A New Way to Measure Prodromal Synucleinopathy: What the PSRS Adds for RBD Clinical Trials

Study Structure

1. Title

The Prodromal Synucleinopathy Rating Scale in REM Sleep Behavior Disorder: A New Multidomain Tool for Measuring Early Disease Burden

2. Highlights

The Prodromal Synucleinopathy Rating Scale (PSRS) showed good-to-excellent interrater and intrarater reliability in a large multicenter North American REM sleep behavior disorder (RBD) cohort.

PSRS domain scores correlated in the expected direction with independent measures of cognition, mood/behavior, motor impairment, autonomic dysfunction, sleepiness, olfaction, and daily function, supporting preliminary construct validity.

The scale is designed to capture the breadth of prodromal synucleinopathy burden, a clinical phase relevant to future neuroprotective trials in RBD.

These findings support the PSRS as a promising clinician-rated outcome measure, but further validation in longitudinal and interventional studies is still needed.

3. Study Background / Disease Burden

Rapid eye movement sleep behavior disorder is more than a parasomnia. In many patients, especially those with isolated or “idiopathic” RBD, it represents a prodromal stage of an underlying synucleinopathy such as Parkinson disease, dementia with Lewy bodies, or multiple system atrophy. This matters because the clinical syndrome can precede overt neurodegenerative disease by years, sometimes more than a decade.

That prodromal interval is now a major therapeutic target. As disease-modifying trials in RBD move closer to reality, investigators need outcome measures that can detect subtle but clinically meaningful changes across several domains at once. Traditional scales often focus on one axis of disease, such as motor symptoms or sleep disturbance. Prodromal synucleinopathy, however, is more heterogeneous: patients may have early cognitive changes, mood or psychiatric symptoms, axial or appendicular motor abnormalities, autonomic dysfunction, impaired sleep quality, or sensory deficits such as hyposmia.

The PSRS was developed to address this gap. Rather than measuring only one symptom cluster, it aims to quantify the overall clinical burden of prodromal synucleinopathy across seven domains: cognitive (COG), behavioral/psychiatric (PSY), motor-axial (MAX), motor-appendicular (MAP), autonomic (AUT), sleep (SLP), and sensory (SEN). The central question in this study was whether the scale behaves in a clinically credible way in a large RBD cohort and whether different raters can apply it consistently.

4. Study Design

This was a clinicometric validation study performed in the North American Prodromal Synucleinopathy (NAPS) cohort. Investigators analyzed PSRS ratings collected from August 2022 through June 2025 in participants who did not have overt Parkinson disease, dementia with Lewy bodies, or multiple system atrophy. The sample included 348 participants, 79% male, with a mean age of 65.4 ± 10.4 years.

Clinician raters used clinical judgment to score signs and symptoms in each PSRS domain. Individual domains were scored on a scale from 0, indicating no abnormality, to a domain-specific maximum of 2 to 4 points. The total score, or SUM, had a maximum of 25 points. The study assessed two major psychometric properties: construct validity, using correlations with independent external measures of related constructs, and reliability, using 20 case example protocols evaluated by 20 different raters. Interrater and intrarater reliability were estimated with a Bayesian generalized linear mixed-effects model.

The study was not designed to test treatment effects, prognostic performance, or conversion to overt synucleinopathy. Its purpose was foundational: to determine whether the PSRS is a usable, reproducible, and biologically plausible instrument for future trials.

Key Findings

1. Domain-level correlations matched the expected clinical biology

One of the most important signals from this study is that the PSRS did not behave like a random composite score. Its domains correlated with external measures in ways that make clinical sense.

The cognitive domain correlated moderately with the Montreal Cognitive Assessment (r = -0.42) and strongly with the Clinical Dementia Rating-Sum of Boxes (r = 0.77). The negative association with Montreal Cognitive Assessment is expected because higher PSRS scores indicate worse impairment, whereas higher Montreal Cognitive Assessment scores indicate better cognition. The strong positive correlation with Clinical Dementia Rating-Sum of Boxes suggests that the PSRS cognitive domain captures clinically relevant impairment rather than background noise.

The behavioral/psychiatric domain correlated with the Neuropsychiatric Inventory-Questionnaire (r = 0.38), supporting its sensitivity to mood and behavioral burden. While this correlation was more modest than the cognitive associations, it remains consistent with the reality that psychiatric symptoms in prodromal synucleinopathy can be heterogeneous and intermittently expressed.

Motor domains also performed well. The motor-axial domain correlated with the Movement Disorders Society-Unified Parkinson Disease Rating Scale Motor score (r = 0.65), and the motor-appendicular domain correlated even more strongly (r = 0.75). These values suggest that the PSRS can detect motor abnormalities relevant to emerging parkinsonism, including both postural/gait features and limb motor signs.

The autonomic domain correlated with Scales for Outcomes in Parkinson’s Disease-Autonomic Dysfunction (r = 0.39), the sleep domain correlated with the Epworth Sleepiness Scale (r = 0.23), and the sensory domain correlated strongly with the Brief Smell Identification Test (r = -0.66). The sensory result is particularly noteworthy because olfactory dysfunction is one of the best-established prodromal markers in synucleinopathy.

2. The total score tracked global clinical burden and function

Beyond domain-specific validity, the total PSRS score correlated with broader measures of function and severity. The PSRS SUM correlated with the Functional Assessment Scale (r = 0.47), Schwab and England Activities of Daily Living scale (r = -0.58), and Clinician Global Impression of Severity (r = 0.29), with all p values less than 0.0001.

