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.

Disorders herein discussed include disease processes in which there are abnormalities on central speech discrimination tests or in which an abnormality of central auditory processing is a major component.

Auditory agnosia can basically be defined as the impaired recognition of non-verbal sounds and noises. The definition implies that the ability to comprehend speech is retained. Coslett and associates (Coslett, et al., 1984) have differentiated auditory agnosia, or auditory agnosia for nonverbal sounds, from pure word deafness, or auditory agnosia for speech. Patients with pure word deafness have an impaired auditory comprehension of speech but otherwise intact language function, including spontaneous speech, reading, and writing. Comprehension of nonverbal sounds is also intact in pure word deafness. In actuality, agnosia for sounds alone presents a rare clinical picture, and most patients with auditory agnosia have a combination of verbal and nonverbal interpretation deficit (Vignolo, 1982). These patients may present with a sudden inability to understand spoken language, repeat spoken words, or write from dictation. The ability to speak, read, or write spontaneously, however, may be preserved. Thus, the deficit is one of auditory language input, and the patients may appear to be deaf. In rare cases in which agnosia for sounds exists in isolation, and there are a few described, varied environmental sounds (the ringing of a bell or running water) or noises such as a siren cannot be distinguished. In patients with either form of auditory agnosia, the pure tone audiogram is normal or only minimally affected, while binaural speech discrimination tasks are markedly abnormal (Coslett, et al., 1984; Rosati, et al., 1982). In patients with agnosia for sounds, lesions involved the right temporal lobe in both, while the corpus callosum remained intact (Hécaen, 1962). In most other clinical reports of auditory agnosia in which the recognition of verbal and nonverbal sounds was impaired, there were bilateral temporal lobe lesions, usually cerebral infarction. In the case of pure word deafness (auditory agnosia for speech) described by Coslett and associates (1984), the patient suffered bilateral infarction of most of the primary auditory cortex but had partial sparing of the auditory association cortex. Since the interpretation of environmental sounds was intact in this case, it was suggested that the auditory association cortex alone was responsible for the decoding of nonspeech sounds (Coslett, et al., 1984). Several cases of auditory agnosia have been described in which there were deficits in the discrimination of tone duration, auditory sequences, and interaural order. These findings lend support to a role of the cortex in the temporal analysis storage of auditory information (Rosati, et al., 1982). Middle and late components of the auditory agnosia may be absent or morphologically abnormal while the early response (BAER) is entirely normal.

Auditory sensory disturbances

Binaural speech discrimination tests such as distorted speech audiometry may reveal deficits characteristically in the ear contralateral to a temporal lobe lesion. Subtle hearing discrimination difficulties may be observed in unilateral temporal lesions as well. For example, patients with right temporal lobectomies have demonstrated impairment on tests of discrimination of tonal patterns and of tone quality or timbre as measured on the commonly used Seashore Music Test (Duane, 1977). Michel and co-workers (1980) have called the asymmetry of late acoustic evoked potentials occurring with unilateral auditory cortical lesions a form of hemisensory deficit--hemianacusia. In this case, however, the term does not refer to a subjective complaint offered by the patient.

Positive auditory phenomena have also been associated with temporal lobe lesions. These include auditory illusions known as paracusia, in which sound volumes may be altered, changed in tone or timbre, or may even sound strange and disagreeable. An extremely unusual form of positive central auditory phenomenon was reported by Auerbach (1981) in a patient with pure word deafness due to bilateral temporal infarcts. This phenomenon, called central razzle, refers to a bothersome sensation that accompanies auditory stimuli such as voices and music. This noxious sensation was thought to be analogous to thalamic or pseudothalamic pain syndromes, although the patient described did not demonstrate a thalamic lesion radiographically. It was postulated that the lesions of the central auditory in some way interrupt descending efferent inhibitory pathways in the auditory cortices. Auditory hallucinations, including elementary sounds or complex forms (music, voices), may occur with lesions of the temporal lobe, such as brain neoplasms, or may accompany epilepsy of temporal lobe origin. The anatomic locus of lesions causing auditory hallucinations has not entirely been pinpointed, but the superolateral part of the temporal lobe is usually involved in these instances, and if visual hallucinations occur as well, the lesion is felt to lie more posteriorly in the involved hemisphere. Rarely, elementary unformed auditory hallucinations have been described with lesions of the pons, referred to as pontine auditory hallucinosis (Adams and Victor, 1981).

Another auditory sensory disturbance seen variably with acute cerebrovascular disease is the presence of auditory extinction following acute hemispheric damage. This phenomenon may be tested clinically at the bedside wherein the simultaneous presentation of two auditory stimuli is symmetrically presented to each ear of the patient. A positive response consists of failure to report hearing from the side contralateral to the lesion when the ipsilateral side is simultaneously stimulated (DeRenzi, et al., 1984). In DeRenzi and co-workers= series of 144 patients presenting acutely with right or left hemispheric cerebrovascular insults, contralateral extinction was present in about half of the patients tested by DeRenzi, et al., 1984. This sensory phenomenon is most prominent shortly following the acute episode, and gradual recovery was observed in the majority of cases after one month. Rarely, patients showed extinction confined to the ear ipsilateral to the lesion, which is perhaps analogous to the left ear suppression seen with verbal dichotic tests in patients with deep hemispheric lesions such as those caused by multiple sclerosis (Rubens, et al., 1985). These patients are felt to have poor scores in the ipsilateral ear to a hemispheric lesion (or in the case of extinction, ipsilateral extinction) due to interruption of callosal fibers connecting the right temporal lobe (left ear auditory processing area) with the left temporal lobe Adecoding@ area (DeRenzi, et al., 1984; Rubens, et al., 1985).

It is hopeful that functional imaging of auditory cortex utilizing methods such as neuromagnetic imaging, PET, and SPECT may yield additional information on central auditory disorders (Don and Ponton, 1994).

Many of the central auditory tests previously discussed may unmask deficits in patients with brainstem lesions, be they due to cerebrovascular disease, tumor, inflammation, or demyelination. A true intra-axial brainstem lesion will not result in a unilateral hearing loss, although any lesion involving the eighth nerve root entry zone or cochlear nucleus either directly or as a pressure effect can result in unilateral hearing loss for pure tones (Baloh, 1990). Examples of such disease processes include acoustic neuroma, other cerebellopontine (CP) angle tumors, demyelinating disease, and infarction in the lateral pontomedullary region. Also, tinnitus is practically never seen as an isolated sign with intra-axial brainstem disease. Fisher (1967) described 10 cases of sudden deafness due to vascular occulion of the anterior-inferior cerebellar artery (AICA) or internal auditory artery (IAA) unilaterally. The majority of patients, however, also reported dizziness. In the patients described, permanent impairment of hearing was frequent and tinnitus was a rare complaint. There was impairment of labyrinth function in some of these cases as well. It is most likely that the hearing loss was the result of eighth nerve ischemia rather than intrinsic brainstem disease, since the internal auditory artery (a branch of the inferior cerebellar artery) supplies the eighth nerve and labyrinth.

Acoustic neuromas and other CP angle tumors that compress the brainstem can present as unilateral hearing loss. Throughout this discussion we have considered lesions affecting the eighth nerve, such as acoustic neuroma (schwannoma), as peripheral. However, any extra-axial mass in the CP angle region that compresses the brainstem can result in hearing loss and can demonstrate abnormalities on central auditory tests. A patient with a significant mass lesion compressing the brainstem may have symptoms such as vertigo, headache, or incoordination and may manifest multiple cranial nerve dysfunction on examination. Acoustic neuromas account for 80% to 90% of all extra-axial CP angle masses (Jackler and Brackman, 1994). The remaining 10% to 20% of tumors in the CP angle region include meningioma (10% to 15%), epidermoid, metastasis, neurofibroma, aneurysms of the circle of Willis, chordoma, chondroma, arachnoid cyst, and epidural abscess. Rarely, a glomus jugular tumor may extend intracranially into the CP angle region. Also, in rare instances an intra-axial brainstem lesion may extend outward into the CP angle.

Numerous abnormalities on routine audiologic testing have been described in patients with acoustic neuroma, including abnormal adaptation (the inability to maintain response to a continuous pure tone signal), the rollover phenomenon on performance intensity functions (PI) of speech materials (Keith, 1994) and reduced speech intelligibility scores. These patients may demonstrate abnormalities on the acoustic reflex test.

Brainstem audiometry and vestibular tests are extremely useful and sensitive to small and compressive lesions. MRI is the procedure of choice in the radiologic investigation of acoustic neuromas including early intracanalicular lesions.

A variety of intrinsic and extrinsic brainstem lesions can result in bilateral hearing loss. Dix and Hood (1973) have demonstrated a pattern of symmetric hearing loss in each ear of a series of patients with proved lesions of the brainstem. Their series included intrinsic brainstem glioma, cerebellar glioma with compression of the brainstem, neurodegenerative disease involving the cerebellum and brainstem, and diffuse vascular disease. In addition to the pure tone audiographic findings of symmetric loss, these patients often demonstrated loudness recruitment. Loudness recruitment is usually attributed to a retrocochlear lesion, but in this study it was associated specifically with brainstem disease. Furthermore, a pattern of symmetric hearing loss on pure tone audiometry associated with loudness recruitment in patients with brainstem lesions may point to dysfunction above the level of the cochlear nuclei and only when fibers subserving identical frequency bands on both sides of the brainstem are involved (Dix and Hood, 1973). Similar results have been obtained in Luxon=s patients with brainstem disorders (Luxon, 1980). Seventy-five percent of the patients in this series suffered a bilateral hearing loss on pure tone audiometry, which was usually slight to moderate and characteristically involved the higher frequencies. Also, the bilateral deficits were largely symmetric, but a characteristic pattern to the audiogram was not observed. Loudness recruitment was commonly observed in cases of bilateral hearing loss except when a unilateral deficit resulted from lesions involving the cochlear neclei or nerve trunk (Luxon, 1980). Severe, bilateral hearing loss for all frequencies in the hearing range has been described by Keane in the case of locked-in syndrome due to extensive pontine hemorrhage (Keane, 1985).

The hallmark of hearing loss with acoustic neuroma is a slowly progressive unilateral loss accompanied by tinnitus. However in unusual cases, the hearing loss may be sudden or since the tumors usually rise on the vestibular portion of the eighth nerve and are, in fact, vestibular schwanomas, patients with CP angle tumors can present with normal hearing (Novac, 1994).