The effect of polyethylene glycol on mammalian nerve impulses

Polyethylene glycol (PEG) is a polymeric compound used as a vehicle for depot steroid preparations such as methyl-prednisolone acetate and triamcinolone diacetate injected into the epidural or intrathecal space to relieve low back pain. There have been reports of neurodysfunction associated with these injections, and it has been postulated that the PEG vehicle is the offending agent. Studies supporting such a possibility have, however, relied upon concentrations of PEG higher than those clinically (3%) or have used PEG in combination with other drugs. Using an in vitro rabbit sheathed-nerve preparation, we investigated the effects of a 1-hr exposure to different concentrations (3-40%) of PEG in Liley solution on the transmission of impulses of the A, B and C nerve fibers. The 3% and 10% PEG had no effect on mean amplitudes of the compound action potentials (CAPs) nor did they significantly decrease conduction velocity. Twenty percent PEG slightly depressed and 30% markedly decreased CAPs. Both 20% and 30% PEG significantly slowed the conduction velocities of A, B and C nerve fibers. Forty percent PEG abolished CAPs. With washout CAPs recovered to at least 80% of their baseline levels, and conduction velocities returned toward baseline levels. The pH of the Liley solution decreased with an increasing concentration of PEG, from 7.4 in the control Liley solution to 6.45 in the solution of 40% PEG. There was no evidence of an effect of the acidic pH since neither amplitudes nor conduction velocities were affected by the PEG-free Liley solution at pH 6.45 and the degrees of block were the same at pH 7.4 as at the lower pH. Experiments done on desheathed nerves showed no significant change in amplitude or conduction velocity with 3% PEG. Exposure of desheathed nerves to the 20, 30, and 40% PEG showed the same degrees of block as the sheathed nerves. We conclude that PEG, in concentration up to 40%, does not cause neurolysis.