Distortion product otoacoustic emissions as non-invasive biomarkers and predictors of soman-induced central neurotoxicity: a preliminary study.

The organophosphorus nerve agent soman is an irreversible cholinesterase (ChE) inhibitor that can produce long-lasting seizures and brain damage in which the neurotransmitters acetylcholine and glutamate are involved. These same neurotransmitters play key-roles in the auditory function. It was then assumed that exploring the hearing function may provide markers of the central events triggered by soman intoxication. In the present study, distortion product otoacoustic emissions (DPOAEs), a non-invasive audiometric method, were used to monitor cochlear functionality in rats administered with a moderate dose of soman (45 microg/kg). DPOAEs were investigated either 4h or 24h post-challenge. In parallel, the effects of soman on whole blood and brain ChE activity and on brain histology were also studied. The first main result is that DPOAE intensities were significantly decreased 4h post-soman and returned to baseline at 24h. The amplitude changes were well related to the severity of symptoms, with the greatest change being recorded in the rats that survived long-lasting convulsions. The second main result is that baseline DPOAEs recorded 8 days before soman appear to predict the severity of symptoms produced by the intoxication. Indeed, the lowest baseline DPOAEs corresponded to the occurrence of long-lasting convulsions and brain damage and to the greatest inhibition in central ChE. These results thus suggest that DPOAEs represent a promising non-invasive tool to assess and predict the central consequences of nerve agent poisoning. Further investigations will be carried out to assess the potential applications and the limits of this non-invasive method.