Neuroimaging in the evaluation of children and adolescents with intractable epilepsy: I. Magnetic resonance imaging and the substrates of epilepsy

Association of magnetic resonance imaging identification of mesial temporal sclerosis with pathological diagnosis and surgical outcomes in children following epilepsy surgery

OBJECT

Mesial temporal sclerosis (MTS) is widely recognized as a significant underlying cause of temporal lobe epilepsy. Magnetic resonance imaging is routinely used in the preoperative evaluation of children with epilepsy. The purpose of this study was to evaluate the prevalence, reliability, and prognostic value of MRI identification of MTS and MRI findings indicative of MTS in a series of patients who underwent resection of the medial temporal lobe for medically refractory epilepsy.

METHODS

The authors reviewed the medical records and preoperative MRI reports of 25 patients who had undergone medial temporal resections (anterior temporal lobectomy or functional hemispherotomy) for medically intractable epilepsy. The preoperative MRI studies were presented for blinded review by 2 neuroradiologists who independently evaluated the radiographs for selected MTS features and provided a final interpretation. To quantify interrater agreement and accuracy, the findings of the 2 blinded neuroradiologists, the nonblinded clinical preoperative radiology report, and the final pathology interpretation were compared.

RESULTS

The preoperative MRI studies revealed MTS in 6 patients (24%), and histopathological analysis verified MTS in 8 (32%) of 25 specimens. Six MRI features of MTS were specifically evaluated: 1) increased hippocampal signal intensity, 2) reduced hippocampal size, 3) atrophy of the ipsilateral hippocampal collateral white matter, 4) enlarged ipsilateral temporal horn, 5) reduced gray-white matter demarcation in the temporal lobe, and 6) decreased temporal lobe size. The most prevalent feature of MTS identified on MRI was a reduced hippocampal size, found in 11 of the MRI studies (44%). Analysis revealed moderate interrater agreement for MRI identification of MTS between the 2 blinded neuroradiologists and the nonblinded preoperative report (Cohen κ 0.40-0.59). Interrater agreement was highly variable for different MTS features indicative of MTS, ranging from poor to near perfect. Agreement was highest for increased hippocampal signal and decreased temporal lobe size and was consistently poor for reduced gray-white matter demarcation. The sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and proportion perfect agreement were highest for increased hippocampal signal and reduced hippocampal size. An MRI finding of MTS was not predictive of seizure outcome in this small series.

CONCLUSIONS

Mesial temporal sclerosis identification on brain MRI in children evaluated for medial temporal resections has a PPV of 55%-67% and an NPV of 79%-87%. Increased hippocampal signal and reduced hippocampal size were associated with high predictive values, while gray-white differentiation and an enlarged temporal horn were not predictive of MTS. Seizure outcome following medial temporal resections was not associated with MRI findings of MTS or MRI abnormalities indicative of MTS in this small sample size.

Complex Partial Seizures in Children

The treatment of complex partial seizure (CPS) begins with accurate diagnosis. Episodes of staring and unresponsiveness due to nonepileptic causes must be ruled out. Absence seizures (typical and atypical petit mal) must be considered because the treatment strategy and prognosis for these seizures are very different from those for CPS. Diagnostic evaluation should include electroencephalography (EEG) done during both the awake and the sleep states. Standard activation procedures and high-quality magnetic resonance imaging (MRI) of the brain with sequences specifically designed to evaluate mesial temporal structures and subtle regions of cortical dysplasia should be used. Computed tomography is not adequate for brain imaging in the evaluation of persons with seizures. Because most children with CPS will not "grow out of their seizures" and only 50% will have seizures adequately controlled, early aggressive therapy should be pursued. The goal of therapy should be complete freedom from seizures. Although only a few antiepileptic drugs (AEDs) are specifically approved for CPS in children younger than 12 years of age, a variety of pharmacologic options exist. This is because 1) AEDs that are effective in any type of partial seizure are likely to be effective in CPS, 2) AEDs that are effective in adults with partial seizures are effective in children with seizures of the same type, and 3) AEDs that are effective as adjunct therapy can be effective as monotherapy. The choice of an AED should be strongly influenced by side-effect profiles because it is not yet possible to predict which AED will be most effective in an individual child or adolescent. Surgical resection should be considered for children of all ages if seizures are intractable to adequate medical intervention, especially if a structural abnormality is detected on neuroimaging.

Neuroradiologic aspects of West syndrome

Cranial computed tomography (CT) or magnetic resonance imaging (MRI), or both, of 143 patients with West syndrome were retrospectively reviewed. Twenty-four children had normal CT scans. The most striking findings on pathologic scans were cerebral atrophy, ventricular enlargement, and encephalomalacia. Of 78 children who underwent MRI, 14 had normal scans. Cerebral malformations, cerebral atrophy, and delayed myelination were the most common pathologic findings and were observed in 25, 20, and 16 of the cases, respectively. MRI was found to be more informative in 21 of the 27 patients who underwent both neuroimaging methods. In fact, in two patients with normal CT scans, MRI revealed delayed myelination and hyperintensity of the hippocampal area on T2-weighted images in one and increased intensity of basal ganglia on T1-weighted images in the other. On the basis of all data, four patients were classified as cryptogenic and 126 as symptomatic. The most frequent etiologic factors were perinatal asphyxia, cerebral malformations, and infections, encountered in 34, 25, and 24 of the children, respectively. Twenty-one cases could be classified solely on the basis of the data obtained with neuroimaging modalities. The data confirm and provide additional details of the ways in which neuroimaging, especially MRI, is useful in distinguishing between cryptogenic and symptomatic cases of West syndrome.

The practical utility of performing peri-ictal SPECT in the evaluation of children with partial epilepsy

Peri-ictal brain single-photon emission computed tomography (SPECT) is increasingly being established as a useful test in localizing partial epilepsy in adults. However, obtaining an ictal injection and acquiring the SPECT images poses a greater challenge in pediatric patients, and few reports have specifically addressed the practical use of this technique in children. The Mayo Clinic experience of peri-ictal SPECT in the evaluation of children with partial epilepsy is reported here. Peri-ictal SPECT was attempted during 71 admissions involving 59 patients (median age 12 years, range 1 year 6 months-17 years). A peri-ictal SPECT injection was performed on 48 (67.6%) of these admissions in 43 (72.9%) patients, and only two patients could not be scanned. Of the 46 peri-ictal images successfully obtained, 30 (65.2%) were from ictal injection and 16 (34.8%) from post-ictal injections. Forty-two (91.3%) of the successfully obtained SPECT images, in 38 patients (92.3%), were classified as localizing (15 temporal, 24 extratemporal). We conclude that, with the appropriate unit setup and well-trained staff, peri-ictal SPECT scans can be obtained in most pediatric partial epilepsy patients. Moreover, the procedure provides specific localizing information in a high proportion of these patients.