Cytokines, BDNF, and CGRP levels in tear fluid of migraine patients assessed using a novel non-invasive approach

Background: Recent research has focused on identifying innovative, non-invasive sources of migraine biomarkers, such as tear fluid and saliva, that may reflect underlying pathogenetic mechanisms, such as neurogenic inflammation. This study aimed to analyze the levels of interleukin (IL)-1β, IL-20, tumor necrosis factor-α (TNF-α), brain-derived neurotrophic factor (BDNF), and calcitonin gene-related peptide (CGRP) in the tear fluid of patients with migraine. Methods: Consecutive patients with a diagnosis of migraine were included. Tear fluid was collected from migraine patients and healthy controls (HCs) through Schirmer test strips; cytokines (IL-10, IL-1β, and TNF-α) and BDNF were measured using the automated EllaTM (Bio-Techne) multiplex Enzyme-Linked Immunosorbent Assay (ELISA) platform, while CGRP concentrations were quantified using a CGRP sandwich ELISA kit. Clinical characteristics of migraine patients, severity, and disability scores were collected. Results: Fifty-one patients with migraine (9 [17.6%] chronic, 16 with aura [31.4%]) and 17 age-matched HCs were included. Tear fluid CGRP concentrations were significantly elevated in migraine patients (7.4±7.6 pg/mL) compared to HCs (3.1±3.7 pg/mL; p=0.014). In the migraine group, tear CGRP levels were higher in the ictal phase (10.5±7.7 pg/mL) compared to the interictal phase (5.8±7.3 pg/mL) (p=0.021) and in patients with (10.4±9.2 pg/mL) vs. without (6.1±6.4 pg/mL) aura (p=0.042). BDNF tear levels did not differ between patients with migraine and HCs, but were higher in patients with chronic migraine (4.1±7.5 pg/mL) compared to episodic (0.6±2.5 pg/mL; p=0.018). Similarly, TNF-α tear levels did not differ between patients with migraine and HCs, but were higher in patients with chronic migraine (0.15±0.12 pg/mL) compared to episodic migraine (0.05±0.07 pg/mL; p=0.009). TNF-α levels were correlated with IL-10 (ρ=0.426, p<0.001), IL-1β (ρ=0.350, p=0.017), and BDNF (ρ=0.364, p=0.010). Additionally, IL-10 levels were correlated with BDNF levels (ρ=0.481, p<0.001). CGRP levels were negatively correlated with BDNF (ρ=−0.377, p=0.006). Conclusions: Elevated CGRP levels were found in patients with migraine as compared to HCs. Correlations observed among changes in tear fluid IL-10, IL-1β, and BDNF levels suggest common regulatory mechanisms. Furthermore, the increase in both tear fluid BDNF and TNF-α in patients with chronic migraine suggests a role for these biomarkers in the chronicization of the disease. While measuring biomarkers in tear fluid represents a rapid and promising non-invasive approach, it requires supporting evidence from future larger studies.