Re-evaluation of Repetitive Nerve Stimulation Test in Myasthenia Gravis and Myasthenic Syndrome

Suh, Bum Chun; Choi, Byung Ok; Cheon, Hwa Young; Kim, Seung Min; SunWoo, Il Nam

Abstract

BACKGROUND: The repetitive nerve stimulation (RNS) test is a useful tool in the evaluation of neuromuscular transmission disorders. In our laboratory, we frequently use Oh's method, which tests 5 kinds of muscles (flexor carpi ulnaris (FCU), abductor digiti quinti (ADQ), orbicularis oculi, nasalis and trapezius) with 3 kinds of low rate stimulation (LRS) and high rate stimulation (HRS). This method has the advantage of high sensitivity, but is time consuming and painful to patients. So, we tried to reestablish the stage of RNS to overcome this problem and to create a useful test.
METHODS
We analyzed RNS data from 369 patients, retrospectively. The number of patients with myasthenia gravis (MG) was 357 and the number with myasthenic syndrome was 12. We compared the sensitivity of individual muscle as well as individual stimulation rate. And we analyzed the results of MG and myasthenic syndrome to verify the usefulness of HRS.
RESULTS
The sensitivity of RNS (LRS) was 69.7% in MG (generalized symptom 86.4%, only ocular symptom 40.3%). The sensitivity was higher with 3 pps and 5 pps than with 2 pps, while the exclusion of 2 pps did not affect the sensitivity. We found only 3 cases (1.0%) with post-tetanic exhaustion (PTE) in MG patients with negative results on LRS. The distributions of resting CMAP and post-exercise CMAP were different between MG and myasthenic syndrome. In most cases of myasthenic syndrome, the resting CMAP of ADQ and FCU was below 4.0 mV and post-exercise CMAP of ADQ and FCU was above 50%.
CONCLUSIONS
LRS may be done with only 3 and 5 pps, and HRS of the ulnar nerve was helpful only if there was a suspicion of myasthenic syndrome (resting CMAP<4.0 mV or post-exercise CMAP>50%, in ADQ & FCU) or a borderline decremental response in LRS.