In a normal inner ear, sound waves travel from a membrane, the oval window, through the cochlea to another membrane at the far end of the cochlea, the round window. Sound waves need this second window in order to flow through the ear. In superior canal dehiscence, the hole in the canal acts as another window, allowing sound to pass into the canal and tricking it into signaling spinning. SSCD is sometimes called third window syndrome for this reason. The hole also has other serious effects.
The fluid inside the inner ear, the endolymph, is held within the thin lining of the inner ear. Between the inner ear and the skull is a separate fluid, the perilymph. Perilymph fills the bone cavity that holds the inner ear, cushioning it so the delicate membranes do not come in contact with bone. Since the ear senses very tiny fluid motions or pressure changes as spinning, it’s important that pressures between the endolymph and perilymph are kept equal.
The perilymph is similar in many ways to the cerebrospinal fluid (CSF), the fluid around the brain that also cushions it within the skull. There are some very long, narrow tubes connecting the perilymph space to CSF inside the skull that allow only a very slight communication between these spaces so the pressures of the pulsing brain cannot be directly transmitted to the ear. Having a large hole in the canal bypasses these valves, so the perilymph and CSF are easily able to pass pressure waves back and forth from inner ear to brain and vice versa.
This means that pressure changes in the head, from the pulse, breathing, coughing, sneezing, and straining can be sent directly through the hole to the inner ear, causing vertigo. It also means that the pressures between endolymph and perilymph cannot be kept the same, so the rest of the inner ear can also malfunction. For these reasons, large openings that cause many symptoms should be repaired.
A CT scan (CAT scan) can show the missing bone and confirm the leak. VEMP tests use loud sound to test for balance responses; an unusually large response, especially at a lower level of sound than usually causes a response, can indicate a leak. Once this is found, surgery can be considered if your leak is causing symptoms.
One way to fix this is to open the skull and place a patch over the hole. Another method is to sacrifice the canal by plugging it, which can be done by drilling near the ear to locate the canal. Once the canal is plugged or patched, symptoms should resolve. Plugging prevents the affected canal from sensing vertical head motion, but you will still have three other vertical canals to do this work, so it’s hard to detect changes for most people.
Our next post will be an Ask the Doctor segment–let me know your questions!
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