Clinical manifestations

Presentation and course

Central hearing loss may be associated with other central auditory disorders, including dysacusis (ie, the experience of sound as distorted or disagreeable), impaired sound localization, and auditory hallucinations (64; 72). Dysacusis may be experienced as a disagreeable buzzing, banging, rattling, or roaring, and speech may be perceived as if the speaker was speaking a foreign language (72).

Because the acoustic characteristics of sound are not correctly deciphered, patients make errors in sound recognition, are inconsistent in matching identical environmental sounds, and make phonemic substitutions when attempting to reproduce sounds (72). Foreign words and nonsense words are reproduced with particular difficulty. Even among those with normal hearing sensitivity, central auditory processing deficit is associated with greater perceived hearing difficulty (105).

Unilateral brainstem deafness. The spectrum of clinical presentation of anterior inferior cerebellar artery-territory (AICA) infarction includes ipsilateral hearing loss with or without tinnitus as well as a range of labyrinthine, brainstem, and cerebellar symptoms and signs. Other manifestations include ipsilateral Horner syndrome (rare), skew deviation (rare), nystagmus, ipsilateral facial numbness, ipsilateral facial paresis, vertigo, dysarthria, vomiting, unsteadiness, ipsilateral hemiataxia, and contralateral loss of pain and temperature sensation on the limbs and body (03; 39; 87; 04; 65). Occasionally, isolated vertigo or isolated auditory disturbance may occur as transient ischemic attacks preceding AICA-territory infarction, or with partial infarcts (87; 04). Bilateral sudden deafness may occur as a prodrome of AICA-territory infarction in the presence of severe vertebrobasilar occlusive disease (67; 116). Hearing loss occurs in most patients with AICA infarction (over 90%) (66); most commonly this is due to cochlear ischemia (about half of cases) (01; 66), with the majority of the remainder being mixed cochlear and “retro-cochlear” hearing loss and with rare cases of isolated “retro-cochlear” hearing loss; the “retro-cochlear” cases of hearing loss in AICA infarctions may involve the eighth nerve (“neural”) or the cochlear nuclei and associated central auditory structures in the brainstem.

The spectrum of clinical presentation of superior cerebellar artery syndrome includes ipsilateral Horner syndrome, ipsilateral limb ataxia, contralateral superficial sensory loss, vertigo, nystagmus, nausea, and vomiting, but it may rarely include contralateral central hearing loss (due to involvement of the lateral lemniscus carrying decussated ascending auditory information) (81). The reason for such an atypical symptom as unilateral hearing loss with a lesion above the cochlear nucleus is unclear because almost always lesions at this level involve binaural sensory information and so do not cause unilateral hearing loss (25; 38).

Bilateral brainstem deafness. Bilateral brainstem deafness can occur with lesions involving bilateral ascending auditory pathways from the pons through the midbrain (109; 42; 45; 43; 117; 40; 84; 11; 111). Usually, this results from vascular lesions of the pons (24; 11; 111), including hematomas, as well as staged anterior inferior cerebellar artery-territory infarcts (24). Vascular or traumatic lesions of the midbrain may also be responsible, especially with traumatic contusion of the dorsal midbrain by the incisura of the tentorium as a result of extreme acceleration-deceleration forces (42; 45; 43; 47; 46; 117; 40; 84). Sudden bilateral deafness can be a warning sign of imminent brainstem ischemia by occlusion of the basilar artery or both vertebral arteries, with a spectrum of pathology that may include bilateral infarctions of the cochlear and vestibular nuclei (116; 11).

Few cases of the auditory effects of lesions restricted to the inferior colliculi have been reported (45; 10; 74; 43; 47; 46; 117; 40; 09), eg, from infiltration by tumors (40), or from removal of tectal plate gliomas (74), or from trauma (09). Because almost all ascending and descending auditory pathways synapse in the inferior colliculi, bilateral destruction of the inferior colliculi can produce complete deafness. Indeed, some patients experienced complete deafness, and pure-tone audiometry was not obtainable in some reported cases (45; 43). In others, relatively preserved pure-tone hearing was combined with decreased word recognition, severely impaired speech comprehension, or complete word deafness (10; 74; 117; 40). Various cases have also documented sparing of brainstem auditory evoked potentials despite lesions of the inferior colliculi (45; 10; 74; 43; 47; 46; 117). Surgical cases have documented dramatic postsurgical deterioration of speech comprehension with preservation of pure-tone audiometry and brainstem auditory evoked potentials (74).

