The Gut–inner ear axis in sensorineural hearing loss: evidence from Mendelian randomization studies – a systematic review

Abstract Introduction Sensorineural Hearing Loss (SNHL) is a global concern, affecting over 1.5 billion people. This systematic review investigates the emerging “Gut-Inner Ear Axis” by summarizing and evaluating the causal association between specific gut bacterial taxa and SNHL risk. Methods This systematic review (PROSPERO: CRD420251181365), conducted following PRISMA guidelines, searched PubMed, ScienceDirect, and Epitesmonikos for Mendelian Randomization on gut microbiota and SNHL in adults. Study quality was assessed using the JBI critical appraisal tool, and findings were descriptively synthesized based on Odds Ratios (ORs). Result Two two-sample Mendelian Randomization (MR) studies from China ( N = 334,733) were included, both having a low risk of bias. Nine gut bacterial taxa were causally associated with SNHL risk. Five were protective (OR < 1.0): Lachnospiraceae , Intestinimonas , Verrucomicrobia , Flavonifractor , and Streptococcaceae . Four were risk factors (OR > 1.0): Eubacterium , Bifidobacterium , Porphyromonadaceae , and Rikenellaceae . Only Rikenellaceae showed consistent replication as a risk factor across both studies. Conclusion The evidence supports a causal “Gut–Inner Ear Axis” in SNHL. Protective taxa likely function through anti-inflammatory SCFA production and barrier integrity, while risk factors may promote systemic inflammation and BLB dysfunction. High heterogeneity and low replication of taxa limit the findings, underscoring the need for future species-level research and targeted interventional trials.