Chapter 53. The Aging Inner Ear

Presbycusis

• High-frequency hearing loss.
• Reduced clarity of hearing.
• Absence of retrocochlear pathology.

Presbystasis

• Generalized imbalance.
• Absence of vertigo.

According to the World Health Organization, the proportion of population that are elderly is increasing at a rapid rate—by 2025, nearly 1.2 billion people will be over the age of 60! Consequently, the prevalence of age-related auditory and vestibular dysfunction will increase. Genetically determined and environmentally affected, the inner ear, like other organ systems, undergoes degenerative changes with aging. These changes result in a variable functional disability. In the United States, hearing difficulty is reported by 25–30% of people in the age group of 65–70 years and by nearly 50% of those over 75 years of age. It has been estimated that between 1.5% and 3.0% of the total population would benefit from hearing aids. Vestibular dysfunction is also common in the elderly, with reported prevalence of vertigo, dysequilibrium, or imbalance to be as high as 47% in men and 61% in women over the age of 70. The incidence of falling in individuals over the age of 65 is between 20% and 40% in those living at home and is twice as frequent for the institutionalized elderly. These falls are associated with significant morbidity and mortality and constitute one of the leading causes of death among the elderly.

The specialized neural cells of the auditory and vestibular systems are nonmitotic and thus cannot undergo replication and renewal. During the course of a lifetime, DNA transcription errors and insoluble pigments accumulate, and protein synthesis becomes increasingly inefficient. In addition, environmental and external factors such as noise trauma, physical trauma, ototoxic substances, and medications contribute to senescence. More recently, the contribution of genetics to age-related hearing loss is being appreciated.

Age-Related Hearing Loss

Hearing loss in the elderly is multifactorial and is due to the convergence of various risk factors. Presbycusis is the otherwise unexplained, slowly progressive, predominantly high-frequency symmetric hearing loss due to the aging process (Figure 53–1). Progressive high-frequency hearing loss has been clearly documented by numerous studies in populations over the age of 40 (Figure 53–2A). Older patients with presbycusis also have more diminished speech discrimination than younger patients with the same level of pure-tone averages (Figure 53–2B). This suggests that neural processing is affected in addition to end-organ dysfunction.

Central pathology includes increased synaptic time in the auditory pathway, increased information processing time, and decreased neural cell population in the auditory cortex. Thus, the older patient is handicapped by decreased hearing as well as the decreased ability to discriminate between similar words. The ability to discriminate between words further deteriorates in a noisy background. In addition, the ability to identify very small interaural time differences deteriorates. Consequently, there is a decrease in directional hearing, further limiting the understanding of speech.

The hearing loss that occurs with aging is not inevitable. Some individuals reach advanced age and maintain perfectly normal hearing. For example, the Mabaans, a Sudanese tribe who live in an almost silent environment, exercise daily, and abstain from smoking and eating animal fats, have significantly better hearing than age-matched control groups from industrialized areas in the United States. Similarly, other studies have shown that hearing loss is associated not only with noise exposure, but with hyperlipidemia, hypertension, and vascular disease. This has led some clinicians to consider presbycusis as “socioacusis” and to suggest that preventive measures such as limiting exposure to noise may substantially reduce the hearing loss that accompanies aging. Through military, industrial, and recreational (eg, hunting or target practice) activities, men typically receive significantly greater noise exposure than women. Thus, the higher incidence and greater severity of presbycusis in men also argues in favor of the role of environmental causes.

Morphologic studies of human temporal bones have demonstrated an age-related loss of inner and outer hair cells and supporting cells, with the greatest loss being in the basal turn of the cochlea. There is greater loss of outer hair cells compared with inner hair cells; however, these changes have not been directly correlated with auditory function. Age-related loss in spiral ganglion cells, eighth nerve fibers, and neurons in cochlear nuclei have been demonstrated (Figure 53–2C). Some studies have reported changes in the brainstem-evoked response with aging, suggesting alteration at the level of the superior olivary complex, the lateral lemniscus, or the inferior colliculus. Thus, age-related auditory dysfunction results from aggregate deterioration of the entire auditory pathway.

The exact cause of presbycusis remains speculative, in part because of the difficulty in separating the contribution of various etiological factors such as diet, nutrition, metabolism, arteriosclerosis, ototoxic exposure, and noise trauma. Many believe that genetic predisposition alone makes age-related biologic degeneration of the auditory system inevitable. Lifelong acoustic trauma and genetically programmed senescence are the most likely causes of age-related hearing loss. Recently, genome-wide association studies demonstrated that a common allele of GRM7 (gene encoding metabotropic glutamate receptor type 7) contributes to an individual's risk of developing age-related hearing loss.

