Process of Evaluation for Selection of Surgical Candidates

The selection of surgical candidates starts with an exact description of the clinical manifestations of the seizure, called seizure semiology. This is then followed by a general physical and neurological examination, basic and computerized neurophysiological testing (electroencephalography [EEG] and magnetoencephalography [MEG]), structural (magnetic resonance imaging [MRI]) and functional neuroimaging (single photon emission computed tomography [SPECT] and positron emission tomography [PET] scans), and a neuropsychological examination. The results are analyzed to determine if there is evidence of focal, multifocal, or diffuse dysfunction. Further invasive monitoring may be needed, depending on the specifics of each case. The intracarotid amobarbital procedure (Wada test), or invasive EEG monitoring, may be needed to identify the exact location of seizure onset and to map cortical function. However, noninvasive mapping may now be accomplished with functional MRI (fMRI), EEG-guided fMRI, and MEG. The seizure focus itself can consist of several cortical zones, with each modality examining a different cortical zone ( Table 2.2 ).2,3,7,8

Possible surgical procedures include multifocal cortical resection, hemispherectomy, corpus callosotomy, and multiple subpial transection (MST). These procedures are considered with either multifocal or generalized seizures. Multilobar resection or hemispherectomy are considered when the epileptogenic zone is primarily multifocal but unilateral, corpus callosotomy is done for either a bilateral or generalized seizure onset, and MST is considered when the epileptic focus is within eloquent cortex. Neurostimulation techniques, such as vagus nerve stimulation (VNS), can be performed for patients not considered ideal candidates for a focal resection: patients with multifocal seizures, or patients with the epileptic focus within eloquent cortex, for example, or when the patient or family are not interested in resective surgery.

The selection of the appropriate treatment is determined by the presurgical evaluation. This is divided into three phases: the noninvasive presurgical evaluation is phase 1, invasive monitoring is phase 2, and the surgical resection is phase 3. All three phases may not be needed in every patient. There are three major aims:

1. 
   

   To lateralize and localize the epileptic focus

   
   
2. 
   

   To determine the function of the presumed epileptic focus (brain mapping)

   
   
3. 
   

   To determine which surgical procedure has the greatest chance of controlling seizures without causing a neurological deficit

Presurgical Clinical Evaluation

This process begins with an initial outpatient evaluation.9 Several questions are addressed before any invasive procedure:

1. 
   

   Does the child truly have epilepsy?

   
   
2. 
   

   What is its etiology?

   
   
3. 
   

   Is epilepsy surgery warranted for the given seizure type or epilepsy syndrome?

   
   
4. 
   

   Is the epilepsy truly refractory, or are other therapies indicated before any consideration of surgery?

   
   
5. 
   

   What is the possibility of remission?

   
   
6. 
   

   Are the child and family prepared for surgery, including the psychological aspects of the invasive monitoring, the resection, or even its failure? ( Table 2.3 )

We find it best to review this in an outpatient visit before the admission so that the family knows what to expect and we know how to best tailor the phase 1 evaluation to the child’s individual needs and expectations.

The epileptologist must first determine whether the initial diagnosis of epilepsy was correct. Many nonepileptic paroxysmal events are easily mistaken for epilepsy, treated with antiepileptic drugs, and ultimately referred for a pre-surgical evaluation. In a study of 223 children referred to a tertiary epilepsy center, 87 (39%) did not have epilepsy.10 We have seen children referred for refractory epilepsy with other diagnoses, especially pseudoseizures, vasovagal events, periodic movement disorders, and hyperekplexia. Capturing the habitual seizure on long-term EEG monitoring excludes these conditions. Appropriate AED treatment for the seizure type or epilepsy syndrome is needed before considering surgery. If an underlying metabolic or degenerative condition is responsible for the seizures, respective surgery may not be appropriate.

Epilepsy syndromes are divided into benign or malignant. This distinction refers to the ultimate course of the actual seizures in addition to the developmental outcome. The term catastrophic epilepsy applies to early onset epilepsy syndromes in which the outcome is poor unless seizures can be controlled, commonly occurring when a structural lesion causes refractory epilepsy.11 Alternatively, children with benign syndromes, such as rolandic epilepsy, may have seizures that, although difficult to control, will ultimately remit.

The Subcommission for Pediatric Epilepsy Surgery defined refractory epilepsy as the failure of two or three appropriate AEDs, disabling side effects of seizures, or disabling side effects of medications. However, the definition of refractory epilepsy varies ( Table 2.4 ),12–15 and children considered intractable may later achieve seizure control. In a prospective, community-based study of 613 children with epilepsy, 13% of those considered refractory ultimately became seizure free and early remission periods preceded intractability in two thirds, with catastrophic epilepsy as a risk factor.12 Other factors contributing to refractory epilepsy include cryptogenic or symptomatic generalized epilepsy, high initial seizure frequency, early onset epilepsy, and focal EEG slowing.16

The subcommission listed other indications for referral to a center specialized in surgical epilepsy: seizures unable to be classified as a clearly defined electroclinical syndrome, stereotyped or lateralized seizures, other evidence of focality, or a potentially resectable epileptogenic lesion evident on MRI study ( Table 2.5 ). Specific disorders considered for surgical epilepsy syndromes are cortical dysplasia, tuberous sclerosis complex, polymicrogyria, hypothalamic hamartoma, hemispheric syndromes, Sturge-Weber syndrome, Rasmussen syndrome, Landau-Kleffner syndrome, and certain lesions (tumors, infarctions).