Objective: Because some of the axons of the fornix originate in the pyramidal cells of the hippocampus, we hypothesize that neuronal loss within the hippocampus may result in wallerian degeneration and subsequent atrophy of the ipsilateral fornix. Using high-resolution MR imaging, we evaluated the size of the fornix in patients who had lateralizing partial complex temporal lobe seizures after unilateral hippocampal sclerosis. The expectation is that unilateral forniceal atrophy will provide an additional, objective imaging criterion for determining the side of seizure involvement.
Materials and methods: We retrospectively reviewed preoperative MR images of 13 patients with temporal lobe epilepsy who subsequently had unilateral hippocampal sclerosis proved surgically. The images were obtained with specially designed bilateral phased-array coils that permitted high-resolution (512 x 512, 16-cm field of view) T2-weighted fast spin-echo imaging of oblique coronal sections. The width of each rostral crus of the fornix was measured by three neuroradiologists who did not know the patients' history and surgical findings, and consensus measurements were obtained. The side with the smaller fornix was interpreted as abnormal. All 13 patients had lateralizing findings on EEG, unilateral anterior temporal lobectomy, and histologic findings of hippocampal sclerosis.
Results: The fornix was smaller on the side of hippocampal sclerosis in 12 (92%) of 13 patients. The percentage of asymmetry between the fornices (smallest/largest x 100) varied from 41% to 82% (mean, 68%).
Conclusion: Our experience indicates that volume loss in the ipsilateral crus of the fornix can be seen on MR images in patients with unilateral hippocampal sclerosis. The size of the fornices should be analyzed in all patients being examined for temporal lobe epilepsy, as asymmetry in the size of the fornices may reveal the side of hippocampal sclerosis. Such knowledge can assist in the preoperative workup of patients with intractable temporal lobe epilepsy related to hippocampal sclerosis.