ACUTE EFFECTS OF SEMICIRCULAR CANAL DESTRUCTION ON THE COCHLEA, WITH AND WITHOUT PRECEDING PSEUDOMONAS-AERUGINOSA EXOTOXIN-A TREATMENT

Acute electrophysiological and morphological changes in the cochlea fo llowing destruction of the semicircular canals (SCCs) were investigate d to elucidate differences in cochlear vulnerability to surgical proce dure under two conditions. normal healthy condition and a pathological condition induced by Pseudomonas aeruginosa exotoxin A (PaExoA). Freq uency-specific auditory brainstem response recording and examination b y light microscopy (LM) and scanning electron microscopy (SEM) were pe rformed in the acute stage, immediately following, and 2, 5, and 10 da ys after the intervention. All normal healthy rats showed noticeable h earing loss immediately after the operation, predominantly in lower fr equency areas, followed by complete recovery within 5 days. LM reveale d rupture and slight distension of Reissner's membrane. SEM revealed c onsiderable disarray of the stereocilia, especially on the outer hair cells (OHCs) of the third row in the upper half-turns of the cochlea. By contrast, all rats under pathological conditions showed delayed and incomplete hearing recovery from postoperative hearing loss. LM revea led various kinds of cochlear reaction, such as distension of Reissner 's membrane, infiltration of inflammatory cells into the cochlea, and severe inflammatory change. Damage both of inner hail cells in the bas al turn and of OHCs in the upper half-turns was more noticeable, compa red with that under normal conditions. These findings indicate that SC C destruction under pathological conditions induced by PaExoA is detri mental to postoperative hearing recovery, even if the preoperative hea ring level had appeared electrophysiologically normal.