Electrophysiological changes were studied in the albino rat following instillation of Pseudomonas aeruginosa exotoxin A into the middle ear cavity through the tympanic membrane. Hearing threshold was measured by a burst-elicited, frequency-specific auditory brainstem response (ABR) technique prior to exposure, then 24 and 48 h, 5 days, 2 and 4 weeks after the toxin instillation. A single dose (1 microgram/20 microliters) of Pseudomonas aeruginosa exotoxin A raised the ABR threshold over the whole frequency range, by 5-25 dB, particularly in the high tones. All threshold shifts were of combined conductive and cochlear type, reversible, with deterioration starting at 24-48 h and recovery at 2-4 weeks. Effusion of serous fluid occurred at 24 or 48 h, resulting in conductive hearing loss. Latency/intensity curves revealed a cochlear component in addition to conductive hearing loss. Morphological examination by SEM showed slight and inconsistent derangement of OHCs. It is concluded that Pseudomonas aeruginosa exotoxin A causes middle ear inflammation, facilitating penetration to the inner ear and that this toxin also reversibly affects cochlear function.

One group of Sprague-Dawley rats (group A, n = 6) was treated by instilling Pseudomonas aeruginosa exotoxin A (PaExoA), and another (group B, n = 6) treated similarly with Haemophilus influenzae type b endotoxin (HiBEndo). In group A a 20 dB hearing loss was observed, predominantly in the high-frequency region, which was reversible within 1 month. In group B no significant hearing impairment was noted. Between 1 and 6 months later, the lateral and posterior semicircular canals (SCCs) were ablated unilaterally. Control rats (group C, n = 8) were subjected to ablation only. All rats were cochleotomized contralaterally prior to labyrinthine surgery. Frequency-specific evoked potential testing at 2-31.5 kHz tone bursts was performed before and directly after surgery, 6, 24 and 48 hours and 1, 4 and 16 weeks postoperatively. After surgery in 18 rats, thresholds rose immediately, predominantly at 2, 4 and 6 kHz, followed by varying degrees of recovery. Greatest immediate postoperative hearing loss was observed in group A; no rat recovered completely and two rats showed severe permanent threshold elevation. All group B rats recovered completely, except one showing moderate threshold impairment. No permanent hearing loss was observed in group C. This study shows that destruction of SCCs in rats does not necessarily cause permanent hearing loss, even if the fluid spaces are not sealed off. However, previous exposure of the middle ear to PaExoA (but not HiBEndo) renders the cochlea more vulnerable and can result in persistent hearing loss.

Pseudomonas aeruginosa exotoxin A (1 microgram/20 microliters) was instilled into the middle ear cavity in the rat. Morphological changes of the mucosa were analyzed after various intervals using light (LM) and transmission electron microscopy (TEM). An inflammatory cell reaction was seen in which there was a predominance of polymorphonuclear leukocytes (PMNs) and mononuclear cells that were mostly lymphocytes and monocytes/macrophages. Epithelial cells often showed signs of metaplasia and hyperplasia, followed by degradation through necrosis and/or apoptosis, resulting in denudation of the submucosal layer. A characteristic feature was the physical interaction between intraepithelial lymphocytes and epithelial cells. Lymphocytes adhering to the cell coat of epithelial cells showed signs of directed secretion that seemed to end in necrosis or apoptosis of the target cell. These changes occurred simultaneously with phagocytosis of cells and cell debris by PMNs and macrophages. Ultrastructural analysis suggested that intraepithelial, lymphocyte-mediated cytolysis and apoptosis may be important events in degradation of the epithelium following exposure to Pseudomonas sp. exotoxin. These findings indicate that denuding of the lamina propria may facilitate the penetration of toxin into the labyrinth, thus explaining subsequent inner ear damage.

Interleukin-2 (IL-2), one of the chemical mediators produced by helper T cells, activates its target cells via its receptor and induces inflammatory reactions indirectly. In this study, IL-2 was instilled into the round window niche of rats and the electrophysiological effects of inflammation on the inner ear were evaluated by a frequency-specific auditory brainstem response (ABR) technique. ABR threshold curves and intensity-latency (I-L) curves were investigated. In most cases, moderate sensorineural hearing loss (SNHL) was observed in the high-frequency areas (16–31.5 kHz). In the time course investigation of I-L curves, the responses of SNHL type appeared only 1 day after instillation (day 1) gradually becoming more prominent, to become most pronounced on days 5–7. The electrophysiological impairment was reversible within 2 weeks. Differing degrees of responsiveness of effusion production were observed following the instillation of IL-2, varying from pronounced middle ear effusion causing rather severe mixed hearing loss, to complete lack of effusion. Inflammation induced by IL-2 affects cochlear function gradually but reversibly.

An analysis of auditory brainstem response (ABR) thresholds and ABR-based frequency tuning curves was performed in 15 Sprague-Dawley rats exposed to Haemophilus influenzae type b endotoxin; 5 microg/50 microl toxin was instilled every second day, altogether five times, into the middle ear cavity through a small perforation in the tympanic membrane. ABR was measured 48 h after the second application and 24 h, 48 h, 5 days and 10 days after the fifth instillation. Five applications of toxin had no statistical effect on ABR thresholds and no changes in TC configuration were observed. It is concluded that Haemophilus influenzae type b endotoxin, instilled repeatedly through the tympanic membrane into the middle ear, does not affect cochlear electrophysiology.