Occasionally, auditory hallucinations can occur due to damage to brainstem structures involved in hearing, especially with acute lesions of the pontine tegmentum (19; 64). Such cases typically occur in association with central deafness. Clinical resolution of the auditory hallucinations and central hearing loss may accompany overall clinical improvement (eg, with resolution of a pontine hematoma or with antibiotic treatment of a brainstem abscess).

Cortical auditory disorders. Onset of cortical deafness is usually characterized by sudden deafness, which often resolves to the point where patients can hear sounds but are unable to recognize their meaning, or they may react to sounds as with a startle reaction to a clap (72). Most often this is the result of staged bilateral embolic infarcts involving the temporal lobes or less commonly the insula (26; 56; 60; 21; 14; 06; 104; 82; 68; 59; 76). In exceptional cases, bilateral embolic infarcts involving the temporal lobes may present concurrently (107). Cortical deafness may uncommonly be caused by other forms of cerebrovascular disease, delayed traumatic intracerebral hematoma, demyelination, neoplasms, and meningitis (54; 103; 106). For example, a case of a 38-year-old man resulted from a ruptured right internal carotid-posterior communicating artery aneurysm and subarachnoid hemorrhage with resulting extensive infarction in the bilateral primary auditory cortices (as well as other areas) (54). In the majority of cases of cortical deafness, there is bilateral involvement of Heschl gyri, but in exceptional cases, either subcortical areas (presumably affecting the auditory radiations) or the insula is involved instead (26; 85; 52). Cortical deafness from isolated (unilateral) lesions probably does not exist (72).

In a patient with profound cortical deafness who was unable to consciously perceive sound, reaction time was reliably shortened on visual reaction time trials where an unperceived acoustic stimulus was presented, confirming the presence of multisensory response enhancement (18). This suggests that activation from subcortical auditory processing circuits can contribute to cortical or subcortical areas responsible for the initiation of a response, without the need for conscious auditory perception (18). The precise mechanism for this effect, though, is unclear.

Cases reported as cortical deafness (due to a primary central sensory audiologic deficit) overlap clinically with cases of “pure” word deafness and generalized auditory agnosia (14; 72; 53). Pure word deafness (or auditory verbal agnosia) is a selective impairment in the ability to understand speech sounds, even though the patient can hear them; the patient can hear and recognize nonspoken sounds, and spoken language, reading, and writing are preserved (14; 94; 53; 110). Such individuals are unable to understand or repeat spoken words or write to dictation. Generalized auditory agnosia is an impaired ability to interpret both verbal and nonverbal sounds, even though the patient can hear them (14; 53).

Patients with diagnoses of pure word deafness often demonstrate evidence of a more pervasive auditory agnosia when tested formally for linguistic and nonlinguistic sound comprehension and musical abilities (14). Unilateral cases may be more frequently accompanied by aphasic symptoms (94). Because such cases are seldom if ever “pure” when carefully tested (17; 14; 72), Buchman and colleagues suggested the simpler term of “word deafness” be adopted (14). Furthermore, some reported cases show evolution of clinical features from one category to another (06; 72; 82).

Like cortical deafness, word deafness is most commonly a result of bilateral cortico-subcortical lesions of the temporal lobes (36; 110), although there are well-recognized cases in which the lesion involves only the left temporal lobe (36; 94; 71) or left hemispheric subcortical structures (112). Word deafness most frequently stems from strokes causing (usually sequential) bitemporal cortico-subcortical lesions, presumably on a cardioembolic basis (14; 110), though uncommonly the same picture can occur as a result of staged intracerebral hematomas (12). The cases with bilateral lesions may develop with partial resolution of cortical deafness (72; 94) whereas unilateral lesions especially may become evident after clinical improvement following an initial Wernicke aphasia. In childhood, word deafness can be caused by Landau-Kleffner syndrome (acquired epileptic aphasia), and it is often the first symptom of this disease (73). Paroxysmal word deafness secondary to focal epilepsy has also been rarely reported in adults (28). Tumors and neurodegenerative disorders (such as Alzheimer disease, frontotemporal dementias, primary progressive aphasia, Creutzfeldt-Jacob disease) and encephalitis (eg, herpes simplex encephalitis) can also rarely produce cortical deafness and related syndromes (115; 89; 53; 49; 58; 55; 20; 118).

