ESSENTIALS OF DIAGNOSIS
May affect patients of all ages.
For patients who have unilateral hearing loss:
Pure-tone thresholds result in equally diminished air and bone conduction.
Speech discrimination testing less than 90% correct.
Hearing loss is extremely common and encompasses a wide spectrum of disease ranging from a nearly undetectable degree of disability to a profound loss of ability to function in society. Hearing loss can present at any age. One to three of every 1000 newborns in the United States is completely deaf, and more than 3 million children have hearing loss. In the adult population, nearly 10% of the population has some degree of hearing loss. Above the age of 65 years, between 30% and 35% of individuals have a hearing loss sufficient to require a hearing aid. Forty percent of people over the age of 75 have hearing loss.
Hearing loss may result from any pathology along the pathway from the auricle to the auditory cortex. In general, lesions in the auricle, external auditory canal, or middle ear cause conductive hearing loss. Loss of cochlear hair cell function leads to a sensory hearing loss. Neural hearing loss occurs as the result of pathology involving the cochlear nerve, and central hearing loss may result from lesions in the brainstem or auditory cortices. These latter forms of hearing loss are grouped together under the classification of sensorineural hearing loss. Mixed hearing loss is defined by the presence of both conductive and sensorineural elements.
Hearing occurs by air conduction and bone conduction. In air conduction, sound waves reach the ear by propagating through the air, entering the external auditory canal, and setting the tympanic membrane in motion; the movement of the tympanic membrane, in turn, moves the malleus, incus, and stapes of the middle ear. The structures of the middle ear serve as an impedance-matching mechanism, improving the efficiency of energy transfer from the air to the fluid-filled inner ear. Hearing by bone conduction occurs when the sound source, in contact with the head, vibrates the bones of the skull; this vibration produces a traveling wave in the basilar membrane of the cochlea.
Cochlear neurons send fibers bilaterally to a network of auditory nuclei in the midbrain, and impulses are transmitted through the medial geniculate thalamic nuclei to the auditory cortex in the superior temporal gyri. At low frequencies, individual auditory nerve fibers can respond more or less synchronously with the stimulating tone. At higher frequencies, phase locking occurs so that neurons alternate in response to particular phases of the sound wave cycle. Three elements encode the intensity of sound: (1) the amount of neural activity in individual neurons, (2) the number of neurons that are ...