Download The Human Ear

The Human Ear:
The ear is described in three parts, outer, middle, and inner as shown in
figure 1. The outer ear consists of the auricle and the ear canal. The auricle,
the immediately visible part of the ear, collects the sound like a funnel and
transmits the sound through the ear canal to the tympanic membrane in the
middle ear . The tympanic membrane vibrates as the sound hits it; the
vibration is tranmitted through the middle ear space by the three bones,
the malleus, incus and the stapes, or, in English, the hammer, the anvil, and
the stirrups; these are the three smallest bones in the human body. When
the vibration reaches the stapes this results in fluid waves in the inner ear
and sound is amplified by the hydraulic movement of the ear drum that is
relative to the stapes. The hydraulic motion is not at the ear drum, but at the
oval window in the cochlea--Alex Szatmary 5/15/08 8:09 PM The Eustachian
tube that is connected to the middle ear helps to maintain the equalization of
pressure between the middle ear and the outside atmosphere. As vibrations
of the stapes reach the inner ear, the cochlea, which is a spiral chamber, like
a snail, lined with fine hairs, stereocilia, to detect vibrations in the fluid
mediumThis is a sentence fragment--Alex Szatmary 5/15/08 8:12 PM; these
vibrations are from sound conducted through the timpanic membrane and
the bones in the middle ear. The stereocilia then stimulate the auditory
nerve, which sends the signal to the brain.
Figure 1. Anatomy of the human ear.
Causes of Hearing Loss:
There are many causes of hearing loss. Some people lose their hearing
slowly as they age- presbycusis; it happens when the tiny hair cells in the
inner ear are fall out or are damaged. Another cause of hearing loss is
Otosclerosis this is a disease involving the ossicles getting stiff and will not
vibrate in response to sound. Some medications are known for their otoxicity
if used in large quantities; examples of the ototoxic drugs are aspirin, drugs
that are used in chemotherapy regimens, and aminoglycoside antibiotic. An
exposure to extremely loud sound can cause hearing loss. A puncture of the
ear drum, high changes in pressure and a heavy blow on the ear can all
cause a hearing loss.
Symptoms:
A person with a hearing loss will normally ask the person they are
speaking with to repeat themselves, they misunderstand what people say, it
sounds like everyone is mumbling to them, they strain to hear and keep up
with conversations, they have difficulty hearing on the telephone, they have
difficulty hearing environmental sounds like a generator, yelling group of
people etc. They read the lips of the person they are speaking with in order
to follow what the person is saying.
Types of Hearing Loss:
The two major types of hearing loss are conductive and sensorineural
(1). Conductive hearing loss is the kind of hearing loss where sound is not
transmitted efficiently from the auricle through the timpanic membrane, to
the ossicular bones (malleus, incus and the stapes). Conductive hearing loss
could arise as a result of the fusion of the ossicles to other surrounding parts
of the middle ear. Sometimes, it could just be that one of the ossicular bone
is damaged or inactive for example, otosclerosis, the hardening of the stapes
in the middle ear. Other causes of conductive hearing loss are ear infection,
impacted ear wax, or birth defects. Depending on the cause, the condutive
hearing loss is easily treatable.
Sensorineural hearing loss results from damage to the inner ear or the
nerve pathways from it to the brain(1). Most cases of sensorineural hearing
losses are inherited; they may not always be apparent at birth, but they
show up with age. Other causes of sensorineural hearing loss include certain
kinds of antibiotics intravenously (e.g., gentamicin), exposure to loud noise,
brain infection, viral infection in the inner ear, inadequate oxygen at birth.
Generally, the sensorineural hearing loss cannot be surgically corrected, but
can be overcome by the use of cochlea implant.
Mixed hearing loss is a combination of the conductive and the
sensorineural hearing loss (1). When a person has mixed hearing loss, it
means that there are problems in both the middle and inner ear. The
conductive part of the mixed hearing loss can be treated, but the
sensorineural part could be permanent.
