Epilepsy surgery pre-surgical evaluation

All patients should undergo high resolution MRI study to rule out neoplasm, AVM, cavernous malformations, mesial temporal sclerosis or hippocampal lesion. Noninvasive techniques allow localization in the majority of cases.

Neuropsychological assessment is an integral component of the surgical procedure in patients with epilepsy. As no French consensus for neuropsychological assessment was available, the main goal of a work of Brissart et al. was to define French neuropsychological procedure consensus in regard to literature review.

A panel of expert in neuropsychology was created within the framework of the French League Against Epilepsy. A systematic search of publications from 1950 to 2017 listed in PubMed database was conducted leading to a classification of articles according to their level of scientific evidence. French neuropsychological procedure consensus was then carried out with an expert panel of expert.

Low scientific evidence of neuropsychological data was reported. A panel of expert proposed a comprehensive neuropsychological assessment procedure including the exploration of intellectual efficiency, long-term memory, short-term and working memory, attention, executive functions, processing speed and motor skills, language, visual processing, praxis, psychobehavioral, and social cognition.

A common procedure for assessing cognitive and psychobehavioral function is now available in patients with epilepsy undergoing surgical evaluation have been established, they may help to improve the quality of care and the patient experience. This work highlights the need of furthers investigations and the necessity to develop specific tools with normative data ¹⁾

All patients should undergo high-resolution MRI study to rule out neoplasm, AVM, cavernous malformations, mesial temporal sclerosis or hippocampal lesion. Noninvasive techniques allow localization in the majority of cases.

Video-EEG monitoring. Pre-operative long-term inpatient video-EEG monitoring (surface electrodes) to correlate the clinically disabling seizure with appropriate electrical abnormalities and possibly to identify the seizure focus is required.

High-resolution MRI. The imaging modality of choice. Extremely good for detecting hippocampal asymmetry of mesial temporal sclerosis (MTS), and neuronal developmental abnormalities (e.g. cortical dysplasia) that may produce complex partial seizures (CPS) ²⁾.

CAT scan. A seizure focus may enhance with IV contrast shortly following a seizure. Subtle enhancement may be present on the side of the focus on interictal CT scan ³⁾.

PET scan (positron emission tomography). Interictal PET scan using fluorine-18 deoxyglucose (18FDG) shows hypometabolism lateralized to the side of temporal lobe focus in 70%of patients with medically refractory CPS (does not show actual site of origin). Useful when MRI and EEG cannot localize.

SPECT scan (single-photon emission tomography). Used to demonstrate increased blood flow during a seizure to help localize site of onset. [99m] Technetium (Tc) hexamethyl-propylene-amine- oxime (HMPAO) is usually administered immediately after onset of seizure, and the scan may be obtained within several hours ⁴⁾.

MEG (Magnetoencephalography). Functional imaging technique for mapping brain activity by recording magnetic fields created by neuronal activity (electrical current) ⁵⁾. Synchronized neuronal currents induce a weak magnetic field. Clinical uses include detecting and localizing pathological activity in patients with epilepsy and in localizing the eloquent cortex for the pre-operative surgical planning. It requires a magnetically shield room.

Wada test ⁶⁾ AKA the intracarotid amytal test. Localizes the dominant hemisphere (side of language function) and assesses the ability of hemisphere without lesion to maintain memory when isolated. Usually reserved for candidates for large resections ⁷⁾. Each cerebral hemisphere is individually anesthetized via selective carotid catheterization (usually by a neurointerventionalist) and injection of short-acting barbiturate.

Start with angiogram to assess cross flow and to R/O persistent trigeminal artery. Significant cross-flow is a relative contraindication to anesthetizing the side of the dominant supply (patient goes to sleep). The Wada test may be grossly inaccurate with high flow AVM. Also, portions of the hippocampus may be supplied by posterior circulation (not anesthetized by ICA injection). EEG monitoring is usually performed during the test when it is being done for seizure surgery. Patient will show delta waves during the deepest level of anesthesia.

● instruct patient as to what is expected

● catheterize ICA: usually, start on side of the lesion

● have patient hold the contralateral arm in the air, and instruct them to hold it there

● inject 100–125 mg sodium amobarbital (Amytal®) rapidly into internal carotid artery (effect starts almost instantaneously, begins to subside after ≈ 8 minutes; (may subside in ≈ 2 minutes with AVM where flow rates are high)

● determine the adequacy of injection by assessing motor function in the elevated arm (should be ≈ flaccid)

● assess language skills by showing patient pictures of objects and ask them to name each one out loud and remember each one

● assess memory function by asking the patient to name as many of the pictures as they can ≈ 15 minutes after the test: if they have difficulty, ask them to pick out pictures from a group that contains additional ones not shown to the patient

● repeat procedure on other side (use lower Amytal doses with each subsequent injection).

EEG obtained with invasive electrodes. Indications: Lack of lateralizing or localizing electrophysiology in pre-operative evaluation requires invasive electrodes for a better definition of the seizure focus.

Surgical options:

● Depth electrodes

○ Electrodes are placed stereotactically

○ stereoencephalography (sEEG): popularized in Europe by J. Talairach and J. Bancaud during the 1950s for invasive mapping of refractory focal epilepsy. The techniques require the placement of multiple depth electrodes in an orthogonal orientation to localize seizure onset ^{8) 9) 10)}

○ 2-3% risk of intracerebral hemorrhage ¹¹⁾. Risk of infection with depth electrodes ¹²⁾. : 2-10%.

● Subdural grids or strips

○ Grids are frequently used for extra-operative functional mapping (helpful in children or in the mentally retarded). Subdural grid electrodes are placed with a craniotomy.

○ Surface strip electrodes may be placed through a burr hole

○ useful technique for intra-operative functional mapping