An Israeli team reports the case of two women with a normal sense of smell when they are apparently devoid of the olfactory bulb, a structure located at the base of the brain. These are disconcerting clinical observations in that the olfactory bulb is the first relay of sensory information from nasal cavities before it reaches the brain. The olfactory bulb contains glomeruli from which nerve fibers carry the olfactory information to the cerebral level.
Everything is surprising in this study published in the journal Neuron November 6th. Researchers at the Weizmann Institute of Science (Rehovot) were working on another project in magnetic resonance imaging (MRI) when they needed to build a cohort of healthy volunteers (control subjects) with normal olfactory function. Against all odds, they discover on MRI images that a participant apparently does not have an olfactory bulb. This 29-year-old woman is also left-handed.
Scientists then undertake to push further their investigations. They decide to compare the MRI images of this person with those of a patient unable to perceive odors due to a lack of omega-bulbs of congenital origin. The researchers also make other comparisons, with MRI images from young women with no smell disorder and whose olfactory bulbs are perfectly visible on brain MRI but who, like the first patient identified, are left handed. These young women will be a control group. In this cohort, the researchers then identify, to their astonishment, a second person without olfactory bulb. This is a 26-year-old woman, left-handed like the first.
Noam Sobel's team then conducts tests to explore the sense of smell of the two women who feel that they normally perceive odors. At the end of psychophysical tests, designed to evaluate the detection, identification and discrimination of odors, the researchers conclude that the two young women show normal results. The ultra-high resolution 3D MRI also confirms the complete absence of olfactory bulbs in the two women in question whereas these structures, considered essential for the olfaction, appear very clearly on the MRI in the participants of the control cohort. Assuming that the olfactory bulbs of these two women are extremely small (so invisible to the MRI), this would mean that they would occupy a space equivalent to 0.18% of their normal volume and contain only about 10 glomeruli, compared with 5,500 normally.