What is epilepsy?
Epilepsy is a disorder of the nervous system where people experience episodes of abnormal behaviour, sensation, movements, or sometimes even episodes of loss of consciousness. These episodes are also known as seizures or ‘Fits’.
What is the treatment of epilepsy?
Firstly, the neurologists need to confirm that the episodes are really seizures and not just behavioural changes. This can be done by a thorough clinical evaluation and some electrophysiological tests including Electroencephalography (EEG). On confirmation of the diagnosis, patients are treated with antiepileptic drugs (AEDs). Sometimes a single drug may be effective in controlling the seizures, otherwise, a combination of two or more drugs may be tried to control the seizures. Long term treatment may be required and even in some cases, seizures may not be controlled with these treatments.
When to consider epilepsy surgery?
Evaluation for epilepsy surgery is appropriate for anyone with seizures that are focal in origin, are continuing to occur despite treatment with antiepileptic drugs (AEDs), and cause significant impairment in quality of life. Although the exact number of medication trials must be individualized, initial consideration is reasonable after two monotherapy trials with first-line AEDs, and possibly one trial with dual therapy (a combination of two AEDs). Many studies have shown the benefits of epilepsy surgery for seizure control, psychiatric symptoms, and overall quality of life, especially for temporal lobe epilepsy, the most common type of epilepsy in adults. There is even evidence that successful epilepsy surgery may increase life expectancy.
How are the patients evaluated for epilepsy surgery?
The goal of the presurgical evaluation is to determine if the patient has a single epileptogenic focus that is not in the “eloquent” cortex and can, therefore, be resected without causing an unacceptable neurological deficit. The most common location of seizure onset in adults is the temporal lobe, especially the medial temporal lobe (hippocampus). This is also the seizure location most amenable to surgical cure.
The first step is to confirm that the patient’s habitual spells or episodes of abnormal movements or sensations are indeed epilepsy. A significant proportion of patients referred for epilepsy presurgical evaluation do not have epileptic seizures (in some studies, up to 50% of patients undergoing video/EEG monitoring have events other than epilepsy, mostly psychogenic non-epileptic seizures; PNES). Although a detailed history can be very helpful in this regard, video/EEG monitoring of a typical spell is required to make this diagnosis definitively.
What is video EEG and how does it help?
This is the cornerstone of the epilepsy surgery evaluation. EEG is a diagnostic test to identify abnormal electrophysiological discharges from the brain. It helps in lateralization (left vs. right hemisphere) and localization (specific region within one hemisphere) of the brain’s abnormalities. Continuous EEG with synchronous video monitoring is performed in an epilepsy monitoring unit until the patient’s typical spells occur. Supervised medication withdrawal and provocation procedures (e.g. sleep deprivation) are often necessary to help elicit spells more quickly.
What other tests are helpful?
MRI: The presence of a focal epileptogenic lesion on MRI significantly increases the chance of surgical cure. In patients with temporal lobe epilepsy (TLE), special views of the medial temporal lobes with thin oblique coronal cuts often show hippocampal atrophy and increased T2 signal. These findings strongly correlate with pathological evidence of hippocampal sclerosis (neuronal loss and gliosis, also called mesial temporal sclerosis or MTS) and with seizure freedom after temporal lobectomy (approximately 80% likelihood).
Many patients with MTS have a history of febrile seizures in childhood. MRI may also show tumours (usually low grade), vascular malformations (usually arteriovenous or cavernous malformations), or cortical dysplasia. Detection of dysplasia often requires special views and careful examination for ectopic grey matter (heterotopias), cortical thickening, and blurring of the normal grey-white junction.
PET: Positron emission tomography (PET) utilizes an injection of radio-labelled glucose (18FDG) or other radio-labelled chemicals to measure brain metabolism or distribution of certain receptors. Interictal glucose PET usually shows hypometabolism in the seizure focus, especially in TLE. Ictal PET is not practical due to the extremely short half-life of the radiotracers used. PET is most useful in MRI-negative TLE, though it may be helpful in extra-temporal epilepsy as well.
SPECT: Single-photon emission computed tomography (SPECT) utilizes the injection of a radio-labelled tracer of blood flow that binds on first-pass through the brain; thus, it is a snapshot of brain perfusion at the time of injection. The tracer is stable for several hours, allowing delayed imaging. The most useful study for presurgical evaluation is an ictal SPECT, in which the injection is performed as early as possible following ictal onset. The patient can then be scanned within the next few hours. Ictal studies usually demonstrate increased blood flow at the site of seizure onset. Interictal studies often show relative hypoperfusion at the site of seizure onset.
Comparing ictal and interictal studies, including quantitative subtraction and co-registration of the subtraction images with MRI can add additional information.
