Abstracts

We studied stimulation parameters tolerated by patients one week and one year after undergoing vagus nerve stimulator (VNS) pulse generator reimplant.
Patients who had VNS generators replaced due to end of service were assessed for tolerance of their pre-reimplant current. At surgical reimplant stimulation was programmed to the following parameters: current = 0.25 mA, frequency = 30 Hz, pulse width = 250 usec, time on = 7 sec, time off = 30 min. Within the next week the patient[rsquo]s pre-reimplant parameters were programmed. Next the current was increased by 0.25 mA increments to tolerance or pre-reimplant current. Patients that were unable to achieve pre-reimplant current initially were seen for further programming within 1 week and then every 1 to 3 months for the next year.
Pearson[rsquo]s bivariate correlation and Wilcoxon two related samples tests.
Twenty-eight patients were evaluated. Mean VNS therapy duration before reimplant was 39 months. 26 of 28 patients had VNS [ldquo]model 100[rdquo] replaced with [ldquo]model 101.[rdquo] We were able to achieve pre-reimplant current within 1 week post-operatively in 7 patients. We were able to achieve pre-reimplant current at 1year post reimplant in only 4 additional patients. The median current (.87 mA) 1 week after reimplant was significantly lower than baseline (p[lt]0.001). The median current (1.50mA) 1 year after reimplant was significantly lower than baseline (p[lt]0.001). This current increase from 1 week to1 year was significant (p=0.001). No change in seizure rate was observed at 12 months compared with baseline in the 17 patients who did not tolerate their pre-reimplant current. Adverse events limiting current were coughing in 12 patients and throat pain in 9 patients at 1 week and coughing in 9 patients, throat pain in 7 patients and voice change/headache in 1 patient at 1 year. Patients with shorter off times at baseline were less likely to return to their baseline current within 1 week (p=0.009) and 1 year (p= 0.009). Patients with longer durations of therapy prior to reimplant returned to their baseline currents sooner (p=0.02).
Most patients undergoing VNS replacement are unable to tolerate their pre-reimplant current settings, even at followup intervals as long as 1 year. This is most noticeable in patients with shorter off times. The observation that no seizure rate change occurred even when pre-reimplant settings were not achieved by 1 year may suggest: 1) the patient never really needed the previous higher settings, or 2) the earlier treatment at higher settings produced persistant modification of the patients[rsquo] underlying epileptogenic process.