Introduction: The prevalence of post-viral olfactory dysfunction has increased significantly during the COVID-19 pandemic, posing a major challenge for patients and practitioners. While olfactory training (OT) is a common approach to therapy, there has been increasing interest in supplementing therapy with a combination of palmitoylethanolamide (PEA) and luteolin (LUT), which are known for their anti-inflammatory properties. In this study, their efficacy in the treatment of patients with olfactory loss following upper respiratory tract infections, mainly COVID-19, was investigated in an outpatient clinic. Methods: Fifty patients with persistent olfactory dysfunction were randomized to two groups: one receiving OT and PEA-LUT, the other OT alone. Olfactory function was evaluated before and after treatment. Results: The study group showed significant improvements in odor discrimination and overall olfactory function (TDI score) after treatment with PEA-LUT and OT, while the control group did not. However, when clinically meaningful improvements were considered, there was no significant difference between the groups. Conclusion: The present study suggests that while PEA-LUT may have the potential to improve olfactory function in post-viral dysfunction, the additional benefit over OT alone may be limited. These results contrast with some previous studies.

Post-viral olfactory disorders increased significantly during the COVID-19 pandemic [1]. Although new variants are less frequently associated with persistent olfactory dysfunction [2], more patients seek advice for olfactory and taste disorders. This results in the search for new treatment options for patients with post-viral olfactory dysfunction in addition to olfactory training (OT) [3]. A promising approach is the treatment of corona-associated neuroinflammation with palmitoylethanolamide (PEA) and luteolin (LUT) in combination with OT [4‒7].

The rationale behind using PEA-LUT lies in the observation that both the olfactory bulb and higher olfactory centers may show neuroinflammation upon infection with SARS-CoV2 [8]. PEA-LUT has been reported to exhibit a reduction of neuroinflammation by affecting microglia and reducing oxidative stress [9]. This approach could also be useful in support of the regeneration of olfactory function after olfactory loss due to infections of the upper respiratory tract.

A first randomized clinical trial from 2021 reported preliminary results of a study comparing olfactory rehabilitation therapy with an intervention treatment involving PEA-LUT for 12 patients experiencing olfactory dysfunction after COVID-19. It showed a greater improvement in olfactory function in the intervention group [4]. A second article presented the results of a longitudinal study that investigated the effects of Ultra-Micronized PEA-LUT on olfaction and memory in patients with long-term COVID-19. The study revealed a superior effect of the treatment with OT and additional PEA-LUT over PEA-LUT alone [5]. The same study group published an article that discusses a multi-center double-blinded randomized placebo-controlled clinical trial that combined Ultra-Micronized PEA-LUT with OT to treat post-COVID-19 olfactory impairment of 185 patients. In this case, PEA-LUT in combination with OT demonstrated a more significant improvement in the rehabilitation of the sense of smell than OT alone [7]. While the other studies primarily focused on quantitative olfactory impairments, a fourth study examined the impact of PEA-LUT and OT on patients with parosmia after COVID-19 revealing a positive effect of the treatment on quantitative olfactory disorder but not on qualitative olfactory disorders like parosmia [6].

Overall, these studies indicate a positive impact of PEA-LUT in combination with OT on the quantitative olfactory function of patients following post-COVID-19 olfactory dysfunction. However, this therapeutic option has not seen widespread application to date. The study aimed to investigate the use of PEA-LUT as additional treatment in our outpatient olfaction and taste clinic in comparison to OT alone for patients suffering from olfactory loss due to upper respiratory tract infection, mostly SARS-CoV2 infection.

The study design was approved by the Institutional Review Board of the University Clinic of the TU Dresden (EK122032011) and was conducted according to the principles expressed in the Declaration of Helsinki. Possible risks and benefits related to participation in the study were explained to patients during the initial consultation. All participants provided their written informed consent.

Patients

Patients with persistent olfactory dysfunction after COVID-19 were recruited for this study. The evaluation of Covid disease as the cause of the olfactory disorder was evaluated anamnestically. A virological assessment was not carried out after the disease had passed. Exclusion criteria included pre-existing olfactory dysfunction, neurological or psychiatric disorders, previous head and neck surgery or radiation therapy, history of head trauma, chronic rhinosinusitis, and allergic rhinitis. After nasal endoscopy, the olfactory function was assessed psychophysically using the Sniffin’ Sticks test (Burghart Messtechnik, Holm, Germany).

