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.