  Neocortical epilepsy surgery fails to achieve post-operative seizure freedom
in 30-40% of cases. It is not fully understood why surgery in some patients is
unsuccessful. Comparing interictal MEG bandpower from patients to normative
maps, which describe healthy spatial and population variability, we identify
patient specific abnormalities relating to surgical failure. We propose three
mechanisms contributing to poor surgical outcome; 1) failure to resect
abnormalities, 2) failing to remove all epileptogenic abnormalities, and 3)
insufficiently impacting the overall cortical abnormality. We develop markers
of these mechanisms, validating them against patient outcomes. Resting-state
MEG data were acquired for 70 healthy controls and 32 patients with refractory
neocortical epilepsy. Relative bandpower maps were computed using source
localised recordings from healthy controls. Patient and region-specific
bandpower abnormalities were estimated as the maximum absolute z-score, using
healthy data as a baseline. Resected regions were identified from
post-operative MRI. We hypothesised our mechanism markers would discriminate
patient's post-surgery seizure outcomes. Mechanisms of surgical failure
discriminate surgical outcome groups (Abnormalities not targeted: AUC=0.80,
Partial resection of the epileptogenic zone: AUC=0.68, Insufficient cortical
abnormality impact: AUC=0.64). Leveraging all markers together found that 95%
of those who were not seizure free had markers of surgical failure in at least
one of the three proposed mechanisms. In contrast, of those patients markers
for any mechanism, 80% were seizure-free. Abnormality mapping across the brain
is important for a wide range of neurological conditions. Here we demonstrated
that interictal MEG bandpower mapping has merit for localising pathology and
improving our mechanistic understanding of epilepsy.
