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Hearing 

Hearing

Chapter:
Hearing
Author(s):

Linda M Luxon

DOI:
10.1093/med/9780199204854.003.240603_update_001

Update:

Chapter reviewed, minor changes made, further reading updated.

Updated on 28 Nov 2012. The previous version of this content can be found here.
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Essentials

Hearing loss

Hearing loss is the most common sensory impairment. The World Health Organization has estimated that at least 275 million people are affected worldwide, as are 17% of the adult population in the United Kingdom, three-quarters of these being over 60 years of age.

Clinical examination and investigation—examination includes visual inspection of the anatomy of the external ear and tympanic membrane, and tuning-fork tests to distinguish conductive from sensorineural hearing loss in some cases. Audiological investigations (1) quantify audiometric thresholds at each frequency; (2) differentiate conductive from sensorineural defects; (3) differentiate cochlear from retrocochlear abnormality; (4) identify central auditory dysfunction in the brainstem, midbrain or auditory cortex; and (5) identify a nonorganic component.

Epidemiology and causes—(1) prevalence in adults—factors include age, gender, genetic susceptibility, occupational group, ototoxicity, smoking, drinking, obesity, and hazardous noise exposure. (2) Congenital hearing impairment in children—over one-half of cases are explained by factors associated with admission to a neonatal intensive-care unit genetic factors and craniofacial abnormalities. (3) Acquired hearing impairment in children—the commonest cause is a conductive hearing loss due to chronic secretory otitis media; meningitis (particularly meningococcal) is the commonest cause of acquired sensorineural hearing loss in the UK. (4) Many of the preventable causes of hearing impairment remain common in the developing world: consanguineous marriages, birth trauma, childhood infections, noise exposure, and the unlicensed sale of ototoxic drugs.

Treatment—this may involve (1) protection from noise hazards and ototoxic drugs and management of chronic secretory otitis media; (2) auditory rehabilitation—including environmental aids, instruction in communication skills, and (if accepted by the patient) hearing aids; and (sometimes) (3) surgery—restorative in some cases of conductive hearing loss; implantable devices for totally deafened adults and children.

Tinnitus

Defined as a noise in the head or ears lasting for more than 5 min, tinnitus increases in frequency with age, affecting about 20% of people >60 years, although only 4% complain of the symptom. It can be associated with many conditions, frequently in association with hearing impairment.

Management—this is primarily medical, including the following interventions: (1) psychological—explanation of the problem, and if necessary treatment of anxiety/depression; (2) prosthetic—provision of hearing aids noise generators to ‘mask’ tinnitus with desirable environmental noise; and (rarely) (3) pharmacological—intravenous lidocaine (lignocaine) can result in the disappearance or amelioration of tinnitus, but no oral preparation has been found to be equally effective.

Hearing impairments

Our hearing is a choice and dainty sense, and hard to mend, yet soon it may be marred. Blows, falls and noise … all these  …  breed tingling in the ears and hurt our hearing.

Physicians of the Medical School of Salerno

Pathophysiology

For clinical purposes the ear is separated into three parts: the external, middle, and internal ear (Fig. 24.6.3.1). The external ear is important in funnelling sound to the tympanic membrane and in the localization of sound. The middle-ear ossicles connect the tympanic membrane to the oval window of the cochlea, such that sound waves cause displacement within the fluid-filled compartment of the membranous labyrinth. Within the internal ear, the mechanical activity at the oval window is transduced into neural responses by the hair cells of the organ of Corti (Fig. 24.6.3.2).

Fig. 24.6.3.1 Diagram to illustrate the anatomy of the peripheral auditory system.

Fig. 24.6.3.1
Diagram to illustrate the anatomy of the peripheral auditory system.

Fig. 24.6.3.2 Diagram of the organ of Corti.

Fig. 24.6.3.2
Diagram of the organ of Corti.

Disorders of the external and middle ear result in abnormalities of the mechanical transmission of sound from the environment to the internal ear, and give rise to a conductive hearing loss. Common examples include impacted wax, serous otitis media (glue ear), chronic otitis media, and disorders of the ossicular chain, e.g. otosclerosis, and traumatic discontinuity.