Various frameworks have been proposed for considering the deficits associated with word deafness, with many reports assuming impairments of low-level pre-phonetic auditory processes (05; 21), whereas others implicate phonetic processes (101; 17), and still others advocate mixed processes resulting from “an interaction between an impairment of general auditory processes and a deficit at the level of phonetic analysis” in the majority of such patients (94). In 1965, Norman Geschwind suggested a “disconnection” framework for understanding word deafness that accommodated cases with either left temporal or bitemporal lesions: the left temporal lesions were suggested to have destroyed the left optic radiations or the cortical pre-linguistic auditory area and, in addition, destroyed the trans-callosal interhemispheric fibers from the opposite (right) auditory area so that the speech area was effectively disconnected from incoming auditory information (36). Although the disconnection model works fairly well in explaining most cases of word deafness, some cases with unilateral left hemisphere lesions appear to have preserved cross-hemispheric connectivity and, instead, may have a deficit in temporal processing difficulty (108). In 1982, Auerbach and colleagues suggested there may, in fact, be two distinct subtypes of pure word deafness: type 1, in which the deficit was proposed to be pre-phonemic (ie, a general auditory process), related to a temporal auditory acuity disorder, and associated with bilateral temporal lobe lesions; and, type 2, in which there is a presumed deficit in linguistic discrimination and an association with left unilateral temporal lesions (23; 101; 05; 108). Auerbach and colleagues further suggested that Type 2 is a “fragment,” or perhaps forme fruste, of Wernicke aphasia (05). However, a critical analysis by Praamstra and colleagues discounted this framework, arguing based on a detailed and careful analysis that cases are often mixed and that the differentiation into two categories was simplistic and artificial, recognizing, for example, the “improbability of a [strictly prelinguistic] acoustic deficit as the only cause of the syndrome in all bilateral cases” (94).

Because common features can be delineated in reported cases of pure word deafness, auditory agnosia, and cortical deafness (14; 94), Buchman and colleagues have suggested that these disorders form a continuum rather than being three distinct syndromes (14). Others subsequently suggested that the different clinical syndromes caused by cortical auditory lesions form a spectrum of related auditory processing disorders, with syndromic differences resulting from differences in the degree of involvement of the primary cortical processing system, a more diffuse accessory system, and potentially also the efferent auditory system (72). Although all reported cases of word deafness have exhibited additional auditory deficits, word deafness is the most distinctive deficit on clinical examination; for this reason, patients with disorders on this spectrum are often categorized under the rubric of “word deafness” (14).

An odd feature of cortical auditory disorders is so-called “inconsistent auditory behavior” or “auditory behavioral inconsistency” (72). For example, cortically "deaf" patients may complain of soft environmental sounds despite having no reaction to a ringing telephone or even to loud noises. Also, as patients recover from cortical deafness, they often regard themselves as deaf and may deny hearing sounds that they react to. In addition, such patients lack common behaviors of patients with acquired peripheral deafness such as asking for clarification from a speaker. Such inconsistencies may be due to disrupted auditory attentional mechanisms (72).

The inability to interpret nonverbal sounds, but preserved ability to interpret speech, may be a result of a right hemisphere lesion. Amusia is a type of auditory agnosia in which only the perception of music is impaired; this usually occurs with right temporal lesions. For example, resection of right hemisphere gliomas involving Heschl’s gyri can result in impairment of music comprehension (100).

Patients with auditory agnosia may have anosognosia for the deficit (59).

Deficits in higher-level segregation processes have been reported following a right hemisphere lesion affecting nonprimary auditory cortex (41). A 33-year-old woman developed an unusual pattern of acquired central auditory deficits after an infarct in the right middle cerebral artery territory causing damage to the part of the auditory cortex including the planum temporale, as well as involving the posterior insula and inferior parietal lobule, but sparing the medial portion of Heschl gyrus. She reported difficulty segregating speech from noise and segregating elements of music. The deficits were most pronounced for sounds presented to the left ear and affected the segregation of words, music, and more basic abstract stimuli. Clinical tests showed no evidence for abnormal cochlear function. Additional testing demonstrated difficulties with auditory segregation in her left ear that spanned multiple domains, including words-in-noise and music streaming. These symptoms were considered to be analogous to the visual symptom of simultaneous agnosia.

