BDNF levels in T2DM hearing-impaired patients: audiology and inflammation correlates

Purpose To investigate the correlation between serum brain-derived neurotrophic factor (BDNF) and hearing impairment in patients with type 2 diabetes mellitus (T2DM), and to explore its relationship with inflammatory cytokines. Methods This cross-sectional case-control study enrolled 97 T2DM patients (October 2023–June 2024), stratified by pure-tone average (PTA) into Group A (hearing impairment, PTA > 25 dB HL, n = 44) and Group B (normal hearing, PTA ≤ 25 dB HL, n = 53), alongside 57 healthy controls (Group C). Serum BDNF, pure-tone audiometry, and distortion product otoacoustic emissions (DPOAE) were assessed. An independent cohort (30 T2DM, 32 controls; July–October 2024) assessed BDNF and inflammatory cytokines. Results Hearing thresholds at all tested frequencies (500–8000 Hz) were significantly higher in Group A than in Groups B and C (all P < 0.001), with correspondingly lower DPOAE amplitudes. Serum BDNF was similarly reduced in both diabetic groups (Group A: 4.07 ± 1.02; Group B: 4.53 ± 0.94 ng/mL) versus controls (11.77 ± 7.92 ng/mL; P < 0.001). In Group A, BDNF correlated negatively with BMI (r = − 0.531), lipid profiles (TG r = − 0.379, TC r = − 0.398, LDL-C r = − 0.691, HDL-C r = − 0.577), fasting glucose (r = − 0.633), HbA 1c (r = − 0.672), diabetes duration (r = − 0.931), and mean hearing threshold (r = − 0.852; all P < 0.05). In the exploratory cohort, T2DM patients exhibited lower BDNF, elevated pro-inflammatory cytokines (TNF-α, IL-6, IL-1β, MCP-1; all P < 0.001), and reduced IL-10 (P < 0.001). BDNF negatively correlated with pro-inflammatory cytokines and positively with IL-10 only in T2DM (all P < 0.05). Conclusion Serum BDNF is decreased in T2DM and inversely associated with glycemic control, disease duration, and hearing impairment severity. BDNF reduction also correlates with pro- inflammatory changes. Although BDNF reduction is a general feature of T2DM rather than being specific to hearing impairment, within hearing-impaired T2DM patients, BDNF levels correlate strongly with auditory dysfunction, suggesting that BDNF may serve as a potential biomarker for T2DM-related hearing impairment. However, causal inference is limited by the cross-sectional design.