Correlation between rating scales and sleep laboratory measurements in restless legs syndrome.

OBJECTIVES: The aim of this study was to test the external validity of the International Restless Legs Scale (IRLS) by assessment of the correlation between IRLS scores and objective measures of severity such as polysomnography (PSG) and Suggested Immobilization Test (SIT). DESIGNS: Correlation analysis between rating scales for RLS (IRLS and Johns Hopkins RLS Scale--JHRLSS) and sleep laboratory measurements in untreated RLS patients.
METHODS: The study included 30 untreated patients diagnosed with RLS according to the criteria of the International RLS Study Group. Diagnostic procedures included physical exam, laboratory analysis, PSG and a nocturnal SIT. Statistical analysis was performed by means of Spearman's correlations and Kruskal-Wallis test.
RESULTS: IRLS correlated significantly with Periodic Leg Movement of Sleep-index (PLMS), and PLMS-arousal index during PSG as well as with Periodic Leg Movement of Wakefulness (PLMW) during SIT (SIT-PLMW) (all r=0.4; p<0.01). There was no correlation between IRLS and the number of PLMW in PSG (PSG-PLMW) or any other sleep variable during PSG. Nor was any correlation found between IRLS scores and ferritin, age, duration of illness or any other clinical variables.
CONCLUSIONS: This study represents the first demonstration of a correlation between IRLS and objective parameters of motor dysfunction such as PLMS-index or SIT. This finding is particularly relevant for the design of future clinical trials. Furthermore, the association between PLMS and SIT-PLMW supports the view that both PLMS and PLMW might share a common mechanism.