Central presbycusis. Hearing loss is strongly correlated with age, and much of this reflects accumulated cochlear damage with aging (presbycusis). There are, however, central contributions to age-associated hearing impairment associated with declines in auditory processing capability (44; 91; 80; 102; 95). Although there is insufficient evidence to establish the existence of an isolated central form of presbycusis, available evidence does support the existence of multifactorial changes in central auditory processing that contribute to age-associated declines in hearing (44). Speech-in-noise hearing (eg, spoken digit recognition in noise) declines exponentially with age from about age 50 years as opposed to pure-tone audiogram data that show a more linear decline. Age-associated declines in the ability to hear in noisy environments are felt to be a key symptom of “central presbycusis” (119; 29). However, the age-associated decline in speech-in-noise hearing is markedly worse among those with lower cognitive ability (80), so it is not entirely clear whether performance on such tasks is a function specific to central auditory processing or rather a manifestation of the more global impairments in cognition. Unfortunately, studies of the age-associated central contributions to hearing impairment are frequently complicated by confounding with age-associated changes in both peripheral hearing (ie, due to cochlear damage) and cognition, and few studies are available that adequately separate these issues.

Dementia. Hearing loss is associated with the onset of dementia (69; 91). An association between hearing loss and dementia can occur via a number of potential mechanisms, which are not mutually exclusive and which all may contribute to the observed association: (1) hearing loss impairs performance on tests of cognition, resulting in false diagnoses or earlier diagnoses of dementia; (2) hearing loss actually further worsens cognition in those with cognitive impairment; (3) the dementing process impairs central auditory processing in addition to other cognitive functions; (4) the underlying disease process impairs cognition and hearing by separate but mechanistically related processes (eg, cerebral amyloid angiopathy with superficial siderosis). Central auditory dysfunction occurs as part of the dementing process in Alzheimer disease; detectable central auditory dysfunction may precede clinical dementia and might possibly be an early marker of incident dementia (35; 33; 32; 31; 102; 20; 114). Indeed, reduced executive functioning is associated with impairments in central auditory processing (34). Consequently, studies of central auditory processing (eg, with various dichotic behavioral auditory tests) in early cases suggest that such behavioral central auditory tests may be useful diagnostically (33; 30). Slight and moderate peripheral hearing loss markedly increases the risk of environmental sound recognition deficits, suggesting an interaction between peripheral hearing loss and Alzheimer disease pathology (20). Consequently, peripheral age-related hearing loss has been recognized as the modifiable risk factor with the greatest potential impact on the development of dementia (102). However, this relationship may not be causal, and there is no evidence that improving hearing changes the onset or progression of Alzheimer disease. Other studies have found an association between hearing loss in Alzheimer disease and loss of brainstem and cerebellar volume (70).

Prognosis and complications

Central deafness is often a protracted problem. Some patients may regain auditory abilities over months, as occurred in a young man who developed central deafness after a subarachnoid hemorrhage that produced bilateral involvement of auditory structures within the midbrain (84).

Clinical vignette

Case 1. A 60-year-old, right-handed man with previously normal hearing developed sudden deafness (72). On admission 2 weeks later, he was alert, with fluent speech but did not respond to verbal questions or environmental sounds and did not startle to loud noises. Although the patient appeared to be unaware of sound, he occasionally and inconsistently reacted to some sounds. Although he stated he could not hear, he did not request that the speaker either talk with increased volume, repeat what was said, or write things down nor did he tilt an ear toward the speaker. A noncontrast CT scan of the head showed bilateral hemorrhagic infarcts in both temporal lobes involving the superior temporal gyri as well as surrounding hypodensity from the temporal tips to the parietal lobes. Pure-tone audiometry demonstrated pure-tone thresholds to be at least 70 dB HL, above which responses were inconsistent.

After 2 weeks, he had consistent awareness of sounds. Pure-tone thresholds were 30 to 40 dB HL in the right ear and 20 to 25 dB HL in the left ear, although his responses continued to be variable. He was unable to differentiate voices, music, and environmental sounds. All sounds were perceived as disagreeable noise (“hurr”). He did not startle to loud noises and was unable to localize sound.

After 6 weeks, he had a mild fluent aphasia with paraphasic errors on naming. The paraphasic errors tended to be acoustically similar, so for example, he responded “whiskle” for “whistle.” Verbal repetition was severely impaired with phonemic substitutions that he self-corrected by acoustic approximations. In contrast, reading and writing were relatively preserved.

