Abstracts

Olfaction is processed by the limbic system. Imaging of olfaction, therefore, offers a unique approach to diagnose limbic dysfunctions in mesial temporal lobe epilepsy (MTLE). To test this, activation studies were performed with ¹⁵O-H[sub]2[/sub]O-PET during passive smelling of odors, and using smelling of odorless air as the baseline condition., Thirteen patients with left-sided MTLE, 10 with right-sided MTLE, and 21 controls participated. The epileptogenic region was assessed from the consensus of results from ¹⁸F-FDG PET, MRI and ictal electroencephalography recordings. Significant activations in each group, and differences between the groups were tested with random effect analyses (SPM2, T-threshold = 0.01, corrected p[lt]0.05). In addition, we evaluated whether the functional connectivity of the piriform and insular cortex (using the whole brain as search space) differed between patients and controls. Functional connectivity was defined as a significant co-variation between different regions, and assessed during both the baseline condition and odor activation (ANCOVA, SPM2, p[lt]0.05 corrected). Both familiar and unfamiliar odors were used., In control subjects smelling of odors activated bilateral amygdala, piriform, anterior insular and cingulate cortices. An additional activation was observed in Broca[apos]s area when smelling familiar odors. Patients differed in several aspects: 1) They failed to activate the amygdala, piriform and anterior insular cortex ipsilaterally to the epileptogenic side. 2) Patients with LTLE, in addition, failed to activate the Broca[apos]s area when smelling the familiar odors; furthermore, they perceived familiar odors as less familiar than the controls and patients with right MTLE. 3) As opposed to the healthy controls who showed functional connectivity between the two-piriform cortices, and between the right piriform and orbitofrontal cortex during odor stimulation, no such connectivity was detected in patients., During limbic activation patients with documented MTLE fail to recruit several extrahippocampal limbic regions on the side of seizure onset. Imaging of olfaction in MTLE may be used not only to locate the side of seizure onset, but also to trace the patterns of altered neuronal processing during functional activation., (Supported by Swedish Medical Research Council.)