A comparison of initial and permanent surface changes to guinea pig hair cells after acoustic overstimulation.

Groups of pigmented guinea pigs were exposed unilaterally to a 15 kHz pure tone at 133 dB SPL for 7.5 min. The inner ears of one group were examined by light microscopy to count damaged hair cells 3 weeks after exposure. Four other groups were investigated using scanning electron microscopy to assess the progression of initial surface changes after sound exposure. Hair cells were examined at 0, 10 and 30 min and compared with those seen at 3 weeks. Drastic stereociliary disturbances were present immediately after this short sound stimulation. Changes initially affected the first row of outer hair cells, followed by the inner hair cell row, and then spread to all rows at the centre of a damaged area. These changes appeared more advanced in specimens from the later groups. At 3 weeks, primarily phalangeal scars were seen at the main damaged area, with partially degenerating cells at the periphery. The latter still showed stereociliary disturbances, but the types predominating were different from those seen initially. However, no differences were found in the extens of damage when the various exposure groups were compared. No changes due to overstimulation were present in the contralateral ears.