Epilepsy Treatment: Surgery

Dean P. Sarco

Most of the 0.5% to 2% of the population with epilepsy will achieve seizure control with antiepileptic drugs. However, approximately 16% of children with new-onset epilepsy may become medically refractory or may have an epilepsy syndrome for which medical treatment is known to be ineffective.¹ These children should be evaluated at a comprehensive pediatric epilepsy surgery program to evaluate their candidacy for epilepsy surgery, because just over half of them may be surgical candidates. The evaluation to determine candidacy requires accurate classification of a child’s seizures and epilepsy, knowledge of the natural history of the child’s condition, detailed information about past epilepsy treatments, and data acquisition to localize seizures, including video-electroencephalography (EEG) monitoring, functional nuclear studies, and neuroimaging data. Because many types of infantile and early childhood seizures are difficult to classify and of uncertain prognosis, pediatric surgical experience is greatest in older children and adolescents who have focal cerebral lesions or mesial temporal sclerosis. Select infants and younger children, however, may benefit from surgery, particularly when persistent seizures may affect developmental progress, as in the catastrophic epilepsies.

There is no standardized definition of medical intractability. In practice, it is often defined as the failure of an adequate trial of at least 2 appropriate anticonvulsant medications, increased to the limits of tolerability or expected efficacy. The appropriate timing of when to consider surgical options is somewhat subjective. In general, referral should be considered for medically intractable epilepsy which is functionally disabling and impairs quality of life. Children who may be optimal candidates include those with focal epilepsy, limited involvement of eloquent cortical areas, and a focal lesion.

Children differ from adults in terms of etiology of refractory seizures. In adults, mesial temporal lobe epilepsy, frequently associated with hippocampal sclerosis, accounts for most cases. In contrast, children more often have extratemporal or multifocal seizure onset. Pediatric etiologies of intractable epilepsy are diverse and include cortical dysplasia, tuberous sclerosis, hypothalamic hamartoma syndrome, SturgeWeber syndrome, Rasmussen syndrome, infections, trauma, and developmental tumors.

PRESURGICAL EVALUATION

The goal of the initial presurgical evaluation, also termed the Phase I evaluation, is to characterize seizures and localize their onset. No individual study is sufficient for determining epilepsy surgery candidacy or for surgical planning. Rather, the data obtained from different studies are complementary and, overall, must be concordant to proceed with surgery.

At most centers, the basic Phase I evaluation consists of video-EEG monitoring, magnetic resonance imaging (MRI), single-photon emission computerized tomography (SPECT), and 2-fluoro-2-deoxyglucose positron emission tomography (FDG-PET) scans. MEG (magne-to-encephalography) is becoming increasingly available and is being used to characterize inter-ictal epileptiform activity and to provide data complementary to scalp EEG.

Video-EEG is useful for identifying electro-graphic seizure onset, the approximate extent of the epileptogenic network, and the clinical signs of a seizure, also termed semiology. Seizure semi-ology may provide valuable localizing information, depending on the seizure type. MRI is useful for defining structural abnormalities such as hippocampal sclerosis, cortical dysplasia, or developmental tumors. SPECT is a functional neuroimaging technique measuring cerebral blood perfusion. Separate ictal and interictal SPECT scans are performed to measure perfusion in each state and are compared to identify relatively increased localized ictal perfusion, which correlates with increased cortical activity at the ictal focus. The PET scan, a functional neuroimaging technique, measures interictal cerebral glucose metabolism. PET interictal hypometabolism may help to localize the epileptogenic region. The more concordant the semiology, ictal and interictal electrographic, ictal perfusion, interictal metabolic, and structural data are in localizing an area of seizure onset, then the greater the degree of certainty that the ictal onset region and extent of the epileptogenic network have been identified.

The Phase I data are discussed at an epilepsy surgery conference with all relevant providers in attendance, including epilepsy, neurosurgery, radiology, neuropsychology, EEG technicians, social work, and nursing staff. Once a consensus is reached, further evaluation is planned. Some patients will proceed to Phase II evaluation, which involves intracranial video-EEG monitoring to better localize seizure onset and to map eloquent cortical regions essential for motor, sensory, language, vision, and memory functions. Some patients, particularly those with well-defined epileptogenic lesions or temporal lobe abnormalities, may not require intracranial video-EEG monitoring and may proceed to surgery.

