Primary Lateral Sclerosis: Implications for Diagnostic Criteria From a Natural History Study in the Netherlands

Primary lateral sclerosis (PLS) is a rare disease characterized by upper motor neuron (UMN) degeneration. We aimed to elucidate the natural history in patients with UMN syndromes suggestive of PLS and validate the most recent diagnostic (consensus) criteria.

A validation study of a long-term follow-up cohort was conducted, including adults with UMN syndromes and disease durations ≥6 months. Patients were assessed at baseline (T1), at 3 years (T2), and when possible after 13 years (T3). Diagnostic categorization followed the 2020 PLS consensus criteria. Main outcomes included diagnostic classification at follow-up and survival.

The study comprised 86 patients (34 women [40%], mean age 58.9 ± 10.1 years), of whom 43 met the PLS consensus criteria at baseline (6 probable, 37 definite). Eight patients had a disease duration <2 years, and 35 patients presented with UMN symptoms localized to 1 region (1 bulbar, 34 legs). Change of initial diagnosis occurred in 14% of patients with PLS, and 49% of patients presenting with UMN symptoms in 1 region progressed to PLS. Seven patients developed amyotrophic lateral sclerosis (ALS), and for 7 patients, diagnosis was revised to hereditary spastic paraplegia (HSP). Survival was shorter for patients with a disease duration <4 years. In the probable PLS group, 33% converted to ALS. Converters had a steeper Amyotrophic Lateral Sclerosis Functional Rating Scale slope (p = 0.023) and shorter symptom duration (p < 0.001) at inclusion. Of patients presenting with leg symptoms, diagnosis was revised between T2 and T3 in 29%. Introducing a 4-year minimal disease duration for PLS diagnosis and categorization based on regions involved resulted in 86% of PLS diagnoses remaining within the PLS category, 5% transitioning to ALS (slow variant), and 9% to HSP. Survival was longest for patients presenting with symptoms confined to arms and legs or legs only, followed by those with bulbar involvement at baseline, while patients with disease durations between 6 months and 4 years exhibited the shortest survival.

Our findings suggest that a diagnosis of PLS should be deferred until 4 years after symptom onset because shorter durations correlate with higher ALS conversion rates and shorter survival. Categorization by regional involvement may facilitate more effective monitoring of patients with UMN syndromes.