Serum Neurofilament Light Chain Level as an Indicator of Axonal Injury in Parsonage–Turner Syndrome (Neuralgic Amyotrophy)

Background and Aims Parsonage–Turner syndrome (PTS), also known as neuralgic amyotrophy, is a peripheral neuropathy resulting in severe axonal loss. This study aimed to characterize initial elevation and longitudinal trends of serum neurofilament light chain (sNfL), a marker of neuro‐axonal damage, in PTS. Methods This prospective cohort included 29 adults with electromyography (EMG)–confirmed PTS ≤ 6 months from symptom onset. Patients underwent sNfL testing and EMG at baseline (median 86 days from symptom onset) and again at 3‐ and 6‐month follow‐up intervals. Age‐ and BMI‐adjusted Z ‐scores were analyzed. Linear mixed‐effects models assessed associations between sNfL and time from onset, number of nerves involved, EMG metrics, and corticosteroid use. Results Mean sNfL Z ‐scores were significantly elevated (1.64, SD 1.32, p < 0.001) compared with healthy controls at 0 (1.64, p < 0.001) and 3 months (0.49, SD 1.06, p = 0.020). At 6 months, statistically significant elevations were not detected (0.39, SD 0.96, p = 0.106). sNfL declined by 0.17 Z ‐scores per month (95% CI: 0.11–0.23; p < 0.001). On EMG, the presence of nascent motor units, reflecting reinnervation, was associated with lower sNfL ( p = 0.043). Interpretation sNfL elevation was detected in PTS patients within the first 6 months from symptom onset and decreased as reinnervation ensued. These findings suggest sNfL deserves further consideration as a blood‐based biomarker for detection and monitoring of PTS.