Behavioral and electrophysiological recovery following cryogenic nerve injury.

Postthoracotomy pain can be reduced by cryoanalgesia of intercostal nerves. The technique involves focal freezing of peripheral nerves to interrupt pain pathways, producing immediate functional changes that recover as the nerves regenerate. To assess the time-course of functional changes that follow nerve injury, unilateral freeze lesions of sciatic nerve were induced in rats with a cryosurgical unit. The contralateral nerves were used as sham-operated controls. Following nerve injury, behavioral and electrophysiologic tests were repeated to 90 days. The acute effect of nerve injury was a decrease in behavioral measures of hind limb function (P less than 0.05), an increase in electrical threshold to elicit hind limb contraction (P less than 0.005), and an absence of stimulus-evoked compound action potential (P less than 0.005). Morphologic changes included substantial endoneurial edema associated with Wallerian degeneration. Remyelination occurred subsequently during the following 35 days. Although all physiologic measures returned toward normal, nerve conduction velocities were still much slower in the experimental group. In a second study, the long-term effects of cryogenic injury were compared with neurolytic injury with 10% procaine HCl, both of which produced a conduction velocity deficit that persisted at least 90 days after the initial injury. These behavioral and electrophysiologic results complement previous reports of morphologic deficits in the nerves including incomplete recovery of nerve fiber diameter and increased thickness of the perineurial sheath.