A “Double” Third Window Syndrome: The Case of Semicircular Canal Dehiscence in Twin Sisters

Third window lesions are defined as abnormal communications of the inner ear with adjacent spaces, namely the middle ear or cranial cavity. These lesions can occur at multiple anatomic locations, including the superior, posterior, and lateral semicircular canals; vestibule and vestibular aqueduct; and scala vestibuli of the cochlea. Semicircular canal dehiscence (SCD) is the most common condition wherein the temporal bone adjacent to the vestibular apparatus thins or is interrupted completely. Normally, there are two mobile windows: the oval window and round window. SCD results in a third mobile window in the inner ear, which causes an abnormal transmission of acoustic energy (“third-window effect”) towards the vestibular end-organs and causes symptoms. Superior SCD is a well-described pathological condition with potentially debilitating symptoms, instead the dehiscence of the posterior canal is very rare. The diagnosis is difficult and often delayed because the symptoms can be absent or aspecific and confused with other diseases: vestibular disorders, cerebrovascular diseases, trauma, and tumors. We report a rare case about a “double” third window syndrome of twin sisters due to a bone defect of two different semicircular canals. A 34-year-old woman presented to the Otology Department of University Hospital with a worsening of tinnitus, autophony and vertigo. She had been treated for 3 years as if affected to Menière disease. An audiometric exam revealed a right mixed hearing loss with 20–40 dB air bone gap from 250 to 4000 Hz. Vestibular examinations are unchanged compared to the past exams. But Valsalva maneuver induced pathological oscillopsia, vertigo and torsional up-beating nystagmus. Clinical manifestations may be like other diseases, such as Menière disease or perilymphatic fistula, confounding the diagnosis. Moreover, the appearance of the same symptoms in the patient’s twin sister, confirmed by the radiological investigation, suggested the familiarity for semicircular canal dehiscence. Audiometric and vestibular examinations are important even if diagnosis is impossible without radiological investigation. Indeed, we describe the history, clinical profile and management of twin sisters who had similar symptoms (worsening vertigo induced by pressure, autophony, tinnitus and hearing loss) and different bony labyrinth defects. Surgical management resulted in complete resolution in imbalance 3 months after the surgery. This surprising case of homozygous twin sisters suggests that there may be a genetic aspect to the disease.