medical or surgical treatment, but the sensorineural component is not reversible. The pure tone audiometric pattern for a mixed hearing loss is displayed in Figure 5. With a mixed loss, both air and bone conduction thresholds are elevated but bone conduction thresholds are better than air conduction thresholds. The difference between the two thresholds is referred to as the air-bone gap and represents the amount of the conductive component present.
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