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Clinical Evaluation of Hearing Loss

Alyssa R. Terk and John F. Kveton

The number of people with a severe-to-profound hearing impairment in the United States in 2001 ranged from 464,000 to 738,000, with 54% of them over the age of 65.¹ The incidence of significant hearing loss in infants ranged from 1:1000 to 1:2000. The average lifetime cost for one person with hearing loss is estimated to be $417,000 (in 2003 dollars) (includes productivity loss in the workplace and household). This represents costs over and above those experienced by a person who does not have a disability.²

Given the prevalence of hearing loss, a cost-effective and accurate diagnostic approach is essential to assess the degree and nature of hearing loss and to arrive at the underlying cause. Patients may undergo assessment for hearing loss (Fig. 13–1) because of their subjective complaints or because they belong to groups that have been shown to be at risk for hearing loss, such as low birth weight infants, or those receiving ototoxic chemotherapy. In either case, an understanding-of the differential diagnosis of hearing loss is helpful in guiding clinical evaluation.

Differential Diagnosis of Hearing Loss

A critical step in establishing a differential diagnosis is to determine whether the hearing loss is conductive, sensorineural, mixed, or central.

A sensorineural hearing loss (SNHL) may be caused by a lesion of the cochlea or the auditory nerve. A conductive hearing loss is caused by a lesion affecting the mechanism that transmits sound energy from the external environment to the cochlea and may involve the external ear, the tympanic membrane, or the contents of the middle ear. Some patients may have both sensorineural and conductive hearing loss simultaneously, in which the hearing loss is categorized as mixed. Conditions that may be associated with mixed hearing loss include chronic otitis media, acquired cholesteatoma, temporal bone trauma, certain hereditary syndromes, and otosclerosis. Central hearing loss is caused by a lesion along the neural pathway either from the inner ear to the auditory region of the brain or in the brain itself.

History and physical examination findings are extremely important in narrowing the list of clinical possibilities. To refine the differential diagnosis, information from the audiogram as well as data obtained from impedance testing and speech audiometry are used. Further examinations can be performed such as laboratory testing and diagnostic imaging to further narrow the diagnosis.

The differential diagnosis for a patient with unilateral SNHL varies considerably from that for a patient with bilateral slowly progressive SNHL. However, for the purposes of giving a broad overview, differential diagnoses for sensorineural and conductive hearing loss are discussed in this chapter (Tables 13–1 and 13–2).

Sensorineural Hearing Loss

Sensorineural hearing loss can be categorized in different ways. One scheme uses age of onset. Congenital hearing loss is that which is present from birth, whether due to hereditary factors, prenatal exposure or infection, or perinatal events. In contrast, hereditary hearing loss may not be present at birth. More than 50% of congenital hearing loss is due to genetic factors, 25% is acquired, and 25% is of unknown etiology. Causes of congenital SNHL include a variety of genetic disorders, intrauterine infections, exposure to teratogenic agents, prematurity, perinatal anoxia, and hyperbilirubinemia. Some of these factors may result in inner ear aplasia; however, such deformities may also exist without identifiable cause. Although the mechanism is not understood, low-birth-weight infants (<2500 g) are also at increased risk for SNHL as well as problems with figure/ground differentiation and auditory memory.³ Congenital infections that can cause hearing loss include toxoplasmosis, rubella, cytomegalovirus (CMV), herpes simplex virus (HSV), and syphilis.

Delayed-onset hearing loss can be caused by a variety of factors, including infection, trauma, and ototoxic medications. Genetic factors can also cause delayed-onset hearing loss. Hereditary hearing loss may be dominant, recessive, X-linked, or mitochondrially inherited, and may exist with or without abnormalities in other organ systems (i.e., syndromic versus nonsyndromic).

Of the 50 to 60% of congenital hearing loss that is hereditary, approximately 70% is nonsyndromic, of which 80% is inherited in an autosomal recessive manner.⁴ Many loci have been mapped, including autosomal recessive, autosomal dominant, and X-linked loci. The gene coding for connexin 26 (Cx26) is located at locus DFNB1 on human chromosome 13q12. The connexin gene codes a protein called gap junction protein β2 (GJB2). Connexin 26 and connexin 30 as well as other connexins fit together to compose gap junctions. These are expressed in the cochlea where they are thought to aid in returning potassium ions to the endolymph following hair cell stimulation. The most common mutation in connexin 26 is the 35delG mutation with a carrier rate estimated at 2.5% in the general population. Other mutations have been found in specific ethnic groups, such as 167delT in Ashkenazi Jews and 235delC in Korean and Japanese populations. Genetic mutations in the connexin 30 gene have also been implicated in some nonsyndromic hearing loss.

