Methylprednisolone shortens the effects of bupivacaine on sensory nerve fibers in vivo.

Perineural administration of corticosteroids is frequently applied in the treatment of a variety of chronic pain conditions. Methylprednisolone selectively inhibits the transmission of nerve impulses in C-fibers whereas A-fiber activity is unaffected. In the present study the effect of a mixture of 0.05 ml of methylprednisolone (40 mg/ml) and 0.05 ml of bupivacaine (5 mg/ml) was compared to that of 0.05 ml bupivacaine (5 mg/ml) using a plantar nerve block model in the rat. The conduction of impulses in electrically stimulated A-fibers of the plantar nerve was monitored by a bipolar volley recording from the sciatic nerve. Impulse conduction in electrically stimulated C-fibers was studied through a C-fiber evoked segmental flexion reflex. The function of both the A-fibers and the C-fibers exposed to the methylprednisolone-bupivacaine mixture showed a less profound block with signs of earlier recovery than those exposed to plain bupivacaine. The A-fibers recovered somewhat faster than the C-fibers. It is postulated that methylprednisolone adjuvant to bupivacaine affects the intra-axonal uptake of bupivacaine in C-fibers but not in A-fibers by some unknown mechanism. The effect seems to be longer lasting in C-fibers than in A-fibers.