Simulated laughter immediately improves lipid layer thickness and tear meniscus height.

PURPOSE: Dry eye disease is a common condition associated with mental health. Daily simulated laughter for eight weeks has been shown to alleviate symptoms and signs of dry eye disease. However, the underlying mechanisms remain unclear. This study aimed to evaluate the immediate effects of laughter on tear film lipid layer thickness and tear meniscus height. METHODS: This was a single-center, self-controlled before-after trial. Participants aged 18 to 45 years with ocular surface disease index scores above 13 and non-invasive tear film break-up time below 8 s were included in the dry eye disease group. The healthy group comprised individuals with no history or current diagnosis of dry eye disease. Baseline tear film lipid layer thickness (LLT) and tear meniscus height (TMH) were first obtained with the participant's facial muscles relaxed. Participants subsequently performed a 20 s simulated laughter; LLT was measured during the laughter, and TMH was acquired immediately upon completion. RESULTS: A total of 71 participants (mean age 28.5 ± 6.1 years; 55% [39/71] female) were enrolled, including 40 patients with dry eye disease and 31 healthy controls. A 20 s laughter increased the TMH (0.27 ± 0.11 mm vs. 0.23 ± 0.08 mm, P < 0.001) and improved the LLT-Min (56 ± 21 nm vs. 48 ± 18 nm, P < 0.001), LLT-Max (74 ± 20 nm vs. 66 ± 22 nm, P < 0.001), and LLT-Average (63 ± 21 nm vs. 55 ± 21 nm, P < 0.001). CONCLUSIONS: Laughter immediately increases TMH and tear film LLT in both dry eye disease patients and healthy individuals, suggesting that mechanical eyelid compression and other physical effects may contribute to the beneficial effects of simulated laughter in dry eye disease.