Assessing extraocular muscle activity by recording ocular vestibular evoked myogenic potentials (oVEMP) is a promising test for isolated ocular myasthenia gravis (MG), report researchers.
The test, which records activity of the inferior oblique muscle in response to vibratory stimulation of the forehead, was as sensitive for isolated ocular MG as it was for generalised MG, making it potentially better than traditional diagnostic tests, which tend to miss isolated ocular MG, say Konrad Weber (University of Zurich, Switzerland) and study co-authors.
In an accompanying editorial, Sashank Prasad (Harvard Medical School, Boston, Massachusetts, USA) and G Michael Halmagyi (University of Sydney, New South Wales, Australia) note that although sensitivity may increase further as the test is refined “oVEMP already shows promise as a new diagnostic test for [ocular] MG.”
But they add that “cohort studies are necessary to assess its value in clinical practice, where a broader range of diagnostic possibilities leaves the clinician uncertain.”
The study included 13 patients with isolated MG, 14 with generalised MG and 28 healthy control participants. Between the second stimulation and the average of the fifth to ninth stimulations, the patients had an average response reduction of 21.5%, compared with just 2.8% in the controls.
If just one eye was affected, the researchers found the ideal decrement cutoff to be 15.2%, giving a sensitivity of 89% and a specificity of 64%. When both eyes were affected, so only the one with the smallest decrement was considered, a cutoff of 20.4% gave the best sensitivity and specificity, of 63% and 100%, respectively.
Using the unilateral cutoff, sensitivity was comparable for ocular and generalised MG patients, at 92% and 86%, respectively, and the same was true for the bilateral cutoff, with corresponding values of 62% and 64%.
The patients had previously undergone other diagnostic tests, all of which were more successful in those with generalised MG. For example, acetylcholine receptor antibodies were detected in 93%, compared with 54% of ocular MG patients, 100% versus 43% had a positive repetitive nerve stimulation test and 100% versus 78% had a positive edrophonium test.
“Compared to other diagnostic procedures, oVEMP has the unique advantage of providing direct evidence of the myasthenic process in affected extraocular muscles”, write Weber et al in Neurology.
They add that it “is a simple, noninvasive, and fairly inexpensive tool that can be linked to any standard EMG equipment and therefore easily implemented in any clinical electrophysiology unit.”
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