Objective: To develop standardized, quantifiable criteria when using neurophysiologic techniques to diagnose SPSD. Furthermore, we desired two levels of diagnostic certainty: Neurophysiology Possible for high sensitivity, and Neurophysiology Probable for high specificity.

Background: Stiff-person Spectrum Disorders (SPSD) are difficult to diagnose. Neurophysiologic testing can be a helpful diagnostic aid, although better standardization of this technique is needed. Mayo Clinic neurologists have been studying the acoustic startle reflex (ASR), exteroceptive reflexes elicited with medial plantar nerve stimulation (ER), and electromyographic assessment for continuous motor unit activity (CMUA) in lumbar paraspinals in patients with suspected SPSD for over thirty years.

Method: In an exploratory cohort, we evaluated 97 cases referred to the Mayo Clinic Movement Disorder laboratory from 2013-2020 who had undergone neurophysiologic testing- 21 with SPSD, 76 with non-SPSD defined by the referring physician using clinical criteria. We studied the neurophysiologic results of cases and developed quantifiable criteria to aid in diagnosis.

In a confirmatory cohort, we then applied these criteria to evaluate 172 unique cases referred to the lab from 2019-2023 (22 with SPSD; 150 with non-SPSD) to determine the criteria’s accuracy.

Results: In the exploratory cohort we found a very large variation amongst the ASR, ER and CMUA results in the SPSD cases. Therefore, we designed criteria applying points to each of the neurophysiologic methods. By summing up the points, we could categorize the patients into normal, Neurophysiology Possible or Neurophysiology Probable SPSD.

We applied these criteria to the 172 unique cases in the confirmatory cohort. Neurophysiology Possible Criteria produces Sensitivity = 86.4% and Specificity = 86.7%. Neurophysiology Probable criteria had a Sensitivity = 31.8%, but a Specificity = 99.3%.

Conclusion: We have developed standardized quantifiable neurophysiology criteria to aid in the diagnosis of SPSD. Neurophysiology Possible criteria can give further diagnostic certainty in those already suspected to have SPSD clinically, while Neurophysiology Probable criteria can aid in clinically ambiguous cases, especially those that are seronegative for the autoantibodies commonly associated with SPSD.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/neurophysiologic-testing-aids-diagnosis-of-stiff-person-spectrum-disorders/