Superior Canal Dehiscence Linked to Abnormal ECoG Readings

While Paul Kileny, MD, PhD, of the University of Michigan Medical Schoo, was treating patients with superior semicircular canal dehiscence (SSCD), he noticed something a bit strange: The patients had abnormal readings on electrocochleographic tests. And not just some of them-all of them did.

The observation led a team of researchers in the university’s Department of Otolaryngology to do a retrospective study, led by H. Alexander Arts, MD, Professor of Neurosurgery and Otolaryngology, to look more closely. They validated the trend, and found that patients who had the disorder corrected reverted to normal readings on the tests conducted during surgery.

Abnormal readings in electrocochleographic (ECoG) tests, which measure the ear’s electrical response to sound, have traditionally been identified with Ménière’s disease, a disorder than can cause vertigo, buzzing in the ear, and hearing loss. But researchers say the findings might be cause for ear surgeons to think twice when they see a patient with these abnormal readings.

The team examined records from patients with 15 ears that had been diagnosed with superior canal dehiscence, a rare disorder first described only a decade ago. Almost across the board, the patients had abnormal readings on the ECoG test.

An abnormal reading is defined as a ratio of the summating potential to the action potential of greater than 0.4. In 14 of the ears with SSCD, the reading was greater than 0.4. In the other, it was right at 0.4.

We found, remarkably, that every single patient who had this superior semicircular canal dehiscence problem has a very abnormal electorocochleogram, said Dr. Arts, who presented preliminary findings at the 43rd annual meeting of the American Neurotology Society.

In superior semicircular canal dehiscence, the temporal bone overlaying that canal is either thin or missing. With the gap acting as a kind of third window to the inner ear, the disorder leads to hearing and balance problems. Those who have it suffer from oversensitivity to sound. They also exhibit lower thresholds on the vestibular-evoked myogenic potential tests.

It seems to be an acquired disorder because no children have been found to have it. But the cause is unknown.

Dr. Arts said that a busy neurotologist will see only about two or three case of the disorder per year. We’re just starting to look for it in earnest, he said.

In Ménière’s disease, the ECoG readings tend to fluctuate with the course of the disease and are more likely to be abnormal during periods when patients exhibit symptoms, the researchers noted in their study.