We evaluated functional and morphological changes after trans-tympanic laser application using several different powers of photobiomodulation (PBM). The left (L) ears of 17 rats were irradiated for 30 min daily over 14 days using a power density of 909.1 (group A, 5040 J), 1136.4 (group B, 6300 J), and 1363.6 (group C, 7560 J) mW/cm(2). The right (N) ears served as controls. The safety of PBM was determined by endoscopic findings, auditory brainstem response (ABR) thresholds, and histological images of hair cells using confocal microscopy, and light microscopic images of the external auditory canal (EAC) and tympanic membrane (TM). Endoscopic findings revealed severe inflammation in the TM of C group; no other group showed damage in the TM. No significant difference in ABR threshold was found in the PBM-treated groups (excluding the group with TM damage). Confocal microscopy showed no histological difference between the AL and AN, or BL and BN groups. However, light microscopy showed more prominent edema, inflammation, and vascular congestion in the TM of BL ears. This study found a dose-response relationship between laser power parameters and TM changes. These results will be useful for defining future allowance criteria for trans-tympanic laser therapies.
Keywords: Cochlea; Laser therapy; Safety.