Age-Related Balance Disturbance

Degenerative changes and atrophy have been noted throughout the vestibular apparatus, including the otoconia, vestibular epithelium, vestibular nerve, Scarpa's ganglion, and cerebellum. In the otolithic organs (the utricle and saccule), statoconia progressively demineralize and fragment, resulting in a decreased responsiveness to gravity and linear acceleration. The migration of degenerated otoconial debris into the dependant ampulla of the posterior semicircular canal may result in positional balance disturbances (cupulolithiasis or benign paroxysmal positional vertigo). After age 70, there is also a 20% decrease in the number of hair cells in the maculae of otolith organs and a 40% decrease in cristae of the semicircular canals. Type I hair cells are affected more than Type II hair cells.

In the sensory epithelium, there is accumulation of inclusion bodies, lipofuscin, and vacuoles. Atrophy and scar formation are also present in the sensory epithelia. A reduction in the number of ganglion cells in the Scarpa's ganglion occurs earlier, by age 60. Beginning at age 50, there is a loss of nerve fibers between the vestibule and the Scarpa's ganglion. The greatest loss occurs among the thick myelinated fibers of the cristae. Lipofuscin accumulation in the vestibular nuclei has also been observed. In the cerebellum, there is loss of Purkinje cells beginning in the fifth decade. Presbyastasis is a dysequilibrium that occurs with aging and should be used only as a diagnosis of exclusion.

Presbycusis

Classically, four types of presbycusis have been defined: sensory, neural, metabolic or strial, and conductive (Table 53–1). These types may occur in isolation or in combination.

Sensory Presbycusis

Sensory presbycusis is audio-metrically characterized as bilateral, symmetric high-tone hearing loss with an abruptly sloping threshold pattern that begins in middle age. Speech discrimination is directly correlated with the preservation of high-frequency hearing. Histologically, there is a loss of both hair cells and supporting sustentacular cells isolated to the basal turn of the cochlea. The initial flattening of the organ of Corti is followed by secondary neural degeneration. The middle and apical turns of the cochlea containing the speech frequencies are usually spared. These pathologic changes are similar to those seen with noise trauma.

Neural Presbycusis

Neural presbycusis is characterized by a loss of cochlear neurons involving the whole cochlea and is associated with a significant loss of speech discrimination. The loss of speech discrimination is more profound than would be predicted on the basis of a pure-tone threshold level alone. Although it may occur at any age, hearing difficulty is not noted until the neuronal population falls below a critical number. A downward-sloping audiogram with a variable slope is characteristic. It has been shown that the magnitude of speech discrimination loss directly correlates with the extent of cochlear neuronal loss in the region corresponding to the speech frequencies in the cochlea.

Strial Presbycusis

Strial presbycusis is characterized by a flat pure-tone audiogram with excellent speech discrimination. The stria vascularis is a metabolically active region of the cochlea that is responsible for the secretion of endolymph and the maintenance of ionic gradients across the organ of Corti. In strial presbycusis, a slowly progressive hearing loss begins in middle age. Pathologically, there is a patchy atrophy of the stria vascularis in the middle and apical turn of the cochlea, without loss of cochlear neurons. Strial atrophy may also involve the entire cochlea. The magnitude of atrophic changes correlates directly with the level of the hearing loss. The quality of endolymph is thought to be affected by strial degeneration, resulting in a loss of energy available to the end organ.

Conductive Presbycusis

Changes in the mechanical characteristics of the basilar membrane have been suggested as causative of the gradually sloping high-frequency hearing loss of middle age. Cochlear conductive presbycusis lacks discernible pathological changes within the inner ear. Without confirmation from direct micromechanical measurements, cochlear conductive presbycusis remains a theoretical category of presbycusis. The speech discrimination is said to be diminished in relation to the magnitude of pure-tone loss.

Noise Trauma

Noise trauma, in addition to presbycusis, is an important cause of sensorineural hearing loss in the elderly. Exposure to sounds greater than 85 dB for prolonged periods of time is potentially injurious to the cochlea and may result in a high-frequency biased hearing loss that is typically maximal at 4000 Hz (Figure 53–3). With continued acoustic trauma, the hearing loss progresses to involve the primary speech frequencies and therefore further affects speech communication. Because of the similarities between noise-induced hearing loss and presbycusis, assessing the relative contribution of each to auditory dysfunction in the elderly is often difficult. Preventive measures, including monitoring noise levels in the workplace, wearing earplugs and ear-muffs, and avoiding loud noise exposure, should aid in diminishing noise-induced hearing loss.

