Rosacea and seborrheic dermatitis differentially respond to lactic acid sting and capsaicin tests in Chinese women

Although rosacea and seborrheic dermatitis share some symptoms of sensitive skin, whether they respond differently to lactic acid sting and capsaicin tests, common tests for diagnosis of sensitive skin, is unknown.


| INTRODUC TI ON
Sensitive skin is a skin condition that is hypersensitive to stimuli which do not cause any skin symptoms in normal skin.[7] Both capsaicin test (CAT) and lactic acid sting test (LAST) are commonly used to assess the skin sensitivity to stimuli.CAT can evaluate skin neurosensory responses to topical capsaicin.Because of the differences in skin physiology and pathophysiology among various skin conditions, cutaneous responses to capsaicin and lactic acid can vary with skin disorders even they share similar clinical features.
Rosacea and seborrheic dermatitis are common inflammatory dermatoses on the face.][10] Rosacea is often manifested by transient flushing, persistent erythema, telangiectasia, papules, pustules, and phymatous changes on the face. 11It is generally accepted that rosacea is caused by multiple factors including neurovascular dysfunction, innate and adaptive immune dysregulation in individuals with genetic background under endogenous and exogenous stimuli. 12Recently, evidence showed that defective epidermal permeability barrier in patients with rosacea can aggravate cutaneous inflammation. 8,13In addition, evidence also suggests a pathogenic role of TRPV1 in rosacea. 14In contrast, the factors involved in the pathogenesis of seborrheic dermatitis include Malassezia infection, defects in host immunity, increased sebaceous gland activity, defective epidermal permeability barrier function, inflammatory response, as well as exogenous risk factors. 15In addition to the face, other sebumenriched sites such as scalp and chest are often involved. 16Taken together, this line of evidence indicates that rosacea and seborrheic dermatitis share some clinical features, but with distinct pathogenesis.
In the present study, we compared basal epidermal permeability barrier function, erythema and cutaneous responses to LAST and CAT in individuals with rosacea versus those with seborrheic dermatitis, possibly elucidating additional pathomechanisms of these two conditions.

| Participants
A total of 60 patients with rosacea, 20 patients with seborrheic dermatitis and 40 normal controls were included in this study at dermatology department of our hospital from October 2020 to October All participants gave written informed consent, and the study was approved by the Institutional Review Board (approval ID: 2020-L-39).

| Measurement of epidermal permeability barrier function and erythema index
All participants were instructed to wash the face with warm water, and use sterile gauze to dry the skin and then rest for 15 min prior to the measurements.Neither topical medication nor skin care products were allowed to use on the face 12 h prior to the clinical visit.

| Lactic acid sting test
Lactic acid (concentration ≥ 98%, sigma Aldrich) was diluted into 10% lactic acid (V/V) with distilled water.A filter paper with a diameter of 0.8 cm was soaked with 50 μL of 10% lactate acid solution and placed on the left nasolabial fold.Afterwards, sting sensation scores were recorded at 0, 2.5 and 5 min, respectively, according to fourpoint method: 0, no sting sensation; 1, mild sting; 2, moderate sting; and 3, severe sting.Cumulative scores at 2.5 min and 5 min ≥3 were diagnosed as lactic acid sting test positive (LASTP), otherwise considered as lactic acid sting test negative (LASTN). 17

| Demographic characteristics of participants
All participants were females, aged 14-69 years.There were no significant differences in age among these three groups (Table 1).

| Response to LAST and CAT
We first compared the positive rates of LAST and CAT among three groups.The positive rates of LAST in individuals with rosacea were significantly higher than that in either normal controls (p < 0.0001) or subjects with seborrheic dermatitis (p < 0.05; Figure 1).Similarly, the positive rates of CAT were higher in subjects with rosacea than in normal controls and subjects with seborrheic dermatitis (p < 0.001 for both).Moreover, the rates of both LAST and CAT positive were also significantly higher in individuals with rosacea than in normal controls (p < 0.0001) and subjects with seborrheic dermatitis (p < 0.01), while the rates of both LAST and CAT negative were lower in subject with rosacea than in normal controls and those with seborrheic dermatitis (Figure 1).Surprisingly, the positive rates of LAST, CAT or both LAST and CAT did not differ between normal controls and subjects with seborrheic dermatitis although individuals with seborrheic dermatitis exhibit symptoms of sensitive skin. 18 next compared the LAST and CAT scores among the three groups.As shown in Figure 2, individuals with rosacea showed significantly higher LAST and CAT scores in comparison to either those with seborrheic dermatitis or normal controls.Moreover, CAT scores were higher in subjects with rosacea than those with seborrheic dermatitis (p < 0.0001).However, LAST scores in subjects with seborrheic dermatitis did not differ significantly from that either in normal controls or rosacea patients.These results demonstrate that subjects with rosacea are more sensitive to stimulation with either LAST or CAT than normal individuals or those with seborrheic dermatitis.

