Imaging Spectrum of Cortical Dysplasia in Children

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Introduction

Surgical therapy is an increasingly recognized option for treatment of children with intractable seizures.1, 2 It is estimated that up to 30% of patients become resistant to optimized medical therapy and are potential surgical candidates.2 Criteria for this distinction have varied, but most agree that failure of 2 appropriate antiepileptic drugs to adequately control seizures, with more than one seizure/month over an 18-month–2-year period, constitute intractability.2 Additional important clinical features indicating that the patient may be a surgical candidate include stereotyped, lateralizing seizures with focality of semiology, a localized “epileptogenic zone” by scalp electroencephalography (EEG), magnetoencephalography (MEG), or surface EEG, and a localized anatomical target on magnetic resonance imaging (MRI).2 In fact, the presence of an imaging abnormality is one of the key findings that predict resistance to medical therapy and may be a critical finding in determining suitability for surgical therapy.3 Interestingly, it is thought by many pediatric epileptologists that younger children may in fact be better candidates for epilepsy surgery than adults because of their increased neuroplasticity and the detrimental effect of repeated seizures on normal neurologic development.4

Cortical dysplasia is the most common neuropathology noted after surgical resection for intractable epilepsy in children, and recent clinicopathologic classification systems have had a significant effect on the treatment and understanding of this disorder.5, 6 Some forms of focal cortical dysplasia (FCD) (such as type IIb) have typical imaging manifestations, whereas other types such as isolated type I cortical dysplasia have variable and often nonlocalizing imaging features. As imaging findings indicating cortical dysplasia may significantly affect selection for potential surgery, radiologists involved with the workup of these patients must be familiar with the imaging spectrum of FCD, especially in children. In this article, we review the current pathologic classification and imaging findings in cortical dysplasia with an emphasis on the pediatric population.

Section snippets

Pathologic Classification

FCD was first described and popularized as a distinct clincopathologic entity by Taylor et al.7 They described abnormal cell morphology and altered cortical lamination in brain specimens from surgery for intractable epilepsy. This “Taylor type” dysplasia was found subsequently to have typical imaging features on MRI, greatly facilitating preoperative imaging diagnosis.8 Other less severe dysplasias and “mild” malformations of cortical development were also identified histopathologically in

Imaging Evaluation

MRI of the brain provides the critical initial anatomical evaluation in children with epilepsy. In multiple studies of both children and adults with intractable epilepsy and FCD, the identification and resection of a defined imaging abnormality (lesion) is one of the best predictors of seizure freedom after surgery.15, 17 Application of a sensitive dedicated seizure protocol MRI examination is therefore critical in evaluating patients with intractable seizures18, 19, 20 (Table 2). Imaging at

MRI Findings

A variety of abnormal findings on MRI have been described in patients with intractable seizures and cortical dysplasia identified at subsequent histopathology. The prevalence of abnormal findings on MRI, however, is very variable in the literature and is heavily dependent upon surgical selection, MRI technique, expertise of imaging evaluation, range of histopathology, and cohort demographics. MRI findings may be completely normal in 20%-46% of cases (as an overall group).29 Key imaging findings

Type I FCD (Isolated)

Type I FCD, as described previously, indicates isolated alterations in radial or tangential cortical lamination (or both), without another associated lesion. The histopathologic distinction between lamination types is aided by adherence to careful specimen orientation and application of immunohistochemical techniques (eg, NeuN staining)5 but still needs evaluation to determine its widespread clinical feasibility.39 In many prior imaging-based studies nonisolated FCD type I associated with other

Future Directions in Imaging or Adjunctive Imaging Tests

Given the difficulty in identifying FCD on MRI and the importance of identifying a potentially surgically amenable lesion for seizure freedom, multiple adjunctive techniques have been described to improve sensitivity including voxel-based morphometry of both T1- and T2-FLAIR sequences compared with normative groups19, 55 (Fig. 9), diffusion tensor imaging,19, 56 sulcal morphometry,32 and ultrahigh field imaging.57, 58 Alternative PET agents59 and SPM analysis of PET data25 have also been used

Conclusions

FCD is the primary pathology identified in pediatric patients undergoing surgery for intractable epilepsy. MRI is the key diagnostic test for clinical detection; however, imaging appearance is variable. Knowledge of current classification schemes of FCD and the differing expected imaging appearances of subtypes are of significant importance to the radiologists, as identification of an MRI-defined lesion may affect patient selection, surgical performance, and postoperative outcome in these

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      The most sensitive non-invasive in vivo imaging technology for detecting FCD is MRI. Traditional clinical scans (<7 T) typically involve the collection of T1- and T2-weighted, and T2 fluid-attenuated inversion recovery (FLAIR) MRI data [9–18]. T1-weighted, T2-weighted and FLAIR images have been used to estimate the volume, thickness and shape of features in the brain, based on having defined FCD lesion anatomical boundaries via changes in image intensity and contrast [13–15].

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      One of the most common and consistent techniques for diagnosing FCD is the use of MRI. Studies have correlated MRI abnormality-driven resection with a reduction in frequency of seizures (Leach et al., 2014; Mühlebner et al., 2012), particularly with complete resection of the epileptogenic region determined by MRI (Chen et al., 2014; Jin et al., 2016; Rowland et al., 2012). MRI findings in FCD include a blurring of the gray and white matter boundary, increased white matter signal, the transmantle sign (an extension of the white matter signal in the direction of the lateral ventricle), and a reduction in focal gray and white matter volume.

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      During review of participant MRIs, FCD classification as “definite”, “probable”, or “possible” and lesion location were determined. FCD was defined as a localized disorder of the cortex and/or subjacent white matter typical of pathologically proven FCD seen in patients with drug-resistant epilepsy undergoing surgical resection (Blümcke et al., 2011; Leach et al., 2014a, 2014b). MRI characteristics of FCD, adapted from those previously developed and reported by our group (Leach et al., 2014b), included these features: 1) localized increased cortical signal without other known cause, 2) localized increase in cortical thickness, 3) ill-defined or irregular cortical-white matter junction, 4) localized subcortical signal located at the bottom of a sulcus, 5) asymmetric gyral pattern and/or depth, 6) transmantle signal changes related to a gyrus, and/or 7) subcortical heterotopic gray matter.

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