4. Discussion
This study found that old age, sex (male), DM, subjective olfactory
dysfunction, and objective diagnosis of olfactory dysfunction were
associated with the objective test-based diagnosis of gustatory
dysfunction by the univariate analysis, and old age, sex (male), and
objective diagnosis of olfactory dysfunction were significant factors in
the multivariate analysis. Additionally, it found that in subjects aged
60 years or older, the threshold score was more significantly correlated
with the objective diagnosis of gustatory dysfunction rather than the
other subsets in the olfactory function test or a diagnosis of olfactory
dysfunction based on the TDI score. It has been reported that subjective
recognition of gustatory function did not correspond to the gustatory
function test-based objective gustatory function.6Similarly, subjective recognition of gustatory dysfunction was not
associated with the objective diagnosis of gustatory dysfunction in this
study. Furthermore, the characteristics associated with subjective
gustatory dysfunction differed from those with objective gustatory
dysfunction (Table 2, Supplementary Table 1). Therefore, other
referencing characteristics, except for patient discomfort, associated
with objective gustatory dysfunction are important for clinicians to
determine the application of the gustatory function test.
Variable factors, including endocrinological problems, are known to be
associated with gustatory dysfunction.12, 15 It was
found that the prevalence of xerostomia was 46.09% among diabetic
patients, and salivary flow rates were lower in DM patients than in
non-DM patients.16 DM disturbs the hemostasis of the
oral cavity by altering salivary function and composition even in
well-controlled patients and increases the risk of burning mouth
syndrome.17 Although the exact pathogenic mechanisms
have not been identified, these factors could affect the gustatory
function in diabetic patients. We suggest that care should be applied to
DM patients regarding gustatory dysfunction regardless of subjective
complaints of gustatory dysfunction.
It has been reported that with the increase in age, gustatory function
tends to decrease, and studies have reported an age-related decrease in
taste function.8 This study also found that age was
significantly associated with objective gustatory dysfunction. The
interesting finding of this study was that the association between
objective olfactory function and objective gustatory dysfunction was
different between subjects aged less than 60 years old and subjects aged
60 years and older (Table 3). In the older patients (age ≥ 60), the
threshold score of the olfactory function test was significantly
associated with objective gustatory dysfunction rather than other
subsets and the final diagnosis of olfactory dysfunction. However, in
patients under 60, the final diagnosis of olfactory function based on
the TDI score was significantly associated with objective gustatory
dysfunction. Among the olfactory function subsets, performance on the
odor identification tests is dependent on verbal abilities, and the
results can be influenced by cognitive and language
functions.18 Although this study excluded patients
previously diagnosed with cognitive impairments, there is a possibility
that in the older subjects, naturally occurring cognitive impairments
could have affected the odor identification test, causing low
identification, TDI scores, and the final diagnosis of olfactory
dysfunction. Therefore, we suggest that in older patients, the threshold
subset score should be carefully reviewed when interpreting olfactory
function tests, and clinicians should consider performing a gustatory
function test regardless of the patient’s subjective symptoms.
In this study, the prevalence of objective gustatory dysfunction in
patients subjectively complaining of chemosensory dysfunction was 17.8%
(39 out of 219 patients). In a previous study, Deems et al. reported
that the prevalence of taste loss was 8.7% among patients with
complaints concerning smell and taste.9 These studies
imply that a few subjects who complained of chemosensory discomfort were
objectively diagnosed with gustatory dysfunction. However, the results
of these studies on gustatory dysfunction are heterogeneous, which might
be due to the heterogeneity of the applied gustatory function test. This
study applied the chemical gustatory function test based on the various
concentrations of five taste solutions and the application of the
solutions. The study by Deems et al. utilized a whole-mouth test, which
used suprathreshold concentrations of liquid taste
solutions.9 Filter paper discs/strips impregnated with
a taste solution are also frequently utilized in other
countries.19 Although these chemical gustatory
function tests are regarded to be ‘objective’ function tests, there is a
possibility that these are not really ‘objective’ tests. These tests
enable the numerical measurement of gustatory function and are
objectively compared with a patient’s subjective complaints. However,
subjective factors, such as a patient’s will, could be involved during
the test procedure. Furthermore, there was a previous study where the
correlation level was low even among the currently applied ‘objective’
gustatory function tests.20 Although more objective
gustatory function tests, such as a functional MRI or gustatory evoked
potentials, have been introduced, they cannot be commonly applied in the
usual clinical field.9,17 Future studies with larger
populations based on a single gustatory function test procedures are
needed to suggest further the actual prevalence and characteristics of
objective gustatory dysfunction in patients with subjective chemosensory
impairments.
This study has several limitations. First, this was a retrospective
study based on electronic medical records. Second, this study did not
consider all possible candidate factors that could have affected
objective gustatory dysfunction. Although the study tried to collect a
lot of information, including previously diagnosed medical history and
smoking history, other factors, such as burning mouth syndrome and
previous medication histories, were not evaluated. Finally, a chemical
gustatory function test, which only diagnosed the patient’s quantitative
function, was applied. Since the currently applied gustatory function
test sums up the score of five taste solutions, the final diagnosis was
based on the summed recognition threshold score. Therefore, qualitative
gustatory dysfunction, such as parageusia, was not considered in this
study.