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The validity and responsiveness of the Japanese version of the International Restless Legs Syndrome Study Group Rating Scale (IRLS)
Yuichi Inoue1, Yasunori Oka1, Hiroaki Matsuda2, Tatsuo Kagimura2, Tadashi Kusunoki3, Kenji Kuroda4, Koichi Hirata5, Taneyoshi Nozawa6
1Japan Somnology Center, Neuropsychiatric Research Institute, 2Nippon Boehringer Ingelheim Co. Ltd., 3Japanese Society for Pharmacoepidemiology, 4Hannan Hospital, 5 Dokkyo University School of Medicine, 6Toranomon hospital, Showa University School of Medicine

Objective: To verify the validity and responsiveness of the Japanese version of IRLS, and to estimate the relationship between IRLS and Pittsburgh Sleep Quality Index (PSQI).

Background: Restless legs syndrome (RLS) is a disorder frequently associated with severe sleep disturbance. The IRLS has been widely used to evaluate the severity of RLS. However, the utility of the Japanese version of IRLS and its relation to the PSQI estimating subjective sleep disturbance has not been studied.

Methods: This study was designed as a sub-study of a drug efficacy trial comparing pramipexole (PPX) with placebo. The IRLS were self-reported by 59 patients in the screening phase. Factor analyses were made on both IRLS items alone and IRLS items together with PSQI items to evaluate the construct validity. The responsiveness of IRLS to PPX treatment was confirmed by evaluating the correlations between the changes in the IRLS total score for 6 weeks treatment with PPX and clinical global improvement (CGI-I) and patient global improvement (PGI-I) as well as the changes in the PSQI total score.

Results: A statistically significant correlation was found between the total scores of IRLS and PSQI. Factor analysis for the ten IRLS items alone identified one factor with a factor loading of 0.65 to 0.92. Moreover, analysis for the IRLS items and the PSQI items suggested the existence of three potential factors: factor1 relevant to RLS symptom itself and disturbance in both initiating and maintaining sleep, factor2 relevant to daytime consequence, and factor3 relevant to nocturnal sleep duration (Table1). The correlations of CGI-I and PGI-I with the change in IRLS total score were 0.648 and 0.666, respectively. The correlation of the change in the total scores of PSQI and IRLS was 0.727 (Figure1).

Conclusions: The concurrent validity, construct validity and responsiveness of Japanese version of IRLS could be considered adequate. The scale is highly applicable to both evaluation of severity of RLS and drug effectiveness assessment.

Table 1. Factor loadings of PSQI items and IRLS items

 

Factor1

Factor2

Factor3

Communality

PSQI

1

sleep quality

0.485

0.062

0.313

0.494

2

sleep latency

0.327

-0.222

0.121

0.139

3

sleep duration

0.141

-0.005

0.655

0.525

4

habitual sleep efficiency

0.134

-0.302

0.777

0.718

5

sleep disturbance

-0.148

0.100

0.321

0.09

6

daytime dysfunction

-0.044

0.585

-0.224

0.353

RLS

1

Discomfort in legs/arms

0.970

-0.031

-0.12

0.836

2

Need to move

0.976

-0.167

-0.25

0.714

3

Relief from moving around

0.304

0.140

0.287

0.311

4

Sleep disturbance

0.471

0.199

0.166

0.438

5

Tiredness/sleepiness

0.104

0.702

0.037

0.574

6

Severity as a whole

0.772

0.123

0.052

0.727

7

How often

0.312

0.260

0.155

0.305

8

Average severity

0.513

0.167

0.270

0.559

9

Impact on daily affairs

-0.048

0.771

0.082

0.584

10

Mood disturbance

0.419

0.441

-0.035

0.507

Factor loading was calculated by principal factor analysis with oblique promax rotationfigure 1

Figure 1. Relationship between change in IRLS total score and change in PSQI total score six weeks after starting treatment with pramipexole