Antioxidant treatment reduces blast-induced cochlear damage and hearing loss

Exposure to blast overpressure has become one of the hazards of both military and civilian life in many parts of the world due to war and terrorist activity. Auditory damage is one of the primary sequela of blast trauma, affecting immediate situational awareness and causing permanent hearing loss. Protecting against blast exposure is limited by the inability to anticipate the timing of these exposures, particularly those caused by terrorists. Therefore a therapeutic regimen is desirable that is able to ameliorate auditory damage when administered after a blast exposure has occurred. The purpose of this study was to determine if administration of a combination of antioxidants 2,4-disulfonyl α-phenyl tertiary butyl nitrone (HPN-07) and N-acetylcysteine (NAC) beginning 1 h after blast exposure could reduce both temporary and permanent hearing loss. To this end, a blast simulator was developed and the operational conditions established for exposing rats to blast overpressures comparable to those encountered in an open-field blast of 14 pounds per square inch (psi). This blast model produced reproducible blast overpressures that resulted in physiological and physical damage to the auditory system that was proportional to the number and amplitude of the blasts. After exposure to 3 consecutive 14 psi blasts 100% of anesthetized rats had permanent hearing loss as determined at 21 days post exposure by auditory brainstem response (ABR) and distortion product otoacoustic emission (DPOAE) testing. Animals treated with HPN-07 and NAC after blast exposure showed a significant reduction in ABR threshold shifts and DPOAE level shifts at 2–16 kHz with significant reduction in inner hair cell (IHC) and outer hair cell (OHC) loss across the 5–36 kHz region of the cochlea compared with control animals. The time course of changes in the auditory system was documented at 3 h, 24 h, 7 day and 21 day after blast exposure. At 3 h after blast exposure the auditory brainstem response (ABR) threshold shifts were elevated by 60 dB in both treated and control groups. A partial recovery of to 35 dB was observed at 24 h in the controls, indicative of a temporary threshold shift (TTS) and there was essentially no further recovery by 21 days representing a permanent threshold shift (PTS) of about 30 dB. Antioxidant treatment increased the amount of both TTS and PTS recovery relative to controls by 10 and 20 dB respectively. Distortion product otoacoustic emission (DPOAE) reached a maximum