Discussion
Olfactory disorders can be classified as conductive, sensorineural, or due to an impairment in the olfactory central nervous system. The vast majority of olfactory dysfunction occurs as a result of upper respiratory tract infection (18–45%), sinonasal disease (7–56%), or head trauma (8–20%), whereas the prevalence of congenital anosmia is low (0.4%).2
Synopsis of key findings
Clinical investigation of olfactory disorders involves examination for chronic nasal obstruction, with or without associated infection, as well as the performance of olfactory functional tests. In the cases presented here, nasal endoscopic examination was unremarkable, but olfactory functional tests revealed anosmia. Chemical and electro-gustatory test results indicated normogeusia.
Imaging is not routinely performed for olfactory dysfunction, as findings are negative in most cases. Imaging may be beneficial in certain inflammatory, structural, neurodegenerative, traumatic, and neoplastic conditions. MRI scans can confirm congenital agenesis of olfactory bulbs in cases such as those with Kallmann syndrome.3,4 We performed MRI scans for these 6 patients because detailed histories and physical examinations were negative. MRI results were suggestive of congenital agenesis of the olfactory bulb.
Agenesis of the olfactory bulb does not necessarily mean olfactory dysfunction. A recent study revealed that human olfactory function can be normal without an olfactory bulb apparent on structural MRI.5 Therefore, objective tests for olfaction are needed. We recorded OERPs alongside the functional MRI scan to provide a highly objective assessment of olfactory function. Functional MRI can identify areas in the brain activated in response to olfactory stimulation, and OERPs can indicate residual olfactory function.6 In our study, 5 patients exhibited no activation in response to olfactory stimulation across the whole brain when family-wise error was corrected for (P = 0.05). No OERPs were observed in response to olfactory stimulation. These functional MRI findings and OERP results provide objective evidence for anosmia in these patients. Our study has clinical significance as objective olfactory tests such as these have not previously been described in patients with congenital anosmia.
Agenesis of the olfactory bulb is conventionally associated with Kallmann syndrome. However, Powell recently reported that congenital anosmia is more frequently an isolated diagnosis.7 We therefore assessed the baseline levels of follicle-stimulating hormone, luteinizing hormone, and testosterone to eliminate the possibility of Kallmann syndrome, although signs of puberty were not delayed or absent in these patients. Endocrinological results were within normal limits.
Clinical applicability of the study
In the absence of other structural and hormonal abnormalities, patients with congenital anomalies of the olfactory bulb can be diagnosed with isolated congenital anosmia. Patients with congenital anosmia should be carefully investigated to exclude more serious conditions such as Kallmann syndrome before making this diagnosis. However, agenesis of the olfactory bulb identified on structural MRI does not necessarily mean olfactory dysfunction. Therefore, to further evaluate congenital anosmic patients, simultaneous functional MRI and OERP recording can detect activated areas in the brain and help to provide high objective evidence for the diagnosis of anosmia.