Presbystasis

Vertigo is the cardinal symptom of vestibular disease. Although it is usually described as a rotatory sensation, it may take the form of any illusion of movement such as rocking, ground rolling, or a sense of falling forward or backward.

Dysequilibrium is a sense of poor coordination with erect posture or during a purposeful movement. Vertigo is usually episodic; dysequilibrium is typically continuous. The term imbalance implies an orthopedic (eg, hip disease) or neurological (eg, hemiparesis) problem. Dizziness is an all-encompassing term used by the patient and may include vertigo, dysequilibrium, or imbalance. It may also be used to denote a light-headed feeling, as in postural hypotension or hypoglycemia, or to indicate an inability to concentrate.

Equilibrium problems are common in the elderly. Like the auditory system, the vestibular and balance systems also undergo degenerative changes, resulting in significant clinical disability. An estimated 50–60% of elderly patients living at home and 81–91% of patients in an outpatient geriatric clinic complain of dizziness. By age 80, one in three people will have suffered a fall associated with significant morbidity. Vestibular symptoms precede these falls in more than half of the patients. The diagnostic evaluation of elderly patients complaining of dizziness yields a specific diagnosis in less than a third of the patients.

Patient Evaluation

A thorough vestibular evaluation begins with a complete history, a general physical examination, and a specialized neurotological examination.

Imaging Studies

In the presence of asymmetric or sudden hearing loss or vestibular symptoms with associated neurological findings, a magnetic resonance imaging (MRI) scan with contrast enhancement is indicated to rule out retrocochlear pathology. When inner ear malformations or superior canal dehiscence is suspected, computed tomography (CT) of the temporal bone may be revealing.

Special Tests

Further evaluation may include electronystagmography, CT scanning, and MRI. Electronystagmography is a graduated series of evaluations of the vestibular and vestibuloocular systems that includes caloric responses. It may be useful in establishing the degree of vestibular function in an ear, determining the side of the pathology, and differentiating central from peripheral diseases. Posturography assesses the ability of the subject to maintain balance with changing visual and somatosensory input.

Rotational testing is available to evaluate the vestibuloocular reflex. After age 70, the elderly exhibit a decline in caloric response. The relative energy required to maintain balance on the posturography test increases linearly with age until age 70. Studies of the vestibuloocular reflex in the elderly have shown a decreased sensitivity and shorter time constants over a wide range of frequencies of rotational stimuli. Overall, aging affects the vestibular, visual, and proprioceptive information available for central processing, as well as the ability of the central nervous system to process the sensory information and effect motor response.

Hearing Loss

Ototoxicity

Not all hearing loss in the elderly is presbycusis. Ototoxic drugs such as aminoglycoside antibiotics, loop diuretics, and antineoplastic agents (especially cisplatin) may contribute to hearing loss in the elderly. Patients especially at high risk for injury to the auditory system from ototoxic drugs include those with a preexisting hearing loss, those undergoing simultaneous treatment with multiple ototoxic drugs, and those with renal insufficiency. The risk of ototoxic injury can be significantly reduced by monitoring ototoxic exposure with serial audiometry. Of course, serum peak-and-trough levels should be measured to establish the lowest possible dose compatible with therapeutic efficacy. Substitution with nontoxic therapy, whenever feasible, is paramount for prevention.

Sudden Sensory Hearing Loss

Sudden loss of hearing in one ear is a relatively common occurrence in the elderly. Most cases are the result of thrombotic or embolic obstruction of the internal auditory artery. Although complete losses seldom recover, most partial losses experience some degree of spontaneous improvement within a few weeks to months. Empirical therapy with oral prednisone appears to be of some benefit. Although most sudden losses are idiopathic and presumably vascular, other etiologies, such as acute endolymphatic hydrops, perilymphatic fistula, tertiary syphilis, brainstem ischemia or infarction, demyelinating disease, and vestibular schwannoma, should be considered.

Asymmetric Hearing Loss

Most hearing losses in the elderly are bilateral and symmetric. Unilateral or asymmetric sensorineural hearing loss is atypical and demands further investigation to exclude disease of the central auditory system, such as vestibular schwannoma. The most common symptoms of vestibular schwannoma are sensorineural hearing loss, tinnitus, and dysequilibrium.

