Olfactory function in focal epilepsies: Understanding mesial temporal lobe epilepsy beyond the hippocampus

Several lines of research have linked olfactory regions with the pathophysiology of focal epilepsies. Among those regions, the piriform cortex represents the major part of the primary olfactory cortex. According to these data, we raised the hypothesis that in patients with mesial temporal lobe epilepsy associated with hippocampal sclerosis exists an interictal dysfunction of olfactory processing that could be more significant compared to patients with extra-hippocampal focal epilepsy and healthy controls. This could be the consequence of a dysfunctional epileptogenic network that extends beyond the hippocampus and affects other structures, including the piriform cortex. To test this hypothesis, we evaluated the olfactory function with the Sniffin' Sticks test in 32 patients with mesial temporal lobe epilepsy associated with hippocampal sclerosis, 30 patients with extra-hippocampal focal epilepsy, and 22 healthy controls. Compared to the other study groups, patients with temporal lobe epilepsy due to hippocampal sclerosis showed a basal olfactory dysfunction characterized by an impairment in odor discrimination and odor identification. We also found that high seizure frequency had a strong correlation with the evaluated olfactory tasks. Our results are consistent with neuroimaging and neuropathological data that establish a link between olfactory regions and the pathophysiology of temporal lobe epilepsy.