Progressive hearing damage after exposure to repeated low-intensity blasts in chinchillas

Hearing damage caused by blast waves is a frequent and common injury for Service members. However, most studies have focused on high-intensity blast exposures that are known to cause moderate to severe traumatic brain injury (TBI), and fewer studies have investigated the progressive hearing damage caused by low-intensity blast exposures (below mild TBI). In this paper, we report our recent study in chinchillas to investigate the auditory function changes over the time course after repetitive exposures to low-intensity blast. Two groups of chinchillas (N = 7 each) were used in this study. Group 1 was for an acute study with 2 blasts and Group 2 for progressive study with 3 blasts on Day 1 and observed for 7 days. Animals in both groups were exposed to blast overpressures of 21-35 kPa (3-5 psi or 180-185 dB SPL) at which the eardrum was usually not ruptured. One ear was left open while another ear was protected with an earplug. Blast overpressures were monitored at the entrance of the ear canal (P0) and near the eardrum in the canal (P1). Auditory brainstem responses (ABRs), distortion product otoacoustic emissions (DPOAEs), and middle latency responses (MLRs) were measured after each blast series in the acute group and on Days 1, 4, and 7 in the progressive group. Results show that hearing damage was induced in both ears after blast exposure on Day 1 and more damage was observed in open ears than plugged ears. Seven days after the three-blast series, the ABR threshold in open ears was still 7-20 dB higher on average than prior to the blasts. The MLR wave amplitude shifts were observed in both open and protected ears, which indicated central auditory damage. With the protection of an earplug, hearing thresholds had recovered to the pre-blast level by Day 7. Using this chinchilla blast model, acute and progressive hearing damages were quantified in both open and protected ears following repeated low-intensity blast exposures.