II Treatments
The Bone-Anchored Hearing Aid (BAHA):
BAHA consists of a small titanium fixture, a percutaneous abutment (screw),
and an electronic sound processor as shown in figure 2. (2). The titanium
fixture is implanted into the mastoid bone (skull) during surgery. The
titanium fixture is allowed to bond with the mastoid ( bonding takes a few
months) bone before the screw and the processor is attached to it, so that it
. After several months, the titanium fixture bonds with the mastoid bone
tissue; this process is called osseointegration. The percutaneous abutment
and the titanium fixture secure the sound processor. when the sound
processor detects a sound, it transmits the sound vibrations through the
abutment to the titanium fixture. The sound from the titanium fixture causes
a vibration in the skull and stimulates the nerve fibers in the inner ear to
allow hearing.
BAHA is used for patients with chronic ear infection of the middle and
outer ear, which is caused by conductive and mixed hearing loss. These
patients cannot use the regular hearing aid that is connected to the ear
canal, because the ear canal will be irritated; in order to avoid the irritation
of the outer and middle ear, otologic surgeons recommend the BAHA because
it bypasses the outer and middle ear to get to the inner ear. In a case where
the patient has malformed inner and middle ear from birth, the BAHA can
also be used.
When a patient gets a hearing aid, they expect to get immediate results
after the surgery. After the implantation of BAHA, the surgeons don’t attach
the abutment and the processor to the titanium alloy, because it has to bond
with the skull. This is not very good because it takes some months before
bonding occurs. After the surgery, Since the abutment and the processor are
not yet attached, the patient will be as good as deaf. Another downside of
the BAHA is that it could be cumbersome; the patient cannot lay on that part
of their head on a flat surface without having to detach the abutment and the
processor. It is ok to detach them and relax, but in case of emergency, the
patient will not respond fast because they are temporarily deaf. While
detaching and attaching the abutment and the processor either or both could
be misplaced.
Figure 2. Bone anchored hearing aid
Ossicular Reconstruction:
This method of treating hearing loss can involve reconstruction of the
ossicles, depending on what part of it is damaged. When the stapes are
damaged, the Otologic surgeons may replace them with the patient’s incus.
In the case where the more than one ossicular bone is damaged, either an
autograft or a homograft is done (3). A homograft involves extracting a bone
from genetically non-identical member, while an autograft is when a bone is
extracted from one part of a person’s body and used on the same person.
There are two major methods that are used during the ossicular
reconstruction. In one of the methods the ossicular bones are joined together
using a teflon cup and a shaft. The shaft fits into the drilled holes in the
homograft or the autograft incus or malleus head; the teflon cup is placed on
the stapes capitalum, and the ossicle is placed under the tympanic
membrane. In the other method, only a teflon shaft is used. The shaft is
fitted into a hole in the incus or malleus head, and the base of the shaft is
placed on a footplate. When the bones are connected, the ossicle medial is
placed onto the tympanic membrane.
Over a decade ago, the choice to correct conductive hearing loss is bone
reconstruction (3), but because of the problems associated with them, they
are no longer commonly used. The common problem with the bone
reconstruction is that when the bones are removed at the time of revision
surgery, erosion and thinning of the bone occurs. If the thinning and erosion
continues, the amount of sound waves that are being transmitted from the
timpanic membrane to the cochlea will reduce, there by leading to a hearing
loss all over again. In the case where a homograft is done, there is concern
with disease transmission.Is there risk of rejection? -Alex Szatmary 5/15/08
11:11 PM To avoid disease transmission, an autograft can be done, but the
patient would have to endure the pain from the extraction point and the ear
surgery.Are there other reasons, e.g., length of time in surgery? -Alex
Szatmary 5/15/08 11:13 PM
This section is very much improved. Great! -Alex Szatmary 5/15/08 11:12
PM
Reference:
1)“an overview of Hearing loss”, Patrick J. Antonelli.2002
2) “Bone anchored hearing aid” University of Maryland medical
center.www.umm.edu/otolaryngology/baha.htm. 2002
3)“current use of implants in middle ear surgery”, Goldenberg A. Robert,
Emmet R. John. Pages 145-152. 2001.
4)Farrier JB, ossicular repositioning and ossicular prosthesis in
tympanoplasty. Arch otolaryngol 1960; 443-449
(5) “Required Biocompatibility Training and Toxicology Profiles for Evaluation of
Medical Devices” http://www.fda.gov/cdrh/g87-1.html