Early Pathophysiological Findings in Experimental Concussion in Rabbits

Early neurophysiological changes and pathological findings were studied following experimental cerebral concussion in rabbits. The animals were lightly anesthetized and subjected to occipital trauma with a pendulum impactor. Experimental concussion was defined as fulfillment of at least two of the three following criteria: apnea for more than 10 seconds, absence of the light reflex for more than 10 seconds, and bradycardia or tachycardia (less than 50% or more than 10% of the preimpact rate, respectively). There were 13 cases of lethal concussion, 16 of nonlethal concussion, and 15 of nonconcussion. The resultant head acceleration ranged from 250 to 1660 G and its duration was 0.8 to 5.5 msec. During the first 30 seconds after impact, electroencephalographic activity showed little change even in cases of lethal concussion, but slowed over the next several minutes due to respiratory suppression. The duration of apnea in cases of nonconcussion was under 9 seconds and, in cases of nonlethal concussion, averaged 21 seconds. Five of the rabbits with lethal concussion were apneic throughout the observation period, and the remaining eight exhibited apnea for an average of 46 seconds after impact. The light reflex was absent for less than 9 seconds in nonconcussion cases and for an average of 71 seconds in cases of nonlethal concussion. One animal with lethal concussion exhibited absence of the light reflex throughout, and the remaining 12 animals for an average of 39 seconds. Bradycardia was seen in 15 cases of nonlethal and two cases of lethal concussion. Six rabbits with lethal concussion had tachycardia. Two animals without concussion showed bradycardia and the remainder showed no change. In cases of nonconcussion there was little hemorrhagic change in the brainstem and upper cervical region. In nonlethal concussion, gross hemorrhagic change was observed in the pons in 25% of cases, in the medulla in 56%, and in the cervical cord in 19%. For lethal concussion the rates were 75%, 83%, and 75%, respectively. Among rabbits with fatal concussion, microscopic parenchymal perivascular hemorrhage was seen in 56% in the pons and in 78% in the medulla, but was not observed in the brainstem of rabbits with nonlethal or nonconcussion. Brainstem pathological changes were well correlated with the severity of concussion. Hemorrhagic changes were seen in the cerebrum in 47% of animals and in the cerebellum in 72%, and there was no correlation with the severity of injury. The