A CACOPHONY OF BODILY FUNCTIONS: Superior canal dehiscence syndrome (SCDS) is caused by a small hole in the bone covering part of the inner ear (in blue). Such a hole results in distortion of hearing and, often, impaired balance. Image: COURTESY OF ERICSPHOTOGRAPHY, VIA ISTOCKPHOT.COM
It sounds like something out of an Edgar Allan Poe tale of horror.* A man becomes agitated by strange sounds only to find that they are emanating from inside his own body—his heart, his pulse, the very movement of his eyes in their sockets. Yet superior canal dehiscence syndrome (SCDS) is a very real affliction caused by a small hole in the bone covering part of the inner ear. Such a breach results in distortion of hearing and, often, impaired balance.
The human ear consists of three parts. The outer ear includes the ear lobe and external auditory canal, which funnels sound waves toward the eardrum (or tympanic membrane) allowing it to vibrate. The middle ear converts sound waves that vibrate the eardrum into mechanical vibrations for the cochlea, the hearing part of the inner ear. This area, however, also includes of a system of three fluid-filled semicircular canals in each ear—superior, posterior and horizontal—responsible for giving the brain information about angular motion of the head. SCDS can occur when some part of the bone protecting the superior semicircular canal is missing.
Whereas it is difficult to know exactly how prevalent SCDS is, several reported cases define how it impacts the lives of those suffering from the disorder. Stephen Mabbutt, a 57-year-old Englishman who suffered from SCDS for six years, described "hearing his eyes scratching like sandpaper every time they moved in their sockets." He returned to work earlier this month after successful surgery to plug a pin-size hole in the bone covering the semicircular superior canal in one of his ears. Toby Spencer, a 41-year-old IT professional from Skowhegan, Maine, described similar symptoms as Mabbutt as well as the feeling that loud noises made him feel as though he was losing his balance. Spencer had surgery in April to correct the problem. [Read more about inner ear ailments and remedies: "Regaining Balance with Bionic Ears"]
Musician Adrian McLeish suffered from the disorder for more than two decades, even having to give up his livelihood playing the French horn, before in 2008 seeing Lloyd Minor, provost and senior vice president for academic affairs at Johns Hopkins University in Baltimore. McLeish sought out Minor, a professor of otolaryngology—head and neck surgery—because he is the lead author on the study in 1998 providing the first description of SCDS in Archives of Otolaryngology—Head & Neck Surgery.
McLeish described (video) not only hearing a buzzing, distorted version of his own voice but also being able to feel the vibrations of the sound whenever he spoke. Essentially, the missing piece of bone in McLeish's left ear was allowing the superior semicircular canal to respond to sound and pressure in abnormal ways. It was enough for the canal to act as an amplifier for every sound in his body. McLeish's horror story came to a happy ending soon after meeting with Minor, who corrected the problem and allowed the musician to resume his career.
Scientific American spoke with Minor about the nature of SCDS, the toll it takes on those who suffer from it and why it's best to plug (rather than cover up) such an opening.
[An edited transcript of the interview follows.]
What is superior canal dehiscence syndrome (SCDS)?
Superior canal dehiscence syndrome is a disorder caused by an opening in the bone that should cover the inner ear's top balance canal (called the superior semicircular canal). The inner ear consists of a bony labyrinth that has two parts: the cochlea, which enables us to hear, and the vestibular labyrinth, which enables us to keep our balance. Located within the labyrinth are three semicircular canals that act as angular accelerometers for the head. When these are working properly, they allow us to maintain a steady gaze and keep images stable on our retinas even when our heads are moving.
The inner ear is a closed system surrounded and protected by the petrous portion of the temporal bone. In superior canal dehiscence, a portion of the bone covering the superior balance canal is missing. Without this bone, the membranous canal is exposed to the overlying dura mater membrane of the brain's temporal lobe. Mechanical stimuli such as sound and pressure can then cause motion of the fluid in the membranous canal resulting in abnormal neural activity from the sensory receptor cells associated with the canal. This can lead to hearing loss, abnormal eye movements and a sudden sensation of movement (vertigo) as a result of loud noises or pressure, such as coughing or sneezing.
How were you able to first identify SCDS?
It was a study of the eye movements. The eye movements of this disorder, which is evoked by sound and pressure, are quite distinctive. Because the superior semicircular canal is affected, the eye movement is typically vertical-torsional, meaning the eye closest to the affected ear will twitch in a direction up and away from that ear (toward the middle of the face) in response to loud noises or by stimuli that change middle ear or intracranial pressure (such as coughing, sneezing or straining).
Can someone be born with SCDS or is this something that afflicts people only later in life?
Our hypothesis is that about 1 or 2 percent of the population fails to develop a normal thickness of bone overlying the superior canal. If you have a normal thickness of bone, which is about 0.6 or 0.7 millimeter covering the superior canal, that's very unlikely to ever erode. If you started out with 0.1 or 0.2 millimeter of bone, yes, that can be eroded over time for a number of reasons: pressure from the temporal lobe sitting on top of it, changes in intracranial pressure or maybe by trauma. The median age of onset is in the early 40s, and it does tend to affect men and women equally. But you rarely see it in kids. That suggests that the opening probably doesn't develop until later in life.
*Editor's Note (7/30/13): The misspelling of Edgar Allan Poe's name has been corrected.