The Third Window & Superior Semicircular Canal Dehiscence
Superior Semicircular Canal Dehiscence (SCD) is (essentially) a break in the petrous portion of the temporal bone between the superior canal and the brain. Rosowski (2012) reports that the "hallmark" of SCD is vertigo produced secondary to (typically) loud sounds, or vertigo, which appears to be "pressure induced." In these cases, high resolution CT scans can often help confirm the diagnosis. Of note, some patients with SCD do present with hearing loss and no complaint of vertigo. SCD patients with vertigo as their chief complaint may present with mild low frequency air-bone gap (at 250 and 500 Hz).
In general, for patients with SCD and hearing loss only, surgery is not offered (as surgical repair involves opening the brainstem). However, for patients with debilitating vertigo and hearing loss, patching or plugging (i.e., repairing) the bony opening has been shown (in some cases) to reduce the vertigo and improve hearing.
Rosowski notes in a recent study of otologic patients with conductive hearing loss (and with intact tympanic membranes), greater than 15 percent were eventually diagnosed with "third window" disorders. Other semicircular canals have demonstrated dehiscences and other "third windows" are known to occur in enlarged vestibular aqueduct syndrome and Paget disease and more.
For More Information, References, and Recommendations
Chi FL, Ren DD, Dai CF. (2009) Variety of Audiologic Manifestations in Patients with Superior Semicircular Canal Dehiscence. Otology & Neurotology. 31(1):2-10.
Rosowski J. (2012) Peripheral Anatomy and Physiology- Outer and Middle Ear. In Translational Perspectives in Auditory Neuroscience. Editors: Kelly L. Tremblay and Robert. F. Burkard. Published by Plural Publishing.