Olfactory bedside test. A simple approach to identify temporo-orbitofrontal dysfunction

I. Savic, S. Y. Bookheimer, I. Fried and J. Engel Jr
Department of Neurology, University of California, Los Angeles, USA.

BACKGROUND: Olfactory memory and discrimination are processed by the anteromesial temporal cortex and the orbitofrontal cortex. Both functions may therefore be impaired in limbic epilepsy. METHODS: Twenty-seven patients with mesial temporal lobe seizures (MTLS), 10 patients with neocortical seizures (NS), and 10 matched healthy control subjects underwent evaluation for olfactory quality discrimination (OD) and delayed recognition memory (OM). All patients were referred for presurgical evaluation. The olfactory tests were performed in a same-different paradigm with 10 seconds (OD) and 60 minutes (OM) between presentations of the odors, using the standardized University of Pennsylvania Smell Identification Test. The presentations were monorhinal in the OD and birhinal in the OM tests. The results were related to regional glucose metabolism measured with fludeoxyglucose F 18 positron emission tomography. RESULTS: Patients with MTLS had an impaired OD ipsilateral to the epileptogenic region (P < .001) and a higher total number of errors (including both tests) (P = .002). They also had lower OM scores, but not significantly lower than those of patients with NS (P = .05). The combined OM and OD tests correctly identified patients with MTLS with a sensitivity of 85% and a specificity of 90%, offering a correct lateralization in 74% of patients. Patients with MTLS whose OD was more impaired than OM differed from those with more impaired OM by having a significant hypometabolism not only over the neocortex of the epileptogenic temporal lobe (P = .02) but also in the ipsilateral anterior (P = .008) and orbitofrontal cortex (P = .007) (2-way analysis of variance). CONCLUSIONS: Tests of olfactory function are useful in distinguishing between NS and MTLS. The impairments of OM and OD can be dissociated in pathological states and therefore mediated by different structures.

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