After olfactory testing, patients were randomized to one of two treatment groups in a 2:1 design: the study group received OT and additional PEA-LUT medication for an average of 3.4 months, while the control group received OT only. Olfactory function was then re-evaluated using the same psychophysical olfactory tests (see Fig. 1).

Fig. 1.

Consort diagram showing enrollment, intervention, and follow-up of patients participating in the clinical study. PEA-LUT, palmitoylethanolamide and luteolin.

Fig. 1.

Consort diagram showing enrollment, intervention, and follow-up of patients participating in the clinical study. PEA-LUT, palmitoylethanolamide and luteolin.

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Olfactory Testing

Olfactory function was assessed psychophysically using the Sniffin’ Sticks test at both baseline and follow-up visits, which is a well-established and validated test for the assessment of olfaction [10]. As described in detail previously [11, 12], patients underwent three distinct 16-item olfactory subtests for each of the following functions: odor threshold (T), odor discrimination (D), and odor identification (I). The scores from these three subtests were combined to calculate the composite TDI score, with a maximum possible score of 48. Anosmia was defined as a score of 16 or lower, while a score below 30.75 indicated hyposmia.

Olfactory Training

At the initial visit, all participants were given instructions on how to perform OT. Each patient was provided with four glass jars, each containing one of the following odorants: phenyl ethyl alcohol (rose), eucalyptol (eucalyptus), citronellal (lemon), and eugenol (clove). Following a well-established routine, they were instructed to inhale each odorant for 30 s, both morning and evening, while fully concentrating on the odor [13, 14].

Palmitoylethanolamide and Luteolin

Patients of the study group received daily treatment with PEA-LUT, an oral supplement. This single-dose supplement containing PEA 700 mg and Luteolin 70 mg (Glialia®, Epitech Group SpA, Milano, Italy) was supposed to be administered before breakfast each day. This corresponds to the same dose used in earlier studies by the Italian research group [7].

Statistical Analysis

Data analysis was performed using SPSS Statistics software (version 28/29, IBM, Armonk, NY, USA). Continuous variables are presented as mean ± standard deviation (SD), and a significance level of p < 0.05 was considered statistically significant. Unpaired two-tailed Student’s t tests were used to assess the statistical significance of continuous data. For categorical data, χ2 and Fisher’s exact test were used to compare between groups. Whenever appropriate, a non-parametric test (Mann-Whitney U test or Wilcoxon test) was applied. To control data for age and duration of olfactory regression analysis has been applied before t test assessment.

A total of 63 patients were screened for participation in this longitudinal case-control study. Thirteen patients had to be excluded because they did not meet the inclusion criteria (8 did not exhibit olfactory loss due to upper respiratory tract infection, and 5 had olfactory function in the normosmic range [12]). Hence 50 patients with olfactory loss due to upper respiratory tract infection were enrolled in this study. Most of them complained about olfactory loss after COVID-19 (88%). Psychophysical olfactory testing was performed at the first outpatient visit. After consenting to participate, patients were randomly assigned to two different study groups. Both groups were instructed to perform OT as part of their daily routine. The study group (n = 34) additionally received PEA-LUT for daily single-dose use while the control group (n = 16) executed OT only. After four to 6 months, the patients revisited the outpatient clinic for a follow-up visit. For a second visit, 32 patients attended (study group n = 18, control group n = 14) and were included in the final analysis. Twenty-two of the included patients were women (69%) and 10 men (31%). The gender distribution between the groups was balanced (χ12 = 0.23, p = 0.63, φ = −0.085). Also, regarding age, the two groups were similar (study group 54 years ±11, control group 46 years ±13, p = 0.06). Regarding the duration of olfactory function before treatment, the study group exhibited a significantly longer duration of olfactory loss compared to the control group (study group M = 21.6 ± 17 months, control group M = 12.8 ± 5 months, Kolmogorov-Smirnov p = 0.089, U = 64.5, Z = −2.35, p = 0.019 using Mann-Whitney U test).

In the olfactory assessment before the intervention, a significant difference in olfactory threshold (study group M = 3.2 ± 2.6, control group M = 5.0 ± 2.4, Kolmogorov-Smirnov p = 0.14, U = 75.0, Z = −1.968, p = 0.049 using Mann-Whitney U test) could be observed while all other parameters of assessment were not significantly different (see Table 1). Although the age difference between the groups was insignificant, there was a clear discrepancy between the final groups included in the analysis. The duration of the olfactory disorder was also identified as a potential confounding variable. The data were therefore reanalyzed using regression analysis and the residuals of the regression were compared using a t test. The results of the independent t test show no significant differences between the groups regarding the odor threshold, after controlling for age (F = 1.047, p = 0.3, Cohen’s d = 2.233). The results of the regression analysis indicate that age has a significant impact on the initial odor threshold (p = 0.008), while group membership (p = 0.4) and duration of complaints (p = 0.3) do not have a significant impact. The results for all other initial parameters remained insignificant.