Infectious agents and inflammatory processes can lead to SNHL. The infectious agents include both bacteria and viruses. Some individuals with otitis media may develop SNHL through round window membrane permeability to bacterial toxins.5 Tuberculosis may be associated with otitis media as well as SNHL.^6,7 The spirochetes causing both syphilis and Lyme disease can involve the perilymph, resulting in sudden or fluctuating SNHL. In one study, Lyme titers were positive in 20% of patients with sudden SNHL.⁸

Cytomegalovirus, HSV, and human immunodeficiency virus (HIV) can lead to SNHL. In HIV, the hearing loss may result from the opportunistic infections characteristic of the acquired immunodeficiency syndrome, ototoxic medications used to treat infections, or possibly direct effects of the virus upon the cochlea.⁷ Specifically, nucleoside analog reverse transcriptase inhibitors reduce mitochondrial DNA content, which may contribute to auditory dysfunction in older patients with HIV-1 infection.9

Approximately 5 to 35% of patients are left with SNHL following bacterial meningitis.¹⁰ Since the introduction of the Haemophilus influenzae type B vaccine, most cases of bacterial meningitis are caused by Streptococcus pneumoniae. Bacterial meningitis is the most common cause of acquired SNHL in children (85%).^11,12 Inflammatory processes can be associated with SNHL. These include systemic diseases such as Cogan’s syndrome, relapsing polychondritis, Wegener’s granulomatosis, lupus erythematosus, Takayasu’s disease, and giant cell arteritis. In addition, primary autoimmune hearing loss may present without systemic manifestations.¹³

SNHL may result from primary tumors arising within the temporal bone as well as from metastatic lesions. The most common neoplasm causing SNHL is vestibular schwannoma. Squamous cell cancer and paragangliomas arising within the temporal bone may also cause SNHL, though they are more commonly associated with conductive hearing loss.¹⁴ Metastatic lesions to the temporal bone include those with a proclivity for bony metastases such as carcinoma of the breast, lung, and prostate.¹⁵

Associations have been suggested but not conclusively proven between several metabolic disorders and SNHL. These include diabetes mellitus, in which microangiopathy has been postulated to lead to auditory damage; hypothyroidism; and hyperlipoproteinemia. Hypertension as well as vascular and hematologic problems are also thought to cause hearing loss. Decreased blood flow to the inner ear may result from narrowed blood vessels, as in atherosclerosis or diabetes mellitus (DM), or from abnormal flow or coagulability, as in sickle cell and hypercoagulable disorders. Platelet dysfunction, thrombocytopenia, and hypocoagulable states can also lead to hemorrhage within the inner ear, thus causing hearing loss.^16,17 A rare maternally inherited form of deafness and diabetes (MIDD) has also recently been discovered, in which a mitochondrial gene defect results in type 2 diabetes and bilateral SNHL presenting at an average age of 35 years.¹⁸

Trauma can cause SNHL in several ways. Both longitudinal and transverse fractures of the temporal bone can disrupt the labyrinth, although this is more likely to occur with the latter. Perilymphatic fistula can result from both blunt injuries to the head and barotrauma. Head trauma may also cause concussive injury to the inner ear or traumatic brain injury with central hearing loss. Surgical trauma may result in SNHL. Excessive noise exposure damages the outer hair cells of the cochlea, causing hearing loss with a specific pattern on audiogram. Other conditions known to lead to SNHL include Meniere’s disease, cochlear otosclerosis, multiple sclerosis, ototoxic drugs, radiation injury, and aging (presbycusis).

Conductive Hearing Loss

Congenital causes of conductive hearing loss include ossicular fixation and ossicular malformations, as well as atresia or stenosis of the external auditory canal. These anomalies may exist in conjunction with other craniofacial abnormalities and possibly with SNHL.

Hereditary delayed-onset conductive hearing loss can be caused by otosclerosis, which is thought to be an autosomal dominant disease with incomplete penetrance.^19–21 Osteogenesis imperfecta tarda, another autosomal dominant disorder, leads to delayed-onset conductive hearing loss by causing stapes fixation or ossicular fracture. Other hereditary causes of conductive hearing loss include osteopetrosis, Marfan’s disease, Paget’s disease, and Crouzon’s syndrome. The presence of congenital cholesteatoma in the middle ear can also cause conductive hearing loss.

The most common cause of conductive hearing loss is otitis media with effusion, which can be either acute or chronic. Cholesteatoma or granulation tissue caused by chronic otitis media can interfere with the transmission of sound to the oval window, sometimes even causing ossicular erosion. Other sequelae of infection that may cause conductive hearing loss include tympanosclerosis and atelectasis of the middle ear. Another infectious cause of conductive hearing loss is severe otitis externa, in which edema of the ear canal interferes with hearing. Trauma may also lead to conductive hearing loss, caused by tympanic membrane (TM) perforation, hemotympanum, or ossicular discontinuity.

Rarer causes of conductive hearing loss include neoplasms involving the external auditory canal or middle ear. These may be benign or malignant. A more recent discovery is the superior canal dehiscence syndrome causing a conductive hearing loss in the face of intact middle ear function and normal stapes reflexes.²² Lastly, mechanical occlusion of the external auditory canal from cerumen, foreign bodies, or large exostoses can also cause conductive hearing loss.

History

The history is the first and most important step in the clinical evaluation of hearing loss. The history may identify individuals with hearing loss and can direct the clinician to the appropriate testing.

History in Infants and Children

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