Effects of subarachnoid lidocaine, meperidine and fentanyl on somatosensory and motor evoked responses in awake humans.

Although the effects of local anaesthetics (LA) on motor and sensory transmission in the spinal cord have been described, the effects of opioids are controversial. Our aim was to evaluate the action of clinically relevant doses of subarachnoid (SA) meperidine (MP) and fentanyl (FN), on somatosensory (SSEP) and cortical motor evoked responses (CMER) in awake subjects. Thirty ASA I-II patients scheduled for infra umbilical surgery received SA (N = 10/group): 1 mg/kg lidocaine (LD), 1 mg/kg MP or 25 mu g FN. SSEP elicited by stimulation of the posterior tibial nerve at the ankle, and cortical motor evoked response at rest (r-CMER) and during facilitation (f-CMER) were obtained prior and 30 min after treatment. Conduction at the proximal segment of the motor nerve (F-wave) was evaluated by stimulation of the posterior tibial nerve at the popliteal fossa. Motor/sensory block and side effects were clinically assessed. LD completely abolished SSEP and CMER. At the same dose, MP abolished SSEP in 40% of the patients, while r-CMER and f-CMER were absent in 70% and 30%, respectively; in addition, the F-wave was absent in 50% of the patients. Fentanyl induced small changes in the latencies of SSEP and F-wave; however, a 28% decrease in the amplitude of the f-CMER (P<0.05) was observed. Pruritus was present in 60% of patients in the FN group (P<0.006). Our results show that while LD and MP block sensory and motor conduction at the spinal roots, FN seems to decrease the excitability of the spinal interneurons in the corticospinal tract.