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resolution and allows selection of trajectories that avoid crossing pial boundaries, thus
reducing the risk of intracerebral haemorrhage. Depth electrodes may be placed either
orthogonally or radially, the electrodes having between six and ten contact points at 1 cm
intervals, to allow recording along the length of the electrode from both deep and superficial
points.

In addition to using frame-based stereotaxy, depth electrodes may also be placed using image
guidance systems. These have the advantage that the scan may be acquired pre-operatively,
either with or without fiducials, at a time more convenient to the patient and the radiology
department. It also obviates the need for the application of the stereotactic frame and therefore
both simplifies the operation and reduces operating time. The insertion of the electrodes may
then be either freehand following the trajectory delineated by the image guidance system, or
alternatively they may be introduced using an electrode carrier stabilised to the Mayfield head
holder.

In contrast to depth electrodes, subdural strips and grids do not broach the pial boundaries
and potentially pose less risk of haemorrhage or cortical damage. Subdural strips can be
placed through simple burr holes and used to localise and lateralise both temporal and extra-
temporal epilepsy. Subdural grids can record from a larger area of contiguous cortex and are
frequently used when epileptogenic lesions are adjacent to eloquent cortex. A wider area of
cortex is covered by both strips and grids than by depth electrodes, however if the
epileptogenic lesion is situated deep in the cerebral cortex the grid recordings need to be
interpreted with care. Similarly, the disadvantage of using depth electrodes is that the area of
the brain sampled is usually small and unless seizure onset is seen in a specific electrode or
group of electrodes little conclusion can be made regarding the epileptogenic zone. This
demonstrates the importance of having a clear plan and objective prior to implantation.

Implantation of a subdural grid over eloquent cortex allows an estimation to be made of the
anatomical relationship between the epileptogenic zone and the functional cortex. This allows
construction of a homunculus of motor and sensory cortex as well as the mapping of receptive
and expressive speech areas. The paradigm for cortical stimulation needs to be adjusted in
infants because the thresholds required differ when myelination is immature. As well as direct
cortical stimulation, somatosensory evoked potentials can also be used to determine the
central sulcus.

The duration of invasive monitoring depends very much on the seizure frequency, the success
of any planned stimulation, and patient compliance. In current clinical practice depth
electrode implantation is used mainly to determine laterality in seizures of temporal lobe
origin and in MRI negative frontal lobe cases, whereas subdural grids are most commonly
used for MRI-positive extra-temporal epilepsy and in cases were the presumed seizure onset
zone is close to eloquent function. Several weeks of depth electrode recording may be
necessary to build a true picture of the patient’s seizures and to establish the number of
seizures. In contrast, subdural grid recordings seldom extend beyond 10–14 days as the
seizure frequency is often higher in these patients, as are the inherent risks of infection.

Invasive monitoring may be terminated at any stage if a clinically significant adverse event
is recorded. The risks from monitoring procedures are intracranial haematoma formation as
a result of the primary procedure and infection as a consequence of the wires passing through
the scalp. These risks can be reduced by careful intra-operative technique and appropriate
post-operative nursing care. The use of antibiotics during invasive recording is controversial.

At the end of the invasive monitoring period the data collected are evaluated and the
suitability for surgery reassessed. Regrettably a certain percentage of patients will undergo
invasive recording but not be deemed suitable for resective surgery, either because the