Disorders of the internal ear and cranial nerve VIII characteristically give rise to a sensorineural hearing loss, in which the perception of both bone- and air-conducted sounds is reduced and the appreciation of the intensity of sound and the frequency resolution of complex sounds are impaired. Many conditions may affect the cochlea, ranging from inherited, congenital, or iatrogenic nonsyndromal or syndromal malformations to ototoxic damage (aminoglycoside, antimalarial, loop diuretics, chemotherapeutic drugs), ischaemia including vertebrobasilar ischaemia, diabetic vasculitis, infections (mumps, rubella, syphilis, cytomegalovirus), autoimmune disorders, degenerative disorders, trauma, and idiopathic conditions such as Ménière’s disease. Much doubt has been cast on so-called ‘presbyacusis’, which may merely reflect an accumulation of toxic/traumatic insults to the ear over many years, and recent advances in molecular biology and genetics have shown the role of genetic mutations/deletions in late-onset/progressive hearing impairments. Sudden sensorineural hearing loss, usually of cochlear origin, most commonly results from viral, vascular, or autoimmune disease. Pathology of cranial nerve VIII leading to auditory neuropathy and hearing impairment may present as an isolated phenomenon, but has also been defined in genetic disorders, including spinocerebellar degenerations, trauma, cerebellopontine angle tumours, bony disorders such as Paget’s disease, infective disorders (meningitis), and inflammatory conditions (sarcoidosis).

Central auditory disorders may be developmental or acquired in origin and present with difficulties hearing in background noise and discriminating degraded speech (e.g. over a loudspeaker), and with sound localization, often with a normal or near-normal audiogram. Unilateral neurological pathology rarely gives rise to an audiometric hearing impairment as a consequence of bilateral representation of each cochlea at every level of the central auditory pathway above the cochlear nuclei. Rarely bilateral brainstem pathology may present as a symmetrical sensorineural hearing loss, whereas bitemporal cortical pathology may give rise to cortical deafness or auditory agnosia.

Clinical examination

Clinical examination requires examination of the anatomy of the external ear to define visible signs of congenital ear disease (pits, tags, nodules, or malformations) and evidence of other craniofacial features suggestive of syndromal hearing impairment. In addition, a detailed examination of the tympanic membrane is required to define the presence of pathology within the middle ear. Wax or debris obstructing the external auditory meatus should be removed by or under the supervision of a clinician with experience in this field. Syringing should never be undertaken in the presence of an infection or if there is any possibility of a tympanic membrane perforation. Tuning-fork tests remain the most valuable clinical test of auditory function and frequently enable a clinician to distinguish a conductive from a sensorineural hearing loss (Fig. 24.6.3.3). The tests are based on two physiological facts: first, the inner ear is normally more sensitive to sound conducted by air than to that conducted by bone; second, in the presence of a purely conductive hearing loss, the affected ear is subject to less air-conducted environmental noise, making it more sensitive to bone-conducted sound. A general medical and neurological examination is mandatory to define syndromes and the plethora of general medical conditions associated with hearing impairment. A detailed vestibular assessment is also of value.

Fig. 24.6.3.3 Diagram to illustrate the Weber tuning fork test in (a) a normal individual, (b) a case of unilateral sensorineural hearing loss, and (c) a case of unilateral conductive hearing loss, in which the sound is heard more effectively in the affected ear because of the lack of masking by environmental sounds. s, sound heard; •, tuning fork.

Fig. 24.6.3.3
Diagram to illustrate the Weber tuning fork test in (a) a normal individual, (b) a case of unilateral sensorineural hearing loss, and (c) a case of unilateral conductive hearing loss, in which the sound is heard more effectively in the affected ear because of the lack of masking by environmental sounds. s, sound heard; •, tuning fork.

Investigations

A battery of audiological tests is required to:

  • quantify audiometric thresholds at each frequency

  • differentiate a conductive from a sensorineural hearing loss

  • differentiate a cochlear from a retrocochlear abnormality

  • identify central auditory dysfunction in the brainstem, midbrain, or auditory cortex

  • identify a nonorganic component.

Each test can be defined as being subjective (dependent on patient cooperation) or objective (independent of patient cooperation) in terms of providing auditory data. Two pathophysiological phenomena are of importance in the differentiation of a cochlear sensorineural hearing loss from a nerve VIII or cochlear nuclei dysfunction:

  • Loudness recruitment is defined as an abnormally rapid increase in loudness, with an increase in intensity of the stimulus, and is characteristic of disorders affecting the hair cells of the organ of Corti, but is absent in the pathology of nerve VIII.