Table 1.

Demographics, clinical information, and details of olfactory assessment before and after four to 6 months of therapy for all participants who attend both visits

OT + PEA-LUTOTSignificance level
Female, n (%) 13 (72.2) 9 (64.3) n.s. 
Age, years 53.6±11 45.5±13 n.s. 
Duration of hyposmia, months 21.6±17 12.8±5 p = 0.019 
Duration until follow-up, months 5.2±1.25 5.1±0.7 n.s. 
Pre-therapeutic threshold 3.2±2.7 5.0±2.4 p = 0.049 
Pre-therapeutic discrimination 8.5±3.2 9.1±2.8 n.s. 
Pre-therapeutic identification 8.2±3.3 9.5±2.8 n.s. 
Pre-therapeutic TDI 19.9±7.6 23.6±6.0 n.s. 
∆ threshold 0.9±3.2 0.3±2.4 n.s. 
∆ discrimination 2.6±3.4 1.4±3.1 n.s. 
∆ identification 1.3±2.9 0.64±2.8 n.s. 
∆ TDI 4.9±6.5 2.4±5.6 n.s. 
OT + PEA-LUTOTSignificance level
Female, n (%) 13 (72.2) 9 (64.3) n.s. 
Age, years 53.6±11 45.5±13 n.s. 
Duration of hyposmia, months 21.6±17 12.8±5 p = 0.019 
Duration until follow-up, months 5.2±1.25 5.1±0.7 n.s. 
Pre-therapeutic threshold 3.2±2.7 5.0±2.4 p = 0.049 
Pre-therapeutic discrimination 8.5±3.2 9.1±2.8 n.s. 
Pre-therapeutic identification 8.2±3.3 9.5±2.8 n.s. 
Pre-therapeutic TDI 19.9±7.6 23.6±6.0 n.s. 
∆ threshold 0.9±3.2 0.3±2.4 n.s. 
∆ discrimination 2.6±3.4 1.4±3.1 n.s. 
∆ identification 1.3±2.9 0.64±2.8 n.s. 
∆ TDI 4.9±6.5 2.4±5.6 n.s. 

n.s., not significant.

After 3.4 ± 1.2 months of treatment with PEA-LUT, the second visit with a follow-up examination took place on average 5.2 ± 1 month after the first examination. The study group improved significantly regarding TDI (pre-therapeutic TDI = 19.9 ± 7.6 and post-therapeutic TDI = 24.8 ± 8.8, sign test p = 0.031 n = 18) and subtest of odor discrimination (pre-therapeutic D = 8.5 ± 3.2 and post-therapeutic D = 11.1 ± 3.6, sign test p = 0.049 n = 18), while olfactory threshold and odor identification did not change significantly. The control group did not significantly improve on any of the olfactory tests after OT. However, there was no significant difference in the change in the TDI and its subscores between patients who received PEA-LUT in addition to OT and those who did not. A one-way MANOVA found no statistically significant difference between the change of the three subscores of TDI and combined TDI (F(1, 30) = 1.31, p = 0.26 η2 = 0.042, Wilk’s Λ = 0.736) (see Fig. 2).

Fig. 2.

Effect of OT alone versus the effect of PEA-LUT and OT on TDI score to the first and the second visit of patients displayed in individual lines, Box-Whisker-Plot, and frequency curves.

Fig. 2.

Effect of OT alone versus the effect of PEA-LUT and OT on TDI score to the first and the second visit of patients displayed in individual lines, Box-Whisker-Plot, and frequency curves.

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For the calculation difference in improvement between the groups from the first visit to the second visit again the covariables age and duration of complaints were eliminated. Regression analysis revealed no significant influence of age or duration of complaints on the improvement from the first to the second visit. No group difference could be displayed (∆ T p = 0.12, ∆ D p = 0.7, ∆ I p = 0.9, ∆ TDI p = 0.8) after controlling for age and duration of complaints.

Considering that a 5.5-point improvement in the TDI score represents a significant clinical improvement in a participant's olfactory function [11], the groups were further analyzed. Seven patients improved in the study group (53.8%) and 6 patients improved in the control group (46.2%), so again no significant difference was found between the two groups.

