Diagnostic Principals in Neuro-otology: The Auditory System
B. Todd Troost and Melissa A. WallerOUTLINE
INTRODUCTION
Neuro-otology is a subspecialty that includes disorders of the peripheral and central auditory and vestibular systems. The majority of neuro-otologists have come from the field of otolaryngology with the focus on the ear with its vestibular and auditory functions without reference to central auditory processing within the central nervous system. A variety of causes of dizziness and disequilibration are accompanied by auditory complaints and, thus, neurologists who choose to deal with dizzy patients should be familiar with both vestibular and auditory disorders.
In the assessment of hearing, abnormalities of the auditory system may be a manifestation of a systemic and possibly life-threatening disorders. The examiner should obtain a history of past, present, and familial audiologic and otologic complaints. One of the best techniques for determining whether there is an auditory disorder is to ask family members or co-workers whether there appears to be difficulty in hearing. The patient often may be unaware of hearing dysfunction, particularly if it is unilateral. The first few minutes spent talking with the patient or relatives will help determine the direction the inquiry should take. Subsequent examination of the patient and preliminary audiologic findings, if any, determine how inclusive examination should be and what subsequent tests should be ordered. It is important to remain aware that audiologic tests do not always provide an exact diagnosis. Results of the audiologic test battery must be integrated with the neurological, otoneurological, and radiological information to determine the most accurate diagnosis.
BASIC ANATOMY AND PHYSIOLOGY RETURN TO OUTLINEThe auditory system differs significantly from the visual and somatosensory pathways in that there is no large direct pathway from peripheral receptors to the cortex. Rather, information ultimately reaching the auditory cortex undergoes significant reorganization as it passes through the brainstem (Moore, 1994). A general conclusion reached from work on the anatomic and chemical composition of the auditory pathway is that inhibition plays an extremely important role at all levels of the system in shaping the exquisitely precise responses of central neurons. One implication of these complexities of central organization relates to the placement of CNS stimulating devices and the therapy of auditory disorders (see section on Cochlear Implantation later). Detailed description of peripheral and central auditory pathways is found in standard texts (Baloh and Honrubia, 1990; Jackler and Brackmann, 1994).
Peripheral Auditory System RETURN TO OUTLINESounds that reach the ear set the tympanic membrane in motion and this motion is conducted to the fluid of the cochlea by the three ossicles of the middle ear. The middle ear function as an impedance transformer. It improves the transmission of sound to the cochlear fluid. This improvement in transmission is mainly the result of the large ratio between the area of the tympanic membrane and that of the stapes footplate. Two small muscles are attached to the ossicles. One, the tensor tempani, is innervated by the trigeminal nerve and pulls the tympanic membrane inward when it contracts. The tympanic membrane is stretched and thereby attenuates sound transmission for low-frequency sounds. The other muscle, the stapedius muscle, is attached to the stapes and pulls the stapes in the direction perpendicular to its normal motion in response to sound. The stapedius muscle is innervated by the facial nerve and its contraction also decreases the middle ear=s ability to conduct low-frequency sounds. In humans, the stapedius muscle contracts in response to a strong sound, the acoustic reflex. Motion of the fluid in the cochlea sets the basilar membrane into motion. The sensory cells (hair cells) that are innervated by the fibers of the auditory nerve are located along the basilar membrane and convert the motion of the basilar membrane into a neural code. As a result of the hydromechanical properties of the cochlea, a sound gives rise to a traveling wave on the basilar membrane. The distance the wave travels before it reaches its peak amplitude is a direct function of the frequency of the sound. Because tones of different frequency give rise to maximal vibration amplitudes at different locations along the basilar membrane, the spectral components of a complex sound are separated along the basilar membrane according to frequency. Thus, the cochlea is a frequency analyzer. Since the cochlear hair cells located along the basilar membrane are innervated by fibers of the auditory nerve, the discharge pattern of individual auditory nerve fibers can be expected to reflect the vibratory pattern of the basilar membrane and therefore process frequency. A certain frequency exists at which the threshold of an auditory nerve fiber is lowest. The range of sound frequencies important to humans is well above threshold; in addition, the sounds that are significant to human communication are not pure-tones, but complex sounds that contain many spectral components. The discharges of single nerve fibers are phase-locked to the waveform of sounds within their response areas which is believed to be the basis for the temporal hypothesis for frequency discrimination in the auditory system originally known as the volley theory. Recent evidence has accumulated indicating that temporal coding plays an important role in the coding of frequency or spectral composition of sounds. In theory, temporal coding would be just as efficient in carrying information about a sound=s frequency or spectral composition to the brain as coding by the place principal which assumes that specific nerve cells are activated by tones of specific frequency or specific spectral components of a complex sound. However, because of the temporal jitter that occurs in synaptic transmission, the temporal code is converted to a place code at a peripheral location along the ascending auditory pathway. The anatomical location of such a conversion has not yet been identified but may be in the cochlear nucleus or other nuclei of the ascending auditory pathway.
