Salivary Melatonin Levels in Comparison with Disease Features and Antihistamine Use: Prospective Follow-Up of Atopic Dermatitis Patients

Background: Atopic dermatitis (AD) is a common inflammatory dermatosis often accompanied by itching and sleep disturbances. Various treatment options exist, including antihistamines (AHs), even though current clinical recommendations do not mention them. Methods: This prospective exploratory study measured and recorded salivary melatonin at two timepoints (T1 and T2) in three groups of participants: those not treated with AHs, those who discontinued H1-AHs, and those for whom H1-AHs were introduced. Also assessed were disease severity (Scoring Atopic Dermatitis/SCORAD and Eczema Area and Severity Index/EASI), pruritus severity (Peak Pruritus Numeric Rating Scale/NRS), and sleep quality (Pittsburgh Sleep Quality Index/PSQI). Results: The EASI scores were significantly inversely correlated with salivary melatonin levels (p = 0.009) at the first timepoint (T1), but not at the second timepoint (T2). At T2, SCORAD correlated with EASI, NRS, and PSQI (ρ = 0.304–0.842; p ≤ 0.022). EASI correlated with NRS and PSQI (ρ = 0.281–0.453; p ≤ 0.035). Those who discontinued AHs showed increased AD severity indicators (SCORAD, EASI, NRS), whereas participants for whom AHs were introduced showed decreases in the same scores. After adjusting for baseline values, a significant main effect was detected for the treatment group for T2 SCORAD scores (p < 0.001, η2 = 0.493). Pairwise comparisons confirmed significant differences among all treatment arms/measures (all p values ≤ 0.023). Changes in melatonin levels between the three groups were not significant when baseline SCORAD values were controlled for. Conclusions: Our exploratory study observed an inverse association between melatonin levels and AD severity at T1, but this was not replicated at T2. This association should be interpreted with caution due to study limitations. Changes in AD severity and pruritus scores following AH initiation or discontinuation were observed, but the study’s non-randomized design and baseline differences between groups preclude a definitive causal inference, and further research on this topic is needed.