The sensitivity and significance of lateralized interictal slow activity on magnetoencephalography in focal epilepsy

Abstract

Objective: Asymmetric large-amplitude slow activity is sometimes observed on interictal electroencephalography (EEG) in epilepsy. However, few studies have examined slowing during magnetoencephalography (MEG) recordings, which are performed primarily to localize interictal spikes. Also, no prior investigations have compared the sensitivity of MEG to scalp EEG in detecting slow rhythms.

Methods: We performed a retrospective cohort study of focal epilepsy patients who received MEG followed by surgical resection at our institution. We examined MEG, simultaneous EEG, and long-term EEG recordings for prominent asymmetric slow activity (delta-range, 1-4 Hz), and evaluated post-operative seizure outcomes.

Results: We studied 132 patients with ≥ 1 year post-operative follow-up (mean, 3.6 years). Mean age was 27 (range, 3-68) years, and 55% of patients were male. Asymmetric large-amplitude slow wave activity was observed on interictal MEG in 21 of 132 (16%) patients. Interictal slowing lateralized to the hemisphere of resection in all but one (95%) patient. Among the 21 patients with interictal MEG slowing, 11 (52%) individuals had similarly lateralized EEG slowing, 7 patients had no EEG slowing, and 3 had bilateral symmetric EEG slowing. Meanwhile, none of the 111 patients without lateralized MEG slowing had asymmetric EEG slowing, suggesting significantly higher sensitivity of MEG versus EEG in detecting asymmetric slowing (χ(2)=63.4, p<0.001). MEG slowing was associated with shorter epilepsy duration with an odds ratio of 5.4 (1.7-17.0, 95% confidence interval). At last follow-up, 92 (70%) patients were seizure free (Engel I outcome), with no difference in seizure freedom rates between patients with (71%) or without (69%) asymmetric MEG slowing (χ(2)=0.4, p=0.99).

Significance: MEG has higher sensitivity than scalp EEG in detecting asymmetric slow activity in focal epilepsy, which reliably lateralizes to the epileptogenic hemisphere. Other uses of MEG beyond spike localization may further improve presurgical evaluations in epilepsy.

Keywords: Epilepsy surgery; Epileptogenic zone; MEG; Magnetic source imaging; Slowing.

Figure 1

Example simultaneous MEG/EEG recordings in a patient with asymmetric MEG slowing. Recordings were performed in a 33-year-old male with a seven-year history of drug resistant epilepsy and a normal MRI. MEG revealed large-amplitude slow (delta) activity in the right temporal lobe that was not appreciated on simultaneous EEG recordings (shown on the right), or on long-term scalp EEG recordings (not shown). Enlarged 1-s epochs showing a few temporal lobe channels that do not (bottom left) or do (bottom right) exhibit slow activity are also provided. No interictal spikes were captured during the MEG session. After a long-term intracranial monitoring study, the patient underwent tailored right temporal lobe resection, and remained seizure free (Engel class I outcome) four years after surgery.

Figure 2

Increased sensitivity of MEG for asymmetric slowing compared to EEG. Among 132 patients, asymmetric slowing was identified on MEG recordings in 21 (15.9%) individuals. Among these 21 patients with MEG slowing, asymmetric EEG slowing was observed in 11 (52.3%) individuals, while 10 (47.7%) patients had either no EEG slowing or bilateral EEG slowing. Zero (0%) of the 111 patients without MEG slowing showed asymmetric slowing on EEG. This results in a significantly higher sensitivity of MEG than EEG for asymmetric slowing (χ² = 63.4, p < 0.001).