Causes of taste disorders are multitudinous and can be attributed to the common cold, zinc deficiency, olfactory dysfunction, and systemic, oral, iatrogenic, idiopathic, and psychogenic diseases [8]. In our department, the subjective symptom classification of taste disorder is classified as described in Table 2, whereas the cause was classified as described in Table 1. Conventionally, most of the subjective symptoms of taste disorder include hypogeusia, with other studies reporting a few occurrences of phantogeusia, different taste, and dissociable taste disorders, which are types of taste disorders. Our department has reported that phantogeusia accounts for around 40% of the subjective symptoms, followed by hypogeusia [5]. Phantogeusia has been treated as a psychogenic disease, given its specific complaint emphasizes psychogenic involvement. However, the rates of oral candidiasis at the Department of Oral Medicine, Hokkaido University Hospital has been quite high [5]. Similarly, this report administered an antifungal drug as the initial drug to seven cases. Patients who visit the dentistry department undergo examination for oral candidiasis to determine the cause of dysgeusia. In the future, as the social structure becomes more complex and the population ages, we expect an increase in the number of patients coming in for complaints of subjective abnormal taste due to two factors, psychogenic and candida. Therefore, it is necessary to consider oral disease as a possible cause without being bound by psychogenic prejudice for cases complaining of constant bitterness or saltiness despite nothing in the oral cavity. Moreover, there are cases in which it is difficult to diagnose and identify the cause due to various complaints from patients. The reason is that there are cases where taste disorder is suspected to be caused by multiple predispositions. Moreover, given that the symptoms of dysgeusia are not cause-specific, the therapeutic effect cannot be evaluated unless confirmed by a taste test or similar methods. Considering the numerous difficult cases, our department systematically treats dysgeusia, as shown in Fig. 1. Treatment for the majority of these issues is well established, but effective options for managing idiopathic and psychogenic taste disorders remain to be unidentified [5]. Within the realm of otolaryngology, zinc replacement therapy has traditionally been recommended for patients with idiopathic and psychogenic dysgeusia [11–13], which is why our department has actively administered polaprezinc. Unfortunately, improvement rates have not exceeded 22%.
With this background, one contemporary investigation by Takahashi et al. [1] reported that EL was successful at correcting or improving abnormal taste functions emerging spontaneously, whereas another study by Shimura et al. [4] found that EL specifically stimulated the gustatory cortex. Moreover, unlike other benzodiazepine drugs and polaprezinc [16, 17, 18, 19], EL has been known to act rapidly and last for quite a long time (i.e., half-life of 122 h), prevents or treats anxiety, and appears to have minimal side effects (e.g., less memory loss and drowsiness) [14, 15]. Furthermore, El is safe for older individuals and has been well documented for treating anxiety and dental psychosomatic disorders [1–3]. Thus, we believed that EL would be an ideal therapy for the cases presented to our hospital.
In this study, there was an overall improvement rate in taste disorders of 63% (31/49): 55% (12/22) for patients with idiopathic dysgeusia and 70% (19/27 for patients with psychogenic dysgeusia. In addition, 78% (12/31) of those for whom EL was effective experienced improvement within 2 weeks of administration. Surprisingly, no significant differences in improvement rates by age, gender, duration of the disease, subjective symptoms, cause of dysgeusia, period of EL administration, length of time to experience an effect from EL, serum zinc values, or severity of taste disorder were observed. Prior research, in particular, has demonstrated that upon treatment, the duration of a disease decreases significantly after ≥ 6 months and it takes a while for patients to experience improvements [11], yet for our cohort, there was no significant difference in improvement rates between patients with a course of > 1 year vs. <1 year. This finding bears witness to the expediency of EL over other medications commonly employed in such instances.
Concerning lack of improvement or side effects, the incidence rate was 27% (15/49). Five patients chose to discontinue EL treatment in favor of different drugs because they experienced no alteration in their condition, and the rest predominantly had mild sleepiness. Written on the medical packaging for EL are the main side effects, including psychiatric disorders (e.g., drowsiness and reduced cognition and concentration) and central and peripheral nervous system disorders (e.g., stabilization and speech problems, and head sensations), and a minor side effect, that is, loss of taste. Importantly for the latter, the frequency of loss of taste is described as < 0.1%, but we could not locate evidence of this phenomenon in the literature nor did anyone in our cohort complain of this. A mere amount of 1 mg of EL was administered to our patients because the drug’s manufacturers state that the daily dosage for insomnia is 2 mg. Hence, this would likely induce strong sleepiness. Going forward, healthcare professionals should be diligent in informing their patients regarding the aforementioned side effects and monitoring them carefully after EL administration. One limitation of the current study is the small number of cases. We plan to accumulate more cases in the future and consider undertaking a prospective study.