In both cases, additional studies may be useful to further define seizure etiology and onset, including higher-field-strength MR imaging, magnetoencephalography, magnetic resonance spectroscopy, EEG source localization, and intraoperative electrocorticography to identify areas of epileptiform activity. Additional studies may aid in localizing eloquent cortical areas, including the intracarotid amobarbital procedure (Wada test) to lateralize language and memory dominance; functional MRI to localize somatosensory, language, memory, and visual function; somatosensory- and visual-evoked potentials to assess somatosensory and visual pathways; visual field testing to assess function; and functional cortical stimulation of intracranial electrodes to localize eloquent regions. The neuropsychological evaluation is also an important component to obtain a presurgical baseline that may be useful for future comparison, to describe a child’s developmental and psychosocial state, and to potentially help lateralize or localize specific areas of dysfunction.

TYPES OF EPILEPSY SURGERY

Types of epilepsy surgery may be broadly divided into potentially curative and palliative techniques. Potentially curative surgery involves resection or disconnection of the epileptogenic neuronal network with the goal of seizure freedom. Children not considered appropriate candidates for resective surgery may be palliative surgery or vagal nerve stimulation candidates, where the aim is to limit the severity or frequency of seizures, without the expectation of complete seizure freedom.

Focal cortical resection is the most common procedure performed in patients with intractable epilepsy. If a lesion is evident on brain imaging studies, good results may be obtained by lesionectomy. Often, additional cortex that is electrically potentially epileptogenic may be included in the resection. Many of the anterior temporal lobectomies performed in children with uncontrolled partial seizures have been for tumors or cortical dysplasia. Mesial temporal sclerosis is a less frequent etiology in young children than in older adolescents and adults.

Hemispherectomy is indicated for patients with seizures arising from multiple regions of a hemisphere that is severely dysfunctional by other measures as well. These children typically have a preexisting hemiparesis limiting function, and may have hemisensory loss and hemianopia. In such patients, neurologic function is not significantly worsened by hemispherectomy, which may either be anatomical (near complete removal of the hemisphere) or functional (partial removal of the hemisphere and complete disconnection of the remaining structures). Causes of these severe unilateral epilepsy syndromes include Rasmussen syndrome, SturgeWeber syndrome, hemimegalencephaly, and large predominantly unilateral destructive processes occurring early in development.

Corpus callosotomy, a palliative procedure, is indicated for children with severe atonic and secondarily generalized seizures that result in frequent injury from falls during seizures. Most children have a static epileptic encephalopathy, such as Lennox-Gastaut syndrome, and significant cognitive impairment. Typically, the anterior two thirds of the corpus callosum is disconnected first, with the callosotomy completed if seizure control remains unsatisfactory.

OUTCOMES

In general, predictors of a favorable outcome include focal seizure onset, focal pathology such as the presence of an epileptogenic lesion, and the potential for a complete resection. Reported outcomes after epilepsy surgery are comparable between adults and children, with overall seizure freedom rates ranging from 37% to 91%, depending on various factors.^2-5 Higher seizure freedom rates are reported with temporal lobe and lesional epilepsy. An extratemporal focus and nonlesional epilepsy generally portend a less favorable prognosis however surgery may still be considered worthwhile in many cases.

With focal resections, seizure freedom rates are highest with temporal lobectomy (60% to 78%).⁶ Extratemporal resections are generally associated with lower seizure freedom rates, with larger pediatric series reporting rates of approximately 55%.^2,3 Of note, the lower seizure freedom rates with extratemporal resections still exceed the estimated 4% chance of seizure freedom that may be obtained from a third or multiple antiepileptic drugs (AEDs).⁷ Also, many children experience a reduction in seizures which deems surgery worthwhile, despite them not being completely seizure free.

Hemispherectomy outcomes are related to the degree of disconnection and underlying etiology. Children with developmental brain malformations have lower seizure freedom rates (37% to 58%) than those with acquired etiologies (67% to 88%).⁶

With corpus callosotomy, approximately 60% to 80% will have up to a 50% reduction in drop seizures, and 10% to 20% may experience seizure freedom with AEDs.^8-10 Complete arrest of drop seizures may occur in up to 90% with complete callosotomy, and in 67% by partial callosotomy.¹¹

REFERENCES

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