The combination of subdural and depth electrodes for intracranial EEG investigation of suspected insular (perisylvian) epilepsy

Purpose: We present two methods of implantation for
the investigation of suspected insular and perisylvian
epilepsy that combine depth and subdural electrodes to
capitalize on the advantages of each technique.
Methods: Retrospective study of all intracranial EEG
studies that included insular electrodes from 2004–
2010. Patients were divided according to the implantation
scheme. The first method (type 1) consisted
of a craniotomy, insertion of insular electrodes after
microdissection of the sylvian fissure, orthogonal
implantation of mesiotemporal structures with neuronavigation,
and coverage of the adjacent lobes with
subdural electrodes. The second method (type 2)
consisted of magnetic resonance imaging (MRI)–
stereotactic frame-guided depth electrode implantation
into insula and hippocampus using sagittal axes,
and insertion of subdural electrodes through burr
holes to cover the adjacent lobes. The combined
implantations were developed and performed by one
neurosurgeon (AB).
Key Findings: Nineteen patients had an intracranial study
that sampled the insula, among other regions. Sixteen
patients were implanted using the first method, which
allowed a mean of 4, 5, 20, 15, and 42 contacts per patient
to be positioned into/over the insular, mesial temporal,
neocortical temporal, parietal, and frontal areas, respectively.
The second method (three patients) allowed a
mean of 8, 7, 16, 6, and 9 contacts per patient to sample
the same areas, respectively. The four patients in whom
transient neurologic deficits occurred were investigated
with use of type 1 implantation.
Significance: Combined depth and subdural electrodes
can be used safely to investigate complex insular/perisylvian
refractory epilepsy. Choice of implantation scheme
should be individualized according to presurgical data and
the need for functional localization.