see also Pediatric Epilepsy Surgery.
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) 1).
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 2).
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 3).
MEG (Magnetoencephalography). Functional imaging technique for mapping brain activity by recording magnetic fields created by neuronal activity (electrical current) 4). 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 5) 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 6). 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.
● 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 7) 8) 9)
○ Surface strip electrodes may be placed through a burr hole
○ useful technique for intra-operative functional mapping
The current practice under which patients with refractory epilepsy are surgically treated is based mainly on the identification of specific cortical areas, mainly the epileptogenic zone, which is believed to be responsible for generation of seizures. A better understanding of the whole epileptic network and its components and properties is required before more effective and less invasive therapies can be developed.
Despite significant underutilization of surgical treatment for drug-resistant epilepsy, no studies have quantified patient desire for surgery within a representative population.
An online survey was administered to all clients connected with a core epilepsy community access center. It obtained information about demographics, clinical characteristics, knowledge of epilepsy surgery, and interest in receiving surgery before and after receiving risk/benefit information about it.
Of 118 potential respondents, 48 (41%) completed the questionnaire, of which 67% had failed more than two AEDs and 78% experienced seizures in the past year. Eleven ( 26%) were uninterested in receiving surgery at baseline, and this decreased significantly to 7 (16%) following knowledge translation regarding the benefits (p = 0.001). Significance was lost with subsequent complication rate information despite fewer respondents still being uninterested compared to baseline (20% vs. 26%). Having experienced seizures within the past month was correlated with being interested in or undecided regarding surgery at baseline and following all steps of knowledge translation. Subjects had conservative views regarding the benefits of surgery and largely overestimated the risks.
A significant portion of those with active epilepsy in the community do not desire surgical treatment. Passive knowledge translation regarding the risks and benefits enhanced optimistic attitudes and mobilized interest within a subset of participants. Preexisting views regarding the risks of surgery were exaggerated, and analysis suggests that these views can be modified with information about the benefits of surgery. However, exaggerated risk perceptions return following crude descriptions of the risks, underlying the importance of sensitive counseling from primary care physicians 12).
In Epilepsy surgery where resective surgery is not indicated, deep brain stimulation (DBS) may be an effective alternative. The majority of available literature targets the thalamic nuclei (anterior; centromedian), subthalamic nucleus, hippocampus, and cerebellum.
Data show DBS may be a safe and effective treatment option for refractory epilepsy 13).
Surgery is a safe and effective option for some patients, however the opportunity exists to develop less invasive and more effective surgical options. To this end, multiple minimally invasive, image-guided techniques have been applied to the treatment of epilepsy. These techniques can be divided into thermoablative and disconnective techniques. Each has been described in the treatment of epilepsy only in small case series. Larger series and longer follow up periods will determine each option's place in the surgical armamentarium for the treatment of refractory epilepsy but early results are promising 14).
Several palliative neuromodulation treatment modalities are currently available for adjunctive use in the treatment of medically intractable epilepsy. Over the past decades, a variety of different central and peripheral nervous system sites have been identified, clinically and experimentally, as potential targets for chronic, nonresponsive therapeutic neurostimulation. Currently, the main modalities in clinical use, from most invasive to least invasive, are anterior thalamus deep brain stimulation, vagus nerve stimulation, and trigeminal nerve stimulation. Significant reductions in seizure frequency have been demonstrated in clinical trials using each of these neuromodulation therapies 15).
Engel J Jr, Van Ness PC, Rasmussen T, Ojemann LM: Outcome with respect to epileptic seizures, in Engle J Jr (ed): Surgical Treatment of the Epilepsies, ed 2. New York: Raven Press, 1993, pp 609–621