Two important studies of VNS (Ben-Menachem 1994, Handforth 1998) both found a significant reduction in seizure frequency. These were double-blind studies conducted at several medical centers.
- Each of the patients studied had a VNS implant, but some had only low-level stimulation. The low level settings acted as a placebo or comparison group.
- These studies showed that patients receiving high-level stimulation had a mean decrease in frequency of seizures of about 25% after 3 months. About one-third of the patients actively treated with VNS experienced a reduction in seizure frequency of at least 50%.
- In addition, a 1994 long-term study (George 1994) showed a cumulative improvement in efficacy at 1 year.
In 2000, a large prospective study of VNS (DeGiorgio 2000) was reported. This study involved 195 patients over a 15-month period at 20 medical centers.
- The study consisted of an initial 3-month randomized, double-blind phase, during which patients received either low stimulation or high stimulation, followed by a 12-month period in which all patients received high-level stimulation.
- The median reduction in seizures at 3 months, after completion of the initial double-blind study, was 34%.
- At 12 months, the reduction in seizure frequency was 45%. Furthermore, at 12 months, 35% of the 195 subjects had a greater than 50% reduction in seizures and 20% had a reduction greater than 75%. At 3 months, only 11% of the subjects had experienced a reduction in seizures of more than 75%, so the investigators concluded that the efficacy of VNS improves during 12 months.