Functional MRI (fMRI): fMRI can be used for the non-invasive motor, sensory and language mapping, and is most commonly used as part of surgical planning. FMRI can detect focal changes in blood flow and oxygenation levels that occur when an area of the brain is activated. Possible future applications of FMRI include localizing epileptiform discharges or seizures (EEG/fMRI), and lateralizing memory function.
Magnetoencephalography (MEG): MEG is a relatively new diagnostic technique. MEG is similar to EEG, but it detects magnetic rather than electric signals from the brain. MEG can sometimes detect epileptiform discharges in patients with normal scalp EEGs and can be considered complementary to EEG. It can also be used for functional mapping, including language mapping. Localization of extratemporal interictal EDs with MEG may be of particular importance in the evaluation of patients with extratemporal, neocortical epilepsies. Concordance between MEG and intracranial monitoring data has positive predictive value for epilepsy surgery outcome.
What are the surgical treatments of Epilepsy?
Epilepsy surgery can be divided, based on the goals of the operation, into curative and palliative procedures. Curative procedures include lesion resection, lobectomy, corticectomy, and some cases of hemispheric surgery and multiple subpial transections. The primary goal of curative surgery is to stop seizures and permit the patient to be able to lead a normal life, preferably of all antiepileptic medications.
By definition, palliative procedures seldom result in cessation of seizures. These surgeries may prevent the occurrence of a particularly morbid type of seizure such as drop attacks, or lessen the frequency or severity of seizures. Palliation may be a desirable result in patients with seizure-related injuries or with a predominance of one seizure type that can be eliminated with surgery. Examples of palliative surgery include some cases of hemispheric surgery, multiple subpial transections, and disconnection procedures, including corpus callosotomy. There is a continuum between likely curative and likely palliative procedures, and patient and family expectations should be adjusted accordingly. For example, the procedure of choice for a patient with invasive monitoring-documented medial temporal lobe epilepsy (TLE) who has no magnetic resonance imaging evidence of medial temporal sclerosis (MTS) is resection of the anteromedial temporal lobe through one of a variety of surgical methods. The likelihood of seizure freedom in these patients is approximately 60%, in contrast to 80-90% seizure freedom if MTS is present on MRI.
What should be the goal of epilepsy surgery?
The goal of epilepsy surgery is either to define and resect an area of epileptogenesis (seizure focus) or to disrupt the spread of seizure activity and thus reduce the likelihood of seizures or prevent certain seizure types. Most surgical candidates suffer from partial seizures, and many have epilepsy due to definable structural abnormalities. The location and nature of these lesions dictate the type of surgery performed and the expected outcome.
The concept of surgically remediable syndromes is important when considering patients for surgical evaluation. These are syndromes that respond poorly to medical therapy and well to surgical treatment. Patients with these problems may be considered for surgical intervention earlier than with some other cases of partial epilepsy. Medial temporal lobe epilepsy is the most common of these syndromes. Classically, it consists of a history of a complex or atypical febrile seizure in early childhood, the onset of recurrent seizures in late childhood or adolescence, complex partial seizures, and evidence of hippocampal sclerosis on MRI. Lesional epilepsy, caused by lesions such as vascular malformations, low-grade glial tumours, or cortical dysplasia, also responds well to surgical therapy in most cases.
What is the appropriate timing of epilepsy surgery?
There is gathering evidence that early surgical intervention is favourable for a variety of reasons. Becoming seizure-free at a younger age may lessen the cognitive, behavioural, and psychosocial problems experienced by epilepsy patients, potentially improving societal integration. Additionally, because it is possible that continued seizures may result in progressive neurological damage over time, surgery has the potential to be neuroprotective in contrast to the effect of continued medically refractory seizures. Also, if a hemispheric procedure is required or if “eloquent” areas of the brain are within the epileptogenic zone, the potential for recovery of language and sensorimotor function is better when patients are younger.
Is epilepsy surgery safe? What can be potential complications?
Epilepsy surgery is an overall safe surgery. Mortality or life risk following the procedure is comparatively lower than any other standard neurosurgical procedures.
Typical complications may include visual field defects and memory deficits. Functionally significant visual field deficits are uncommon using the various techniques that spare all but the anterior 3-4 cm of the lateral temporal lobe; such deficits are more frequent after more extensive resection on the non-dominant side. Measurable worsening of verbal memory can result after language-dominant temporal lobectomy. In most cases, this risk is determined by preoperative functioning. In most cases, because MTS is associated with preoperative verbal memory deficits, patients who undergo temporal lobectomy show little or no significant deterioration of memory. Complications of callosal sectioning can be divided into early (or transient) and permanent effects. The early disconnection syndrome can include autism and inability to initiate movement of one or more limbs. This can occur after partial or complete callosotomy, lasts one to several weeks, and then resolves. The more severe, complete disconnection syndrome is rare after partial callosotomy but more common after complete sectioning.