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epileptogenic zone could not be satisfactorily determined, because multiple sites were found,
or alternatively because the epileptogenic zone was situated in eloquent cortex. If neither a
resective nor a functional procedure is thought possible then the electrodes are removed and
the epilepsy is then managed medically.

Invasive intracranial EEG studies are time consuming, expensive, have an inherent
complication risk and require numerous personnel. Despite this, the number of patients
undergoing these procedures is increasing in recent years. This is being driven by newer
imaging modalities such as PET (positron emission tomography), SPECT (single-photon
emission computed tomography), MEG (magnetoencephalography) and EEG-fMRI,
suggesting possible focal targets for patients with epilepsy. At this time the sensitivity and
specificity of these investigations is still to be ascertained and intracranial EEG is needed to
prove or disprove the suggested seizure onset zone.

Surgical resection

Epilepsy surgery may be divided into two major categories: resective and functional. The aim
of resective surgery is to remove the epileptogenic zone and render the patient seizure free.
Based on the discussions at the presurgical meeting, a risk:benefit analysis for each individual
patient is determined and the exact nature of the surgical procedure is explained and discussed
with the patient in detail. Patients and their families or carers are given both verbal and written
information, as well as counselling, so that they are fully informed before written consent is
obtained. Once consent is given the surgeon can embark on surgery with a clear clinical
objective and surgical strategy.

The surgical techniques employed in epilepsy surgery are relevant to all branches of
neurosurgery, with newly-developed technology being particularly useful in this type of
surgical intervention. In addition to the basic principles of resection to preserve pial
boundaries first described by Sir Victor Horsley a century ago it is also essential to respect
the anatomical planes in both the deep and superficial cortex. Stereotaxy or image guidance
assists with localisation while accurate tissue removal is facilitated by high quality operating
microscopes and the use of the ultrasonic aspirator. At low power the aspirator allows
removal of gliotic, tumour and dysplastic tissues while at the same time preserving the pia.
The newly available use of interventional MRI allows documentation of lesion resection prior
to the termination of any surgical procedure and also allows the surgical navigation software
to be recalibrated during the operation, making the procedure more accurate.

Lesionectomy
The increased anatomical resolution afforded by MRI means that many more cortically-based
lesions, which give rise to epilepsy, are identified. Small lesions such as cavernomas, focal
areas of cortical dysplasia, and indolent tumours such as dysembryoplastic neuroepithelial
tumours are recognised as highly epileptogenic and resection of these lesions, particularly
when they are extra-temporal, is associated with a high rate of freedom from seizures. As
with all resective surgery, success depends on the complete resection of the epileptogenic
zone. What may not be clear purely from imaging is the extent to which the tissues
surrounding an area of structural abnormality may be contributing to the epileptogenic zone.
The extent of perilesional resection is determined by visual inspection and intra-operative
electrocorticography and may be further facilitated by the use of image guidance or intra-
operative imaging.

Outcome studies have shown that, when the cortical lesion lies within the temporal lobe,
resection of the lesion alone results in a significantly poorer outcome than in extra-temporal
cases. It is probable that this inferior outcome is a result of the proximity of the cortical lesions
to the mesial temporal structures and associated dual pathology, i.e. the presence of