As there were 7 anosmic patients and 11 hyposmic patients in the intervention group (defined by an initial TDI below 16 [12]), we performed a Fischer exact test for a significant clinical improvement of at least 5.5 points. This showed no significant difference in clinical improvement between the subgroups (anosmic 3/7 clinically improved, hyposmic 4/11, p = 0.583).

The present study investigated the use of PEA-LUT in combination with OT to treat post-viral olfactory loss, which has become a widespread problem, particularly among COVID-19 patients. Olfactory loss due to upper respiratory tract infections, although known for a long time among specialists, has become a significant problem during the COVID-19 pandemic. This has led to an increased demand for treatment options. OT is already being used [3], but there is interest in whether the addition of PEA-LUT can accelerate olfactory recovery.

PEA and LUT are both well-tolerated with no reported toxicity [9, 15]. The combination of PEA-LUT is considered a promising treatment option due to its reported ability to reduce neuroinflammation and oxidative stress in the olfactory centers of the brain and the olfactory bulb. Previous studies have shown positive results in improving the sense of smell in patients who received these substances in combination with OT [4‒6]. PEA is a fatty acid amide synthesized and hydrolyzed by microglia. It can downregulate mast cell activation, leading to a reduction in neuroinflammation. In addition, PEA attenuates the activation of M1-microglia in the brain while enhancing the activation of the M2-phenotype, resulting in a reduction of inflammation [16, 17]. Luteolin primarily exerts its anti-inflammatory effect by regulating transcription factors such as NF-κB, STAT3 and AP-1. These regulatory actions mediate anti-inflammatory and antioxidant effects, safeguarding neural tissue from inflammation [18]. The exact pharmacokinetics of PEA-LUT in the olfactory bulb have not yet been investigated and therefore remain speculative.

The present study compares patients who receive OT alone with patients who receive PEA-LUT in addition. The results indicate that the group receiving PEA-LUT showed a significant improvement in olfactory function measured through TDI, especially regarding discrimination while the control group did not. However, there was no significant difference between the groups in terms of clinically meaningful improvement in olfactory function. These results suggest that PEA-LUT in combination with OT may lead to some improvement in olfactory function in patients with post-viral olfactory loss. However, the additional benefit of OT alone appears to be minimal and thus contrasts with the studies already published strongly [4‒6].

It is important to note that this exploratory study may have some limitations that might have influenced the contrasting results, including a small sample size as a main limitation. Methods need to be established to avoid high dropout rates and to keep participants motivated. Possible variations could be a monthly telephone update or an online diary to track progress. Also, the variability in the duration of olfactory loss before treatment and the study groups had different baseline TDI scores. After the elimination of the dropouts, there was an unequal distribution of anosmic and hyposmic patients. Compensation by block randomization of anosmic and hyposmic patients could avoid such a bias in follow-up studies. Further research is needed to better understand the possible effects of this treatment option and to determine its suitability for broader use in clinical practice. These studies should also be placebo-controlled and not open-labeled, like this study with an exploratory design. Overall, the study provides insight into the limited efficacy of PEA-LUT in the treatment of olfactory loss in COVID-19 patients. It underscores the need for further research to clarify its role in clinical practice.

In summary, a recommendation to use PEA-LUT should be limited to patients with COVID-19-related olfactory loss, if at all appropriate. In any case, patients must be informed that the individual benefit beyond olfactory training is unlikely to be clinically relevant and that they will have to bear the costs of treatment with PEA-LUT themselves.

We are indebted to Agnieszka Sabiniewicz for her help with data analysis and presentation.

The study design was approved by the Institutional Review Board of the University Clinic of the TU Dresden (EK122032011) and was conducted according to the principles expressed in the Declaration of Helsinki. Possible risks and benefits related to participation in the study were explained to patients during the initial consultation. All participants provided their written informed consent.

The authors have no conflicts of interest to declare.

J.G. was supported with a scholarship for the habilitation from the Medical Faculty Carl Gustav Carus in Dresden with full funding of their position for 1 year up to and including 03/24.

Conceptualization: T.H. Data collection: L.C., T.H., and A.H. Data analysis: J.G. and T.H. Writing, review, and editing: J.G., T.H., L.C., and A.H. All authors have read and agreed to the published version of the manuscript.

The data underlying this article cannot be shared publicly due to due to the data protection law of the country of origin. All data generated or analyzed during this study are included in this article. Further inquiries can be directed to the corresponding author.

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