| TWEL levels and erythema index are associated with cutaneous response to LAST and CAT
Previous studies showed that epidermal permeability barrier is a determinant of cutaneous response to LAST and CAT. 19,20Thus, we determined next whether the differences in LAST and CAT among three conditions reflect in different TEWL rates.Indeed, baseline TEWL rates were higher in subjects with rosacea than in normal controls (p < 0.0001), while TEWL rates did not differ significantly between subject with seborrheic dermatitis and normal controls or individuals with rosacea (Figure 3A).Moreover, both LAST and CAT scores correlated positively with TEWL rates (p < 0.0001; Figure 3B).
Together, these results demonstrate that subjects with rosacea display higher TEWL rates, which likely contribute to the high positive rates and severity of LAST and CAT.
Since sensitive skin can display erythema, 21 we compared erythema index among the three groups, and analyzed correlation of erythema index with LAST and CAT scores.Erythema indexes were significantly higher in individuals with rosacea (476.78 ± 14.02) than in normal controls (284.31± 8.69, p < 0.0001 vs. rosacea and seborrheic dermatitis) or in individuals with seborrheic dermatitis (402.80 ± 18.77, p = 0.0053 vs. rosacea).Interestingly, erythema index correlated positively with CAT scores (p < 0.0001), but not with LAST scores (p = 0.0842; Figure 3C).These results indicate that erythema index is associated with cutaneous response to CAT.

| DISCUSS ION
Previous studies demonstrated that the prevalence of sensitive skin symptoms is higher in subjects with rosacea, 22 while LAST and CAT are commonly used to determine whether individuals have sensitive skin. 23,24In agreement with previous studies, 25,26 we demonstrated here that the positive rates of both LAST and CAT were significantly higher in subjects with rosacea than in normal controls.Likewise, excessive blood flow, 28,29 which can contribute to increased cutaneous response to both LAST and CAT in individuals with rosacea.
The higher positive rate and scores of CAT in individuals with rosacea can be due to the neural hyperreactivity.The burning intensity caused by the capsaicin is associated with the baseline current perception threshold (CPT) values. 30Individuals with rosacea have higher expression levels of TRPV1 mRNA compared with normal controls. 31Capsaicin can activate TRPV1 channel on the skin, resulting the release of neurogenic inflammatory factors, including calcitonin gene-related peptide (CGRP) and substance P (SP), which in turn increase inflammatory cytokines and nitric oxide, leading to the development of neurogenic inflammation, accompanied by uncomfortable sensation such as burning and erythema. 32Hence, in addition to permeability barrier dysfunction mentioned above, neural hyperreactivity can contribute to the higher CAT positive rate and scores in subjects with rosacea.
TEWL rate is widely accepted as an indicator of epidermal permeability barrier function.A higher TEWL rate indicates a poorer permeability barrier. 33We showed here that baseline TEWL rates were significantly higher in subjects with rosacea than in normal controls, consistent with previous study. 34But baseline TEWL rates in subjects with seborrheic dermatitis were moderately higher than that in normal controls, in contradictory to previous report. 35This discrepancy can be attributed to subjects' gender difference between the present study and others.All participants in the present study were females while the other study included both males and females.Females generally exhibit higher TEWL rates than males. 36us, difference in TEWL may not be observed in females with versus without seborrheic dermatitis when normal females already have higher baseline TEWL.The differences in TEWL among three groups and the neurovascular hyperreactivity can contribute to the differences in cutaneous responses to LAST and CAT.This assumption is supported by the following evidence, (1) individuals with LAST positive exhibited higher TEWL rates than those with LAST negative 37 ; (2) individuals with CAT positive exhibited higher TEWL rates than those with CAT negative. 20(3) TWEL values were negatively correlated with CPT values. 38(4) The baseline CPT values were lower in both LAST and CAT positive group than that in both LAST and CAT negative group. 30Defective epidermal permeability barrier not only enhances substance penetration, but also induces inflammation, making the peripheral nerves hypersensitive to stimuli such as lactic acid and capsaicin. 39Therefore, the different responses to LAST and CAT could be attributable to the differences in epidermal permeability barrier and the neurovascular hyperreactivity between rosacea and seborrheic dermatitis.However, further studies are needed to validate this speculation.
In conclusions, the present study demonstrates the cutaneous responses to LAST and CAT differ between individuals with rosacea and seborrheic dermatitis, likely due to the differences in the neurovascular hyperreactivity and epidermal permeability barrier function between these two skin conditions.These results further suggest the importance of improving epidermal permeability barrier in the management of skin conditions with sensitive skin symptoms such 2021.Because epidermal function varies with gender and more females than males seek skin care, only females were enrolled in the present study.The diagnosis of rosacea was made in accordance with the diagnostic criteria for rosacea established by the National Rosacea Society Expert Committee(NRSEC). 11The inclusion criteria included: (a) the clinical manifestations of patients are mainly erythema or flushing.(b) patients aged at least 14 years old; (c) patients with no topical treatment for ≥1 month.The exclusion criteria included patients with other facial skin diseases, such as acne, melasma, facial eczema, and so on.Seborrheic dermatitis was diagnosed according to its typical clinical manifestations.Patients with any other facial skin disorders were excluded.Normal controls had no facial skin disorders nor inflammatory dermatoses such as psoriasis and eczematous dermatitis.