  • Abnormal auditory adaptation is a decline in discharge frequency with time, observed after an initial burst of neural activity in response to an adequate continuing stimulus applied to the organ of Corti. This phenomenon is characteristic of nerve VIII and brainstem auditory dysfunction.

Pure-tone audiometry is the most widely available, subjective, quantitative test of auditory thresholds. Electronically generated pure tones are delivered by earphones and the individual is required to respond to the quietest tone, at given frequencies between 125 and 8000 Hz in each ear. The sound may be delivered by air conduction (AC) or, if the tones are delivered by a bone vibrator on the mastoid process, by bone conduction (BC). In the latter test condition, as the intra-aural attenuation for a bone-conducted sound is negligible, masking of the ear not under test with narrow-band noise is mandatory. Bone-conduction thresholds significantly better than air-conduction thresholds indicate a conductive hearing loss, whereas similar bone-conduction and air-conduction thresholds are characteristic of sensorineural hearing loss (Fig. 24.6.3.4).

Fig. 24.6.3.4 Pure-tone audiograms showing both air- and bone-conduction thresholds and illustrating a right conductive hearing loss (a) and a left sensorineural hearing loss (b).

Fig. 24.6.3.4
Pure-tone audiograms showing both air- and bone-conduction thresholds and illustrating a right conductive hearing loss (a) and a left sensorineural hearing loss (b).

The stapedius muscle in the middle ear contracts bilaterally in response to loud sound directed into either ear. Using an impedance bridge, the minimum intensity of sound at a given frequency required to produce contraction of the stapedius muscle, and thus a movement of the tympanic membrane, can be measured (the acoustic reflex threshold). This objective measure enables recruitment and abnormal auditory adaptation to be measured, and allows assessment of middle-ear, cochlear, nerve VIII, and brainstem auditory function.

Otoacoustic emissions (OAEs) are weak signals that can be recorded in the ear canal and are the result of contractile properties of the outer hair cells of the cochlea. Measurement of OAEs thus provides objective information about cochlear function and has become the mainstay of universal neonatal hearing screening. Suppression of the OAEs by delivery of sound to the contralateral ear allows a measure of efferent auditory function and is of particular diagnostic value in neurological diseases affecting auditory function

Speech audiometry is a subjective test requiring the individual to repeat standard lists of words delivered at varying intensities through headphones. The responses are scored and provide an assessment of auditory discrimination. They are of particular value in assessing the efficacy of hearing-aid provision.

Electrophysiological tests provide the major objective means of assessing auditory function and siting pathology in the auditory system. Electrocochleography enables the measurement of the electrical output of the cochlea and cranial nerve VIII in response to an auditory stimulus, whereas brainstem auditory-evoked responses are of particular value in discriminating between cochlear and nerve VIII cochlear nuclei dysfunction. Recordings are obtained by averaging a series of time-locked responses generated by the major processing centres of the auditory system in response to a repetitive sound stimulus (Fig. 24.6.3.5). Analysis of the waveform must be undertaken in conjunction with knowledge of the pure-tone thresholds if appropriate, and valid conclusions about auditory function are to be obtained. Cortical- or late-evoked auditory responses are the most effective method of defining auditory threshold at each frequency in an uncooperative patient, and are essential in legal cases in which a nonorganic loss should always be excluded.

Fig. 24.6.3.5 Illustration of auditory-evoked brainstem responses showing normal waves I, II, III, and V from the right ear (RR) and delayed waves III and V from the left ear (LL), in a case of a left acoustic neurinoma.

Fig. 24.6.3.5
Illustration of auditory-evoked brainstem responses showing normal waves I, II, III, and V from the right ear (RR) and delayed waves III and V from the left ear (LL), in a case of a left acoustic neurinoma.

Vestibular investigations and imaging are frequently required to confirm a diagnosis, e.g. congenital inner-ear anomaly, Menière’s disease, and acoustic schwannoma, whereas cardiac, renal, gastrointestinal, endocrine, and metabolic investigations may be highly relevant in specific cases, e.g. Jervell–Lange–Neilsen syndrome, aminglycoside ototoxicity, collagen vascular disease, ulcerative colitis, Pendred’s syndrome, and autoimmune disorders.

