騒音性難聴におけるdipの位置とその成因についての一考察

抄録

It has generally been agreed that the industrial deafness is characterized by the hearing loss of dip type at 4 kHz, so-called c⁵ dip, in the early stage of the deafness. When the hearing tests were to be made at each 1 kHz on either side of 4 kHz, however, the hearing pattern of so-called c⁵ dip which was shown clearly on the octave audiogram disappears so often and the accurate locations of the dip could occur at 5 kHz or 6 kHz. In short, it was reconfirmed through this observation that the early dip was found not only at 4 kHz but also frequently at 5 kHz or 6kHz, as already reported by some investigators. Then, it is indispensable to measure the hearing thresholds at 5kHz and 6kHz for the early diagnosis and control of the industrial deafness.
Furthermore, it should be emphasized that the term "c⁵ dip"is replaced by the the term "high frequency dip". As the origin of inducing high frequency dip, the vaso-constriction of cochlear blood vessels which present the sympatheticotonic condition by chronic exposure to the intense sounds seems to be the most possible factor.
At present, the vascular hypothesis has been put forward as the most reliable one, attributing the origin of the dip to the chronic blood flow disturbance at the operating area of basilar membrane which corresponds with the dip frequency. From the anatomical viewpoint, it would possibly be suggested that the blood flow disturbance is liable to occur at the anastomosing region between the cochlear proper artery and the cochlear branch of vestibulo-cochlear artery. And moreover, it has already been clarified by the physiological analysis that the controlling area of this anastomosing region for blood supply correnponds with the operating area of basilar membrane for 4 kHz, 5kHz and 6 kHz. Depending on the above-mentioned findings, it seems to be reasonable to speculate that the high frequency dip originates from the blood flow disturbance at this anastomosing area of the cochlear arterioles.