Case ReportFunctional MRI characteristics of a focal region of cortical malformation not associated with seizure onset
Introduction
Malformations of cortical development (MCD) result in a wide range of alterations of the cerebral cortex, including focal cortical dysplasia (FCD), periventricular heterotopias, subcortical band heterotopia (SBH, also known as “double cortex” syndrome), polymicrogyria, and hemimegalencephaly [1], [2]. FCD is the most common localized malformation of this kind [2], with a spectrum of histopathological abnormalities ranging from architectural abnormalities, including those associated with giant neurons, to dysmorphic neurons and balloon cells [3]. FCD is further divided into two main types, each associated with specific anatomical, clinical, electrographic, and imaging characteristics [3], [4]. Type II (Taylor-type) FCD is characterized by architectural abnormalities with grossly dysmorphic cellular elements accompanied by an increase in excitatory amino acid neurotransmission and an overall decrease in intralesional and perilesional inhibition [5]. This normally results in a high degree of intrinsic epileptogenicity, which is usually intractable to medical therapy [6], [7]. Type II FCD includes two main subtypes, FCD type IIA with dysmorphic neurons and FCD type IIB with dysmorphic neuronsand balloon cells [3]. Dysmorphic neurons are associated with a higher degree of epileptogenicity [7] and greater chance of detection by MRI [3], [8]. Ictal behavior and the capacity for the heterotopic tissue to support function may differ in the different subtypes of type II FCD. In one recent study, the type IIA portion of the perirolandic FCD contained the epileptogenic zone and supported some motor function. In contrast, the type IIB portions containing balloon cells were functionally silent and were not associated with seizure onset [9]. We describe a case of intractable epilepsy that demonstrates that fMRI can be used for better electroclinical, imaging, and histopathological correlations of MCD. We aimed to determine whether or not this patient’s MCD supported any cognitive functions that would normally be expected to be subserved by that region of the cortex and the surrounding perilesional cortex.
Section snippets
Case history
S.B.M., a 21-year-old, right-handed, university student, developed medication-resistant epilepsy at age 16. His first two seizures occurred within 3 months of each other and consisted of partial seizures with secondarily generalized tonic–clonic activity. He was treated with carbamazepine after his second seizure, had one complex partial seizure 5 months later, and then had recurrent but similar complex partial seizures twice a week 15 months after starting treatment. These partial seizures
Methods
A 4-T Siemens–Varian fMRI scanner with echo-planar imaging (EPI) was used to measure the blood oxygenation level-dependent (BOLD) response under various task conditions. Block designs were used in all tasks. Tasks were chosen to activate language and related networks [14], [15], [16], [17], motion processing in area MT+/V5 [18], inferior and anterior temporal and lateral occipital regions related to object recognition and semantic processing [19], [20], [21], and medial temporal regions related
Motion perception
Linear correlational analysis was used to determine regions of significant activation relative to the motion stimulus. Robust activation was observed in the right MT+/V5 complex in response to moving stimuli, whereas a much smaller region of activity was observed in the left MT+/V5 (Fig. 2b and Table 2). In addition, a second region of activation in the extrastriate occipital cortex of the right hemisphere was observed, with no such corresponding region of activation in the left hemisphere. No
Discussion
Functional MRI in a wide range of tasks demonstrated that the region of MCD in S.B.M.’s left temporal cortex did not support cognitive functions that would be expected to rely to some degree on this region of the brain [14], [19], [21], [22], [23], [25], [29], [30], [32], [33], [34], [35]. Taken together with the findings from subdural electrodes indicating that the region of MCD was also not the site of seizure onset for S.B.M. (Fig. 6), these results suggest that regions of dysplastic tissue
Acknowledgments
This work was supported by a Physician’s Services Incorporated Grant to S.M. and J.D. and NSERC CRC and Discovery grants to J.D. We thank Dr. Jody Culham and Kenneth Valyear for the control data on the motion and object recognition tasks.
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