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Last update: 10/2008 What is Hearing Loss?Hearing loss is very common, especially in the older population. This page provides a general overview for the diagnosis and evaluation of hearing loss. Anatomy of the Ear The ear is divided into the outer, middle, and inner (Figure 1). The outer ear includes the auricle, the external auditory canal, and the lateral surface of the tympanic membrane (TM). The middle ear includes the medial surface of the TM, the ossicular chain, the eustachian tube, and the tympanic segment of the facial nerve. The inner ear includes the auditory-vestibular nerve, the cochlea and the vestibular system (semicircular canals). The auditory nerve, also called the cochlear nerve, transmits sound to the brain. Types of Hearing LossThere are three “pure” types of hearing loss encountered commonly in clinical practice: sensorineural, conductive, and central. A fourth type, denoted “mixed,” is a combination of sensorineural and conductive hearing loss. Disorders of the middle ear are generally responsible for conductive losses; disorders of the cochlea or cochlear nerve are generally responsible for sensorineural hearing loss; and disorders of the brainstem or brain are generally responsible for central hearing loss. Sensorineural Hearing LossSensorineural hearing loss is the most common type of hearing loss, occurring in 23% of population older than 65 years of age. The term “sensorineural” is used to indicate that there is either a cochlear or an eighth nerve lesion. The diagnosis of a sensorineural pattern hearing loss is made through audiometry, which shows a significant hearing loss without the “air-bone gap” that is characteristic of conductive hearing disturbances. Common causes include advanced age, where the hearing pattern is often called presbyacusis (see following section); Meniere’s disease; ototoxic medications (such as high-dose aspirin or certain strong diuretics); immune disorders; and noise exposure (see following section). Head trauma can cause both temporary and permanent hearing loss. Trauma is more likely to affect hearing in patients who suffer a temporal bone fracture or are older in age (Bergemalm, 2003). Infections, such as meningitis, are a common cause of hearing loss in children where it occurs in approximately 20% of those with streptococcus pneumoniae meningitis (Dichgans et al, 1999). Postmeningitic hearing loss can be due to lesions of the cochlea, brainstem and higher auditory pathways, but usually is related to suppurative labyrinthitis (cochlear damage). Tumors are a very rare cause of sensorineural hearing loss. Examples are acoustic neuroma and glomus tumor.
Noise Induced Hearing LossNoise induced hearing loss is a permanent hearing impairment resulting from prolonged exposure to high levels of noise. One in 10 Americans has a hearing loss that affects his or her ability to understand normal speech. Excessive noise exposure is the most common cause of hearing loss. Twenty-five percent of the work force in the United States is regularly exposed to potentially damaging noise (Suter and von Gierke, 1987). Because of occupational risk of noise induced hearing loss, there are government standards regulating allowable noise exposure. People working before the mid 1960s may have been exposed to higher levels of noise where there were no laws mandating use of devices to protect hearing. Refer to our Noise Induced Hearing Loss article for more information. Congenital Hearing LossThere are also many inherited causes of hearing loss. Much research has focused on the detection of genetic causes of hearing loss, and many of them can now be diagnosed at an early age (Kochhar, 2007; Kokotas, 2007; Xing, 2007; Robin, 2005). Treatments are mainly aimed at preventing further damage and making do with what is left. If the loss is minor, then avoidance of noise and ototoxic medications may be an appropriate treatment. If the loss is significant, then a trial of a hearing aid is often worthwhile. Occasionally, persons with acquired deafness can be treated surgically with a surgically implanted device, a cochlear implant. Nongenetic Causes of Neonatal Hearing LossChildren who are born prematurely or contract certain infections in utero are at a higher risk of hearing loss (Roizen, 2003; Hille, 2007). Risk factors that correlate with hearing loss in premature infants include hypoxia, mechanical ventilation, hyperbilirubinemia, very low birth weight, and the use of ototoxic medications. These conditions are relatively common in premature infants, and the majority do not go on to develop hearing deficits. TumorsWhile very unusual, acoustic neuromas or metastatic cancer (particularly breast) can be a source of hearing loss. Typically there is a combination of a hearing problem or either dizziness or imbalance. Audiometry (see Figure 2) generally shows an asymmetrical sensorineural hearing loss. Sudden Hearing LossSudden hearing loss (SHL) is defined as greater than 30 dB hearing reduction, over at least three contiguous frequencies, occurring over 72 hours or less. It occurs most frequently in the 30- to 60-year age group and affects males and females equally. Refer to our Sudden Hearing Loss article for more information. PresbyacusisPresbyacusis is defined as hearing loss associated with degenerative changes of aging. There are four types:
Sensory presbyacusis is caused by loss of sensory elements in the basal (high-frequency) end of the cochlea with preservation of neurons. These patients have symmetrical, high-frequency sensorineural hearing loss (as shown in Figure 3). Pathology shows loss of hair cells. Neural presbyacusis is caused by loss of cochlear neurons. These patients have poorer discrimination than patients with sensory presbyacusis. Striatal presbyacusis is caused by loss of the stria vascularis with aging. Patients have a flat or slightly sloping hearing loss with good speech discrimination. Mixed presbyacusis is also possible. Cochlear conductive presbyacusis is caused by thickening of the basilar membrane caused by deposition of basophilic substance. This diagnosis is made on postmortem (Harris, 1998). Because presbycusis occurs in some individuals as they age but not in others, genetic or environmental factors must also play a role in its development (Liu, 2007). Medical conditions that are common in the elderly, such as diabetes and stroke, may also increase the risk of hearing loss with aging (Maia, 2005; Helzner, 2005).
Conductive Hearing LossIn conductive hearing loss, the second most common form of hearing loss, sound is not transmitted into the inner ear. Diagnosis is ordinarily made via observation of an “air-bone gap” on audiometry, meaning that hearing is superior when sound is transmitted in such a way that it bypasses the middle ear ossicular chain.
Characteristically, hearing aids work well for conductive hearing loss. Causes include a buildup of ear wax, a foreign body in the ear canal, otosclerosis, tympanosclerosis (as shown in Figure 4), external or middle ear infections (otitis externa or media), allergy with serous otitis media, trauma to the ossicular chain such as temporal bone fracture, tumors of the middle ear, erosion of the ossicular chain (cholesteatoma) and perforation of the tympanic membrane. Central DeafnessCentral deafness is extremely rare compared to conductive and sensorineural. A discussion of central deafness is found here.
AcknowledgmentsImages marked as copyright Northwestern University were developed with the support of a National Institutes of Health (NIH) grant to Northwestern University Dept. of Otolaryngology, and are used with permission.
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