Download Neuro-sensory hearing loss

Chapter 4
The
Ears
The ear is concerned with two functions,
hearing and the sense of balance. The latter
function can, to a very large extent, be
replaced by an alternative mechanism,
namely, that which depends upon vision. and
on stretch receptors in muscles and tendons,
the impulses from which reach the
cerebellum (the portion of the brain at the
back of the cranium).
Hence, even if the internal ear on each side
is completely destroyed, the sense of
balance is not lost; indeed, the only serious
defects suffered by the patients are (apart
from the hearing loss) that he is unable to
swim under water, particularly at night, and
has difficult in walking in the dark as there
is loss of "gravitational sense".
During the acute phase of the inflammation of the
labyrinth, the patient may be extremely dizzy and
unable to walk at all. However, when the acute
phase has settled, the disabilities due to loss of
sense of balance owing to labyrinthine diseases
are not frequently noticed as a serious disability by
the patient. Claims for damages owing to
permanent loss of sense of balance following
internal ear disease are rare and will therefore be
considered no further.
The function of hearing is subserved by the
external ear, the external auditory means, the
tympanic membrane and three ossicles that join
the tympanic membrane to the cochlea. In the
cochlea, sounds are "translated" into nerve
impulses which are conducted by the means of the
auditory nerve to the brain. Interference with the
function of any part of this chain will lead to loss
of hearing of one degree or another.
Auricle
The auricle (or pinna) is one visible part of
the ear. In many animals the auricle is
mobile and is used as a cup to concentrate
sound waves impinging on it. In man this
function has been lost and the contribution
to hearing by the auricle is not great.
The auricle is commonly injured in motor accidents
and during fighting. The common injuries are
lacerations, sometimes associated with complete loss
of a portion of the pinna, and crushing injuries, such
as commonly occur at wrestling. In the former
instance, if there has been no tissue loss a very
satisfactory cosmetic result is frequently obtained. In
crushing injuries, blood vessels are frequently
ruptured and blood may then accumulate between the
skin and the auricular cartilage. If such haematoma is
not completely reabsorbed, it is replaced by fibrous
tissue and leads to an ugly lumpiness of the ear
commonly referred to as "cauliflower ear".
In severe burns, the ears are frequently affected,
often with considerable destruction of auricular
tissue. Cosmetic deformity may be very serious.
Where an ear or a large portion of an ear has been
lost, naturally coloured prostheses may be
employed which closely resemble a normal ear,
plastic surgical reconstruction of ears is
unsatisfactory, although small portions of soft
tissue may be replaced where losses are not great.
External auditory meatus
The external auditory meatus is the canal leading
from the auricle to the middle ear. It is not
commonly severely injured, and as a general rule,
even when it is injured, healing is complete
without residual loss of function. Where there has
been extensive loss of tissue, due. for example, to
burning, the opening may be narrowed to such a
degree that there is interference with the function
of hearing; in addition, debris may accumulate.
The external auditory meatus is one of
the parts of the body where foreign
bodies, including insects and seeds, may
easily become lodged. Although an
employer may be liable for treatment of
such a condition, residual disability after
removal of a foreign body is rare, and
litigation therefore does not commonly
arise
The only other condition of the external
auditory meatus which may become the
subject of litigation is external otitis. This
is essentially a dermatological condition
affecting the skin of the auditory canal, and
a worker may claim that the condition
arose from the handing of irritating or dirty
materials.
Middle ear
The middle ear is that
portion of the hearing
mechanism which
consists of the tympanic
membrane, the three
middle ear ossicles, and
the cavity in which they
lie, together with
accessory structures.
The most common
disease affecting this
area is infection (otitis),
which may be
complicated by
infection of the mastoid
air cells (mastoiditis).
However, both
conditions are rarely
the subject of litigation,
and will be considered
no further.
SECRION 2
AETIOLOGY OF IMPAIRED HEARING
External ear
Loss of hearing due to injury to the external
ear and canal arises in significant degree
only when there is narrowing or obstruction
of the external auditory meatus. Medicolegally this is important in cases where
wounding or burning of this area has
occurred.
Middle ear
Most conditions affecting the
middle ear, which includes the
tympanic membrane and the
ossicular chain, are not traumatic
in origin and are not commonly
directly involved in litigation.
However, they are of great importance in
that they may produce impairment of
hearing in their own rights, and where the
causation of such middle ear disease is the
result of a "naturally occurring " condition.
Such hearing losses as arise have to be
deducted from other causes of hearing loss
due to, for example, excessive noise, where
a claim for workers' compensation or
damages is involved.
The tympanic membrane may be ruptured
by an explosive blast. This may lead to
hearing losses and may also permit the entry
of infecting organisms, thereby leading, in
some cases, to otitis media. Similarly, the
drum membrane may be ruptured by a blow
on the ear, but only rarely do foreign bodies
produce such defects. On rare occasions
blasts and hard blows on the head will cause
dislocations of the ossicular chain.
