Quantitative assessment of differential sensory blockade after lumbar epidural lidocaine.

A cutaneous current perception threshold (CPT) sensory testing device measures both large and small diameter sensory nerve fiber function and may be useful in evaluating differential neural blockade. Eight subjects received both lumbar epidural saline and lumbar epidural lidocaine. Five milliliters of normal saline was administered and the CPTs were measured. After the saline, 10 mL of 2% plain lidocaine was administered. CPTs, and sensation to touch, pinprick, and cold were subsequently measured. Saline had no effect on any measurements. Lidocaine caused an increase in all CPTs at the umbilicus and the knee reaching a statistical significance at 5 Hz for the umbilicus only. The great toe showed a slight increase of the 5 Hz stimulus and no increase of the 2000 or 250 Hz stimulus. There was a significant decrease in touch, pinprick, and cold sensation at the umbilicus and knee and a significant decrease in the cold sensation at the great toe. There was no effect on any measurements made at the mastoid. Epidural lidocaine resulted in a differential neural blockade as measured by a CPT monitor but not with crude sensory measurements.