Exteroceptive Reflexes in Stiff Person Syndrome, Other Neurological Conditions and Healthy Controls

Background Stiff person syndrome (SPS) is a rare disabling neurological condition with overlapping symptomatology with more common neurological conditions. Exaggerated exteroceptive reflex mechanisms are thought to play a role in its pathophysiology. Objectives We aim to describe the results of neurophysiological exteroceptive reflex testing in subjects with SPS, healthy controls, and other neurological conditions causing axial and lower limb muscle stiffness, pain and spasms. Methods We performed a prospective cohort study, of 59 participants with a variety of neurological conditions, healthy controls and 11 with SPS. Retrospective data from participants with SPS who had prior exteroceptive reflex testing performed in an equivalent manner were also included in our analysis. Reflex responses were recorded by simultaneous surface electromyography (sEMG) of axial and lower limb muscles following repeated stimulation of the median and tibial nerves. Results Latencies following repeated activation of median and tibial nerves demonstrated a moderate to very strong correlation at most EMG recording sites within individuals. Using a mixed effects logistic model shorter mean latencies were observed in those with Parkinson's disease, healthy controls, low back pain and serotonergic/noradrenergic antidepressant use compared with SPS following initial stimulation and when the shortest latency at each site within individual participants were analyzed. No study group differed significantly from SPS in the duration of responses following stimulation nor the presence or absence of after‐bursts. Conclusion The use of exteroceptive testing as performed in this study did not differentiate SPS from other neurological conditions or healthy controls.