The Third Mobile Window Syndrome: A Clinical Spectrum of Different Anatomical Locations—Characterization, Therapeutic Response, and Implications in the Development of Endolymphatic Hydrops

<b>Background/Objectives:</b> Multiple dehiscences of the otic capsule can exhibit behavior similar to Ménière's disease, not only from a clinical perspective but also in the results of audiovestibular tests. The main objective of this study is to characterize third mobile window etiologies from an audiovestibular perspective, while also evaluating the therapeutic response to four different treatment protocols. Furthermore, we aim to explore a potential association with the development of radiologically defined endolymphatic hydrops (EH). <b>Methods</b>: This is a retrospective cohort study conducted from 2017 to 2024 at a tertiary-level otology and otoneurology unit. All patients underwent pure tone audiometry, vHIT, cVEMP, and oVEMP. Some of these patients, selected under rigorous inclusion criteria based on clinical and audiometric findings, were subjected to a 4-h delayed intravenous gadolinium-enhanced 3D-FLAIR MRI. <b>Results</b>: We obtained a sample of 86 patients, with a mean age of 52.2 ± 7.64 years: 62.76% were female (n = 54) and 37.21% were male (n = 32); 88.37% (n = 76) were diagnosed with superior semicircular canal dehiscence syndrome (SSCDS), while 11.62% (n = 10) had other forms of otic capsule dehiscence. The most common symptom observed was unsteadiness (44%). While surgery is the only curative treatment, other medical treatments, such as acetazolamide, also helped reduce symptoms such as autophony, falls, instability, and vertigo attacks, with a relative risk reduction (RRR) exceeding 75% (95% CI, <i>p</i> < 0.05). The results of the MRI in EH sequences indicate that 7.89% of the patients diagnosed with SSCDS also developed radiological EH, compared to 40.00% of the patients with other otic capsule dehiscences, a difference that was statistically significant (<i>p</i> = 0.0029. <b>Conclusions</b>: Otic capsule dehiscences are relatively unknown conditions that require clinical diagnosis. Although VEMP testing is useful, imaging studies are necessary to localize and characterize the defect, most commonly found in the superior semicircular canal. We should consider these dehiscences in cases where there is a suspicion of EH development. Further research, including in vivo neuroimaging studies using hydrops sequences, is required to better understand their relationship to potential Ménière's disease.