The human auditory system is complex, yet highly ordered. It should be noted that the central auditory system, at least histologically, arises at the neuroglial-neurilemma junction of cranial nerve VIII within the internal auditory canal. The first-order neurons of the auditory system are cells of the spiral ganglion situated within the modiolus or central core of the cochlea. In humans, there are approximately 32,000 myelinated cochlear nerve fibers. The cochlear nerve occupies the anterior-inferior portion of the internal auditory canal and the vestibular nerve occupies the posterior half. The facial nerve or cranial nerve VII is located in the anterior-superior quadrant of the internal auditory canal. A tonotopic relationship is preserved throughout the entire auditory nervous system. Figure 1 represents a summary schematic diagram of the central pathways.
The lateral lemniscus is the principal ascending auditory pathway in the brainstem. The lateral lemniscus originates laterally to the superior olivary complex, but at the level of the inferior colliculus it lies at a more dorsal portion in the brainstem (see Figure 1). There are diffuse cellular groups within this bundle known to constitute the ventral and dorsal nuclei of the lateral lemniscus. Projections from these cells proceed to the midbrain and terminate in the inferior colliculus. The inferior colliculus, located in the midbrain tectum, serves as a relay center for all of the ascending and descending auditory fibers. Ascending fibers and some fibers in the lateral lemniscus constitute the afferent bundle known as the brachium of the inferior colliculus. These fibers synapse in the medial geniculate body of the thalamus. Interaural time intensity comparisons occur at the inferior colliculus so that it also plays a role in auditory localization. In addition to receiving ascending afferent fibers from the lateral lemniscus and the contralateral inferior colliculus, each colliculus receives descending projections from the ipsilateral medial geniculate body and the auditory cortex. The auditory cortex projects fibers bilaterally to the colliculi. A small portion of fibers pass from the inferior to the superior colliculus which might provide an anatomic connection enabling reflex circuits between auditory and visual systems.
The medial geniculate body is situated on the caudal aspect of the thalamus and is the last major relay station for ascending auditory fibers before they reach the cortex. There is a tonotopic arrangement in the medial geniculate body in which low frequencies are represented laterally and high frequencies are located medially in the principal division. The main projection of the medial geniculate body is to the superior temporal convolution or transverse gyrus of Heschl via the geniculotemporal (auditory) radiations. At the subcortical level, the auditory radiations can be seen in the sublenticular portion of the internal capsule. The medial geniculate body also sends fibers to other thalamic nuclei and may play a part in a regulatory feedback system, with descending projections to the inferior colliculus, the nucleus of the lateral lemniscus, the trapezoid body, and the superior olivary nucleus.
The primary auditory cortex corresponding to Brodmann areas 41 and 42 lies on the transverse gyrus of Heschl on the dorsal surface of the superior temporal convolution. Brodmann=s area 41 is a primary auditory reception area and receives its projections from the pars princapalis of the medial geniculate body. Areas 42, 52, and 22 lie immediately adjacent to the primary auditory cortex and are auditory-association areas (Figure 2). These association areas receive signals from the primary auditory cortex and send projections to the occipital, parietal, and insular cortex. Tonotopic organization of the auditory cortex is particularly impressive. In the simplest analysis, high frequencies are represented anteriorly and low frequencies posteriorly in the auditory cortex. Each auditory cortical area is reciprocally connected to a homotypic area in the contralateral hemisphere via projections in the corpus callosum. In addition, auditory-association areas connect with other sensory-association areas concerned with somatesthesia and vision. They also send projections that converge in the parietotemporal language area. It appears that the higher level of integration in the association areas is responsible for more complex interpretation of sounds. These properties of the auditory cortex may explain why patients with hemispheric lesions have little difficulty with hearing as measured by pure tone audiometry. However, such patients may have impaired ability to discriminate the distorted or interrupted speech patterns and have difficulty focusing on an isolated speech sample when a competing message is introduced.
There is a descending efferent auditory pathway that parallels the afferent pathway and is influenced by ascending fibers via multiple feedback loops. The specific function of this system in audition is not well understood, but clearly modulates central processing and regulates the input from peripheral receptors in a fashion similar to the role played by the efferent vestibular system.
It should be apparent from this brief overview of the peripheral and central auditory systems that the pathway is a complex multisynaptic system throughout which tonotopic organization is preserved. Maintenance of this tonotopic organization allows the cochlea to be represented at each synaptic locus and at various areas on the auditory cortex. A cardinal feature of the auditory nervous system is the extensive binaural representation of acoustic information at various levels resulting from the interaction of neural input from both ipsilateral and contralateral pathways.
