Background: The Eustachian Tube (ET) is a mucosa-lined connection between the nasopharynx and the middle ear cavity. It is believed to have three functions: 1) ventilation of and pressure equalization in the middle ear cavity, 2) mucus drainage from the middle ear, and 3) protection against sound and infection from the nasopharynx1. In adults, Eustachian Tube Dysfunction (ETD) can cause complaints from one or both ears. For many years, various definitions of ETD have been used, impairing the opportunity to compare studies. However, in 2015, an international consensus on definition, types, clinical presentation, and diagnosis of ETD was published by Schilder et al2, which has been adopted by all the Scandinavian countries. The symptoms include pressure (fullness), and/or pain in the ear, muffled hearing, and overall discomfort. Furthermore, chronic ETD can result in tympanic membrane retraction, atelectasis of the middle ear cavity, and ultimately formation of cholesteatoma3. Unfortunately, the symptoms of ETD are multiple and inaccurate giving rise to varying estimates of the prevalence. As an example, a study in UK found a 0.9 % prevalence of ETD4. In addition, clear guidelines on diagnostics and treatment are not currently available due to the fact, that no objective test for detection of ETD exists. In the need of a symptom scoring system, the patient reported Eustachian Tube Dysfunction Questionnaire (ETDQ-7) has been developed and validated in English5. Yet, translation into other languages as well as validation in other settings are necessary in order to substantiate the applicability of ETDQ-7. ETD is associated with a lack of opening of ET. It is believed that the length, diameter and angle of ET influences its ability to open regularly, thus affecting its function. A short, narrow and angled ET may predispose to ETD. However, the imaging available to visualize ET are not accurate enough to diagnose ETD. A direct test of the function of ET is not available. Tubomanometry is a relatively new method developed to directly test the opening of ET, but is yet to be validated6. Both non-surgical and surgical treatment options to improve the function of the Eustachian Tube are available. Non-surgical management includes pressure equalization methods (e.g. the Valsalva maneuver), antihistamines, treatment with decongestants, and nasal douching with a saline solution. Surgically, ventilation tubes are often used to treat ETD. In case of adenoid hypertrophy obstructing the pharyngeal opening of ET, adenoidectomy is recommended. Balloon Eustachian Tuboplasty (BET) was introduced in 2010 by Ockermann et al7. BET is a non-invasive procedure performed under general anesthesia. During the procedure, a catheter is inserted either endonasally or transtympanic into ET, and a balloon is inflated with water for approximately two minutes. Various heterogeneous studies have shown a short-term effect of BET, but long-term effects are unclear8. In summary, despite the assumption of being a common condition, the field of ETD suffers from lack of precise definition, diagnostic criteria, identification of underlying causes as well as purposeful treatment, and prognostic factors. Especially, long-term effects of BET need further investigation. Therefore, in an effort to fill out the gap of knowledge about ETD, the following specific aims are proposed: