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Hyperacusis (also spelled hyperacousis) is a health condition characterized by an over-sensitivity to certain frequency and volume ranges of sound (a collapsed tolerance to usual environmental sound). A person with severe hyperacusis has difficulty tolerating everyday sounds, some of which may seem unpleasantly or painfully loud to that person but not to others.
It can be acquired as a result of damage sustained to the hearing apparatus, or inner ear. There is speculation that the efferent portion of the auditory nerve (olivocochlear bundle) has been affected (efferent meaning fibers that originate in the brain which serve to regulate sounds). This theory suggests that the efferent fibers of the auditory nerve are selectively damaged, while the hair cells that allow the hearing of pure tones in an audiometric evaluation remain intact. In cases not involving aural trauma to the inner ear, hyperacusis can also be acquired as a result of damage to the brain or the neurological system. In these cases, hyperacusis can be defined as a cerebral processing problem specific to how the brain perceives sound. In rare cases, hyperacusis may be caused by a vestibular disorder. This type of hyperacusis, called vestibular hyperacusis, is caused by the brain perceiving certain sounds as motion input as well as auditory input.
Although severe hyperacusis is rare, a lesser form of hyperacusis affects musicians, making it difficult for them to play in the very loud environment of a rock band or orchestra which previously gave them no problems. It also makes attendance at loud discos or live events difficult for a portion of the population, given that sound levels at such events usually exceed recommended safe levels of exposure. This is a problem which may be caused by genetic differences, stress or ill-health, or by abnormal responses in the tensor tympani and stapedius muscles, which function in the normal acoustic reflex response that protects the inner ear from loud sounds.
40% of tinnitus patients complain of mild hyperacusis.
Hyperacusis was the subject of an ABC News 20/20 segment aired on Feb. 7, 2014. 
A non-profit foundation has been set up to research hyperacusis at hyperacusisresearch.org. 
The most common cause of hyperacusis is overexposure to excessively high decibel levels (or sound pressure levels). Some come down with hyperacusis suddenly by firing a gun, having an airbag deploy in their car, experiencing any extremely loud sound, taking ear sensitizing drugs, Lyme disease, Ménière's disease, TMD/TMJ (Temporomandibular joint disorder), head injury, or surgery. Others are born with sound sensitivity, develop Superior Canal Dehiscence Syndrome, have had a history of ear infections, or come from a family that has had hearing problems. The Diagnostic and Statistical Manual of Mental Disorders (DSM) that is published by the American Psychiatric Association lists hyperacusis as one of the possible signs indicating phencyclidine (PCP or Angel-dust) intoxication.
Causes include, but are not limited to:
In cochlear hyperacusis (the most common form of hyperacusis), the symptoms are ear pain, annoyance, and general intolerance to any sounds that most people don't notice or consider unpleasant. Crying spells or panic attacks may result from cochlear hyperacusis. As many as 86% of hyperacusis sufferers also have tinnitus.
In vestibular hyperacusis, the sufferer may experience feelings of dizziness, nausea, or a loss of balance when sounds of certain pitches are present. For instance, people with vestibular hyperacusis may feel like they are falling and as a result involuntarily grimace and clutch for something to brace themselves with. The degree to which a sufferer is affected depends not only on the overall severity of that person's symptoms but also on whether the person can detect sounds in that frequency range at the volume in question, as well as on the person's preexisting muscle tone and severity of startle response.
Anxiety, stress, and/or phonophobia may be present in both types of hyperacusis. Someone with either form of hyperacusis may develop avoidant behavior in order to try to avoid a stressful sound situation or to avoid embarrassing themselves in a social situation that might involve noise.
A person suffering from hyperacusis might be startled by very low sound levels. Everyday sounds like shutting doors, ringing phones, television, running water, ticking clocks, chewing gum, cooking, normal conversation, eating, dishes, and other sounds will hurt his/her ears.
The most common treatment for hyperacusis is retraining therapy which uses broadband noise. Tinnitus Retraining Therapy (TRT), a treatment originally used to treat tinnitus, uses broadband noise to treat hyperacusis. Pink noise can also be used to treat hyperacusis. By listening to broadband noise at soft levels for a disciplined period of time each day, patients can rebuild (i.e., re-establish) their tolerances to sound. Another treatment method is the Berard Auditory Integration Training. When seeking treatment, it is important that the physician determine the patient's Loudness Discomfort Levels (LDL) so that hearing tests (brainstem auditory evoke response) or other diagnostic tests which involve loud noise (MRI) do not worsen the patient's tolerance to sound. If people have access to medical care, steroids are used to treat hyperacusis within 72 hours of the onset of the condition. Treatment is necessary because sounds over 85db are harmful, and entertainment in some public places, such as live music, have sound levels of around 110 to 120 db, (Examples of sound pressure and sound pressure levels).
- Ludwig van Beethoven suffered from hyperacusis, alongside tinnitus and hearing loss.
- Musician Jason DiEmilio of Azusa Plane suffered from hyperacusis. His story was told in BuzzFeed. 
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- "Molecular Interventions - CLOCKSS". Molinterv.aspetjournals.org. Retrieved 2012-10-29.
- "Decreased Sound Tolerance", by Pawel J. Jastreboff and Margaret J Jastreboff, in: "Tinnitus: theory and management", ed. James Byron Snow, 2004, ISBN 1-55009-243-X