Magnetic resonance imaging in childhood intractable partial epilepsies: pathologic correlations

Febrile seizures and hippocampal sclerosis: frequent and related findings in intractable temporal lobe epilepsy of childhood

The relationship between hippocampal sclerosis, febrile seizures, and complex partial seizures in temporal lobe epilepsy continues to be the subject of great debate in the literature. Hippocampal sclerosis is reported infrequently in young children with temporal lobe epilepsy, a factor that has supported the theory that hippocampal sclerosis develops in later life during the course of recurrent complex partial seizures. In a blinded review of magnetic resonance imaging in 53 children, aged 2-17 years (mean: 10 years) with temporal lobe epilepsy, hippocampal sclerosis was diagnosed in 30 children (57%), concordant with ictal electroencephalographic lateralization in 93% and pathologic diagnosis in all children who had undergone surgery and had hippocampal tissue available for histologic examination. Fourteen of the children (47%) with hippocampal sclerosis were younger than 10 years of age, the youngest being 2 years. Thirty-four children (64%) had histories of neurologic insults prior to the onset of complex partial seizures, including idiopathic febrile seizures in 22. Hippocampal sclerosis was associated with a history of a neurologic insult prior to the onset of complex partial seizures (P < .001) and was not associated with age at onset of temporal lobe epilepsy, age at magnetic resonance imaging, duration of epilepsy, or presence of secondarily generalized seizures. These findings suggest that hippocampal sclerosis is underdiagnosed in children and is the cause and not the consequence of temporal lobe epilepsy.

Frontal and central lobe focal dysplasia: clinical, EEG and imaging features

The authors studied 11 patients with developmental malformations and seizures originating from the frontal and central regions. Patients with centrally located seizures had primary involvement of the face or mouth; clonic activity involving the limb was also seen. Seizures among those with frontal lesions were primarily unilateral or bilateral tonic motor. Secondary generalized tonic-clonic seizures preceded by focal manifestations occurred infrequently in those with central localization, but were not rare in the frontal group. MRI revealed abnormalities in 10 patients, nine of whom underwent surgical resection with good results. Focal cortical dysplasia may be the underlying epileptogenic abnormality in young patients with clinical features suggestive of central or frontal cortical involvement.

Clinical applications: MRI, SPECT, and PET

MRI, PET, and SPECT are all used to image abnormalities in the epileptic brain. Comparison of the techniques is difficult because they measure different aspects of the epileptic process--structure, metabolism, and perfusion. SPECT is the only one that can be systematically applied during seizures, while all three are used to image interictal abnormalities. Literature review suggests that of interictal techniques, PET has the highest diagnostic sensitivity in temporal lobe epilepsy (TLE) (84% vs. 66% for SPECT, 55% for qualitative MRI, 71% for quantitative MRI) while SPECT has the highest sensitivity in extratemporal epilepsy (ETE) (60% vs. 43% for MRI and 33% for PET). The highest diagnostic sensitivity and specificity were achieved by ictal imaging with SPECT (90% in TLE, 81% in ETE). The techniques, however, were not always redundant. One reason for the wide discrepancy of results in TLE and ETE might be the differing pathologic substrates. A literature review of imaging findings associated with mesial temporal sclerosis (MTS), developmental lesion or tumor as the underlying abnormality associated with epilepsy supports this explanation. PET and MRI are much more sensitive to MTS than SPECT (100%, 95% vs. 70%). On the other hand, in developmental lesions the three techniques are equally sensitive (88-92%) and in tumors, MRI was most sensitive (96%) and SPECT least (82%). A study at NIH explains the differing sensitivities: using PET to measure both blood flow and metabolism revealed discrepant findings in the same patients. Preliminary evidence also indicates that the distribution of hyperperfusion on ictal SPECT can differentiate subtypes of TLE. Combining the results of refined imaging techniques holds great promise in epilepsy localization and diagnosis.

Curvilinear reconstruction of 3D magnetic resonance imaging in patients with partial epilepsy: a pilot study

In an attempt to better delineate the abnormalities associated with focal cortical dysgenesis, we performed curvilinear reformatting of the cortex from 3D magnetic resonance (MR) images. Illustrative patients with partial seizures and conventional orthogonal MRI evaluation show that small regions of cortical thickening suggestive of focal dysplastic lesions may not be recognized. In three such patients the curvilinear reformatting demonstrated two additional focal abnormalities of the cortical gyri and better defined the two focal lesions found on conventional orthogonal MR images. This method promises to be a useful tool in the evaluation of epileptic patients with proven or suspected subtle structure cortical abnormalities, particularly focal neuronal migration disorders where cortical thickening, abnormal gyral pattern, and poor delineation of the gray-white matter transition are the main findings.

MRI in cerebral developmental malformations and epilepsy

Cerebral developmental malformations are increasingly recognized as a major cause of developmental delay and epilepsy. The incidence of these developmental malformations in patients with epilepsy is not known, but epilepsy surgery data suggest that this pathology is commonly seen in children who undergo epilepsy surgery for intractable epilepsy. These malformations can be diagnosed by a combination of clinical, neurophysiological, and imaging techniques. However, imaging techniques such as MRI have been able to provide in vivo recognition of many of these malformations and have contributed to the recognition of specific syndromes. These malformations can be classified on an anatomical basis either into diffuse, unilateral, or generalized. However, a combination of imaging data in conjunction with genetics and embryology may be more appropriate in the future. Further technical developments promise to increase the sensitivity of MRI in detecting these malformations and may help to delineate the possible biology of these disorders.

