A condition characterized by a collapsed tolerance to auditory stimuli. Hyperacusis sufferers may experience ambient noises (e.g. the purr of a dishwasher, the rustle of a newspaper, a dog’s staccato barking) as inner ear pain or pressure. The most common form of hyperacusis, Cochlear Hyperacusis (CH), is caused by damage to the inner ear or the auditory nerve. In certain cases this condition occurs as a cerebral processing disorder, the result of brain damage. The presentation of this disorder is measured by Johnson's Hyperacusis Quotient.

Developed by Dr. Marsha Johnson, an audiologist and hyperacusis researcher from Oregon, Johnson's Hyperacusis Quotient is derived by first comparing a person’s threshold of discomfort, the decibel level at which a person can no longer listen, at different frequencies. This is then compared with the same person’s Loudness Discomfort Level, the level at which sound is merely uncomfortable. Mild hyperacusis patients experience pain at a range of 75-90 decibels; profound victims can be shattered by a sound above 30 decibels.

Hyperacusis often develops gradually, but can be triggered suddenly by exposure to high levels of sound. Temporomandibular Joint Disease (TMJ), Ménière's disease, and Lyme disease have all been indicated as factors affecting the onset of Hyperacusis. Sufferers of Williams Syndrome are more likely to be susceptible to Hyperacusis.

Hyperacusis is often comorbid with tinnitus, an experience of ringing in the ears. The introduction of loud or even ambient sounds that accompany a routine hearing test can increase an individual’s perception of the ringing, and lead to phonophobia, a fear of sound. This may induce the individual to retreat into the protection of ear plugs, leading to a further collapse of tolerance levels.

There is no cure for Cochlear Hyperacusis. A treatment involving a regimen of desensitization to a form of sound known as pink noise is commonly used to stabilize auditory perception.

Much rarer is Vestibular Hyperacusis (VH), in which sounds are incorrectly interpreted as motion and balance cues. A person that suffers from VH might feel as if they are falling in response to exposure to loud, jarring sounds. Nausea, the sensation of spinning, and severe disorientation present in much the same manner as motion sickness, but in response to sounds about the 85 decibel level. Comorbidity with tinnitus is much less prevalent with VH.

Notable Hyperacusis sufferers include Stephin Merritt, of The Magnetic Fields.

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