Objective: To measure McArdle sign (rapidly reversible weakness induced by neck flexion) both qualitatively and quantitatively and to evaluate its specificity and clinical utility for diagnosis of multiple sclerosis (MS). Patients and Methods: In this prospective study, McArdle sign was evaluated by a technician blinded to diagnosis by measuring changes in finger extensor strength in successive trials of neck extension and flexion, first clinically and then with a torque measurement device. We studied 25 healthy controls and 81 patients with finger extensor weakness. Patients were not selected for having McArdle sign. Fifty-two patients had MS, 24 had other myelopathies, and 5 had peripheral nerve lesions accounting for their weakness. The study was conducted between February 1, 2016, and June 30, 2017. Results: The median clinical McArdle sign and the 2 quantitative measures of neck flexion–induced strength reduction were greater in patients with MS than in the other groups (P<.001). Baseline strength did not confound the difference. The area under the receiver operating characteristic curve was 0.84 (95% CI, 0.75-0.93) comparing patients with MS vs healthy controls and 0.84 (95% CI, 0.75-0.93) comparing MS vs patients with other myelopathies. The 2 quantitative and 1 clinical measurement of McArdle sign by the technician who performed the quantitative testing were correlated (r=.57 and r=.58; P<.001), and in turn, the technician's and unblinded referring physician's clinical assessments were correlated (r=.58; P<.001). McArdle sign was evident in some patients who had minor disability and who were in early phases of MS. Conclusion: McArdle sign, when defined as greater than 10% neck flexion–induced reduction in strength, is entirely specific and 65% sensitive for a diagnosis of MS when compared with other conditions that mimic MS-associated myelopathy. It may facilitate diagnosis in certain clinical situations. Trial Registration: clinicaltrials.gov Identifier: NCT03122873.
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