Magnetic Resonance Imaging in Pediatric Epilepsy

Epilepsy is a common neurological affliction of children. It carries an estimated annual incidence of up to 1 per 1,000, and a prevalence of up to 5 per 1,000 children (1). Children suffering from epilepsy have a significantly increased mortality rate (2–4). The major causes of death are the underlying cause of the epilepsy itself, associated neurological compromise, injuries, and status epilepticus (5). Epilepsy was characterized as occasional, sudden, excessive, rapid, and local discharges of gray matter by Jackson (6) in 1873. It now is defined as a chronic predisposition to recurrent unprovoked seizures, which occur as a result of increased abnormal electrical discharges of neurons. An important distinction should be made between epilepsy and acute seizures; the latter typically occur as a consequence of an acute neurological change, such as that in the setting of trauma, infection, ischemia, tumors, and even fevers. CLASSIFICATION OF SEIZURES AND EPILEPSIES On the basis of both clinical and electroencephalographic (EEG) features, seizures are separated into three major categories according to the scheme proposed in 1981 by the International League against Epilepsy (ILAE) (7): Partial (“focal”) seizures, including simple and complex partial seizures without or with secondary generalization Generalized seizures Unclassified epileptic seizures Partial seizures are thought to arise from a localized area of the brain and are separated into simple and complex types. Simple partial seizures do not entail an alteration of consciousness, which is, by definition, an integral part of complex partial seizures. The electrophysiological explanation for this difference between these two subcategories is the lack of extension of abnormal electrical activity beyond neocortical structures in simple partial seizures, in contradistinction to the additional involvement of the limbic system in complex partial seizures. Generalized seizures are caused by abnormal electrical activity that originates simultaneously from both cerebral hemispheres and are accompanied by a loss of consciousness (e.g., absence, myoclonic, tonic, clonic, tonic-clonic, and atonic seizures). As noted earlier, epilepsy is considered a chronic disorder of recurrent seizures. An epileptic syndrome is further defined as an “epileptic disorder characterized by a cluster of signs and symptoms customarily occurring together” (8). For the purposes of clarifying the description, underlying etiology, proper treatment, and prognosis of various epilepsies and epileptic syndromes, an additional revised classification for these disorders was created by the ILAE in 1989 (8). This system divides epilepsies and epileptic syndromes into four groups: “Localization-related” (i.e., focal electrical origin) Primary generalized Undetermined, whether focal or generalized Special syndromes (e.g., febrile seizures) The first two sets are further separated into idiopathic, symptomatic, and cryptogenic subtypes. Idiopathic epilepsies and syndromes usually are associated with characteristic clinical and EEG features, and a specific age of onset. They most likely represent inherited disorders that have no other apparent explanatory cause (e.g., benign Rolandic epilepsy). Symptomatic epilepsy and syndromes are those with identifiable causes (e.g., temporal, frontal, parietal, and occipital epilepsies). Cryptogenic types are presumed to have underlying causes that have not yet been uncovered. CLINICAL APPROACH AND THE ROLE OF NEUROIMAGING In the initial clinical workup of any child referred with possible epilepsy, other disorders that may mimic epileptic seizures must be excluded. Examples of these include tics, breath-holding spells, night terrors, and long QT syndrome (9). This is achieved by obtaining a detailed medical and family history and a careful description of the episodes (seizure semiology), which includes their frequency of occurrence and any potential triggers. A comprehensive neurological examination also is performed, as is further evaluation with interictal and ictal EEG. Inpatient video-EEG surveillance in a specialized monitoring unit may be required. Based on this information, a diagnosis of a specific type of epilepsy or syndrome is possible, and appropriate treatment can begin. If an idiopathic benign epilepsy can be confidently diagnosed, further radiological evaluation of the child may not be required. Benign epilepsies within this group usually have prototypical clinical and EEG characteristics, age of onset, and familial history. Their “benign” designation indicates that they have a good prognosis, they resolve spontaneously, and they are not associated with any definite neurological impairment or imaging abnormality. Examples of such syndromes include benign neonatal convulsions, benign childhood Rolandic epilepsy, and juvenile absence epilepsy (8,10,11). In the absence of a firm diagnosis of a benign epilepsy or epileptic syndrome, neuroradiological examination becomes an essential adjunct to the collection of clinical and electrophysiological data. Indications for additional investigation with magnetic resonance imaging (MRI) include the following (12): Partial seizures or secondary generalized seizures at any age Generalized seizures beginning within the first year of life Seizures refractory to anticonvulsant treatment Any static or progressive neurological or neuropsychological deficit Changes in the seizure pattern The obvious goal of the radiologist is to detect any structural abnormalities, such as malformations, tumors, vascular lesions or sequelae of previous trauma, infection, or ischemia, which may account for the child's epileptic condition. In cases of medically refractory epilepsy, which may be amenable to neurosurgical therapy, the role of the radiologist is even more integral to the multidisciplinary treatment approach to these patients. Surgical treatment of pediatric epilepsy is becoming more popular because of the increasing awareness of the side effects of long-term anticonvulsant therapy and the much higher morbidity conferred by malignant refractory epilepsies in children due to the associated risk of progressive irreversible developmental delay and encephalopathy. The importance of this multidisciplinary collaboration is best illustrated by localization-related epilepsies, which can be treated successfully by surgical resection of epileptogenic tissue via selective temporal, frontal, parietal, or occipital lobectomies. A good outcome in such cases frequently is incumbent upon the radiological documentation of a morphologic abnormality that is associated with, and thus corroborates, the presumed seizure focus identified by the clinical history, neurological examination, and EEG data. Furthermore, the postoperative outcome has been shown to be more dependent on resection of the cortical area of abnormality seen on MRI than the epileptogenic region found by scalp EEG studies (13–16). 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