Magnetic resonance imaging
The common abnormalities identified by MRI in patients with refractory epilepsy are mesial temporal atrophy and sclerosis (MTS), malformations of cortical development, primary brain tumors, vascular malformations and focal atrophic lesions. MRI have to be performed using epilepsy protocol to ensure maximal yield in picking up the abnormalities. Sometimes the presurgical evaluation team may ask for a repeat MRI if they do not agree with the MRI findings. MRI remains one of the strong tools in defining the candidature for surgery.
Other than MTS, malformations of cortical development are being increasingly recognized in patients with refractory epilepsy. They may be focal cortical dysplasia (FCD), lissencephalies, heterotopia, polymicrogyria, schizencephaly. Patients with low-grade primary brain tumors frequently present with seizures. The underlying histopathologies include dysembryoplastic neuroepithelial tumors, ganglio-glioma, gangliocytoma and pilocytic and fibrillary astrocytoma. Newly developed MRI techniques, diffusion-weighted imaging (DWI), diffusion tensor imaging (DTI), tractography improve the sensitivity of MRI and help in surgical planning.
Ictal (during seizure) single photon emission computerized tomography (SPECT) and interictal (in-between seizures) positron emission tomography (PET) remain important imaging tools in the presurgical evaluation of patients with refractory partial epilepsy. SPECT measures blood flow; and comparing interictal and ictal SPECT studies, the increase in blood flow of certain brain regions during the ictal phase with respect to the interictal period can be evaluated. During ictal SPECT, due to epileptic activation, the neurons located in these areas are hyperactive and there is an increase in blood flow as an autoregulatory response. An ictal SPECT displays both the ictal onset zone and seizure propagation pathways. In common practice, the region with largest and most intense hyper-fusion is considered as the ictal onset zone.
18 F-deoxyglucose (FDG) PET measures changes in cerebral glucose metabolism and has higher spatial resolution and more reliable quantitation than SPECT, but the temporal resolution of PET with 18 FDG is unfavourable for ictal studies. PET maps cerebral glucose metabolism using FDG PET and cerebral blood flow using 15 O-labelled water. Regional hypo-metabolism is best analysed with co-registration of PET scans to MR images. The sensitivity of FDG PET is 60-90% for the detection of interictal temporal lobe hypo-metabolism. FDG PET is more useful for lateralizing than localizing the epileptic focus. Patients with MTS have low glucose metabolism in the whole temporal lobe while patients with mesiobasal temporal tumors show only a slight decrease in metabolism.
Functional magnetic resonance imaging
Functional MRI (fMRI) helps to visualize regional brain activity. It provides a reliable way to lateralize language dominance and eliminates the need for invasive intra-carotidamobarbital test (IAT) in 80% or more patients. A series of related tests, such as verbal fluency and language comprehension, are administered during MRI to localise the speech centre. Similarly, in patients with lesions near the motor area (area responsible for power) the relation of the lesion to the motor strip can be defined using fmri. This helps in surgical planning of the excision of lesion.
Role of EEG in Presurgical Evaluation
Non-invasive EEG monitoring (Video telemetry)
Long-term non-invasive video EEG monitoring in presurgical evaluation is performed to differentiate seizure versus non seizure events, classification of seizure types and localization of seizure onset. It is expensive and labour intensive. At least two to five habitual seizures should be recorded after gradual AED withdrawal. The aim is to pin point the onset of the electrical seizure and correlate it with the seizure manifestation (semiology). At Jaslok hospital we perform this in ICU to avoid any untoward complications.
In cases where we cannot conclude from non-invasive EEG we would perform invasive EEG. This are in the form of subdural grids and depth electrode recordings.
The primary goal for neuropsychological evaluation is to characterize the patient’s intellectual level, intelligence quotient (IQ) with the Wechsler Adult Intelligence Scale (WAIS) or a revision of it (WAIS-R), the Minnesota Multiphasic Personality Inventory (MMPI) and the Washington Psychosocial Seizure Inventory (WPSI). An epileptic dysfunction in a silent cortical area will have less influence on IQ area. Discriminative neuropsychology has several tests in store. Neuropsychology provides information about size, location and degree of epileptic dysfunction. Preoperative evaluation assists in predicting epilepsy surgery outcome and thus helps in selecting ideal candidates for surgery. Epilepsy surgery can be performed without any neuropsychology at all, but it helps in the preoperative counselling of the patients and their caregivers. It provides baseline values against which the postoperative values can be compared.
Successful epilepsy surgery in India requires a multidisciplinary team approach with discussion of individual patient presurgical evaluation data in detail in a patient management conference. It will improve patient care and communication among members of the team.