Management

Appropriate management of both acute and chronic hearing loss requires a detailed history and examination to ensure both appropriate management of related general medical and otological conditions and protection from leisure (discothèques), occupational noise hazards, and ototoxic drugs. Conductive hearing loss due to trauma, chronic middle-ear disease, or otosclerosis may be surgically remediable, whereas recent animal studies have highlighted the possible future role of antioxidants in the amelioration of sensorineural hearing loss caused by chemotherapeutic agents, aminoglycoside antibiotics, and noise. Sudden sensorineural hearing loss is deemed to be a medical emergency, but there are no randomized controlled trials confirming the efficacy of the various therapeutic interventions advocated: steroids, antiviral agents, and haemodilution techniques.

Auditory rehabilitation is a problem-solving exercise centred on each individual patient, and depends on assessing both the auditory disability of the individual and the relevance of this to other important people in the patient’s life. Not only auditory impairment, but also communication skills, including lip-reading ability, the use of visual cues, and the level of speech and language, together with psychological and sociological factors, must be considered.

The remedial process may be straightforward in a highly motivated patient in whom there is an uncomplicated hearing loss. However, in the presence of a complicating factor, such as a hearing loss that is difficult to resolve with an aid or arthritis making hearing aid manipulation difficult, the particular problem must be addressed to ensure optimal use of subsequent hearing-aid provision. In patients who have a negative view of hearing aids, environmental aids and instruction in communication skills before the introduction of a hearing aid may facilitate long-term rehabilitation. In general, the provision of a hearing aid is effective only when the patient him- or herself, rather than well-meaning family members, wishes to get help.

Although hearing aids play a pivotal role in audiological rehabilitation, a detailed description of their provision and selection is outside the scope of this short review. For many patients, wearable hearing aids, which bring sound more effectively to the ear, are invaluable, but environmental aids (assisted-listening devices such as amplification systems, alerting warning devices, e.g. flashing lights connected to a doorbell or an alarm clock) may be adequate. In addition, sensory substitution systems, e.g. where visual signals are generated in response to auditory cues such as a telephone or doorbell ringing, or a baby crying, may be helpful to a hearing-impaired person.

The general principles of hearing-aid provision include the fitting of a comfortable earmould, which provides a secure mounting for the aid and a good acoustic connection between the aid and the ear canal. Hearing-aid selection involves matching the amplification required from the aid at specific frequencies with that required by the user. A particular disability experienced in most hearing-impaired people is that of hearing speech in a noisy environment and, although programmable digital processing hearing aids are of some help in this situation, conventional aids provide selective amplification of the frequencies relevant to speech, with minimal amplification at the peak frequency of background noise. Conventional aids may be divided into body-worn and head-worn aids, which can be in spectacles, behind the ear, in the ear, or in the canal in design. The major advantage of body-worn aids is the very high gain and maximum output that can be achieved, whereas the disadvantage is the unsightly nature of the device and the poor microphone placement.

Cochlear implants are electronic devices that convert sound into electrical current for the purpose of directly stimulating residual auditory nerve fibres to produce hearing sensations. The devices are implanted in the cochlea, usually with an electrode array, with an externally worn microphone and processor. Cochlear implants have been used in totally deafened adults and children with good results, and should be considered in all cases of profound acquired hearing loss and in children in whom there is good evidence of auditory nerve preservation in both congenital and acquired hearing impairment. In the bilateral absence of a functional cranial nerve VIII (e.g. neurofibromatosis) brainstem implants have been shown to be of value.

The value of counselling for the hearing-impaired person by a skilled hearing therapist must be emphasized. Such simple hearing tactics as encouraging the individual to ensure that the light is always on the speaker’s face, that he or she places himself so that the better ear is towards the speaker, and sitting close to the sound source thereby minimizing background noise, can greatly improve communication ability. For profoundly hearing-impaired individuals, psychological problems associated with isolation and occupational handicaps are significant and it is therefore essential that psychological, medical, and social support are readily available.

