Effects of intrathecal fentanyl and lidocaine on somatosensory-evoked potentials, the H-reflex, and clinical responses.

Intravenous narcotics increase the latency of somatosensory-evoked potentials (SSEPS), which are decreased but not abolished by epidural local anesthetics. In addition, intrathecal narcotics decrease spasticity in patients with central nervous system disease. This study of the effects of intrathecal fentanyl on posterior tibial SSEPS and the monosynaptic H-reflex arc found that intrathecal fentanyl had no effect on the latency of SSEPS, indicating the effects of narcotics on SSEPS are likely to exist at a supraspinal level. H-reflexes were not affected, confirming the lack of effect on this spinal motor reflex. In the same group of patients, intrathecal lidocaine administered 1 week later completely abolished SSEPS and H-reflexes. Complete suppression of SSEPS corresponded to full motor blockade, but sensation to pain and temperature was already many dermatomes higher than the S1 level. Return of SSEPS occurred with return of motor but not sensory function, indicating the likelihood that SSEPS are carried at least in part by large A-fibers. The study shows that spinal narcotics neither affect the transmission of SSEPS nor decrease the H-reflex, a spinal motor reflex. In addition, changes in SSEPS after intrathecal lidocaine do not correlate with the level of surgical anesthesia.