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Role of neurophysiology in evaluation of patients for epilepsy surgery

Inter-ictal and ictal EEG are pivotal in pre-surgical assessment, in conjunction with
neuroimaging and neuropsychological evaluation, but the importance of neurophysiological
investigation depends in part on the surgical procedure. It is high in resective surgery
(lesionectomy, lobectomy) and multiple sub-pial transection, moderate in hemispherectomy,
and low in functional procedures (callosotomy, vagal nerve stimulation) except to exclude
the option of resection.

Neurophysiological assessment in pre-surgical evaluation is aimed at:

 Ensuring that the individual has epileptic seizures (410% of patients in surgical
programmes have co-morbid psychogenic non-epileptic seizures)

 Characterisation of electro-clinical seizure features, and show concordance with
other data (MRI, functional imaging, psychometry)

 Demonstration of epileptogenicity in the presumed pathological substrate of
refractory epilepsy

 Identification of possible other epileptogenic foci
 Assessment of cortical function when pathology is in or close to eloquent cortex.

While most epilepsy surgery candidates can be adequately investigated by scalp inter-ictal
and ictal EEG, some require invasive neurophysiological studies. The proportion who do in
a given epilepsy surgery centre depends on complexity of case mix, availability of non-
invasive localising investigations such as SPECT, PET, MEG, and fMRI-EEG. Invasive EEG
utilises depth electrodes (rigid or flexible multi-contact wires inserted under stereotactic MRI
guidance, most appropriate for deep lying foci, and with the disadvantage of sampling only
small areas of brain), and sub-dural electrodes (strips or grids, inserted via a craniotomy or
burr hole, and recording from larger superficial cortical regions). Cortical stimulation can be
performed with either type of electrode. Electrode selection and placement is determined by
location of the epileptogenic zone. The usual indications for invasive EEG are dual or
possibly multiple potential epileptogenic pathologies, bilateral hippocampal sclerosis, and
focal lesions in eloquent cortex. Invasive EEG is also undertaken when underlying structural
pathology is not evident on neuroimaging, but a plausible hypothesis as to location of the
epileptogenic region has been generated by other investigations.

Specialised neurophysiological techniques

A number of techniques have been developed to optimise selection of candidates for epilepsy
surgery, and to enhance understanding of the anatomical-pathophysiological basis of
epilepsy. These include analytical methods to study seizure propagation (small time
differences in EEG signals, cross-correlation, chaos theory); source localisation of the
generators of epileptic foci using EEG, magnetoencephalography and combined functional
MRI/EEG; DC recording; measurement of cortical excitability through magnetic brain
stimulation36; and anticipation/prediction of seizures using linear and non-linear analysis of
EEG signals37. These techniques are of considerable theoretical interest, but at present, their
application is largely confined to specific areas of research.

References

1. NICE. The diagnosis and management of the epilepsies in adults and children in primary and secondary care.
National Institute for Clinical Excellence, October 2004 (www.nice.org.uk).

2. BENBADIS SR, TATUM WO. Overinterpretation of EEGs and misdiagnosis of epilepsy. J Clin Neurophysiol
2003;20:42-44.

3. BINNIE CD. Epilepsy in adults: diagnostic EEG investigation. In: Kimura J, Shibasaki H, eds. Recent Advances
in Clinical Neurophysiology. Amsterdam: Elsevier, 1996:217-22.