Tinnitus

Tinnitus may be defined as a perception of sound that originates from within the head rather than from within the external world. Rarely, the sound may have an externally detectable component and is then termed ‘objective tinnitus’ as opposed to the more common ‘subjective tinnitus’. The experience of tinnitus is universal, but the complaint of tinnitus is rare.

Many conditions are associated with tinnitus, but it is frequently, although not always, associated with hearing impairment. The proposed pathophysiological mechanisms include:

  • decoupling of the stereocilia of the hair cells

  • misinterpretation of auditory neural activity by higher auditory centres

  • self-sustaining oscillation of the basilar membrane

  • spontaneous OAEs

  • an abnormality of the spontaneous resting activity of primary auditory nerve fibres, either secondary to the hypo- or hyperexcitability of damaged hair cells or as a direct consequence of the derangement of primary neurons themselves

  • damage to the myelin sheath between auditory nerve fibres, allowing ephaptic transmission (cross-talk) between adjacent nerve fibres

  • derangement of efferent fibres of the vestibulocochlear nerve, producing aberrant auditory behaviour.

A number of studies have demonstrated that tinnitus complaint does not correlate with psychoacoustic features of the tinnitus, but there is a significant correlation between tinnitus complaint and psychological symptoms. Importantly, the onset of tinnitus complaint may be associated with negative life events such as retirement, redundancy, bereavement, and divorce.

The assessment of tinnitus includes a detailed history, clinical examination, and audiometric investigation as outlined for hearing impairment. The most common causes of objective tinnitus include palatal myoclonus, temporomandibular joint abnormalities, vascular abnormalities such as an arteriovenous fistula, and vascular bruits. Rarely, a patulous auditory tube may give rise to tinnitus in which the patient complains of a blowing sound associated with respiration.

Bilateral subjective tinnitus with evidence of a cochlear hearing loss is associated most commonly with presbyacusis, endolymphatic hydrops, vascular labyrinthine lesions, and noise-induced hearing loss. However, it is also common with head injury, whiplash injury, ototoxicity, barotrauma, surgical intervention, and after such simple clinical practices as syringing. Unilateral subjective tinnitus, with or without an associated sensorineural hearing loss, must be fully investigated to exclude an underlying cerebellopontine angle lesion, in particular an acoustic neurinoma.

Management

The primary management of tinnitus is medical, although surgical intervention is required for the correction of arterial stenoses giving rise to bruits and for glomus jugulare tumour and arteriovenous malformations. Destructive surgery, e.g. labyrinthectomy or auditory nerve section, has no place in the management of tinnitus as there is no evidence that destruction of the peripheral cochlear elements brings about improvements in tinnitus complaint.

The medical management of tinnitus can be divided into psychological, pharmacological, and prosthetic intervention. The psychological management includes an explanation of tinnitus, reassurance that the symptom will not progressively deteriorate or indeed remain unchanged, the exclusion of sinister pathology to allay fear, and, if necessary, the appropriate formal psychiatric management of depression/anxiety. In the presence of a hearing impairment, the provision of hearing aids to ‘mask’ tinnitus with desirable environmental noise may be of value. In the absence of such a loss, tinnitus maskers and noise generators have been advocated to promote ‘adaptation’, but it must be emphasized that there is no evidence that tinnitus maskers are superior to placebo devices. Pharmacologically, intravenous lidocaine has been shown to result in the disappearance or amelioration of tinnitus, but no oral preparation has been found to be equally effective. Psychiatric drugs may be required for psychological management, although no single drug has been shown to be uniformly effective.

Tinnitus re-training therapy is a management strategy based on a neurophysiological model of tinnitus. The re-training is a combination of prosthetic and psychological intervention, which in essence provides a structured framework for the various well-established mechanisms of tinnitus management outlined above.

In conclusion, positive reassurance, appropriate psychiatric management, and prosthetic support remain the mainstays of the medical management of tinnitus.

Further reading

Gleeson M, Browning G, Luxon LM (2008). The ear, hearing and balance. In: Browning G, Luxon LM (eds) Scott-Brown’s otorhinolaryngology, head and neck surgery, 7th edition, Vol. 3, Part 19. Hodder Arnold, London.Find this resource:

    Luxon LM (2009). Hearing loss and dysacuses. In: Donaghy M (ed.), Brain’s diseases of the nervous system, 12th edition. Oxford University Press, Oxford.Find this resource: