A test for ocular vestibular evoked myogenic potentials (oVEMP) had a sensitivity of 89% and a specificity of 64% for detecting myasthenia gravis (MG), according to a case-control study of 55 adults published online in Neurology.
“The presence of an oVEMP decrement is a sensitive and specific marker for MG,” said Dr. Yulia Valko at University Hospital Zurich in Switzerland and her associates. “This test allows direct and noninvasive examination of extraocular muscle activity, with similarly good diagnostic accuracy in ocular and generalized MG.”
Myasthenia gravis usually manifests first in the eyes, and early diagnosis and treatment can limit generalization. But nearly half of patients remain undiagnosed a year after onset, partly because standard tests often fail to detect isolated ocular MG, the researchers noted. The recently developed oVEMP test directly measures the activity of the extraocular inferior oblique muscle in response to repeated bursts of vibratory stimulation to the forehead. A decreased response, or decrement, indicates failed neuromuscular transmission, as with standard repetitive nerve stimulation. The researchers evaluated the test in 13 patients with isolated ocular MG, 14 patients with generalized MG, and 28 healthy controls. They defined the oVEMP decrement as the decrease between the second stimulus and the average of the fifth through ninth stimuli (Neurology. 2016 Jan 20. doi: 10.1212/WNL.0000000000002383).
A repetition rate of 20 Hz best differentiated between cases (average decrement, –21.5% plus or minus 29.6%) and controls (–2.8% plus or minus 16.9%), the researchers reported. When at least one eye showed a decrement, the ideal cutoff was a drop of at least 15.2%, which detected MG with a sensitivity of 89% and a sensitivity of 64%. When both eyes were affected, the ideal cutoff for the smallest of the two decrements was at least 20.4%, which yielded a sensitivity of 100% and a specificity of 63%. For both cutoffs, the test was similarly sensitive for detecting ocular and generalized MG, the investigators noted. For the unilateral cutoff, the sensitivity was 92% for patients with isolated ocular MG and 86% for patients with generalized MG. For the bilateral cutoff, specificity was 62% in ocular MG and 64% in generalized MG.
The results provide class III evidence that oVEMP can distinguish between patients with MG and healthy controls, “but future studies will need to confirm its diagnostic utility in clinical practice, where the main challenge is differentiation from patients with other neuro-opthalmologic conditions,” the researchers said. “The possibility to apply fast repetition rates is one important advantage of oVEMP, which is not possible by measuring voluntary saccadic eye movements. As a consequence, oVEMP allowed us to unmask myasthenic decrements even in clinically asymptomatic eyes,” they added.
Because the study used a confirmed diagnosis of MG as a benchmark, all patients were already being treated with cholinesterase inhibitors, the investigators noted. Although they underwent oVEMP testing in the morning before their first dose of medication, the test needs further study in drug-naïve patients, as well as in patients with worse limitations in their upward gaze, they added.
The study was funded by the University of Zurich, the Betty and David Koetser Foundation for Brain Research, the Albert Bruppacher Foundation for Eye Research, and the OPOS Foundation. The investigators had no relevant disclosures.