Nearly one-third of patients with focal epilepsy experience disabling seizures that are refractory to pharmacotherapy. Drug-resistant focal epilepsy is, however, potentially curable by surgery. Although lesions associated with the epileptic focus can often be accurately detected by MRI, in many patients conventional imaging based on visual evaluation is unable to pinpoint the surgical target. Patients with so-called cryptogenic epilepsy represent one of the greatest clinical challenges in many tertiary epilepsy centers. In recent years, it has become increasingly clear that epilepsies that are considered cryptogenic are not necessarily nonlesional, the primary histopathological substrate being subtle cortical dysplasia. This Review considers the application of new advances in brain imaging, such as MRI morphometry, computational modeling and diffusion tensor imaging. By revealing dysplastic lesions that previously eluded visual assessments, quantitative structural MRI methods such as these have clearly demonstrated an increased diagnostic yield of epileptic lesions, and have provided successful surgical options to an increasing number of patients with ‘cryptogenic’ epilepsy.
Introduction
Epilepsy affects approximately 50 million people worldwide. Up to 30% of patients with focal epilepsy have persistent, disabling seizures that are refractory to pharmacotherapy. Uncontrolled epilepsy is harmful to the brain, has devastating socioeconomic consequences, and is associated with increased risk of injury and sudden death.
The two most common drug-resistant focal epilepsy syndromes are temporal lobe epilepsy, which results from mesiotemporal sclerosis, and neocortical epilepsy related to focal cortical dysplasia (FCD), which accounts for more than half of pediatric patients and a quarter of adult patients. In both groups, resective surgery is the most effective treatment to eliminate seizures.
MRI has revolutionized the evaluation and management of drug-resistant epilepsy by allowing reliable detection of the structural lesion associated with the epileptogenic zone, leading to increased rates of successful resective surgery. Several studies have shown that the most important predictor for favorable postsurgical outcome is the complete resection of the abnormality detected by preoperative MRI. Despite technical improvements in MRI hardware and sequences, however, best-practice MRI is unremarkable and thus unable to reveal the potential surgical target in up to 50% of patients with drug-resistant focal epilepsy. This limitation is partly attributable to the anatomical complexity of the neocortex, which leads to difficulties in the localization and delineation of the epileptogenic tissue by MRI.
Surgery in the absence of a visible lesion is currently one of the greatest clinical challenges in many tertiary epilepsy centers, because when patients with cryptogenic epilepsy undergo resective surgery, the outcome is generally poorer than when a lesion is found on MRI. The formulation of an a priori hypothesis about the epileptogenic zone is demanding in these patients because of diverse patterns of seizure semiologies and spread, and nonlocalizing EEG findings in many cases. When surgery is considered, patients with cryptogenic focal epilepsy undergo prolonged EEG recordings with intracranial electrodes a procedure carrying complication rates and risks comparable to those of the surgical process. Even with widespread cortical coverage, sampling errors can occur and the electrodes may need to be removed without a resection having been performed because the epileptic focus cannot be localized. These difficulties should not, however, foster skepticism towards surgery. Indeed, compelling evidence is available from several studies published since 2005, and from a large meta-analysis, indicating that 40–60% of patients with cryptogenic extratemporal epilepsy who undergo surgery could become seizure free or attain good seizure control.
In the past few years, the fact that epilepsies initially considered to be cryptogenic are not necessarily nonlesional has become increasingly clear. Indeed, in 30–50% of those patients who undergo surgery, histological examination of the resected specimens reveals the presence of epileptogenic lesions—mainly dysplasias and, rarely, subtle isolated gliosis. Importantly, retrospective assessment of preoperative MRI scans, often guided by quantitative structural image analysis, can frequently identify a lesion. These findings reinforce the importance of obtaining high-quality images, interpreted by MRI experts, to evaluate and treat patients with cryptogenic epilepsy.
This Review considers the application of new advances in brain imaging, such as MRI morphometry, computational modeling and diffusion tensor imaging (DTI). By revealing dysplastic lesions that previously eluded visual assessments, quantitative structural MRI methods such as these have clearly demonstrated an increased diagnostic yield of epileptic lesions, and have provided successful surgical options to an increasing number of patients with ‘cryptogenic’ epilepsy.
Source: MedScape News Today
