Morphological Changes in the Cupula after Mechanical Rupturing of the Membranous Labyrinth: A Comparison of Each Canal

Morphological changes in the cupula could be confirmed not only after injecting the ototoxic agent into the inner ear, but also after mechanical rupturing of the membranous labyrinth. This study aimed to investigate the differences in cupula changes in each semicircular canal after rupturing the membranous labyrinth. For mechanical rupturing, a fine needle was inserted into the center of the saccule of the bull frog under anesthesia. The degrees of cupula change were observed under a stereoscopic microscope as of 1 or 2 weeks after rupturing. The ampullary nerve compound action potentials (CAPs) in response to mechanical endolymphatic flow were recorded to assess the sensory cell activity. Severe changes were observed in 24.2% of the posterior semicircular canals (PC), 34.8% of the anterior semicircular canal (AC), and 58.4% in the lateral semicircular canal (LC). The changes in the cupulae in all canals developed with time. Especially in the LC at 2 weeks after rupturing, cupulae showed severe morphological shrinkages, and also the deformities of the cupula other than the shrinkage were found in 25.0%. The CAP results were not different among each canal. It has been suggested that an inflammatory change, and/or mixture of endolymph and perilymph in the inner ear caused the morphological changes of cupula, particularly in the LC. Multiple cupula changes in one inner ear were observed in some subjects. One of the reasons why the LC is more susceptible than the AC or PC is that the LC is located closer to the rupturing site. Another possibility is the LC is smaller than the others, thus making it more difficult to maintain its homeostasis. These findings make us reconsider about the etiology and examination of dizziness.