Laird Harrison

February 25, 2015

CORONADO, California — Ocular vestibular-evoked myogenic potentials could be the basis of a new test for myasthenia gravis, according to the results of a recent study.

"We're very excited because it's a simple trick that kind of opens up a new door," lead researcher Konrad Weber, MD, from University Hospital Zurich, told Medscape Medical News.

He presented the finding here at the North American Neuro-Ophthalmology Society 2015 Annual Meeting.

Early intervention can slow or even stop the progress of myasthenia gravis; however, diagnosis can be difficult. Typically, eye dysfunction is the first manifestation of the disease, but eye muscles are hard to access for testing.

Antibody tests produce false-negative results in half of patients with the disorder. Other tests are technically difficult or cumbersome, and some can only be done in specially equipped academic centers.

"This is one of the biggest dilemmas we have," said session moderator Matthew Thurtell, MBBS, from the University of Iowa in Iowa City. "None of the testing we do is good at pinpointing whether a patient has myasthenia," he told Medscape Medical News.

 
We're very excited because it's a simple trick that kind of opens up a new door.
 

To address this problem, Dr Weber and his colleagues examined the use of ocular vestibular-evoked myogenic potentials in 27 people with myasthenia gravis and a control group of 28 healthy people.

Typically, as muscles fatigue in patients with myasthenia gravis, there is a decrease in their response to stimulation. The measurement of these potentials was developed to assess vestibular function, but the researchers adapted the approach to detect a decrement in the extraocular muscles of myasthenia patients.

The researchers applied a vibrating cylinder to the foreheads of study participants as a stimulus and measured the response of the extraocular muscles with electromyography.

The response to 10 stimulus vibrations at three repetition rates — 3 Hz, 10 Hz, and 20 Hz — was evaluated in all study participants.

The difference between the myasthenia and control groups was largest at 20 Hz.

In the myasthenia patients, there was a rapid decline in the amplitude of response from the second to the ninth stimulus. In the control group, the response was steady over the 10 stimulus vibrations.

Sensitivity was greatest in tests of individual eyes, whereas specificity was greatest in tests of both eyes. Ten of 13 patients with isolated ocular myasthenia showed a decrement in response.

Table. Change in Response From Stimulus Vibrations 2 to 9 at 20 Hz

Eyes Change, % Specificity, % Sensitivity, %
One –15.2 79 76
Both –20.4 100 63

 

The researchers conclude that myogenic potentials are useful for detecting myasthenia, especially in its isolated ocular form.

A member of the audience asked about the control population.

"It was a mixed population," said Dr Weber. "They were a tiny bit younger than the myasthenia patients."

Another audience member asked whether the patients were taking pyridostigmine.

"We had a mix of what we could get, but we tried to withdraw them from pyridostigmine overnight," Dr Weber reported.

Any neuromuscular clinic could implement this test, he explained, but he cautioned that this is only a proof-of-concept study. The team has plans to try it on patients who have not yet received a diagnosis to confirm the findings.

This is "phenomenal," said Dr Thurtell. "It's a really creative use of a diagnostic test."

This study was funded by the University of Zurich. Dr Weber and Dr Thurtell have disclosed no relevant financial relationships.

North American Neuro-Ophthalmology Society (NANOS) 2015 Annual Meeting. Presented February 24, 2015.

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