# Ikelos‐Rating Scale: Validation of a Behavioural Severity Marker in REM Sleep Behaviour Disorder

## TL;DR

The Ikelos-RS is a new tool to measure the severity of REM sleep behavior disorder, validated for reliability and connection to other clinical markers.

## Contribution

The Ikelos-RS is introduced as a novel severity marker for REM sleep behavior disorder with demonstrated reliability and validity.

## Key findings

- The Ikelos-RS showed high inter-rater and test-retest reliability.
- It correlated strongly with clinical severity and REM sleep without atonia.
- It also correlated with dopamine transporter binding in brain regions.

## Abstract

The Ikelos‐Rating Scale (Ikelos‐RS) is a new, expert‐interviewed and bedpartner‐reported severity marker assessing frequency and expression of isolated REM sleep behaviour disorder (iRBD), a precursor of clinical α‐synucleinopathies. This study aimed to validate the Ikelos‐RS in 180 patients with three‐night PSG‐confirmed iRBD (68.4 ± 8.3 years; 139 m). Inter‐rater (n = 45) and test–retest reliabilities (n = 25; 174 Ikelos‐RS) were evaluated. For construct validity, correlation analyses were performed with: (1) Clinical Global Impressions‐Severity (CGI‐S; n = 151), (2) REM sleep without atonia (RWA) within videometry (n = 20), (3) RWA at initial diagnosis (n = 131) and changes over time (n = 36), (4) dopamine transporter scintigraphy (DaT‐SPECT) at baseline and changes over time (n = 75). RWA‐ and DaT‐SPECT‐analyses were conducted for the whole sample (‘all’) and after excluding confounders (‘cleaned’). Correlation analyses indicated high inter‐rater (r

s
 = 0.865, p < 0.001) and test–retest reliabilities (r

s
 = 0.900, p < 0.001). Construct validity was supported by associations of Ikelos‐RS with (1) CGI‐S (r

s
 = 0.845, p < 0.001), (2) RWA within videometric analysis (r

s
 = 0.592, p = 0.006) and at baseline (‘all’: r

s
 = 0.274, p = 0.002), (3) DaT‐binding (z‐scores) at baseline in right anterior putamen (AP) (‘all’: r

s
 = −0.319, p = 0.005) and changes over time, most pronounced in right anterior putamen (AP) (‘all’: r

s
 = −0.243, p = 0.035; ‘cleaned’: r

s
 = −0.374, p = 0.008) and left posterior putamen (PP) (‘all’: r

s
 = −0.259, p = 0.025; ‘cleaned’: r

s
 = −0.319, p = 0.024). Given its high reliability and construct validity, demonstrated by associations with the best available severity markers DaT‐binding ratios and RWA, Ikelos‐RS appears to represent a reliable, valid and easy‐to‐use tool for measuring the severity of iRBD. Thus, Ikelos‐RS may prove beneficial in research. Its suitability as a screening tool in older at‐risk populations needs to be proven in future studies.