BRIEFING SHEET - HEARING
DIFFICULTY
Syllabus section 3.5.3 page 37: Responses in the Human. Guidelines page 67.
The senses, with the brain as an interpreting centre. Knowledge of the senses. Study of the
eye and the ear. Corrective measures for long and short sight or for hearing.
This sheet contains both prescribed and unprescribed material.
Sound travels through a medium like air or water as
waves of compression and rarefaction. These waves are
collected by the external ear and cause the tympanic
membrane (ear drum) to vibrate. The chain of ossicles
connected to the ear drum--the incus, malleus, and stapes-carries the vibration to the oval window, increasing its
amplitude 20 times on the way. There the energy causes a
standing wave in the watery liquid (endolymph) inside the
Organ of Corti. (A standing wave is one that does not
move. A vibrating cup of coffee will demonstrate standing
waves.) Many thousands of tiny nerve fibers detect the
highs and lows of the standing wave and transmit their
findings to the brain, which interprets the signals as
sound.
The organ of Corti is the sensitive element in the inner ear
and can be thought of as the body's microphone. It is
situated on the basilar membrane in one of the three
compartments of the Cochlea. It contains four rows of hair
cells which protrude from its surface. Above them is the
tectoral membrane which can move in response to
pressure variations in the fluid- filled tympanic and
vestibular canals. There are some 16,000 -20,000 of the
hair cells distributed along the basilar membrane which
follows the spiral of the cochlea.
CAUSES
Hearing can be interrupted in several ways. The external
ear canal can be blocked with ear wax, foreign objects,
infection, and tumors. Overgrowth of the bone, a
condition that occurs when the ear canal has been flushed
with cold water repeatedly for years, can also narrow the
passageway, making blockage and infection more likely.
This condition occurs often in Northern Californian
surfers and is therefore called "surfer's ear."
The ear drum is so thin a physician can see through it into
the middle ear. Sharp objects, pressure from an infection
in the middle ear, even a firm cuffing or slapping of the
ear, can rupture it. It is also susceptible to pressure
changes during scuba diving.
Several conditions can diminish the mobility of the
ossicles (small bones) in the middle ear. Otitis media (an
infection in the middle ear) occurs when fluid cannot
escape into the throat because of blockage of the
eustachian tube. The fluid that accumulates, whether it be
pus or just mucus and dampens the motion of the ossicles.
A disease called otosclerosis can bind the stapes in the
oval window and thereby cause deafness.
All the conditions mentioned so far, those that occur in the
external and middle ear, are causes of conductive hearing
loss. The second category, sensory hearing loss, refers to
damage to the Organ of Corti and it’s associated nerves.
Prolonged exposure to loud noise is the leading cause of
sensory hearing loss. The cause is often believed to be
prolonged exposure to rock music. Occupational noise
exposure is the other leading cause of noise induced
hearing loss (NIHL) and is ample reason for wearing ear
protection on the job. A third of people over 65 have
presbycusis--sensory hearing loss due to aging. Both
NIHL and presbycusis are primarily high frequency
losses. In most languages, it is the high frequency sounds
that define speech, so these people hear plenty of noise,
they just cannot easily make out what it means. They have
particular trouble selecting out speech from background
noise. Brain infections like meningitis, drugs such as the
aminoglycoside antibiotics (streptomycin, gentamycin,
kanamycin, tobramycin), and Meniere's disease also cause
permanent sensory hearing loss. Meniere's disease
combines attacks of hearing loss with attacks of vertigo.
The symptoms may occur together or separately. High
doses of salicylates like aspirin and quinine can cause a
temporary high-frequency loss. Prolonged high doses can
lead to permanent deafness. There is an hereditary form of sensory deafness and a congenital form most often caused by
rubella (German measles).
Sudden hearing loss--at least 30dB in less than three days--is most commonly caused by cochleitis, a mysterious viral
infection.
The final category of hearing loss is neural. Damage to the acoustic nerve and the parts of the brain that perform hearing
are the most likely to produce permanent hearing loss. Strokes, multiple sclerosis, and acoustic neuromas are all
possible causes of neural hearing loss.
Hearing can also be diminished by extra sounds generated by the ear, most of them from the same kinds of disorders
that cause diminished hearing. These sounds are referred to as tinnitus and can be ringing, blowing, clicking, or
DIAGNOSIS
An examination of the ears and nose combined with
simple hearing tests done in the physician's office can
detect many common causes of hearing loss. An
audiogram often concludes the evaluation, since these
simple means often produce a diagnosis. If the defect is
in the brain or the acoustic nerve, further neurological
testing and imaging will be required.
The audiogram has many uses in diagnosing hearing
deficits. The pattern of hearing loss across the audible
frequencies gives clues to the cause. Several alterations
in the testing procedure can give additional information.
For example, speech is perceived differently than pure
tones. Adequate perception of sound combined with
inability to recognize words points to a brain problem
rather than a sensory or conductive deficit. Loudness
perception is distorted by disease in certain areas but not
in others. Acoustic neuromas often distort the perception
of loudness.
TREATMENT
Conductive hearing loss can almost always be restored to some degree, if not completely.
·
matter in the ear canal can be easily removed with a dramatic improvement in hearing.
·
surfer's ear gradually regresses if cold water is avoided or a special ear plug is used. In advanced cases,
surgeons can grind away the excess bone.
·
middle ear infection with fluid is also simple to treat. If medications do not work, surgical drainage of the ear
is accomplished through the ear drum, which heals completely after treatment.
·
traumatically damaged ear drums can be repaired with a tiny skin graft.
·
surgical repair of otosclerosis through an operating microscope is one of the most intricate of procedures,
substituting tiny artificial parts for the original ossicles.
Sensory and neural hearing loss, on the other hand,
cannot readily be cured. Fortunately it is not often
complete, so that hearing aids can fill the deficit.
therapies. Oral supplementation with essential fatty acids
such as flax oil and omega 3 oil can help alleviate the
accumulation of wax in the ear.
In-the-ear hearing aids can boost the volume of sound by
up to 70 dB. (Normal speech is about 60 dB.) For
complete conduction hearing loss there are now
available bone conduction hearing aids and even devices
that can be surgically implanted in the cochlea.
Prompt treatment and attentive follow-up of middle ear
infections in children will prevent this cause of conductive
hearing loss. Control of infectious childhood diseases such
as measles has greatly reduced sensory hearing loss as a
complication of epidemic diseases. Laws that require
protection from loud noise in the workplace have achieved
substantial reduction in noise induced hearing loss. Surfers
should use the right kind of ear plugs.
Tinnitus can sometimes be relieved by adding white
noise (like the sound of wind or waves crashing on the
shore) to the environment.
Conductive hearing loss can be treated with alternative
therapies that are specific to the particular condition.
Sensory hearing loss may be helped by homeopathic
www.medicinenet.com
www.bioscience-explained.org
www.irishhealth.com
sightandhearing.org
www.wikipedia.org http://medlineplus.gov/
www.vhihealthe.com