Relative Roles of Middle Ear Bulla Volume and Eustachian Tube Function in Diving-related Barotrauma: An Experimental Pressure Chamber Study in Guinea Pigs

Hypothesis: We hypothesized that isolated changes in middle ear air volume do not increase the risk of diving-related middle ear barotrauma, and that Eustachian tube function is the primary determinant of barotrauma development. Background: The relative contributions of middle ear air volume and Eustachian tube function to the development of diving-related middle ear barotrauma remain controversial. This first experimental comparative study aimed to differentiate the effects of middle ear air volume alteration and Eustachian tube function on barotrauma in guinea pigs exposed to high pressure. Materials and Methods: A diving protocol using a pressure chamber was applied to 18 guinea pigs with normal hearing and otoscopic findings (first dive simulation). Ears developing barotrauma were recorded. Three weeks later, after complete recovery was confirmed, bulla obliteration was performed in the left ears of 12 randomly selected guinea pigs (obliteration group), while the right ears underwent bulla opening without obliteration (sham surgery group). After a 2-week wound-healing period, the same diving protocol was repeated (second dive simulation). Otoscopic examination, tympanometry, and auditory brainstem response measurements were then performed, followed by decapitation. In a double-blind design, botulinum toxin was injected into the Eustachian tube muscles on one side of the remaining 6 guinea pigs (botox group), and physiological saline was injected on the contralateral side (saline control group). Results: Some degree of barotrauma was commonly seen in all ears. Within-group comparisons of barotrauma occurrence after the first and second dive simulations showed no significant differences in the obliteration, sham, or saline groups. In contrast, barotrauma developed in all ears in which the Eustachian tube muscles were paralyzed with botulinum toxin. Conclusions: Isolated reduction of middle ear volume by bulla obliteration did not influence the development of diving-related middle ear barotrauma. In contrast, functional blockade of the Eustachian tube resulted in consistent barotrauma, indicating that Eustachian tube function is the primary determinant of susceptibility to barotrauma during acute pressure changes. Middle ear volume may, therefore, be better interpreted as a long-term marker of Eustachian tube function rather than a direct causal factor.