gms | German Medical Science

Objective: In the electrocorticogram (ECoG), high-frequency oscillations (HFO, >500 Hz) are markers for epileptogenic tissue. In intraoperative recordings, clinically relevant HFOs have to be distinguished from artefacts and physiological HFOs. As an example of the latter, the somatosensory evoked potential (SEP) of the medianus nerve elicits a N20 component and a HFO. Here we investigate the feasibility of intraoperative subdural HFO recordings and the temporal and spatial relationship between N20 and HFO.

Method: During neurosurgical interventions in 7 anesthetized patients, the medianus SEP was recorded (11 recordings, 52 channels) from subdural electrode strips to localize the central sulcus. Depending on surgical restrictions, the strips were placed directly on the sensory cortex or the motor cortex or both.

Results: The SEP response consisted of the two distinct spectral components N20 (<250 Hz) and HFO (721 ± 70 Hz) in the time-frequency domain. The HFO maximum amplitude appeared at the same or an adjacent electrode contact as the N20 maximum in 10 of 11 recordings. HFO amplitude was proportional to N20 amplitude over patients. The N20 amplitude decayed with a larger spatial extent (exponential space constant 5.2 ± 4.0 cm) than the HFO amplitude (2.4 ± 1.9 cm, p<0.05). The HFO peak (21.4 ± 1.8 ms) preceded the N20 (23.2 ± 1.4 ms, p<0.05) by up to 8 ms.

Conclusions: HFOs in response to medianus stimulation can be recorded under conditions of anesthesia in the surgery room. N20 and HFO are caused by the same neuronal generator but as distinct processes.