A follow-up CT scan of the head after 3 months showed smaller bilateral lesions of the superior temporal gyri bilaterally.

Case 2. An 11-year-old boy had genetically confirmed MELAS syndrome with microcephaly, small stature, learning disabilities, refractory epilepsy, mild weakness, and fatigue (93). After a nocturnal bout of seizures, he awoke unable to respond to auditory stimuli, even to loud sounds, even though he seemed unaware of this. Brainstem auditory evoked potentials were preserved. Central deafness was, therefore, suspected. MRI showed multiple bilateral stroke-like lesions involving particularly the superior temporal lobes including both primary auditory cortices.

Despite some hearing recovery within 2 weeks following high-dose intravenous L-arginine in addition to his prior treatment with oral citrulline, he sustained permanent residual language and cognitive deficits.

Biological basis

Etiology and pathogenesis

The central auditory system. The auditory system includes multiple interconnected subcortical nuclei and cortical areas (50; 51). In general, because of bilateral projections at multiple levels of the ascending auditory pathways rostral to the cochlear nuclei, a unilateral central hearing loss can only occur from a lesion of the cochlear nuclei; above the cochlear nuclei, bilateral brainstem lesions are necessary for central deafness, in which case the deafness is bilateral (25; 38). Binaural interaction occurs primarily and almost simultaneously at the superior olivary complex, the nuclei of the lateral lemniscus, and the inferior colliculi (79). Binaural information is coupled with spectral cues to produce topographic representations of auditory space (79).

Ascending auditory pathways. The ascending pathways are modifiable early in life but not later. Neonatal, unilateral hearing loss produces a rearrangement of binaural connections in the auditory brainstem, changes the responses of neurons in the inferior colliculi in response to stimulation of the normal ear, and also produces compensatory alterations in the auditory space map in the superior colliculi (79).

There is a “fast” ascending auditory system from the cochlea to the ipsilateral dorsal cochlear nucleus, then decussating and ascending in the lateral lemniscus to the inferior colliculus in the midbrain, then to the medial geniculate body, and then to the primary auditory cortex in the transverse temporal gyrus.

There is also a slower ascending system that involves many more synapses and that may provide more information about the character of the sounds heard, such as location. In the slow ascending auditory pathway, impulses from the cochlea synapse first in the ipsilateral ventral cochlear nucleus and then in the ipsilateral and contralateral superior olivary complex, with the crossing fibers forming the trapezoid body of the pontine tegmentum. Thus, the superior olivary complex, located posterior (dorsal) to the trapezoid body, receives binaural input and is, therefore, the first place in the central auditory system where binaural processing (stereo hearing) is possible. Different parts of this complex serve to measure the inter-aural time difference (the difference in time of arrival of sounds between the ears) and the inter-aural level difference (the difference in sound intensity between the ears). Fibers from the superior olivary complex in the slow ascending auditory system also pass through the lateral lemniscus to the inferior colliculus, the medial geniculate body, and the primary auditory cortex. From the superior olivary complex to the transverse temporal gyri, the pathways incorporate signals pertaining to both ears.

Effectively all auditory afferents travel through the midbrain in the lateral lemnisci, and all auditory afferents synapse in the inferior colliculi before passing to the medial geniculate body in the thalamus. The medial geniculate body, the major auditory nucleus of the thalamus, is important in directing auditory attention. The ventral division of the medial geniculate nucleus projects in a tonotopic mapping to the primary auditory cortex, whereas the surrounding auditory areas receive more diffuse projections from the rest of the geniculate body.

Auditory cortex. The auditory cortex is located bilaterally, on the superior temporal plane, within the lateral fissure; it comprises parts of Heschl's gyrus and the superior temporal gyrus, including planum polare and planum temporale. The auditory cortex is functionally divided into a primary area and surrounding peripheral or “belt” regions. The primary auditory cortex is essential for comprehending speech, but comprehension of nonverbal sounds may be mediated by other cerebral structures, including auditory association cortex (21). The right planum temporale (the cortical area just posterior to Heschl's gyrus within the Sylvian fissure) is apparently critical for sound localization in the presence of multiple distracter sound sources (eg, at a cocktail party) (122).