K
E Y W O R D S capsaicin test, lactic acid sting test, rosacea, seborrheic dermatitis, transepidermal water loss | 3507 HU et al.the right infraorbital area with 10% alcohol swab for 5 times, 50 μL of 0.001% capsaicin was dripped on a two-layer filter paper with the diameter of 0.8 cm and placed on infraorbital region 1 cm away from the nasolabial fold.The intensity of perception of burning was evaluated according to the following scales: (1 point, doubtful, barely perceptible; 2 points, slightly perceptible; 3 points, moderate perceptible; 4 points, strong perceptible; 5 points, pain).The participants with a burning sensation on capsaicin-treated area ≥3 points and the duration >30 s were considered as capsaicin test positive, otherwise the capsaicin test is negative.

2. 5 |
Statistics analysisSPSS 26.0 statistical software was used for statistical analysis.Data are expressed as mean ± SEM.Some non-normal distribution data were presented as the median and quartile.Kruskal-wallis was used to compare difference among three groups.The numeration data were represented by frequency and percentage, either Fisher's exact test or Chi-square test was used in group comparison.

TA B L E 1
Demographic characteristics of participants.Age Normal controls (N = 40) 29.50 (24.25, 40.75)Rosacea (N = 60) 35.00 (25.25, 45.00) Seborrheic dermatitis (N = 20) 26.50 (22.25, 35.00)Significance H = 7.054, p = 0.070 Note: Kruskal-wallis was used to determine the significances.Data are expressed as the median and quartile [Median (P25, P75)].LAST scores and CAT scores were significantly higher in subjects with rosacea than in normal controls.The higher positive rates and scores of LAST and CAT can be attributable to both compromised epidermal permeability barrier and increased erythema.Induction of sting sensation requires substances to penetrate the stratum corneum into the skin.A poor permeability barrier in the stratum corneum can enhance the penetration of substance such as lactic acid and capsaicin.TEWL was higher in individuals with rosacea than in that with seborrheic dermatitis or normal controls.In addition, TEWL correlated positively with LAST scores, consistent with the previous study. 27Therefore, the defective epidermal permeability barrier can account for the higher LAST positive rates and scores of LAST and CAT.Moreover, individuals with rosacea have a higher erythema index, an indicator of inflammation in sensitive skin and F I G U R E 1 Response to LAST and CAT in the three groups.Fisher's exact test was used to determine significances.Data are expressed as percentage.CAT+, CAT positive; LAST+, LAST positive; LAST & CAT+, both LAST and CAT positive; LAST & CAT−, both LAST and CAT negative.F I G U R E 2 LAST and CAT scores among three groups.Kruskalwallis was used to determine significances.Data are expressed as mean ± SEM.F I G U R E 3 (A) Comparison of the baseline TEWL rates in three groups.Kruskal-wallis was used to determine significances.Data are expressed as mean ± SEM. (B) Correlation of LAST and CAT scores with TEWL rates.(C) Correlation of LAST and CAT scores with erythema index.Pearson's test was used to determine significances.R and p values are indicated in the figures.
as rosacea.Additionally, improvement in erythema is also essential to decrease skin sensitivity in subjects with sensitive skin.The sample size of 20 for seborrheic dermatitis is not enough in the present study.Larger sample of skin disorders including rosacea and seborrheic dermatitis are needed in the following study.If necessary, another study will be performed to observe the response to LAST and CAT before and after improvement of epidermal permeability barrier and erythema.