These relationships are clinically important because they indicate that higher PSRS scores are not just statistically associated with isolated symptom measures; they also reflect how patients are functioning in everyday life. In a prodromal population, this is especially relevant. A useful scale must do more than identify disease presence—it should capture cumulative burden that may matter for trial eligibility, enrichment, and longitudinal change over time.

3. Reliability was strong across raters and over time

Reliability is essential for any clinician-rated scale intended for multicenter research. In this study, both interrater and intrarater reliability were good to excellent, with mean values ranging from 0.76 to 0.98. That range is encouraging because it implies that different clinicians can use the instrument consistently, and that individual raters can reproduce their own assessments when presented with the same cases later.

This is not a trivial achievement. Multidomain clinical scales often suffer from ambiguity in item definitions, overlap between domains, or dependence on local interpretation. The PSRS appears to have avoided major reliability pitfalls, at least under the controlled conditions of case-example testing. That makes it a practical candidate for multicenter prodromal synucleinopathy trials.

4. The results support validity, but not yet responsiveness

The overall pattern of correlations and reliability testing provides preliminary support for the PSRS as a valid measure of prodromal synucleinopathy burden in RBD. However, the study stops short of demonstrating responsiveness to change. In other words, we still do not know how well the PSRS detects clinical progression or treatment effect over months to years.

That distinction matters. A scale can be reliable and correlates appropriately with related measures, yet still fail as an endpoint if it does not change meaningfully over time or if its change scores are difficult to interpret. For a future disease-modifying trial, responsiveness, minimal clinically important difference, and sensitivity to domain-specific progression will be as important as baseline validity.

Expert Commentary

This study addresses a major unmet need in prodromal synucleinopathy research. RBD cohorts are increasingly viewed as the most feasible population in which to test preventive or disease-delaying therapies for Parkinson disease, dementia with Lewy bodies, and multiple system atrophy. But trial success depends on selecting outcome measures that reflect the biology of a slowly evolving multisystem disorder.

The PSRS is conceptually attractive because it is multidimensional and clinician anchored. Rather than forcing investigators to rely on a single endpoint such as conversion to overt disease, it offers a structured way to quantify the broader clinical phenotype. This may improve enrichment, phenotyping, and potentially trial sensitivity.

Still, several limitations should temper interpretation. First, the study cohort consisted of participants already enrolled in a specialized prodromal research network, which may reduce generalizability to community practice. Second, the validity evidence is cross-sectional; longitudinal behavior remains unproven. Third, clinician judgment is central to scoring, which is both a strength and a possible source of variability in less experienced hands. Fourth, correlations do not prove that the PSRS captures all relevant biology, especially nonmotor features that may evolve at different rates.

Another practical issue is whether the scale will be feasible in routine clinics outside research settings. A tool that is too time-consuming or complex may have limited real-world adoption. The study does not yet fully answer this implementation question.

Even with those caveats, the overall data are persuasive. The PSRS appears to be more than a theoretical construct; it has begun to show the psychometric profile needed for a serious trial instrument. If future studies confirm longitudinal sensitivity and predictive value for phenoconversion, it could become an important endpoint in preventive neurology.

Conclusion

In this multicenter validation study of 348 participants with RBD and no overt Parkinson disease, dementia with Lewy bodies, or multiple system atrophy, the Prodromal Synucleinopathy Rating Scale demonstrated moderate-to-strong correlations with established measures of cognition, motor dysfunction, autonomic symptoms, sleepiness, olfaction, and function. Reliability was good to excellent across raters and repeated assessments.

These findings provide preliminary evidence that the PSRS is a credible clinician-rated measure of prodromal synucleinopathy burden. Its greatest value may lie in future disease-modifying trials, where a multidomain scale could help capture subtle but clinically meaningful change in patients at risk for synuclein-related neurodegeneration.

The next essential steps are longitudinal validation, assessment of responsiveness to progression and treatment, and testing in broader clinical settings. For now, the PSRS represents a thoughtful advance in the measurement science of prodromal synucleinopathy.

Funding and Clinical Trial Registration

Trial registration: NCT05826457; NAPS Consortium Website: naps-rbd.org/.

Funding details were not provided in the abstract. Readers should consult the full article for sponsor and grant information.

References

Boeve BF, Nie Y, Lu R, et al. The Prodromal Synucleinopathy Rating Scale: An Assessment in Patients With REM Sleep Behavior Disorder. Neurology. 2026;107(1):e218176. PMID: 42302227.

Postuma RB, Iranzo A, Hogl B, et al. Risk and predictors of dementia and parkinsonism in idiopathic REM sleep behaviour disorder: a multicentre study. Lancet Neurol. 2019;18(5):476-485.

Högl B, Stefani A, Videnovic A. Idiopathic REM sleep behaviour disorder and neurodegeneration—an update. Nat Rev Neurol. 2018;14(1):40-55.

International Parkinson and Movement Disorder Society-related prodromal research literature has emphasized the need for biomarkers and multidomain clinical measures in at-risk synucleinopathy populations.

AI Image Prompt

A modern neurology research scene showing a clinician scoring a multidomain prodromal synucleinopathy rating scale for an older adult with REM sleep behavior disorder, with subtle visual elements representing cognition, motor function, autonomic symptoms, sleep, and smell testing; clean hospital setting, scientific infographic style, muted blue and gray palette, high-detail editorial illustration, wide horizontal composition.