An etiological consideration was made on hearing impairment of 22 patients with cerebellopontine angle lesions which were confirmed by surgery. Their audiometric examination results were analyzed, and the diagnostic value of the audiometric examinations was also discussed.
1. The presumable causative factors of the hearing impairment. were described bellow.
The first, direct involvement of the lesion to the acoustic nerve or central auditory pathway, the second, malcirculation in the labyrinth, the third, perilymphatic hypertension or affection on the central auditory pathway following elevated CSF pressure and affection on the perilymphatic fluid following the localized disturbance of CSF flow.
2. In acoustic tumor cases, many patients generally showed 1) unilateral threshold elevation on pure tone audiometry, 2) Békésy's type III and or IV unilateral hearing impairment (fixed frequency), but a few patients showed labyrinthine malfunction only.
3. Patients with other tumors usually showed no remarkable difference between both ears on pure tone audiometry, and also showed Békésy's type III and IV, but one case of meningioma developed around the orifice of the internal ear canal showed the similar results to that of acoustic tumor. Another case of vascular tumor showed an unexpected fluctuation of hearing.
4. Cases of arachnitis were classified into 3 types as follows; Meniérè's disease type, sudden deafness type with vertigo and no hearing impairment. and several various factors were presumed as the causative mechanism of hearing impairment as follows; 1) Inflammatory change of the VIII th cranial nerve. 2) Sudden elevation of pressure by the rapidly developed cyst on the VIII th cranial nerve and the labyrinthine artery. 3) Affection on the perilymphatic fluid.
5. The results of audiometric examinations suggested us the possibility of evaluation of the forms and severity of the lesion, and it was very useful for differential diagnosis. Especially, Békésy's type III and/or IV was very important to give us a clue of the differentiation of the lesions.
6. The results of equilibrium function test of one case of acoustic tumor showing labyrinthine malfunction alone revealed the existence of severe, laterally localized brain stem lesion. It was noteworthy that the comparable consideration between hearing impairment, malequiribrium and neurological examinations would be necessary for the proper diagnosis.
