Abstracts

Rationale: Somatosensory stimulation is used to identify the central sulcus. It can also be used to determine if a neurological disorder, such as epilepsy has reorganized the location of the sensory cortex. This intact sensory cortex after a neurosurgical procedure is vitally important for a good quality of life. Electrical median nerve stimulation is the gold standard to identify the N20 and P35 peak at 20ms and 35ms, respectively, after onset of stimulation to median nerve. MEG can be used to identify the magnetic evoked field responses from electrical stimulation as well as from mechanical stimulation. We investigated the latency and location differences between rapid pressure pad mechanical stimulation and the latency, location, and orientation of electrical stimulation.  Methods: Participants were patients with a diagnosis of drug-resistant epilepsy (DRE) who underwent MEG as part of the standardized pre-surgical work between November 2011 and January 2019 at Henry Ford Hospital. Patients who had SEF study were included. Each patient received 512 stimulation taps of the second digit on each side using air pressure bellows. Taps were presented every 500ms. All epoch were averaged together and filtered 8-40Hz. If data for a stimulation epoch was heavily contaminated with artifact (typically VNS) or no discernable N20m or P35m waves were noted, it was excluded. This study reports the mean latency of the N20m and P35m waves, as well as location and dipole direction for each side stimulated. Results: This study included 58 patients (n = 116 unilateral stimulation epochs) who had mechanical SEF done during MEG acquisition as part of the presurgical epilepsy work up. The mean age was 23 years old (SD = 11) and 63.8% were males. The N20m wave had an average latency of 27.2 msec (SD = 12.5, n = 96), and mapped to the postcentral and precentral gyri in 53.7% and 28.4% of the time respectively. The P35m wave average latency was 49.7 msec (SD = 10.2, n = 80), and mapped to the postcentral and precentral gyri in 55.7% and 27.8% respectively. Mapping outside the two locations was considered abnormal and the dipole direction was not recorded. The N20m and P35m waves had an anterior orientation in 44.8% (n = 58) and 58.1% (n = 43) of the time, respectively. Conclusions: Previous MEG SEF using electrical stimulation identified the first-peak at 20.2 +/- 1.5 ms and the dipole was localized on the posterior surface of the central sulcus. The dipole pointed in the anterior direction (to the nose). The second-peak was seen at 27.9 +/- 3.2 ms and localized on the anterior wall of the central sulcus dipole pointing in the posterior direction. Our results indicate that mechanical stimulation using a tap produced similar average latencies for the N20m response at 27.2 +/- 12.5 ms. Our study using mechanical stimulation is able to distinguish two adjacent cortical sources, located on opposite walls of the central sulcus, and within a few millimeters of each other. This is in contrast to work by Inoue et al (2005) where they found significantly prolonged latencies for N20m and P35m of 57 ms and 82 ms respectively while stimulating at 1-s intervals. A potential explanation is that faster tapping frequency may recruit more myelinated nerve fibers peripherally and thus shorter latencies. Another study by Simões et al (2001) showed even longer latencies, with longer interstimulus intervals of 2-s. As such, rapid mechanical stimulation could be a safe, more comfortable alternative for patients. Funding: No funding