Long-term functional and structural consequences of primary blast injury to the eye

Abstract

Purpose : Patients with primary blast injury (PBI) often present with delayed visual deficits of unknown etiology. In this study, we evaluated longitudinal changes in retinal function and morphology following acoustic blast exposure, using a rat model of PBI.

Methods : Adult male Long-Evans rats (N=10-12/group) were exposed to a single acoustic blast (63 kPa, 195 dB-SPL) at 3.5 mo of age, using a shock tube device. The blast was directed at the right eye only, perpendicular to the body axis, with only the head exposed. Non-blasted, age-matched rats served as controls. Animals were tested longitudinally at 2, 4, 6 and 8 mo post-blast for: retinal function with electroretinography (ERG); contrast sensitivity (CS) and spatial frequency (SF) thresholds with optokinetic tracking (OKT); and retinal structure with spectral domain-optical coherence tomography (SD-OCT). At 8 mo post-blast, animals were sacrificed and tissues were taken for future biochemical and immunohistochemical analyses. Statistical analysis: repeated measures two-way ANOVA (significance: p<0.05).

Results : CS thresholds were consistently decreased in both eyes of blast-exposed animals vs. controls across time (p<0.001), and the blasted eyes had significantly lower CS thresholds than the contralateral eyes (e.g., at 8 mo post-blast: Δ = 45%; p<0.001). Blasted eyes also showed a 20% decrease in SF thresholds compared to contralateral and control eyes (p<0.001), at all time points. ERG scotopic a- and b-wave amplitudes were substantially greater (p<0.001) and b-wave implicit times were increasingly delayed (e.g., 18% at 8 mo; p<0.001) in blast-exposed rats, relative to controls. Similar trends were observed for photopic flicker responses. SD-OCT showed increased total retinal thickness, relative to controls, in both eyes of blast-exposed rats across all time points (5% at 8 mo; p<0.01), with inner nuclear layer and inner/outer segment layers being the major contributors.

Conclusions : PBI can manifest with functional and structural changes to the retina that are counter-intuitive and atypical of common retinal diseases. Here, blast exposure compromised visual function, yet ERG amplitudes were supranormal, with delayed implicit times, suggesting disruption of inhibitory signaling pathways. Rather than degenerative thinning, retinal thickness of blast-exposed eyes was greater than normal. The underlying mechanisms for these changes remain to be determined.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.