Seizure Control After Stimulation of the Vagus Nerve: Clinical Outcome Measures

BACKGROUND: Currently, decreases in seizure frequency are the accepted efficacy outcome measure of therapeutic interventions in the management of patients with epilepsy. In a longitudinal randomized controlled trial of 10 subjects with intractable complex partial seizures who received left vagal nerve stimulation (VNS) to control seizures, it was found that the total number of consecutive seizure-free days is a significant efficacy outcome measure. Unlike measures in which percentage decreases in seizure frequency are calculated, measures of consecutive seizure days indicate improvement in the amount of time for which patients may function at a higher level in activities of daily living. METHODS: Fourteen day blocks of consecutive seizure-free days and 14 day blocks of consecutive days in which subjects had seizures were tabulated. RESULTS: A Pearson correlation coefficient showed that prior to VNS subjects had few, if any, seizure free blocks of time and after VNS they had more blocks of time seizure free r = -1.00 and r = -0.99. The blocks of seizure-free days increased tenfold (mean 0.85 to mean 8.00) from 1991-1995 while mean seizure frequency in those blocks in which subjects had seizures only decreased from (mean 20.14 to mean 17.59) for the same time period. Correlations between total number of seizures after 24 months of VNS and after 50 months of VNS were r = 0.85 showing a consistency in the effect of VNS. CONCLUSIONS: Monitoring the number of consecutive seizure-free days is a significant clinical outcome measure of VNS.