Enhancement of trigeminal chemosensory function in congenital anosmia and its contribution to the perception of odors

Background and purpose: Perception of odorants implicates activation of both olfactory and trigeminal systems, since it is well known that the vast majority of odorants stimulate both olfactory and trigeminal system, and that these two systems closely interact with each other to give the odorous sensation. Patients suffering from congenital anosmia exhibit modifications in their trigeminal response in comparison to controls, and trigeminal system seems to constitute a unique way for people suffering from congenital anosmia to perceive some odorants. The aim of this study was thus to investigate the contribution of the trigeminal system to the perception of odorants in cases of congenital anosmia. Materials and methods: We retrospectively reviewed 25 cases of patients suffering from congenital anosmia. Chemosensory function was investigated using psychophysical testing and electrophysiological testing with recording of chemosensory event-related potentials. An MRI was also performed to evaluate the morphology of their olfactory pathways. Their results were compared to those of 19 patients suffering from acquired anosmia (15 posttraumatic and 4 postinfectious), and to 18 healthy controls. Results: In congenital anosmia, latency of P2 is significantly longer and amplitude of P2 is higher than in healthy controls. Also, we found a clear correlation between the orthonasal olfactory function and the trigeminal event-related potentials in the group of congenital anosmia. However, this correlation is absent in cases of acquired anosmia and in healthy controls. Conclusion: In congenital anosmia, trigeminal system contributes to the perception of odorants, while it is not the case in acquired anosmia or in healthy controls