Destruction or interference with the function of
the tympanic membrane or the middle ear
mechanism produces a very variable amount of
hearing loss, ranging from very little to a
maximum (average hearing loss) of 50-55 decibels
(equivalent to approximately 40 percent hearing
loss in the affected ear). If the hearing loss is
greater than this, it should be concluded that there
is also damage to the cochlea or the auditory nerve.
Otosclerosis is a disease affecting a large number
of people from the third decade of life onwards.
There is progressive impairment of hearing, which
is frequently bilateral. The footplate of the stapes
(one of the osscles in the middle ear) becomes
attached to the oval window, and this union
becomes ossified. In the fully developed condition
the otosclerotic process may also affect parts of
the internal ear and also the canal in which the
auditory nerve runs.
Otosclerosis is not
produced by trauma nor is
there any evidence that it
is an occupational hazard.
It is a common disease,
however, and a workman
in a noisy occupation who
develops the condition
may wrongly attribute it to
this work.
Internal ear
Disease of the cochlea and of the auditory
nerve leads to what is termed neuro-sensory
hearing loss (also described as nerve
deafness and perceptive deafness),
Impairment of hearing arising from this area
can be due to a large number of factors
including congenital anomalies. Only those
causes which are of medico- legal
importance are considered below.
Injury
A blow to the head, whether it results in skull
fracture or not, may produce neuro-sensory
hearing loss. Such losses are most likely to arise
when fractures involve the temporal bone, and
particularly if the fracture line passes through the
cochlea. Almost invariably, in injuries of such
severity there will have been a periods of loss of
consciousness. Deafness induced by head injuries
is usually unilateral, or at least of unequal severity
on the two sides.
Explosive blasts, in addition to
producing injury to the middle ear as
previously indicated, may, probably by
inducing haemorrhage into the internal
ear, produce neuro-sensory hearing
loss. Similarly, pressure changes may
produce similar effects although here
again the important injury is usually to
the middle ear.
Following trauma, the neuro-sensory
hearing loss produced does not stabilise for
at least six months even though at the end of
14 days the amount of hearing loss which
one may reasonably expect will be fairly
clear.
Noise exposure
There is no doubt that exposure to noise
impairs hearing. It is common knowledge that
ringing in the ears (tinnitus) can be induced
by loud noises. Then again after exposure to
a loud noise, such as, machinery in a factory
or a jet aeroplane, a temporary period of
partial hearing loss is almost universally
experienced.
Exposure to noise for prolonged periods may
induce permanent neuro-sensory hearing loss.
Unfortunately, as all individuals in a civilised
society are exposed to noise, it is impossible to
determine precisely what level of noise impairs
hearing,, most probably; all noise is detrimental.
As the result of statistical studies, the following
statements can be made:
(1). Hearing loss will occur in many individuals
exposed for may years to high noise levels(explosions
produce their effects by a blast wave rather than by a
noise wave).
(2). The noise level required to induce hearing loss is
widely held to be 85 decibels per octave band in the
audible frequencies. (The 85 decibel level does not
refer to the overall sound level, which in typical
industry steady noises may be 20 decibels higher ).
(3). When hearing losses due to noise exposure occur,
they are first noticed in the higher frequencies, that is
3000 to 6000 cycles per second.
In industries with a high noise level, for
example, blacksmithing, boilermaking,
grinding work, construction work, mining,
aviation and many forms of factory work, the
employer can reasonably be required to pay
attention to noise reduction, and where he
fails to do so, he might be deemed negligent.
The measures available to reduce the volume
of noise impinging on the ear are:
1). planning the layout of factory.
2). Sound insulation.
3). Design of equipment.
4). Reduction of sound by the use of absorption.
5). The provision of ear muffs or ear plugs for
those exposed to noise.
Where the worker is continuously
exposed to loud noises, regular checks of
hearing should he made so that hearing
losses can be detected at an early stage,
and suitable arrangements made before
serious impairment arises.
Poisons and drugs
A large number of drugs in common use may lead
to serious hearing losses. Well known examples
are streptomycin, kanamycin and quinine.
Salicylate drugs such as aspirin may produce some
hearing loss, although in these cases it is usually
only temporary. Aminoglycosides, for example,
Gentamycin can induce permanent deafness,
accurate doses, controlled by blood level
estimations, are required to diminish the risk.
Hearing loss due to ageing and
degeneration
Hearing loss due to this cause is, of course, not
induced by trauma or occupations, but it is of
medico-legal importance in that a suitable
reduction of detected heating loss must be made
when the patient is over the age of fifty. The usual
loss for presbycusis is 0.5 per cent at the age of 50,
and an additional 0.5 per cent for each year
thereafter.
SECTION 3
MEASUREMENT AND TESTS
OF HEARING LOSS
Measurement of Hearing Loss
Accurate measurement of hearing has been
possible for only 60 years following
developments in the field of electronics.
Prior to this, only the severe grades of
impaired hearing could be detected with any
accuracy.
