The effects of valproic acid and carbamazepine on strength-duration properties of peripheral nerve

OBJECTIVE: To study strength-duration properties of motor and sensory axons to evaluate whether there is a change in current through the persistent sodium (Na+) channels of sensory and motor axons in peripheral nerves of epileptic patients before and after valproic acid (VPA) and carbamazepine (CBZ) treatment due to the presence of similar channels in the CNS and peripheral nervous system (PNS).

METHODS: This study, conducted in Baskent University Faculty of Medicine, Adana, Turkey from January 2011 to February 2012, involved 10 patients with partial epilepsy, 10 patients with primary generalized epilepsy who were not currently prescribed anticonvulsant therapy, and 10 control subjects. Using an electromyography machine, stimulus intensity was performed to produce the target (40% of maximum) compound muscle action potentials and compound sensory action potentials. The currents required for different stimulus durations, 0.05, 0.1, 0.2, 0.3, 0.5, and 1 ms, were produced. Stimulus-response curves were then constructed, and the strength-duration time constants were estimated using Weiss’s formula.

RESULTS: The rheobase of motor and sensory fibres was lower in the control group than the values of patients before and after CBZ and VPA therapy.

CONCLUSION: In the PNS of epileptic patients, CBZ and VPA therapy results in decreased axonal excitability. This method may be used in investigating the underlying pathology of peripheral nerve diseases in vivo.