Prospective Validation of the New PLS Diagnostic Criteria From PLS Natural History Study: EMG and Neurofilament Analyses

Background: Primary lateral sclerosis (PLS) is an ultrarare upper motor neuron syndrome with a prognosis unique from classical ALS. The study of PLS is complicated by its rarity and the difficulty distinguishing PLS from ALS. We present data from a 1-year prospective follow-up study on PLS and efforts to distinguish it from ALS. Methods: Seventy-six PLS participants enrolled in this prospective natural history study. EMG studies, blood neurofilament light chain levels (NfLs), and demographic characteristics were obtained at baseline. At 1-year follow-up, repeat EMG studies were conducted to determine which participants fulfilled criteria for ALS. Baseline characteristics were then compared to determine features that predict reclassification. Results: Seventy participants completed 1-year follow-up. Five of the 70 were reclassified to ALS (7.1%). Those reclassified had higher trends in baseline blood NfL levels (91.4 vs. 34.0 pg/mL, p = 0.13) and shorter symptom duration (39 vs. 69 months in the PLS group, p = 0.15). Reclassification was noted in both probable and definite PLS participants. All cases with a symptomatic duration of less than 2 years retained the PLS phenotype (5 of 5). NfL levels over 90 pg/mL predicted reclassification with 94% specificity and 60% sensitivity. No other features predicted reclassification to ALS. Conclusions: In our population, reclassification of PLS to ALS occurred at a low frequency at 1 year follow-up (7.1%). Baseline NfL was the strongest predictor in differentiating UMN dominant ALS from PLS at 1-year follow-up. Based on our data, we propose EMG and NfL criteria for enrollment in future PLS trials.