Validation of the RBD Symptom Severity Scale in the North American Prodromal Synucleinopathy Consortium

Background and Objectives REM sleep behavior disorder (RBD) is a parasomnia characterized by dream enactment. The International RBD Study Group developed the RBD Symptom Severity Scale (RBDSSS) to assess symptom severity for clinical or research use. We assessed the psychometric and clinimetric properties of the RBDSSS in participants enrolled in the North American Prodromal Synucleinopathy (NAPS) Consortium for RBD. Methods NAPS participants, who have polysomnogram-confirmed RBD, and their bedpartners completed the RBDSSS (participant and bedpartner versions). The RBDSSS contains 8 questions to assess the frequency and severity/impact of (1) dream content, (2) vocalizations, (3) movements, and (4) injuries associated with RBD. Total scores for participant (maximum score = 54) and bedpartner (maximum score = 38) questionnaires were derived by multiplying frequency and severity scores for each question. The Clinical Global Impression Scale of Severity (CGI-S) and RBD symptom frequency were assessed by a physician during a semistructured clinical interview with participants and, if available, bedpartners. Descriptive analyses, correlations between overall scores, and subitems were assessed, and item response analysis was performed to determine the scale’s validity. Results Among 261 study participants, the median (interquartile range) score for the RBDSSS-PT (participant) was 10 (4–18) and that for the RBDSSS-BP (bedpartner) was 8 (4–15). The median CGI-S was 3 (3–4), indicating moderate severity. RBDSSS-BP scores were significantly lower in women with RBD (6 vs 9, p = 0.02), while there were no sex differences in RBDSSS-PT scores (8 vs 10. 5, p = 0.615). Positive correlations were found between RBDSSS-PT vs RBDSSS-BP (Spearman r_s = 0.561), RBDSSS-PT vs CGI-S (r_s = 0.556), and RBDSSS-BP vs CGI-S (r_s = 0. 491, all p < 0.0001). Item response analysis showed a high discriminatory value (range 1.40–2.12) for the RBDSSS-PT and RBDSSS-BP (1.29–3.47). Discussion We describe the RBDSSS with adequate psychometric and clinimetric properties to quantify RBD symptom severity and good concordance between participant and bedpartner questionnaires and between RBDSSS scores and clinician-assessed global severity.