The transverse gyrus of Heschl, located bilaterally on the temporal plane within the Sylvian fissure, is an important (but only approximate) marker for the primary auditory cortex. The borders of Heschl’s gyrus do not indicate exact architectonic borders, and the morphology of Heschl’s gyrus varies across individuals and brain hemispheres (97; 22). Partial or complete duplications of Heschl’s gyrus occur in nearly half of all individuals, so such duplications must be considered normal variants (96). It had been thought that the primary auditory cortex occupies only the first (more anterior) division of such duplications of Heschl’s gyrus, but studies have shown that this is not the case; the primary auditory cortex spans both divisions of Heschl’s gyrus in cases with partial or complete duplications (22). The human primary auditory cortex occurs on approximately the medial two thirds of Heschl’s gyrus, with secondary architectonic regions occupying the lateral end of Heschl’s gyrus (98).

According to the cytoarchitectural parcellation of the cerebral cortex published in 1909 by German anatomist and neurologist Korbinian Brodmann (1868-1918), based on the cytoarchitectural organization of neurons observed using the Nissl method of cell staining, primary auditory cortex in the transverse temporal gyri of Heschl corresponds to Brodmann’s areas 41 and 42, and auditory association cortex is located in a portion of Brodmann's area 22 (superior temporal gyrus) and Brodmann’s area 52 (the parainsular area) (13).

Neurons from the primary auditory cortex ultimately project to various other cortical processing areas, including the Wernicke area.

Etiology and pathogenesis

Central hearing loss may occur with lesions of the ascending auditory pathways from the cochlear nuclei (on the dorsolateral surface of the brainstem at the junction of the medulla with the pons) to the auditory cortex in the temporal lobe.

Of the central disorders of hearing, unilateral hearing impairment can occur with focal lesions of the brainstem that involve the cochlear nucleus and its connections. Such disorders occur most commonly with infarction in the territory of the anterior inferior cerebellar artery. The cochlear nucleus projects bilaterally to the superior olive, so unilateral lesions of the central auditory pathways past the cochlear nucleus do not generally produce unilateral hearing loss. Disorders of the superior olive can disrupt sound localization, apparently due to impairment of the central comparison of timing of signals from the two ears (64). Higher relays and some processing of auditory signals occur sequentially in the superior olive, the inferior colliculi, the medial geniculate bodies, the primary auditory cortices (Heschl gyri), and in secondary cortical association areas. Bilateral lesions of the superior olives, inferior colliculi, medial geniculate bodies, or primary auditory cortices can result in central deafness or other central disorders of auditory processing.

Just as congenital strabismus affects the development of the lateral geniculate ganglion and visual cortex, genetic forms of sensorineural deafness are not isolated deficits of the cochlea but, instead, are impairments that extend to the entire auditory system due to alterations to the encoded sensory inputs or involvement of the causal deafness genes in the development or functioning of central auditory circuits (77).

Epidemiology

No general descriptive or analytic epidemiological studies are available for central hearing loss or other auditory disorders due to central nervous system lesions. Central components of hearing loss in the elderly are seldom recognized but are more common than previously appreciated (33; 29).

Prevention

There are no recognized means of preventing central hearing loss, except as might be applicable to the prevention of other causative disorders (eg, stroke).

Differential diagnosis

Confusing conditions

Central hearing loss must be distinguished from conductive hearing loss and from sensorineural hearing loss. Central hearing loss involves brainstem or cortical auditory pathways; consequently, associated neurologic signs and symptoms are typically referable to nearby structures or specific vascular territories. Sensorineural hearing loss involves the cochlea (ie, “sensory”) or eighth cranial nerve (ie, “neural”). Conductive hearing loss involves structures in the external and middle ear responsible for the conduction of sound to the cochlea, where sensory transduction takes place.

Hearing loss may be central, sensorineural, or mixed in patients with infarction of the anterior inferior cerebellar artery. Most cases are sensorineural, despite evident brainstem involvement. Brainstem auditory evoked response studies, otoacoustic emissions, and auditory reflex testing can help make the distinction.

In children, central deafness is likely to be lumped in disorders of language or overall cognitive development (37; 104).

Cortical auditory disorders may overlap with language disorders (ie, aphasias). In addition, there may be an evolution over time, so an initial presentation as Wernicke aphasia may improve leaving word deafness.

Patients with central hearing loss often have inconsistent auditory behavior that may lead to incorrect diagnoses of "functional" or psychogenic hearing disturbances or to suspicion of malingering (72).