The initial screening test for the evaluation of asymmetric hearing loss is the auditory brainstem response (ABR), which records the changes in the electroencephalogram evoked by sound stimulation. Five waves may be observed in the first 10 ms, corresponding to the activation of the eighth cranial nerve (Wave I), the cochlear nucleus (Wave II), the superior olive (Wave III), the lateral lemniscus (Wave IV), and the inferior colliculus (Wave V). Absent ABR response or interaural latency differences in Wave V >0.3 ms are suggestive of retrocochlear pathology and warrant further radiological evaluation. Gadolinium-DTPA-enhanced MRI scanning is the gold standard for evaluating diseases involving the cerebellopontine angle and the internal auditory canal. MRI scanning may also detect brainstem pathology, such as multiple sclerosis or infarction, which can mimic the clinical presentation of vestibular schwannoma.

Other Types of Hearing Losses

Less common causes of sensorineural hearing loss in the aged are numerous and include metabolic derangements (eg, diabetes, hypothyroidism, hyperlipidemia, and renal failure); infections (eg, measles, mumps, and syphilis); autoimmune disorders (eg, polyarteritis and lupus erythematosus); physical factors (eg, radiation therapy); and hereditary syndromes (eg, Usher syndrome). The identification of metabolic, infectious, or autoimmune sensory hearing loss is especially important because these hearing losses are occasionally reversible with medical therapy.

Balance Disturbance

Vertebrobasilar Insufficiency

In the elderly, vertebrobasilar insufficiency is an important cause of vertigo and dysequilibrium. It usually results from arteriosclerosis with insufficient collateral circulation, but may also be due to compression of vertebral arteries by cervical spondylosis, postural hypotension, or the subclavian steal syndrome. The full-blown clinical presentation of vertebrobasilar ischemia includes vertigo with head motion (especially looking up), dysarthria, numbness of the face, hemiparesis, headache, and diplopia. Less frequently, visual disturbances occur including oscillopsia, field defects, transient blindness, cerebellar ataxia, and dysphagia; drop attacks may also occur, reflecting ischemia of the brainstem and cerebellum. Vertigo or dysequilibrium may occur without other neurological signs or symptoms. A definitive diagnosis may be established by four-vessel cerebral angiography, but is seldom indicated. Presently, there is no effective medical or surgical treatment for vertebrobasilar insufficiency, although rehabilitative measures may be beneficial.

Systemic Disorders

A plethora of systemic disorders may affect equilibrium and balance in the elderly, including cardiovascular disease, cerebrovascular disease, peripheral vascular disease, neurological disorders, visual impairment, metabolic disease, and musculoskeletal problems. Therapeutic drugs are frequently responsible for dysequilibrium and postural instability, especially the antihypertensive, antidepressant, and sedative-hypnotic classes.

Peripheral Vestibular Disorders

A host of peripheral vestibular disorders may cause vertigo, including benign paroxysmal positional vertigo (BPPV) or cupulolithiasis, labyrinthitis, vestibular neuronitis, Meniere syndrome, labyrinthine concussion due to trauma, superior canal dehiscence, and perilymph fistulas, among others. In younger patients, BPPV is usually secondary to trauma, whereas in the elderly it is usually a result of degenerative processes. Patients complain of intermittent, irregular episodes of vertigo precipitated by rapid head motion. Vestibular suppressant medications are of limited usefulness except during periods of exacerbation. The severity of symptoms may diminish with repetition because of habituation. Patients usually respond to vestibular exercises, and spontaneous resolution occurs within 1 year in most cases.

Meniere Syndrome

Meniere syndrome is characterized by episodic severe vertigo, fluctuating sensorineural hearing loss, tinnitus, and ear “fullness.” Pathologically, there is distention of the endolymphatic system throughout the inner ear, presumably due to dysfunction of the endolymphatic sac. The clinical course is highly variable, with clusters of severe episodes interspersed with periods of remission of variable duration. Management may include a sodium-restricted diet, diuretics, vasodilators, vestibular suppressants, and, occasionally, surgery to decompress the endolymphatic system.

Acute Labyrinthitis

Probably a viral infection of the inner ear, acute labyrinthitis, causes both severe vertigo and hearing loss. Typically, it runs its course over a period of 1–2 weeks, although residual hearing loss and the periodic recurrence of vertigo are common sequelae. Vestibular neuronitis also presents with vertigo similar to labyrinthitis, but is unaccompanied by auditory symptoms.

