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.

Study Results

The patients in the study were typical SSCD patients, with typical air-bone gap readings-the largest gaps were at lower frequencies.

The VEMP readings were also typical-an average of 68 dB in the ears with SSCD, compared with an average of 82 dB in the ears that were not affected.

On ECoG tests, the average score for ears with the disorder was 0.77. The six ears without it had lower scores-three lower than 0.4 and two at 0.41 and 0.45.

One ear had a relatively high reading-0.65-but the researchers found with CT scans that the bone overlaying the canal was thin on that side.

The SP/AP ratios improved after surgery, according to the records from the four patients for which those pre- and post-scores were available.

One patient’s ratio dropped from 0.62 to 0.25, another dropped from 0.84 to 0.36, another fell from 0.56 to 0.32, and sanother from 1.48 to less than 0.1.

ECoG was continuously performed during the canal-occlusion procedure for one of the patients. The results show a steady improved as it was performed-scores fell at one point from 0.86 to 0.69 to 0.68 before the occlusion then moved to 0.2, 0.28, and 0.32 afterward.

The VEMP thresholds also improved, rising an average of 17.5 dB in the four patients for whom pre- and post-procedure readings were available.

Implications of the Results

Dr. Arts said he hopes the findings leads to better understanding of the disorder. I’m really hopeful that this helps us understand what we’re measuring in electrocochleography and that it will help us understand what’s really going on in SSCD, he said.

But he hopes there’s a more basic scientific benefit as well. I hope it will lead to a better understanding of the physiology of the inner ear, he said.

Jay T. Rubinstein, MD, PhD, Professor of Otolaryngology and Bioengineering and Director of the Virginia Merrill Bloedel Hearing Research Center at the University of Washington, said the study will have an immediate effect at his clinic.

He said he is going to start performing ECoG tests on all patients undergoing procedures for SSCD, because it seems to be a reliable indication that the procedure has been successful.

I do quite a few of these surgeries, he said. I’m now going to start doing this on all of our patients. It’s not that critical for making the diagnosis, although it helps, but intraoperatively it seems to suggest that you’ve closed the dehiscence.

He said the findings also might mean that fewer CT scans will be needed to get at the root of SSCD cases.

If it turns out that this is, in fact, a reliable measure, it will help avoid doing a lot of CT scans, Dr. Rubinstein said. We see a lot of patients with atypical readings, and it might be due to this. And a CT scan is not an insignificant radiation dose.

He said the findings might shed new light on cases in which the typical diagnosis has been Ménière’s disease.

I don’t know that it’s really going to teach us that much about the inner ear, per se, but it could explain a lot clinically, he said. It could be that a lot of people [with abnormal ECoG readings] didn’t have Ménière’s disease, they really had this.

It just helps with our diagnostic precision to just help present this thing better-again, if this pans out in a bigger population of patients.

©2009 The Triological Society