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.

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.

Auricle

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.

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.

External auditory meatus

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.

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.

Middle ear

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. Medico-legally this is important in cases where wounding or burning of this area has occurred.

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.

Middle ear

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 lossIf 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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