Bast's valve: what we know and what we don't

To synthesize current knowledge on the utriculo-endolymphatic valve (UEV, Bast's valve)-a flap-like structure at the junction of the utricular and endolymphatic ducts and to address the central question of whether it represents a passive anatomical remnant or an active regulator of endolymphatic homeostasis with implications for disorders such as Ménière's disease.

A critical narrative review of anatomical, imaging, experimental, computational, developmental, comparative, and pathological studies was conducted. Literature was identified through PubMed/MEDLINE, Embase, Web of Science, Scopus, and citation tracking. Sources included human temporal bone reconstructions, animal models, finite element simulations, hydrostatic measurements, and clinical reports examining associations between valve morphology and endolymphatic hydrops.

The UEV is consistently identified across species and developmental stages, with asymmetric morphology favoring directional flow. Advanced imaging (micro-CT, SR-PCI) and reconstructions confirm its structural plasticity, while animal and computational models support a check-valve role in modulating endolymph exchange. Pathological observations implicate valve dysfunction in hydrops distribution in MD, though clinical correlations remain inconsistent. No in vivo studies have captured valve dynamics, and interindividual variability remains underexplored.

The UEV is an evolutionarily conserved feature of the vestibular labyrinth, likely contributing to inner-ear hydrodynamics. Whether it functions as an active regulator or a passive remnant remains unresolved. Progress will depend on ultra-high-resolution imaging, computational modeling, and prospective clinical studies to clarify its role and potential value for diagnosing and managing hydrops-related disorders.