Using a library of almost 1,000 skull bones collected for more than 30 years at the Johns Hopkins Medical Institutions, researchers have identified a consistent congenital basis for a rare but troubling disorder they discovered in which loud noises cause dizziness. Their findings were presented at the Association for Research in Otolaryngology's February meeting in St. Petersburg Beach, Fla.
Called superior canal dehiscence syndrome, the disorder is caused by a hole, or dehiscence, in part of the skull's temporal bone that overlies an inner ear balance canal. Lloyd Minor, of the Johns Hopkins Center for Hearing and Balance, first discovered the disorder in 1995 and described it--along with a treatment--in 1998.
"Patients show specific symptoms, including vertigo--the sense that the room is moving--in response to loud music, the noisy environment of a sports event or the dial tone of a telephone," he says.
The bone collection used in his study, thought to be the most extensive of its kind in the world, was preserved by George T. Nager, past chair of the School of Medicine's Otolaryngology Department. All specimens came from cadavers with varying medical problems. Minor and colleagues used them to study the possible cause and incidence of the disorder. "We found that if the temporal bone in one ear was thin, the second ear showed the same thinness, an indication that the weakness is likely to be congenital if not inherited," says Minor, associate professor of otolaryngology-head and neck surgery, biomedical engineering and neuroscience. His team examined 989 specimens and found four with holes and 18 with thin sections in the bone.
One of Minor's first SCDS patients described seeing stationary objects move back and forth when he sang in the shower. Eye tests showed that tones played in his ear stimulated circular and upward eye motion. (A video of the eye movements can be seen on the Center for Hearing and Balance Web site, www.bme.jhu.edu/labs/chb.) These movements were in the plane of the superior semicircular canal, or top balance canal, so Minor and colleagues used a special CT scan to view the canal. There they found a tiny hole in the temporal bone.
Historically, the first description of eye movements evoked by sound came from Pietro Tullio, a turn-of-the century biologist, who studied the inner ear, or vestibular system, of pigeons. By drilling holes in their balance canals and using loud noises to stimulate eye movements, he provided the first evidence that balance disturbances are linked to the direction of eye and head movements.
Roughly half of the 15 SCDS patients Minor has diagnosed and treated can point to a specific event that brought on the symptoms. One fell off a bulldozer, one strained to pull a root in her garden and heard a loud "pop" in her ear, and violent coughing brought on the symptoms in another.
Treatment ranges from simple lifestyle adjustments, like wearing ear plugs, to surgery, depending on the degree of incapacitation. Minor's surgical approach is to plug the hole or cover it with a mixture of muscle tissue and bone dust. The material hardens to seal the canal and symptoms are reduced or cease altogether.
Minor and his team also have created an animal model--in a chinchilla--with SCDS to improve treatment techniques. "This is a classic example of many different investigations being completed in the right place at the right time," he says. "Without the bone collection, expertise and patients at Hopkins, it would have been difficult to take our initial finding this far."
Since Minor began describing SCDS, patients have been diagnosed elsewhere in the United States and abroad, including four patients in Australia and three in Sweden. The National Aeronautics and Space Administration has shown interest, as balance in humans is affected in microgravity.
This research was funded by the National Institutes of Health, American Otological Society and National Space Biomedical Research Institute. Nager and John Carey, a postdoctoral fellow in Minor's lab, were co-investigators in the study.