Page 458 - ILAE_Lectures_2015

Chapter 47

Methods of epilepsy surgery

ANDREW W. McEVOY1 and WILLIAM J.F. HARKNESS2

1Institute of Neurology, University College, London, National Hospital for Neurology and
Neurosurgery, Queen Square, London, and 2Great Ormond Street Hospital for Children,
London
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Pre-surgical evaluation

In order for any epilepsy surgery programme to be effective there has to be a critical mass of
staff with the necessary expertise in order to carry out the appropriate evaluations in patients
in the pre-operative period, and the post-operative follow up. In both adults and children,
following pre-operative evaluation it is essential that the information acquired is critically
appraised in a multidisciplinary meeting, not only to determine the suitability of the patient
for surgical intervention, but also to attempt to assess the potential risks and benefits of
surgery. The meeting should be structured to ensure that the information obtained is carefully
assessed and any shortfall in the information identified.

A principal aim of pre-surgical evaluation is to determine the epileptogenic zone and the
relationship of this zone to eloquent areas of the brain. The epileptogenic zone is the area of
the brain which gives rise to seizures, and the removal of which results in the patient
becoming seizure free. No single pre-operative investigation can determine the epileptogenic
zone with complete reliability and even when various investigative modalities are combined
there may be a variable degree of congruence. When pre-operative investigations have a high
degree of concordance it may be possible to recommend immediate surgery with predictable
levels of benefit and risk. However, if pre-operative investigations are discordant surgery
may be rejected in favour of gathering further information using invasive studies.

Intracranial EEG recording

The aim of invasive EEG recording is to acquire neurophysiological data to support or
disprove a hypothesis regarding the site of onset of seizures. The type of intracranial
recording depends on the suspected pathophysiological substrate of the epilepsy and its
location. Invasive electrodes may be placed either within the brain parenchyma, in the
subdural space, or in the extradural space. Electrodes may be used both for recording and for
stimulation, allowing assessment of the relationship between the epileptogenic lesion and
eloquent cortex.

The first brain electrode implantation took place in the early 1940s, followed in 1946 by the
introduction by Spiegel and Wycis of the first stereotactic instrument for human use.
Placement of electrodes was initially determined by pneumoencephalography. Angiography
was also used in order to avoid major vascular structures when planning electrode trajectories.
The additional use of contrast ventriculography allowed the positioning of multiple-depth
electrodes in both hemispheres when a wide area needed to be sampled and this approach is
still favoured in some centres.

Contemporary frame-based stereotaxy uses either CT or MRI to determine intracranial
targets. For depth electrode implantation MRI offers the advantages of high anatomical