RESTLESS LEGS SYNDROME (RLS) is a disorder characterized by unpleasant leg sensations and an irresistible urge to move the legs that occurs mainly at night. The prevalence of RLS in Western populations is estimated at 5–15%,[2-4] and a relatively smaller prevalence of the disorder in the Japanese population has been reported.[5, 6]
The International Restless Legs Syndrome Study Group Rating Scale for restless legs syndrome (IRLS) was developed as a useful tool for assessing the subjective severity of the disorder, and has become the first-choice method to evaluate the severity of RLS in Western countries. Reports have confirmed the high reliability, validity, and responsiveness of the original English version of IRLS.[7, 8] However, utility of the Japanese version of IRLS (J-IRLS) has not yet been elucidated. Moreover, IRLS contains only one item for evaluating the insomnia symptom (item 4) even though insomnia is the most problematic symptom of RLS. In addition, the relation between the findings of IRLS and widely accepted scales for assessing subjective sleep disturbance, such as the Pittsburgh Sleep Quality Index (PSQI), has not been well elucidated.
To clarify these issues, the present study was designed to ascertain the test–retest reliability, validity, and responsiveness of J-IRLS and to estimate the correlation between J-IRLS and PSQI-J.
- Top of page
The value of ICC for sum score of J-IRLS in the present study was similar to that of the first validation report on the original version of IRLS made by IRLSSG, suggesting that test–retest reliability of J-IRLS is stable over a certain time period.
Although the above report confirmed the test–retest reliability of the sum score of IRLS, this is the first report to estimate the test–retest reliability of the 10 items of J-IRLS. Herein, all Kappa statistics for 10 items showed >0.4 of the values, suggesting that reliability of the J-IRLS 10 items is moderate or good. Among lower items constructing the J-IRLS, ‘need to move’, ‘relief from moving around’, ‘sleep disturbance’, and ‘mood disturbance’ had moderate inter-rater reliability (weighted Kappa, 0.493–0.590).
When evaluating internal consistency of J-IRLS, Cronbach's alpha was >0.90, consistent with the above-mentioned first validation report on IRLS. Therefore J-IRLS is regarded as having good internal consistency.
Regarding the concurrent validity of the sum score of J-IRLS examined by comparing its value with CGI-S, the correlation coefficient in this study was consistent with that of the previous validation report.
In this study, we found the two-factor structure for the J-IRLS 10 items, as reported previously.[8, 15] The factors relevant to RLS symptoms mainly occurring at night and relevant to the impact of RLS symptoms on daily life were statistically distinguished. The reason for the discrepancies of results for item 3, ‘RLS relief from moving around’, among our findings and those of Allen et al. and Abetz et al. remains unclear. However, considering that tiredness and sleepiness (item 5) as well as mood disturbance (item 10) of RLS have been speculated to emerge as a daytime consequence of chronic sleep deprivation caused by the disorder,[16, 17] it seems reasonable that these items belong to the symptom impact domain. The values of ICC for these factors were also similar to the total score and were stable.
This study revealed that both J-IRLS and PSQI-J included some uncorrelated items, and that the total scores of J-IRLS and PSQI-J showed significant but small correlation. This phenomenon could be ascribed to the difference in the target domain between these two parameters. When evaluating factor structure of the items of PSQI-J together with those of J-IRLS, we found three distinct factor structures. Ten items of J-IRLS were included in the above-indicated RLS symptom domain and symptom impact domain. Cole et al. previously reported that items of PSQI could be divided into three divisions. In this study, item 1 of PSQI-J (sleep quality) was classified as a symptom-related factor, item 7 (daytime dysfunction) as a symptom impact factor, and items 3 (sleep duration) and 4 (habitual sleep efficiency) appeared as a third factor indicative of sleep efficiency – as consistent with Cole's report. This finding implies that additional use of PSQI-J among RLS patients may elicit further information regarding sleep disturbance that cannot be detected by J-IRLS alone.
The change of J-IRLS total score after treatment correlated well with both CGI-I and PGI, which is consistent with a previous report. The effect sizes of the change of J-IRLS for CGI-I subgroups were also consistent with those reported by Abetz et al. These findings strongly indicate that J-IRLS is a useful tool for evaluating symptom response to RLS-specific treatments.
Our study contained a number of limitations. First, this study was performed as a sub-study of a randomized, double-blind, placebo-controlled clinical trial of pramipexole, potentially leading to selection bias. Second, because the study sample comprised a considerable number of patients having low CGI-S, this phenomenon could lead to the increased weighted Kappa value. Nonetheless, J-IRLS demonstrated appropriate reliability and validity to evaluate RLS symptoms compatible with the original version of the scale. Moreover, J-IRLS was shown to have sufficient responsiveness to RLS treatment. Analysis of construct validity revealed that J-IRLS and PSQI-J share common domains for RLS-related symptoms and their impact on daily life; however, only PSQI-J was revealed to be powered to measure sleep efficiency. From these findings, we speculate that J-IRLS is valid for quantifying the severity of RLS in Japanese patients.
- Top of page
This study was supported by Boehringer Ingelheim. The department chaired by Dr Y. Inoue has been donated by Alfresa Pharma, Astellas Pharma, AstraZeneca, Boehringer Ingelheim, GlaxoSmithKline, MSD, Mitsubishi Tanabe Pharma, Otsuka Pharmaceutical, Takeda Pharmaceutical, Pacific Medico, Philips Respironics, Sanofi-Aventis, and Shionogi. The department chaired by Dr Y. Oka has been donated by Philips Respironics. Dr K. Kuroda has no conflict of interest to declare. Dr K. Hirata has received financial support from GlaxoSmithKline. Mr. T. Kagimura is an employee of Nippon Boehringer Ingelheim. This trial was registered with ClinicalTrial.gov (no. NCT00152997).