Cochlear Changes Following Destruction of Semicircular Canal in Healthy and Previously Toxin-exposed Rats: An Electrophysiological and Morphological Investigation

Abstract

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.

The effect of noise on ears with a hole in the vestibule

“…The opposite ear in the same animal was sham-operated. ABR thresholds to broadband clicks (in this and other studies) and tone bursts at low and high frequencies (in other studies) did not change as a result of the introduction of holes in the wall of the inner ear, for example: in the superior SCC [9,14], in the wall of scala vestibuli of the first cochlear turn [15], and following ablation of SCCs [16].…”

“…The results of the control experiments (drilling a hole, but without noise exposure and ABR thresholds assessed a week later) show that the elevated thresholds in the ear with the hole were not likely to be due to the 5 s drilling procedure, not to possible critical loss of perilymph at the hole, and not to deterioration of the inner ear following the week-long presence of a hole in the vestibule, with possible entry of toxic agents into the inner ear, since there was no change in ABR threshold in these ears. Stenqvist et al [16] also did not see permanent (weeks to a month) threshold elevations following ablation of SCCS without sealing the labyrinth.…”

“…Evidence for this comes from results which showed that following complete ablation of the lateral and superior SCCs, with aspiration of perilymph and profuse loss of perilymph, without sealing off of the labyrinth, there is no permanent hearing loss in rats [16].…”

Hearing Preservation With Labyrinthine Ablation in Otitis Media

“… 102 Unlike toxins isolated from pathogens in acute otitis media, exotoxin A may exert lasting, deleterious effects on the fenestrated inner ear. Stenqvist and colleagues 103 studied the hearing of rat ears that were fenestrated after exposure to either H influenzae endotoxin or P aeruginosa exotoxin A. Ears that were exposed to exotoxin A were much more vulnerable to permanent hearing loss with semicircular canal transection—even 4 weeks after exposure—than ears that were exposed to endotoxin or control ears.…”

“…Ears that were exposed to exotoxin A were much more vulnerable to permanent hearing loss with semicircular canal transection—even 4 weeks after exposure—than ears that were exposed to endotoxin or control ears. 103 The significance of exotoxin A exposure remains to be seen, as transection of the semicircular canals has been possible in ears likely harboring P aeruginosa . 96,100,101 The observations of Stenqvist et al 103 do, however, mandate further investigation into these P aeruginosa –specific toxins and infections.…”