Abstracts

There is somewhat contradictory evidence for whether malformations of cortical development (MCD) may support motor, visual or cognitive function. Factors that may determine whether or not function is retained are the type of malformation and the stage of brain development at which the malformation occurs. To date, severity of MCD has not been examined as a potential causative factor. To address this possibility we examined one type of MCD, focal cortical dysplasia (CD), and compared two subtypes, distinguished by severity as defined by histopathology of surgical specimens. Severity of CD was based on detailed histopathological analysis and cases were classified into mild (n=9) and severe (n=14) subtypes. Mild CD was defined by evidence of dyslamination, and severe CD by dyslamination as well as the presence of balloon cells and/or giant dysmorphic neurons. Cases were relevant to the question when the area of CD overlapped with the expected area of cortical representation of function; hence the number of cases per type of activation paradigm varied. Cortical representations of functions were established with fMRI paradigms for language (n=7), motor (2), visual (6) and auditory function (6). Altered cortical specification for function was defined as absence of activation at expected sites, shift within the same hemisphere, transferring to the contralateral hemisphere or bilateral representation. For language paradigms, activation was absent at expected sites in 2/7 patients, with 1 showing intrahemispheric shift and 4 showing bilateral activations. For motor, visual and auditory tasks, activations at expected sites were absent on the epileptogenic side in 9/14 patients, the rest showing intrahemispheric or bilateral scatter. There was no difference between the two CD groups in terms of the number of cases showing altered cortical specification. Our findings indicate altered cortical specification in almost all patients with CD. Absence of activation correlates with the epileptogenic region, but the rates of altered specification are no different between the two grades of CD. The abnormal hemisphere often retains significant language function even with severe grades of CD, a finding relevant to the planning of surgical strategies.