TYPES OF HEARING LOSS 

Hearing loss can result from a lesion anywhere within the auditory system (Nadol 1993). An abnormality within the outer or middle ear results in a conductive loss of hearing due to an inefficient transmission of sound to the inner ear system. When the loss of hearing is due to pathology in the cochlea or along the VIII 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 these organic types of hearing loss, one should also consider functional hearing loss. The diagnosis of functional hearing loss is made 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 11):

Another finding of conductive loss is that speech discrimination is relatively unimpaired. A patient with a conductive loss has good discrimination ability provided the speech signal is of sufficient intensity.

Frequently, the patient with a conductive loss of hearing complains of tinnitus, which may be localized in one ear, in both ears, or unlocalized in the head. In the case of a conductive impairment, the tinnitus tends to be of relatively low pitch.

Sensorineural Hearing Loss 

Sensorineural hearing loss occurs with pathology in the inner ear or along the nerve pathway from the inner ear to the brainstem. Hearing loss from cochlear disorders alone is termed sensory loss. As mentioned elsewhere, there exists some ambiguity among audiologists, neurologists, and otologists concerning what is a retrocochlear and what is a central problem. For the purposes of this discussion, we will define retrocochlear as an abnormality between the cochlea and the brainstem (see below).

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, inutero 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 alway 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 (12).

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 Loss 

Mixed 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 13.

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.

Sensory Versus Neural Lesions 

The problems of differentiating cochlear dysfunction from VIII nerve lesions have received major emphasis during the past several years. In fact, this area has been emphasized to the extent that some audiologists have limited their concept of differential audiology primarily to those tests that assist in localizing the defect within the sensorineural mechanism. The neurologist's interest in sensorineural hearing loss is with regard to the possibility of a cerebellopontine angle tumor. Although many referrals for audiological evaluation are made for this reason, we must emphasize that even the more sophisticated special auditory tests cannot determine the specific pathology underlying the disorder. An MRI may indicate the presence of an abnormality somewhere in the nervous system, but it does not necessarily define the nature of the pathology. The audiological tests, however, highlight patterns of auditory behavior that are generally associated with cochlear or neural involvement.

Routine pure tone and speech testing can yield valuable information on the site of lesion during the initial phase of the differential audiologic study. For example, a pure tone configuration, which is often seen in patients with a presumptive diagnosis of Meniere's disease, is a unilateral hearing loss most pronounced in the low frequency range. In sharp contrast, patients with VIII nerve lesions frequently present a unilateral hearing impairment most evident in the high frequencies and poor speech discrimination. Although such generalizations may describe a substantial number of cases falling into these two categories, numerous exceptions are encountered with either cochlear or neural pathology. Measures, such as tone decay, acoustic reflex measures, acoustic reflex decay, and speech discrimination at high intensity levels must be used to distinguish between VIII nerve, extra-axial and intra-axial brainstem dysfunction.

Central Auditory Disorders 

As would be anticipated, lesions within the central auditory system are difficult to detect or localize. In fact, many central auditory dysfunctions will not be demonstrated by conventional audiologic measurements. Individuals with known lesions in the central auditory tracts may not manifest any significant hearing loss when tested by conventional pure tone audiometry (Benjamin and Troost 1988). Total removal of one hemisphere of the brain in humans has not resulted in any major change of auditory sensitivity in either ear. Central disorders of hearing are quite unusual. When accompanied by other neurologic signs and symptoms, a central diagnosis is suggested. Normal measures mentioned previously, such as tone decay or acoustic reflex, strongly suggest an eighth nerve lesion. One excludes eighth nerve lesions as a separate category and concentrates on the central auditory brainstem and hemispheric pathways. Neuroimaging procedures such as MRI may help to localized the abnormality.