This article is from the Tinnitus FAQ, by firstname.lastname@example.org (Mark Bixby) with numerous contributions by others.
Unfortunately, because hyperacusis is so rare, little research has been
done so little is known about it. The onset is usually caused by exposure
to loud noise (either prolonged or a single episode) or a head injury. Some
experts speculate that the cause is damage to the auditory nerves.
Currently, a popular theory is that there has been a breakdown or
dysfunction in the efferent portion of the auditory nerve. Efferent meaning
fibers that originate in the brain which serve to regulate or inhibit
incoming sounds. If the cause would be damage to the auditory nerve then
why does hyperacusis most often show up in patients who have little or no
discernable hearing loss? One possibility is that the efferent fibers of
the auditory nerve are selectively damaged even though the hair cells that
allow us to hear pure tones in an audiometry evaluation remain intact. The
real problem is that no one clearly understands how the brain interprets
sound. Medicine has much to learn about the auditory system before
hyperacusis and many other auditory problems can be fully understood. Other
contributing causes of hyperacusis are thought to be Temporomandibular
Syndrome (TMJ), Williams Syndrome, Bell's Palsy, Meniere's Disease and
Tay-Sachs Disease. Also as many as 40% of all autistic children are
sensitive to noise, however their condition is called hyperacute hearing.
Autistic children currently receive Auditory Integration Therapy (AIT) to
resolve their sound sensitivities. These treatments do not work on
hyperacusis and can actually worsen our condition - particularly the
tinnitus because it is administered at uncomfortably loud sound levels.