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.