What are the symptoms of superior canal dehiscence syndrome?

Bob

Superior canal dehiscence syndrome is a newly described syndrome of chronic disequilibrium (poor balance) and vertigo (an abnormal sense of motion -- most commonly a spinning sensation, but vertigo can also apply to other movement sensations, such as bobbing or rocking).

In a 1998 article, a group of otolaryngologists at Johns Hopkins University School of Medicine reported on eight patients with "vertigo, oscillopsia and/or disequilibrium related to sound, changes in middle ear pressure, and/or changes in intracranial pressure." Oscillopsia is a symptom in which one's visual field oscillates (swings, bobs, rocks). Thus, these patients were experiencing sensations of imbalance, abnormal head motion and abnormal motion of their visual fields in response to loud noises and stimuli that increase middle ear pressure or intracranial pressure (pressure within the skull). Some examples of such stimuli: nose-blowing, sneezing, coughing, forcefully "popping" one's ears, straining to lift a heavy object or straining at a bowel movement.

You may be wondering what "superior canal dehiscence" means. "Superior canal" refers to the superior semicircular canal, a tiny fluid-filled channel that is part of the inner ear. As the name would suggest, the path traced by this canal is roughly semicircular. There are three semicircular canals in the inner ear. Altogether, the inner ear looks a great deal like a child's drawing of bagpipes. Instead of pipes, imagine three semicircular tubes coming out of the bag. The tubes are arranged at right angles to one other. This assemblage of fluid-filled tubes and chambers is encased in bone, which is called the temporal bone, and it makes up a portion of the skull base.

The semicircular canals contain sensory cells that detect motion of the fluid within each canal. Thanks to the geometric arrangement of the three canals, we are able to sense angular motion of our head. Depending on the type of head motion (for example, a nod vs. a back-and-forth head shake), the fluid in each canal will move to a different degree, so the sensory cells in each canal will be stimulated to a different degree. Your brain uses this information to determine, among other things, how best to move your eyes in relation to your head movements. Derangements of this system result in vertigo and oscillopsia.

Normally, there is a layer of bone separating the topmost portion of the superior canal from the cranial cavity. Dehiscence in this context means "an absence of bone," a gap in the bone that normally separates the canal from the cranial cavity. This abnormality is usually detected on a CT scan. When there is a dehiscence in the bone separating these two fluid compartments, then motion within the cranial cavity will be transferred to the canal. The two spaces do not actually share fluids -- membranes are present that isolate the two compartments. But the compartments may still bump into each other, causing fluid to move within the superior canal. When that happens, the brain assumes that its owner has just moved his or her head, and causes the eyes to move so as to track with this presumed head motion. This is a mistake -- the head has NOT really moved. Indeed, the brain is receiving discordant information: "My inner ear tells me I just moved, but that disagrees with the information I'm receiving from my eyes and from the postural sensors in the muscles of my neck." The result is the sensation of abnormal head motion (vertigo) and eye movement (oscillopsia).

The good news is that the superior canal dehiscence syndrome is fixable. A surgeon can plug the canal, forever stopping any fluid movement within it. He or she can also "resurface" the dehiscence (patch it, in other words). The bad news: These operations require a craniotomy. (A "window" of skull bone is removed, temporarily, to provide access to the relevant anatomy.) Thus, these are not minor operations. The appropriate specialist for this problem is a neurotologist, also known as a skull base surgeon.