Mesial temporal sclerosis: pathogenesis and significance

Mesial temporal sclerosis (MTS) is a common pathologic finding in patients with temporal lobe epilepsy. Rarely MTS can be detected in children during the first decade of life, but is not commonly found until adolescence. Although the etiology of MTS remains controversial, there is now a considerable amount of evidence demonstrating that MTS is both a result and a cause of seizures. Clinical studies suggest that prolonged seizures or complicated febrile seizures may result in MTS. A variety of epileptogenic agents administered to adult animals have resulted in MTS and spontaneous recurrent seizures. The mechanism of the lesions is due to excessive excitability secondary to release of excitatory amino acids, primarily glutamate. Glutamate, acting at a number of subreceptors on the postsynaptic membrane, leads to prolonged depolarization of neurons and results in the entry of cytotoxic amounts of calcium. Interestingly, the same agents that produce MTS in adult animals do not produce MTS in immature animals. Clinical and experimental evidence suggests that although prolonged seizures or complicated febrile seizures can place a child at risk for MTS, a period of time is required for the lesions to develop fully.

The relative contributions of MRI, SPECT, and PET imaging in epilepsy

Functional and structural neuroimaging techniques are increasingly indispensable in the evaluation of epileptic patients for localization of the epileptic area as well as for understanding pathophysiology, propagation, and neurochemical correlates of chronic epilepsy. Although interictal single photon emission computed tomography (SPECT) imaging of cerebral blood flow is only moderately sensitive, ictal SPECT markedly improves yield. Positron emission tomography (PET) imaging of interictal cerebral metabolism is more sensitive than measurement of blood flow in temporal lobe epilepsy. Furthermore, PET has greater spatial resolution and versatility in that multiple tracers can image various aspects of cerebral function. Interpretation of all types of functional imaging studies is difficult and requires knowledge of time of most recent seizure activity and structural correlates. Only magnetic resonance imaging (MRI) can image the structural changes associated with the underlying epileptic process, and quantitative evidence of hippocampal volume loss has been highly correlated with seizure onset in medial temporal structures. Improved resolution and interpretation have made quantitative MRI more sensitive in temporal lobe epilepsy, as judged by pathology. When judged by electroencephalography (EEG), ictal SPECT and interictal PET have the highest sensitivity and specificity for temporal lobe epilepsy; these neuroimaging techniques have lower sensitivity and higher specificity for extratemporal EEG abnormalities. Regardless of the presence of structural abnormalities, functional imaging by PET or SPECT provides complementary information. Ideally these techniques should be used and interpreted together to improve the localization and understanding of epileptic brain.

Operative strategies for patients with cortical dysplastic lesions and intractable epilepsy

Cortical dysplastic lesions (CDLs) are usually identified by magnetic resonance imaging (MRI). Clinical, electrographic and histologic findings suggest that focal CDLs (FCDLs) are highly epileptogenic, often involve the rolandic cortex, and can present variable degrees of histopathologic abnormalities. An ictal or "ictal-like" bursting pattern of electrographic activity was recorded over dysplastic cortex in 65% of our patients. Resective surgery can eliminate or significantly reduce seizure frequency in many medically intractable patients, depending on lesion location, degree, and extent of histopathologic abnormalities. Best results are achieved when complete or major excision of both the MRI-visible lesion and the cortical areas displaying ictal electrographic activity can be performed. This is more likely when the degree of histopathologic abnormality is mild to moderate or when the lesion is in a temporal lobe. More severe histopathologic abnormalities and central insular or multilobar lesions usually lead to less favorable results: either major excision of the visualized lesion is impractical or the lesion is microscopically more extensive than shown by MRI. Multilobar resection or hemispherectomy for patients with infantile spasms associated with CDLs and for patients with hemimegalencephaly are often associated with dramatic improvement in seizure control. Callosotomy can be performed in selected patients with diffuse CDLs who have intractable drop attacks.

Magnetic resonance imaging in developmental disorders of the cerebral cortex

Developmental disorders of the neocortex are commonly associated with epilepsy. The development of magnetic resonance imaging (MRI) has advanced our understanding of these disorders by permitting accurate recognition and clinical correlation during life. These disorders have multiple etiologies and are dependent on the time of injury to the developing nervous system. MRI has permitted the classification of these malformations in three major groups: generalized disorders, unilateral hemispheric, and focal disorders. Generalized disorders include lissencephaly, pachygyria, band heterotopia, and subependymal heterotopias. Hemimegalencephaly comprised the unilateral disorder. Focal lesions include focal cortical dysplasia, polymicrogyria, schizencephaly, and focal subcortical heterotopias. The information provided by MRI, in conjunction with the clinicoelectrographic features, is extremely important in the recognition of these syndromes and for the appropriate medical and surgical management of those patients with epilepsy.