Diagnostic workup

The diagnosis of central hearing loss is often made by noting an inconsistency between relatively normal performance on pure-tone audiometry with poor performance on so-called speech testing. Speech audiometry tests include assessment of speech reception thresholds (SRTs), speech detection thresholds (SDT), and word recognition (16). The SRT is the lowest intensity level at which a patient can correctly repeat half of a series of common bisyllabic words from a closed set that are spoken with equal emphasis on both syllables (so-called “spondee” words, like “baseball,” “airplane,” and “playground”), preferably using recorded materials rather than monitored live voice (ie, spoken by the audiologist, processed, and presented at a specific intensity). The SRT should correspond closely to the pure-tone average (PTA): assuming that a patient fully understands the audiologist’s directions, a SRT substantially better than the PTA suggests possible malingering or exaggeration of hearing loss whereas a SRT that is substantially worse than the PTA may indicate hearing loss with severe distortion of speech sounds (16). In certain circumstances (eg, a patient has difficulty understanding speech or is hesitant to guess when uncertain), the audiologist may use a limited set of test words that are first practiced with the patient, and in this situation, the SRT more closely approximates the threshold of the best component frequency used in the computation of the PTA, rather than the PTA itself (16). When SRT cannot be measured (as in infants), SDT may be used instead: SDT is the lowest intensity level at which a patient can detect the presence of speech, and it generally corresponds to the best test frequency threshold (16). Word recognition (or “speech discrimination”) measures a patient’s ability to repeat a list of monosyllabic words at a suprathreshold level (approximately 40 dB above the SRT, or at a comfortable listening level if this is too loud for the patient) (16).

Although the pure-tone audiogram provides some information on hearing sensitivity, it does not provide information concerning central auditory processing or the auditory processing of real-world signals (eg, speech or music) (86).

With central deafness, pure-tone testing may demonstrate severe-to-profound hearing loss bilaterally, but otoacoustic emissions will be normal, and generally waves I and II of brainstem auditory evoked response studies will have normal latencies. With cortical deafness, acoustic reflexes and brainstem auditory evoked responses will generally be within normal ranges for both ears (83).

If there is an inconsistency between relatively normal performance on pure-tone audiometry with poor performance on speech testing, a central auditory disorder should be considered, and brain imaging should be obtained (preferably with MRI) even if no focal symptoms or signs are evident.

Various tests of central auditory ability present different binaural challenges or various distortions of the presented sounds (eg, frequency or temporal distortions). For example, speech-in-noise (SIN) tests have been developed that may help identify patients with central hearing loss (91; 80). Dichotic speech listening tests have also been used to assess central auditory processing, challenging the auditory system and cognitive functioning (eg, attention focusing and use of working memory). The Dichotic Digits Test uses single-syllable numbers presented simultaneously in each ear, usually in double-digit or triple-digit pairs. Other central auditory processing tests include adaptive tests of temporal response (ATTR), the time-compressed speech test (TCS), Dichotic Sentence Identification (DSI), and the Synthetic Sentence Identification-Ipsilateral Competing Message (SSI-ICM) tests.

Brainstem auditory evoked response (also known as auditory brainstem response) testing and acoustic reflexes may provide further insights on the integrity of the auditory pathways and the involvement of specific auditory-pathway structures (08).

In addition to auditory brainstem responses, auditory evoked potentials include middle-latency responses, and long-latency cortical responses, which may be delayed or absent with involvement of auditory the cortex (57; 15). These may be useful in selected cases, although there have been conflicting reports concerning the effects of temporal lobe lesions on middle-latency responses (92; 61; 90; 99; 57; 88; 52). In patients with unilateral temporal lobe lesions involving auditory the cortex, the amplitude of Pa and the Na-Pa complex are often reduced over the involved hemisphere but are intact in patients with anterior temporal lobectomy and in patients with cortical lesions that do not affect the temporal lobes (57). However, some patients with temporal lobe lesions, including one reported case with auditory agnosia and bilateral temporal lobe lesions, have normal middle-latency responses (92). In patients with bitemporal lesions, long-latency cortical responses have been reported to be either normal, delayed and of smaller amplitude, absent unilaterally, or absent bilaterally (02; 78; 92; 05; 75; 121; 113; 94).

Management

Central hearing loss is generally not relieved by hearing aids but should be treated with a program aimed at optimizing other communication abilities (29). For example, patients with word deafness may benefit from learning sign language, which allows them to effectively work around their impairment.

Outcomes

Outcomes generally depend on the underlying disease process.