B. Sensory Hearing Loss: Sensory hearing loss results from deterioration of the cochlea, usually due to loss of hair cells from the organ of Corti. Among the many common causes are noise trauma, ototoxicity, and aging (presbyacusis). Sensory hearing loss is not correctable with medical or surgical therapy but often may be prevented or stabilized.

C. Neural Hearing Loss: Neural hearing loss occurs with lesions involving the eighth nerve, auditory nuclei, ascending tracts, or auditory cortex. It is the least common clinically recognized cause of hearing loss. Examples include acoustic neuroma, multiple sclerosis, and cerebrovascular disease.

Conductive losses in adults are most commonly due to cerumen impaction or transient auditory tube dysfunction associated with upper respiratory tract infection. Persistent conductive losses usually result from chronic ear infection, trauma, or otosclerosis.

Sensorineural losses in adults are common. A gradually progressive, predominantly high-frequency loss with advancing age is typical though not invariable. Other than aging effects, common causes of sensorineural loss include excessive noise exposure, head trauma, and systemic diseases such as diabetes mellitus.

In a quiet room, the hearing level may be estimated by having the patient repeat aloud words presented in a soft whisper, a normal spoken voice, or a shout. Tuning forks are useful in differentiating conductive from sensorineural losses. A 512-Hz tuning fork is employed, since frequencies below this level elicit a tactile response. In the Weber test, the tuning fork is placed on the forehead or front teeth. In conductive losses, the sound appears louder in the poorer-hearing ear, whereas in sensorineural losses it radiates to the better side. In the Rinne test, the tuning fork is placed alternately on the mastoid bone and in front of the ear canal. In conductive losses, bone conduction exceeds air conduction; in sensorineural losses, the opposite is true.

Formal audiometric studies are performed in a soundproofed room. Pure-tone thresholds in decibels (dB) are obtained over the range of 250–8000 Hz (the main speech frequencies are between 500 and 3000 Hz) for both air and bone conduction. Conductive losses create a gap between the air and bone thresholds, whereas in sensorineural losses both air and bone conduction are equally diminished. The threshold of normal hearing is from 0 to 20 dB, which corresponds to the loudness of a soft whisper. Mild hearing loss is indicated by a threshold of 20–40 dB (soft spoken voice), moderate loss by a threshold of 40–60 dB (normal spoken voice), severe loss by a threshold of 60–80 dB (loud spoken voice), and profound loss by a threshold of 80 dB (shout). The clarity of hearing is often impaired in sensorineural hearing loss. This is evaluated by speech discrimination testing, which is reported as percentage correct (90–100% is normal). The site of the lesion responsible for sensorineural loss—whether it lies in the cochlea or in the central auditory system—may be determined with auditory brain stem-evoked responses.

Every patient who complains of a hearing loss should be referred for audiologic evaluation unless the cause is easily remediable (eg, cerumen impaction, otitis media). Audiologic screening is not recommended for adults with apparently normal hearing unless they are exposed to potentially injurious levels of noise or have reached the age of 65, after which screening evaluations should be done every few years.

Patients with hearing loss not correctable by medical therapy may benefit from hearing 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 are now becoming available that promise substantial improvements in speech intelligibility, especially under difficult listening circumstances.

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 for telephone communication. In individuals with profound sensory deafness, the cochlear implant—an electronic device that is surgically implanted to stimulate the auditory nerve—offers socially beneficial auditory rehabilitation to most adults with acquired deafness.