Relationship between changes in the cochlear blood flow and disorder of hearing function induced by blast injury in guinea pigs

Abstract

The auditory system is the most susceptible to damages from blast waves. Blast injuries always lead to varying degrees of hearing impairment. Although a disorder of the cochlear blood flow (CoBF) has been considered to be related to many pathological processes of the auditory system and to contribute to various types of hearing loss, changes in the CoBF induced by blast waves and the relationship between such changes and hearing impairment are undefined. To observe the changes in the cochlear microcirculation after exposure to an explosion blast, investigate the relationship between changes in the CoBF and hearing impairment and subsequently explore the mechanism responsible for the changes in the CoBF, we detected the perfusion of the cochlear microcirculation and hearing threshold shift after exposure to an explosion blast. Then, an N-nitro-L-arginine-methyl ester (L-NAME, NO synthase inhibitor) solution and artificial perilymph were applied to the round window (RW) of the cochlea before the blast exposure, followed by an evaluation of the CoBF and hearing function. The results indicated that the changes in the CoBF were correlated to the strength of the blast wave. The cochlear blood flow significantly increased when the peak value of the blast overpressure was greater than approximately 45 kPa, and there was no significant change in the cochlear blood flow when the peak value of the blast overpressure was less than approximately 35 kPa. Following local administration of the NO synthase inhibitor L-NAME, the increase in the CoBF induced by the blast was inhibited, and this reduction was significantly associated with the hearing threshold.

Keywords: Cochlea; blast injury; blood flow; guinea pigs; hearing function.

Figure 1

The pathological changes of the inner ear. Haematoxylin-eosin, original magnification ×100. SV, stria vascularis. A, B, C, D and E: show the pathological changes of the four turns of the cochlea at each distance, respectively. The arrow indicates the rupture of the stria vascularis in the second turn.

Figure 2

Time course of the cochlear blood flow (CoBF) measured by LDF at different distances. The y axis depicts variations in the percentage of CoBF compared with the initial baseline levels, and the x axis depicts the time. The values are means ± S.E.M.

Figure 3

Correlation between the maximum values of the CoBF baseline and peak values of the blast overpressure. The correlation is significant at the 0.01 level (2-tailed) (analysed by SPSS 17.0.0).

Figure 4

Time course of the hearing threshold shift. The y axis depicts the hearing threshold shift in dB SPL, and the x axis depicts the time. The values are means ± S.E.M. The difference between each distance is significant (P<0.05).

Figure 5

Percentage changes in the CoBF following the administration of 2 μl of 1% L-NAME in the RW. The CoBF decreased after the RW infusion, reached its lowest point at 2 hour, and then recovered after 3 hour. The data points are means±S.E.

Figure 6

Percentage changes in the CoBF at 3 distances after the explosion of 600 mg of RDX following the administration of 2 μl of 1% L-NAME (group A) or artificial perilymph (group C) in the RW. The data points are means±S.E. The time course from RW infusion until 3 hour after the explosion is displayed. The results indicated significant differences at 1, 2 and 3 hours after the explosion between groups A and B at distances of 0.5 meter and 0.6 meter and at 1 and 2 hours after the explosion between groups A and C at the distance of 0.7 meter (*, P<0.05).