Dynamic stretch correlates to both morphological abnormalities and electrophysiological impairment in a model of traumatic axonal injury

In this investigation, the relationships between stretch and both morpholog ical and electrophysiological signs of axonal injury were examined in the g uinea pig optic nerve stretch model. Additionally, the relationship between axonal morphology and electrophysiological impairment was assessed. Axonal injury was produced in vivo by elongating the guinea pig optic nerve betwe en 0 and 8 mm (N-total = 70). Morphological damage was detected using neuro filament immunohistochemistry (SMI 32). Electrophysiological impairment was determined using changes in visual evoked potentials (VEPs) measured prior to injury, every 5 min for 40 min following injury, and at sacrifice (72 h ). All nerves subjected to ocular displacements greater than 6 mm demonstra ted axonal swellings and retraction bulbs, while nerves subjected to displa cements below 4 mm did not show any signs of morphological injury. Planned comparisons of latency shifts of the N-35 peak in the VEPs showed that ocul ar displacements greater than 5 mm produced electrophysiological impairment that was significantly different from sham animals. Logit analysis demonst rated that less stretch was required to elicit electrophysiological changes (5.5 mm) than morphological signs of damage (6.8 mm). Moreover, Student t tests indicated that the mean latency shift measured in animals exhibiting morphological injury was significantly greater than that calculated from an imals lacking morphological injury (p < 0.01). These data show that distinc t mechanical thresholds exist for both morphological and electrophysiologic al damage to the white matter. In a larger context, the distinct injury thr esholds presented in the report will aid in the biomechanical assessment of animate models of head injury, as well as assist in extending these findin gs to predict the conditions that cause white matter injury in humans.