Symposium: new data for noise standards. V. Current status of labyrinthine cryosurgery.

Laboratory investigations on the squirrel monkey demonstrated that cryosurgery of the labyrinth was able to attenuate consistently the sensitivity of the vestibular end organ to caloric stimulation. The post surgical rate of decline in semicircular canal function was found to be directly related to the temperature and exposure time employed. For a given exposure time, decreasing cryosurgical temperatures provided loss of function in fewer days. When temperature was held constant, increases in duration of cryosurgical application produced a more rapid decline in semicircular canal function. Examination of the temporal bone of these animals showed that morphological alterations were restricted to the place of contact of the cryosurgical probe on the horizontal semicircular canal. These changes appeared in the form of fibrosis and bony obliteration in the perilymphatic space. Although this blockage could conceivably inhibit the movement of endolymph within the horizontal canal, the posterior and superior canals should remain functional. The absence of nystagmus in response to either caloric or rotational stimuli suggested that cryosurgery did more than just produce an obstruction of the horizontal canal. A series of experiments were then performed on the vestibular end organ of the pigeon, in an attempt to study further the effects of cryosurgery of the labyrinth. In order to eliminate the tissue reaction observed in the monkey, the site of cryosurgical application was moved from the thinned horizontal canal to the bony crista ampullaris, and the cryoprobe temperature adjusted to produce the same intralabyrinthine temperature that had eliminated semicircular canal function in the earlier studies. Using this new technique, a normal rotatory reaction was observed after the horizontal crista was cooled to -30d and -40d C for nine minutes. The animals exposed to -70d and -90d C for nine minutes demonstrated marked reduction in rotatory reaction three weeks after the cryosurgery. Rotatory responses were abolished in those animals which were exposed to -120d and -190d C for nine minutes. Pigeons exposed to -30d and -40d C for nine minutes failed to show any structural changes in the horizontal crista, and surrounding area. Pigeons exposed to -70d C showed the presence of exudate in the endolymphatic space and shrinkage of cupula. Cooling the horizontal crista to -90d C produced partial degeneration of the specialized cells and the sensori-epithelia. The membranous wall of the horizontal canal was ruptured (one of four animals) and the cupula detached. Pigeons exposed to -120d and -190d C demonstrated severe degeneration of the sensori-neural and specialized cells. Ruptures of the membranous wall were present in every specimen. The cupula was not present and exudate was commonly found in the endolymphatic space. A definite correlation seems to exist between histologic and functional changes of the semicircular canals following cryosurgery. It appears that the critical threshold dosage necessary to alter the morphology and function of the semicircular canal system is approximately -70d C for nine minutes (intralabyrinthine temperature +18d C) in the experimental animal. Temperature measurements were performed on two patients with Meniere's disease prior to performing a total labyrinthectomy. The cryosurgical probe was positioned on the thinned bony wall of the horizontal canal at a temperature of -160d C. The temperature of the horizontal crista ampullaris in both patients ranged from +17d to +20d C. These intralabyrinthine temperature measurements correlate well with the experimental data. The clinical use of labyrinthine cryosurgery began in May of 1965. The surgical technique has been described in previously published reports. Using a temperature of -160d C, a cryogenic application is made to the thin bony wall of the horizontal semicircular canal for a total of six minutes (three applications of two minutes each). Since cryosurgery has a dual objective of relieving vertigo and preserving hearing, a case is considered successful only if both objectives have been accomplished. There are 134 cases in which an adequate follow-up has been maintained for one year or more. The results indicate that 100 operations (75 percent) were successful, and seven patients (5 percent) were considered as being improved. Twenty-seven procedures (20 percent) were considered surgical failures, due to inadequate relief of the vertigo attacks. Since experimental animals were rendered unresponsive to caloric stimulation following cryosurgery, it seemed reasonable to assume that the successful clinical cases could be attributed to an attenuation of the sensitivity of the semicircular canal system. Although this change did occur in the majority of the operated cases, 36 percent of the successful cases demonstrated little or no change in the postoperative caloric tests. A change in the sensitivity of the semicircular canal system, therefore, could not completely explain the mechanism by which cryosurgery produces its effect. Based upon the more recent experimental observations, it is entirely possible that changes in the dark and light cells, or the creation of an otic-periotic shunt secondary to rupture of the membranous labyrinth may explain a successful clinical result when little or no change has occurred in the postoperative caloric test.