Proton MR-Spectroscopy in Focal Cortical Dysplasia (FCD)

Background/Purpose:

Focal cortical dysplasia is probably the most common cause of therapy-refractory epilepsy in children, and also a frequent cause of epilepsy in adults. Pathologically an FCD is a malformation of cortical development; several subtypes can be differentiated. The FCD subtype with balloon cells (FCD type IIb), which we have investigated, is considered a distinct clinical, neuropathological, and MRI entity. MRI characteristics are cortex thickening and a funnel-shaped subcortical hyperintensity (in T2 and FLAIR) tapering towards the lateral ventricle. Lesions can be subtle and easily overlooked. Purpose of the study was to spectroscopically measure metabolite concentrations in patients with epilepsy due to an FCD and to characterize the metabolic pattern of FCD IIb in order to achieve a basis for future differential diagnoses.

Materials and Methods:

Examinations were performed on a clinical Philips Achieva 3.0TX MR-System. Fourteen patients (3m/11f, age: 3m-59a, seizure frequencies ranging from 10/day to 1/year) with typical MRI-findings of FCD IIb lesions were included. At present FCD IIb was confirmed in 10 of these cases by histopathology post-surgery. MRI-guided localization and size of the target volume were planned to include the dysplastic cortex and the adjacent hyperintense areas of the subcortical lesion. A second volume was placed in the contralateral hemisphere in a corresponding region of normal-appearing brain tissue. Spectra acquisition included water suppressed proton spectra (TE/TR 30,140ms/2000ms) and non-water suppressed spectra for absolute quantification of proton metabolites using the water peak as an internal reference.

Results:

Compared to the normal appearing hemisphere, matched pair analysis in FCD IIb patients showed a highly significant increase in choline (37%, p=0.003) in 13/14 cases. This was associated with a less pronounced, but also significant decrease in NAA (-13%; p=0.001) concentration in all patients. Despite a significant mean myo-inositol increase (40%; p=0.021), individual findings were ambiguous. While total creatine increase correlated with seizure frequency, changes in NAA, choline, and inositol concentration did not.

Conclusion:

We found a distinct and characteristic metabolic pattern even in rather discrete FCD lesions. The spectroscopic findings may support the diagnosis of FCD IIb also when structural MRI is equivocal.