An epileptic seizure is the external manifestation of a functional cerebral disorder that can affect 10% of the world's population. Virtually any brain abnormality can irritate vulnerable neurons and produce epileptic seizures. The nature and characteristics of the seizure depend on the part of the brain involved in the disturbance.
Most epileptic patients achieve good seizure control through treatment with antiepileptic drugs. Around 20% to 30% of sufferers will continue to have epileptic manifestations despite drug treatment. The majority of these patients will have symptomatic or cryptogenic location-related epilepsy, which may be treated successfully with surgery.
The prime aim of epilepsy surgery is to address specific disorders responsible for triggering seizures in order to bring symptomatic relief without causing another disability.1 This means that out of the large and heterogeneous group of disorders linked to epilepsy, those syndromes in which seizures start at a single point (focus) in the brain are of most interest for surgery. These syndromes are subclassified according to the site of the focus (frontal, temporal, parietal, and occipital lobe epilepsies) and are further divided according to etiology.
A clear distinction is made between epileptic seizures and epilepsies. Seizures are classified according to clinical seizure type and EEG expression (ictal and interictal) and are defined as electroclinical seizure types. Syndromes are defined as an epileptic disorder characterized by a cluster of signs and symptoms customarily occurring together.2
The International League Against Epilepsy (ILAE) and the International Bureau for Epilepsy (IBE) have proposed new definitions for epileptic seizures and epilepsies. They define an epileptic seizure as "a transient occurrence of signs and/or symptoms due to abnormal excessive or synchronous neuronal activity in the brain," and epilepsy as "a disorder of the brain characterized by an enduring predisposition to generate epileptic seizures, and by the neurobiologic, cognitive, psychological, and social consequences of the condition." Their definition of epilepsy requires patients to have undergone at least one epileptic seizure.3 Epilepsies are classified into syndromes by the ILAE and the IBE according to seizure types, etiology, patient's age at onset, EEG changes, and MRI/CT findings.
A ROLE FOR MRI
Advances in MRI have changed the workup of epilepsy dramatically. The role of radiologists was previously limited to ruling out gross pathology, such as occupying lesions, as the cause of seizures. Structural epileptogenic abnormalities can now be detected, localized, and differentiated on MRI, and a proper preoperative evaluation performed.
The limbic system is anatomically complex, and its small, deep structures are difficult to image. Improvements to the spatial and temporal resolution of MRI have made it easier to identify and delineate the cause of epilepsy in most patients. This is especially relevant for patients with drug-resistant focal epilepsy. Good agreement between MRI and EEG data can improve the likelihood of surgical success. The probability of a seizure-free outcome for patients with temporal lobe epilepsy is 82% if the lesion is concordant with EEG results. This drops to 56% for patients with an unremarkable MRI. In frontal lobe epilepsy, the probability of an excellent outcome is 72% with a concordant lesion and 41% when no abnormality is detected.4,5
