The authors examined 101 cases who suffered from the inner ear deafness due to the exposure to intense sound especially from the viewpoint of its occurrence and progress of these cases.
First the cases were classified into three groups according to the condition of the occurrence and progression. The deafness occurred very suddenly in 42 cases (first group) and slowly in 47 cases (second group) and the deafness occurred suddenly or slowly and it progressed for a long period after the cessation of the exposure in 12 cases (third group).
Between these three groups the clear differences were noted in the clinical picture especially the bilaterality of the impairement, the shape of the audiogram and the grade of the hearing loss. In the first group, many cases showed the unilateral deafness or the bilateral deafness without symmetri- city but in the second and third group the cases showed dominantly the bilateral and symmetrical impairment. In the shape of audiogram, first group cases signify the variety of its appearance but in the second and third group the shapes of the high tone deafness was dominant. Further more from the viewpoint of the grade of the hearing impairment, the first group showed variety of impairment but in the second group most of the cases showed the hearing loss of below the 30 db level as the average of the speech range and in the third group the most cases showed the hearing loss of between 30 to 6C db level.
These observations lead the authors to summa-rize the conclusions of this report as follows.
(1) Three groups classified by the authors in this paper were assumed as three different manife-stations of three different injuring mechanism of the so-called acoustic trauma.
(2) In the first group cases it is assumed that the sound is acting as a force and so it is very adequate to consider that the ear exposed to these intense sounds in accidents will always suffer from inner ear injuries.
(3) In the second and third group cases the hereditary vulnerability must play a great role in the occurrence and progression of the deafness.
(4) It is logical to assume that the second critical intensity level is the border line of the sound intensity that injures the inner ear as a force.
(5) Sometimes the inner ear deafness due to the acoustic trauma will progress after the cessation of the exposure.