The standards of normal hearing have been
determined by means of tests conducted on
individuals between the ages of 15 and 35
in whom clinical examination has shown no
aural abnormality. In order to understand
the meaning of test results, a number of
terms require definition.
Threshold of hearing
The threshold of hearing is the minimum intensity
of sound audible at a particular frequently. The
variation of audibility with different frequencies is
very large. Thus, a normal individual can hear a
pure tone of 2000 cycles per second quite easily
when the sound pressure level is only 20 decibels,
but at a frequency of 20 cycles per second
(approximately the pitch produced by the lowest
not on the piano ), 20 decibels is inaudible.
The hearing level for speech is, of course,
the most important one, but unfortunately,
owing to the complexities involved in
testing reactions to speech, it cannot be used
as a method of accurate measurement;
instead a number of pure tones of different
frequencies are employed.
Air conduction
Air conduction refers to the ability of an
individual to appreciate sound produced in
the ordinary way and transmitted through
air.
Bone conduction
Bone conduction refers to the ability of
an individual to appreciate sounds
which are applied directly to a cranial
bone
Conductive hearing loss
If an individual has poor air conduction, but good
bone conduction, there is a defect in the acoustic
transmission through the outer ear or the middle
ear. Such a condition is referred to as conductive
hearing loss.
Neuro-sensory hearing loss (Nerve
deafness or perceptive hearing loss )
This condition is hearing loss due to
diseases of the cochlea or of the auditory
nerve.
Mixed hearing loss
This exists when both neuro-sensory and
conductive hearing losses coexist.
Tests for loss of hearing
Ordinary clinical testing
Is used for the detection of gross auditory
impairment. The individual is asked when
he is no longer able to hear a tuning fork or
the ticking of a wrist watch.
Rinne test
Normally a vibrating tuning fork will be
heard twice as long by air conduction as by
bone conduction. If bone conduction is
better, the patient has a degree of air
conduction hearing loss.
Weber test
A vibrating tuning fork is placed in the centre of
the patient's forehead. If there is a conduction type
of hearing loss in one ear the note will be heard
better in the ear with impaired function. If there is
nerve deafness confined to one ear, the note will
be heard better by the normal ear.
Audiometry
The audiometer is a very sensitive electronic
instrument which must be carefully checked from
time to time and correctly calibrated. Tests must
be performed in non-echo, sound-proofed rooms,
so that all external noises are avoided. Finally, the
technician or physician performing the test must
be well versed in its technique.
Right
Audiometry
ear
Before a patient is submitted to audiometry, it
must be determined that he was not
subjected to loud noises for at least two days
before testing, as temporary deafness may
be induced in this way, thereby, giving a false
result. The precise technique used by
different laboratories varies, but as a general
rule reading are made at 500, 1000, 2000,
3000 and 4000 cycles per second, but other
frequencies may also be chosen.
Similarly, the threshold for hearing is recorded in
some laboratories when, after repeated testing,
half the stimuli are heard by the individual tested,
whereas in other laboratories a threshold is
determined at that point where two out of every
three stimili are heard. These results are then
recorded on a graph and compared with the
normal. An assessment then can be made of the
residual disability.
SECTION 4
RESIDUAL DISABILITY DUE
TO IMPAIRED HEARING
Before an assessment of residual disability for
legal purposes can be made, a number of factors
have to be taken into consideration.
(1). Diagnosis or the cause of deafness. Where
there is more than one cause, for example, if
a man with a perforated ear drum is also
suffering from nerve deafness due to a noisy
occupation, the relative importance of each
contributory cause must be made clear.
(2). The age of the patient must be taken into
consideration. Presbycusis (loss of hearing
occurring with advancing years), must be
taken into consideration. It is generally
accepted that beginning at the age of 50, 0.5
per cent is subtracted from the percentage
hearing loss, and an additional 0.5 per cent is
subtracted for each year thereafter. Thus at the
age of 60, the percentage heating loss would
be (even without any injury) 5.5 per cent.
(3). According to the law in different states or
countries, an assessment is made either of
monaural or binaural deafness, sometimes
both are required.
(4). The laws relating to workers compensation
and the assessment of hearing loss vary widely
in different parts of the would and sometimes
between different states in the same country,
and the scientific bases accepted by different
courts of low also vary. Hence, the figures
given here and the particular method of
assessing hearing loss is merely one of many
and will not be the one universally used.
If an assessment is required of the deafness
remaining in each ear, the calculation is simple.
Where an assessment of binaural deafness is
required, various " loading " methods are used.
They differ widely and no finality has been
reached.
Here is one method commonly employed:
The hearing loss present in the better ear is
multiplied by four, the reading obtained from the
worse ear is then added, and whose result is
divided by five.
To quote a specific example, let us say that
the percentage loss, making due allowance
for age and other caused of hearing loss not
involved in the litigation, is right ear 20 per
cent and left ear 40 percent. The figure of
20 per cent is multiplied by four, yielding
80 per cent to which is added 40 per cent,
making a total of 120 per cent. When this
figure is divided by five the result for
binaural loss is 25 per cent.
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