NEUROLOGIC EVALUATION OF THE RAT DURING SCIATIC-NERVE BLOCK WITH LIDOCAINE

Background: Quantitative behavioral testing is necessary to establish a reproducible measure of differential functional blockade during regi onal anesthesia, Methods for assessment of the neurologic status (ment al status, posture, gait, proprioception, motor function, autonomic fu nction, and nociception) in veterinary neurology were adapted for the rat and used to monitor functional changes separately during a sciatic nerve block. Methods: Sprague-Dawley rats were acclimated to laborato ry routine before the study so that lidocaine (0.1 ml, 1%) could be in jected near the sciatic notch without any chemical restraint, The onse t, duration, and magnitude of functional losses were monitored, Propri oceptive integrity was evaluated by assessing the response to tactile placing and the hopping response. Extensor postural thrust, a test for postural reactions in small animals, was assessed on a digital balanc e and found adequate for quantifying motor function. Analgesia was ass essed by measuring withdrawal response latencies to noxious thermal st imulation (51 degrees C) and to superficial and deep noxious pinches, Autonomic function was monitored by measuring skin temperature. Contra lateral limb function was used as an internal control, and injection o f saline was used as an external control in separate, control animals. Results: Onset of postural and gait abnormalities were observed as ea rly as 40 s after injection. On each occasion proprioceptive impairmen t was detected first, followed by impairment of motor function and noc iception. Complete absence of proprioception occurred from 10 to 30 mi n (n = 9) and of motor function at 30 min after injection (n = 10); bo th functions were fully recovered by 120 min, A unilateral increase in skin temperature on the foot was detected by 1 min; had reached its m aximum change, 5.3 +/- 0.7 degrees C, at 10 min; and had returned to c ontrol levels at 60 min after injection (n = 12). Withdrawal response to cutaneous or superficial pain was absent in all ten animals from 5 to 30 min whereas the response to deep pain was absent in all ten anim als at 20 min only. The response to noxious stimulation recovered at 9 0 min, Attention was paid to the temporal relation of the impairment o f various functions. Conclusions: Quantitative observations of the ons et, offset, and intensity of differential functional impairment or blo ck overtime will make it possible to establish the doses and condition s for local anesthetics that result in differential nerve block and wi ll permit comparison of these changes among different drugs and ''clin ical'' protocols.