Timothy C. Hain, MD
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Central hearing loss is generally thought to be extremely rare compared to the sensorineural or conductive types of hearing loss, but recent studies has shown that central components to hearing loss are much more common than previously appreciated (Gates et al, 1996). The diagnosis is usually not made by the pure tone audiogram, which in fact, is often normal. Rather patients usually have poor scores on speech reception threshold (SRT), or word recognition scores (WRS) portions of the audiogram. Rather than relying on the audiogram, this diagnosis is best made by combining an audiogram demonstrating good pure-tone recognition, with specialized aphasia testing. Inconsistency between pure-tone recognition and speech is another way to recognized central impairment.
Patients with central hearing loss typically have inconsistent auditory behavior, that may cause them to be misdiagnosed as having "functional" or psychogenic hearing disturbances. Cortically "deaf" patients may have reactions to environmental sounds, despite absence of reaction to loud noises. As in patients with cortical visual disturbances, patients may consider themselves "deaf" in spite of having reactions to sounds in the room.
We will next consider the sites of central deafness syndromes.
Brainstem lesions are a rare source of central deafness. Uh and associates (1997) reported a case where complete deafness resulted from a head injury in which there were injuries to the inferior colliculi, bilaterally. In general, bilateral lesions are necessary for central deafness. Unilateral lesions to the inferior colliculus does may not elicit any auditory consequences however (Bognar et al, 1994). Hu and associates (1997) reported a case where complete deafness resulted from a head injury in which there were injuries to the inferior colliculi, bilaterally. Occasionally auditory hallucinations can occur due to damage to brainstem structures involved in hearing such as the superior olive (Casino and Adams, 1986; Lanska et al, 1987).
Moving up to the cortex, it has been recently appreciated that the cortical representation of hearing is highly complex, contains considerable parallel processing, and involves at least 4 cortical levels including 15 or more areas in the brain (Kaas and Hackett, 1998). The diagram above shows some of the most important areas. The medial geniculate body (not shown) is the major auditory nucleus of the thalamus. Parts of the medial geniculate are hypothesized to function in directing auditory attention. It sends output to primary auditory cortex, also known as the transverse temporal gyri of Heschl (Brodman's areas 41 and 42, see figure 3), and association auditory cortex (areas 22 and 52, not shown). The medial geniculate also sends output to auditory motor cortex which controls body responses in response to sound. Auditory cortex is functionally divided into three areas including a primary area, AI, a secondary area, AII, and a remote projection region, Ep. Authorities vary in assigning area 42 to primary or secondary auditory cortex. The ventral medial geniculate projects almost entirely to AI, while the surrounding auditory areas receive projections from the rest of the geniculate body. Once again, as with the lower auditory systems, tonotopic relationships are maintained.
Pure word deafness is a subtype of central deafness. This disorder is defined as disturbed auditory comprehension without difficulties with visual comprehension. Patients characteristically have fluent verbal output, severe disturbance of spoken language comprehension and repetition, and no problems with reading or writing. Noverbal sounds are correctly identified. The lesion is classically postulated to be a disruption in connections between the dominant Heschl's transverse gyrus and the medial geniculate as well as callosal fibers from the opposite superior temporal region. It commonly presents initially as a Wernicke's aphasia, which on recovering, difficulties in auditory comprehension persist. While usually caused by a stroke, pure word deafness can arise from other causes of focal cortical lesions such as tumors.
Auditory agnosia, another rare subset of central deafness, is typified by relatively normal pure tone hearing on audiometry, but inability to interpret (recognize) nonverbal sounds such as the ringing of a telephone. Inability to interpret nonverbal sounds but preserved ability to interpret speech may be a result of a right hemisphere lesion alone. Amusia is a particular type of auditory agnosia in which only the perception of music is impaired. Again, right sided temporal lesions are thought to be the cause.
Cortical deafness is essentially the combination of word deafness and auditory agnosia. It is characterized by an inability to interpret either verbal or nonverbal sounds with preserved awareness of the occurrence of sound (as for instance by a startle reaction to a clap. In most instances, the cause is bilateral embolic stroke to the area of Heschl's gyri. It usually results from bilateral lesions and happens when remaining normal auditory cortex is destroyed. It begins as a sudden deafness, which evolves into a picture where patients can hear sounds but are unable to recognize their meaning. Relatively few cases of this disorder have been studied. Note that this syndrome might be difficult to distinguish from a brainstem lesion such as described above. Mendez and Geehan have reviewed this syndrome (1988).
Auditory hallucinations, consist of an illusion of a complex sound such as music or speech. Auditory hallucinations are classically found in schizophrenia, however they can also be a result of brain damage. In this context they most commonly occur as a result of an injury to the superior temporal auditory association areas. Penfield discovered that stimulating this area induces an auditory sensation that seems real to patients. Auditory hallucinations can also occur as a result of a temporal lobe seizure. Occasionally auditory hallucinations can occur due to damage to brainstem structures involved in hearing such as the superior olive (Casino and Adams, 1986; Lanska et al, 1987).