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388                                          THE EAR

In many cases such as those exposed to aircraft engine noise, a charac-
teristic hearing loss is observed at a frequency of 4000 c.p.a. which appar-
ently bears no relation to the frequencies of greatest intensity produced by
aircraft. Such loss may be reversible if continued prolonged exposure is
avoided. It would appear that the ears of some persons are more easily
fatigued and more susceptible to acoustic trauma than those of others, and
it has been suggested that the relation between liability to auditory fatigue
and traumatic deafness might be employed as a test for predisposition to
the latter. There is no indication that ultrasound constitutes a hazard to

TREATMENT. Treatment in all these cases is unavailing. Further trauma
should be avoided whenever possible, but as a prophylactic measure ear
protectors should be worn during exposure to the noises. The great difficulty
is to get individuals to use protectors regularly. Artillerymen may wear
obturators which reduce concussion during gunfire but permit of hearing
instructions. It must be admitted, however, that such measures do not
afford really adequate protection.

Caisson Disease. Caisson workers are liable to nerve deafness if compression
or decompression is carried out too rapidly. The symptoms manifest them-
selves after the individual has left the caisson; the onset is acute, the
deafness being accompanied by giddiness, tinnitus and vomiting. The patho-
logical changes are due either to haemorrhage into the labyrinth or to air
emboli forming in the blood and reaching the labyrinth. Airmen, moun-
taineers and divers are liable to similar lesions.


Labyrinthitis secondary to middle ear inflammation has already been dealt

Meningitic Neurolabyrinthitis. Although the majority of cases of labyrin-
thitis result from middle ear suppuration, a considerable minority are due
to leptomeningitis. Meningitic neurolabyrinthitis is a frequent cause of
deafness, e.g. deafness after epidemic cerebrospinal or tuberculous menin-
gitis is due to this cause. Measles and pneumonia may also be followed by
meningitis and secondary neurolabyrinthitis. In acquired syphilis, and
mumps, leptomeningitis is sometimes met with, and is associated with inner
ear deafness, which is probably to be explained by neuritis or neurolabyrin-
thitis. Certain cases of deafness after influenza may also be of meningitic
origin. A systemic infection probably forms the connecting link between
the primary disease and the onset of meningitis. Meningitic neurolabyrin-
thitis is generally, but by no means always, bilateral. The onset is usually
sudden. Irritative symptoms, such as tinnitus, nystagmus and giddiness, are
present but may not be observed owing to the comatose condition of the
patient. In epidemic cerebrospinal meningitis and parotitis, deafness, if it
occurs, usually comes on early in the course of the disease. Deafness due to
meningitic neurolabyrinthitis may be associated with other metastatic
lesions. Hie infection usually passes along the subarachnoid space from the
base of the brain into the internal acoustic meatus, and then along the
nerves and vessels to the labyrinth. In some cases the perilymphatic aqueduct
is the route of invasion, while in others both paths may be involved. As a