Abstracts

Rationale: Motor evoked potential (MEP) amplitude and latency are acquired routinely during neuronavigated transcranial magnetic stimulation (nTMS), a method for functional mapping of the motor cortex prior to epilepsy surgery. While MEP amplitude is routinely used to generate the motor map, MEP latency in patients with focal epilepsy has not been studied systematically. Methods: We obtained nTMS-derived abductor pollicis brevis (APB) MEP latency in children and adolescents with intractable, unihemispheric epilepsy (n=33, age range 9-18 years) to identify expected MEP values in this group, and to test whether APB MEP latency varies as a function of whether it is obtained from the hemisphere containing the seizure focus, or the contra-lesional hemisphere. Results: The 95% CI latency range in the unaffected hemisphere was 21.8-22.1 ms, and the 95% CI latency range in the affected hemisphere was 21.7-21.9 ms. Latency varied predictably as a function of MEP amplitude such that latencies in the top 25th %ile MEP amplitude were ~1 ms shorter than latencies in the bottom 25th %ile group (p<0.01). Notably, in patients without a corticospinal tract lesion, the mean latency of top 25th %ile amplitude MEP was reliably shorter in the hemisphere that contained the seizure focus (21.4±1.2 vs. 22.1±1.5, p<0.01). In patients with a corticospinal tract lesion, 25th %ile amplitude MEP latency distribution in the affected hemisphere was bimodal such that patients with a space-occupying lesion (e.g., tumor) had prolonged APB MEP latency, whereas the remainder had shortened MEP latencies. Conclusions: We thus identify for the first time an MEP latency discrepancy between the epileptic and non-epileptic hemisphere in children with focal epilepsy, where the MEP latency is shortened in the hemisphere that contains the seizure focus. Plausibly, this is due to increased use-dependent myelination or similar process that accelerates signal conduction from cortex to the spinal cord. Funding: No funding