Rehabilitation of Hearing Loss

Hearing Aids

Nearly 30 million people, or 10% of the US population, have hearing problems in one or both ears. In the elderly, the reduced ability to discriminate sounds and to understand speech in a noisy background can be minimized with auditory rehabilitation, usually through amplification. Contemporary hearing aids are comparatively free of distortion and have been miniaturized to the point where they often may be contained entirely within the ear canal. To optimize the benefit, a hearing aid must be carefully selected to conform to the nature of the hearing loss. Digitally programmable hearing aids have recently become available and promise substantial improvements in speech intelligibility, especially under difficult listening circumstances.

Assistive Devices

Aside from hearing aids, many assistive devices are available to improve comprehension in individual and group settings to help with hearing television and radio programs and to assist in telephone communication.

Television Devices

Television devices include headphones that plug into the listening jack of the television, listening loops for use with the telecoil on a hearing aid, and wireless infrared devices that send the television signal directly to the listener via a receiver.

Telephone Amplifiers and Devices

Portable and nonportable telephone amplifiers are available to increase the loudness of the telephone audio signal. Handset amplifiers built directly into the telephone base or earphones are widely available. Telephone devices for the deaf using message screens or paper printouts are available for severe or profoundly hearing-impaired individuals.

Cochlear Implants

The cochlear implant, an electronic device that is surgically implanted to stimulate the auditory nerve, is playing an increasingly important role in the audiologic rehabilitation of the elderly with severe or profound sensorineural hearing loss.

Rehabilitation of Vestibular Dysfunction

Nonsurgical Measures

Pharmacologic Agents

Many drugs have been used for the symptomatic relief of vertigo. The most commonly used drugs are antihistamines, sedative-hypnotics, and anticholinergics. Therapy with a combination of pharmacological agents may be efficacious when single-drug therapy has been ineffective.

Vestibular Suppressants

Vestibular suppressants should be used to lessen the unpleasant sensation and alleviate vegetative symptoms such as nausea and vomiting. However, they should be used only for a short duration of 1–2 weeks because they adversely affect the process of central compensation following acute vestibular disease. In acute, severe vertigo, diazepam, 2.5–5.0 mg administered intravenously, may abate an attack.

Antiemetics

Relief from nausea and vomiting usually requires an antiemetic delivered intramuscularly or by rectal suppository (eg, prochlorperazine, 10 mg intramuscularly, or 25 mg rectally, every 6 hours).

Antihistamines

Antihistamines may be used for less severe vertigo. Examples include meclizine or dimenhydrinate, 25–50 mg administered orally every 6 hours.

Anticholinergic Medications

Transdermal scopolamine, which is in widespread use for the suppression of motion sickness, is also useful in the management of vertigo. In the elderly, however, anticholinergic therapy is frequently complicated by mental confusion and urinary obstruction; the latter is found especially in males. The use of transdermal scopolamine may also be limited owing to the side effects of dry mouth and blurred vision and is contraindicated in glaucoma patients. A therapeutic effect with fewer side effects may be achieved by cutting the patch in half or even to one quarter of its size. Careful hand washing after handling the patches is necessary to prevent inadvertent eye contact, which could result in prolonged pupillary dilatation and possible acute narrow-angle glaucoma.

Exercise and Physical Therapy

After nausea and vomiting have resolved, exercise should be encouraged to enhance central compensation following peripheral labyrinthine dysfunction. Physical activity is the single most important element in functional recovery after acute labyrinthine dysfunction. Patients should be instructed to repeatedly perform maneuvers that provoke “vertigo-up” to the point of nausea or fatigue in an effort to habituate them. Many patients find vestibular exercise programs (eg, Cawthorne exercises) helpful. A formal physical therapy program designed to identify and correct maladaptive compensation strategies may also prove beneficial.

Surgical Measures

Surgical intervention may be helpful in selected patients who continue to have disabling symptoms despite a prolonged and varied course of medical therapy. Surgical therapy may include sectioning of the vestibular nerve in a hearing ear or a labyrinthectomy in a deaf ear.

Hearing loss associated with aging is progressive. However, the rate of progression is variable. Age-related hearing loss usually progresses at a rate of 1– dB/year. Rehabilitation of the older deaf individual is often less than satisfactory. Amplification, though helpful in making sound audible, usually does not adequately address the reduction in clarity. Cochlear implantation offers the hope of restoring audition and clarity to profoundly deaf individuals.

Imbalance can often be stabilized, but normal balance cannot be restored. Physical activity can play a critical role in the functional recovery of patients, allowing them to tend to routine daily activities with greater assurance.