Third Window Syndrome, volume II

Book excerpt: In the initial Research Topic, Third Window Syndrome, we brought together recent discoveries of the mechanisms of the associated spectrum of symptoms, dysfunction, novel diagnostic tools and interventions to identify and resolve Third Window Syndrome. The initial Research Topic was truly global in effort and representation with four continents: Asia, Australia/Oceania, Europe, and North America. However, three were not represented: Africa, Antarctica, and South America. There were 15 countries represented: USA; Denmark; Israel; Korea; Germany; Australia; Switzerland; Belgium; Netherlands; Russia; Sweden; England/United Kingdom; New Zealand, Italy; and Japan. There were 118 authors. There were 20 published studies included in the initial Third Window Syndrome Research Topic and they fell within the following categories: Diagnostic Studies and New Diagnostic Tools; Cognitive or Spatial Orientation; Health Utility Values; Biomechanics and Pathophysiology; Reviews; Sites of Dehiscence – Rare or Never Before Reported; and Surgical Advances. Nearly a century ago, Tullio described the physiologic outcomes of creating a third mobile window in the semicircular canals of pigeons. Since that time, many locations of third mobile windows have been described; however, the sound-induced dizziness and/or nystagmus has been memorialized by the eponym ‘Tullio phenomenon.’ Clinically, the most thoroughly characterized third mobile window is superior semicircular canal dehiscence. In 1998, Minor and coworkers first reported the diagnosis of CT positive superior semicircular canal dehiscence (SSCD). Minor later reported a conductive hearing loss, which was recognized as a pseudoconductive hearing loss (bone-conduction hyperacusis), as well as a reduced cervical vestibular myogenic potential (cVEMP) threshold in patients with superior semicircular canal dehiscence. While superior semicircular canal dehiscence is well-recognized; it has been reported the existence of a CT negative third window syndrome with the same clinical phenotype of superior semicircular canal dehiscence exists. It has been reported that CT negative third window syndrome is associated with a pseudoconductive hearing loss and an abnormally reduced cVEMP threshold, among other objective findings typically found in superior semicircular canal dehiscence patients. The more general term of Third Window Syndrome has gained acceptance because the same spectrum of symptoms, signs on physical examination and audiological diagnostic findings are encountered with superior semicircular canal dehiscence, cochlea-facial nerve dehiscence, cochlea-internal carotid artery dehiscence, cochlea-internal auditory canal dehiscence, lateral semicircular canal-superior semicircular canal ampulla dehiscence, modiolus, “perilymph fistula,” posterior semicircular canal dehiscence, posterior semicircular canal-endolymphatic sac dehiscence, posterior semicircular canal-jugular bulb dehiscence, superior semicircular canal dehiscence-subarcuate artery dehiscence, superior semicircular canal dehiscence-superior petrosal vein dehiscence, vestibule-middle ear dehiscence, lateral semicircular canal-facial nerve dehiscence, wide vestibular aqueduct in children, posttraumatic hypermobile stapes footplate and in patients with CT negative Third Window Syndrome. A common structural finding in all of these conditions is an otic capsule defect that creates a ‘third window.’