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 Loss RETURN TO OUTLINESensorineural 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 Loss RETURN TO OUTLINEMixed 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.
SENSORY VERSUS NEURAL LESIONS RETURN TO OUTLINEThe 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 RETURN TO OUTLINEAs 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.
Hemispheric Lesions RETURN TO OUTLINEDisorders 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 RETURN TO OUTLINEAuditory 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.
Cortical deafness RETURN TO OUTLINEPositive 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).
Brainstem Lesions RETURN TO OUTLINEMany 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).
Ear and head noises, the most common complaints presented to the audiologist or otolaryngologist, are frequently seen by the neurologist as well. As many as 32% of the adult population have tinnitus, with 20% of the population rating their condition as severe (Vernon, 1994). Tinnitus may be considered a significant symptom when its intensity so overrides normal environmental sounds that it invades the consciousness. The patient experiencing tinnitus may describe the sound as ringing, roaring, hissing, whistling, chirping, rustling, clicking or buzzing, or other descriptors. Although most patients report the presence of tinnitus to be constant, others report it to be intermittent, fluctuating, or pulsating. Tinnitus may be perceived as a high - or low - pitched tone, a band of noise, or some combination of such sounds.
The perceived loudness of tinnitus in any patient may be intense enough to be highly debilitating. Most patients with sensorineural hearing loss report tinnitus to be a high-frequency tone, but tinnitus associated with conductive hearing loss tends to be low in frequency. However, knowledge of the pitch of the tinnitus is of little diagnostic benefit other than allowing for the gross dichotomy of conductive versus neural pathology.
The majority of tinnitus sufferers have a concomitant loss of hearing, which may be either conductive or sensorineural. Only a minority of tinnitus patients have audiometrically hearing sensitivity. Tinnitus may precede or follow the onset of a loss in hearing, or the two may occur simultaneously.
Tinnitus is a symptom of an underlying disease or specific lesion when it is perceived above the intensity levels of environmental sounds. It may be the first symptom that brings the patient to a neurologist. The complaint may be an early symptom of a tumor in the internal auditory meatus or in the cerebellopontine angle, a glomus tumor, or a vascular abnormality in the temporal bone or skull. Because tinnitus may be a characteristic symptom of a number of disorders, a complete medical and audiological evaluation is an important initial step in the management process.
It has generally been assumed that the anatomic location of the physiologic abnormality that causes tinnitus is in the ear and, therefore, that tinnitus is associated with pathological processes involving the ear. Studies in animals have shown that when an electrical direct current is passed through the cochlea, spontaneous activity can be reduced in single auditory nerve fibers. These findings led to studies in which an electrical direct current was passed through the cochlea (positive current applied through the round window) in patients with tinnitus. The result in reduction of tinnitus in some patients gave support to the hypothesis that tinnitus is associated with increased spontaneous activity in auditory nerve fibers (Moller, 1994). The fact that tinnitus cannot be masked in the same way as an external sound supports the assumption that tinnitus is not usually associated with the same type of auditory nerve activity as is evoked by sound. Possible central causes of tinnitus such as from decrease central nervous system inhibition are discussed by Moller (1994). The fact that many different hypotheses are currently being considered regarding the anatomic location of the physiologic abnormalities that cause severe tinnitus may reflect the many different causes of tinnitus, or the lack of sufficient knowledge of the disorder. It is clear that the pathophysiology of tinnitus is considerably more complex than previously assumed as there is little direct evidence that known abnormalities of the cochlea are directly involved in the generation of tinnitus.
Subjective tinnitus is a auditory sensation heard only by the patient. It may be present in one or both ears or localized within the head. For most patients, tinnitus is a subjective sensation. This type of tinnitus can result from a lesion involving the external ear canal, tympanic membrane, ossicles, cochlea, auditory nerve, brainstem, and cortex. The most common cause is cochlear disease. Tinnitus associated with Meniere's syndrome is often low-pitched and continuous, and is described as a hollow seashell sound or very loud roaring. Tinnitus with otosclerosis is also low-pitched, is described as a buzzing or roaring sound, and may be continuous or intermittent. Continuous bilateral or unilateral high-pitched tinnitus often accompanies chronic noise-induced hearing loss, presbycusis, and hearing loss due to ototoxic drugs. A number of drugs such as aminoglycosides, quinidine, salicylates, indomethacin, carbamazepine, propranolol, levodopa, aminophylline, and caffeine, may produce tinnitus with or without associated hearing loss (Baloh, 1984).