Hyperacusis is an increased sensitivity to sound (including pain) and a low tolerance for environmental noise. Definitions of hyperacusis can vary significantly; it often revolves around damage to or dysfunction of the stapes bone, stapedius muscle or tensor tympani. It is often categorized into four subtypes: loudness, pain (also called noxacusis), annoyance, and fear. It can be a highly debilitating hearing disorder. There are a variety of causes and risk factors, with the most common being exposure to loud noise. It is often coincident with tinnitus. Proposed mechanisms in the literature involve dysfunction in the brain, inner ear, or middle ear. Little is known about the prevalence of hyperacusis, in part due to the degree of variation in the term's definition. Reported prevalence estimates vary widely, and further research is needed to obtain strong epidemiological data.
Signs and symptoms Hyperacusis symptoms can include an increased perception of the loudness of sounds (loudness hyperacusis), pain (noxacusis/pain hyperacusis/sound-induced otalgia), annoyance, and/or fear in response to sounds by which most people are unaffected. It may affect one or both ears. The majority of patients experience bilateral symptoms but often have one ear that is more affected than the other. Annoyance hyperacusis is often considered synonymous with misophonia. Fear hyperacusis is often considered synonymous with phonophobia. Many researchers more narrowly define hyperacusis to only include loudness hyperacusis and pain hyperacusis. Hyperacusis can also be accompanied by tinnitus. The latter is more common and there are important differences between their involved mechanisms. Hyperacusis can result in anxiety and stress. Avoidant behavior is often a response to prevent the effects of hyperacusis and this can include avoiding social situations.
Loudness hyperacusis Loudness hyperacusis is characterized by an increased perception of the loudness of sounds. It is often associated with certain volumes and/or frequencies. It can occur in children and adults, and can be either "short-term" in a duration of weeks to less than a year before recovery, or, less commonly, "long-term," spanning years and in some cases becoming permanent. Sensitivity is often different between ears.
Noxacusis In some instances, hyperacusis is accompanied by pain, which is known as noxacusis. Noxacusis is characterized by pain resulting from sounds, often initiated at certain volumes or frequencies. Pain can be immediate or delayed, and it sometimes persists for an extended period of time following exposure. Pain can be acute or chronic, and is often described as stabbing, burning, throbbing, or aching. In healthy listeners, pain from sound is not typically experienced until the volume exceeds approximately 120 decibels. Individuals experiencing noxacusis report less improvement over time and fewer benefits from sound therapy compared to individuals with loudness hyperacusis.
Loudness discomfort level The threshold of sound at which discomfort is initially experienced; measured in decibels (dB). These tests may cause a setback.
Setback A setback is a temporary exacerbation of symptoms, a worsening of the perception of loudness or pain from sound, often due to a particular noise exposure. Setback prevention is an important focus among those affected. Efforts to avoid setbacks commonly include using hearing protection and avoiding loud noises. Pain hyperacusis patients experience setbacks more frequently than patients with loudness hyperacusis.
Associated conditions Some conditions that are associated with hyperacusis include:
Causes and risk factors The most common cause of hyperacusis is overexposure to excessively high decibel (sound pressure) levels, which can cause acoustic trauma. An acoustic shock, which can lead to symptoms such as hyperacusis and ear pain, can also occur after exposure to an unexpected moderately loud to loud noise, even if this does not necessarily result in permanent cochlear damage. Some affected people acquire hyperacusis suddenly as a result of taking ototoxic drugs (which can damage the cells responsible for hearing), Lyme disease, Ménière's disease, head injury, or surgery. Others are born with sound sensitivity or develop superior canal dehiscence syndrome. Bell's palsy can trigger hyperacusis if the associated flaccid paralysis affects the tensor tympani, and stapedius, two small muscles of the middle ear. Paralysis of the stapedius muscle prevents its function in dampening the oscillations of the ossicles, causing sound to be abnormally loud on the affected side. Age may also be a significant factor, with younger patients exhibiting more severe hyperacusis. Recently, it has been discovered that individuals with one copy of the GJB2 (Cx26) genetic mutation exhibit hearing that is more sensitive than average, akin to hyperacusis. These individuals appear to be at greater risk for damage from noise. Some psychoactive drugs such as LSD, methaqualone, benzodiazepines, or phencyclidine can cause hyperacusis. An antibiotic, ciprofloxacin, has also been seen to be a cause, known as ciprofloxacin-related hyperacusis. Benzodiazepine withdrawal syndrome is also a possible cause.
Epidemiology Prevalence estimates for hyperacusis vary widely in the literature, and further epidemiological data is needed. No gender differences have yet been established among hyperacusis patients. Hyperacusis appears to be more severe in younger patients.
Possible mechanisms
Loudness hyperacusis As one possible mechanism, adaptation processes in the auditory brain that influence the dynamic range of neural responses are assumed to be distorted by irregular input from the inner ear. This is mainly caused by hearing loss related damage in the inner ear. The mechanism behind hyperacusis is not currently known, but it is suspected to be caused by damage to the inner ear and cochlea.
Noxacusis
Inner ear theory Type II afferent fibers of the cochlear nerve are not responsible for hearing like the type I afferent fibers. They are thought to be cochlear pain neurons. Gain of function of these type II afferent fibers may be caused by a flood of ATP after hair cell damage. Now sensitized, they react to the small amount of ATP released during the normal process of hearing. This may result in pain.
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