Epidemiologic knowledge of RLS has increased considerably recently, in fact, since 2005 epidemiological research on RLS has effectively blossomed.
This synthesis of the literature reveals that RLS prevalence rates are linked to:
1) The way RLS is evaluated. Single-item studies have provided the highest RLS prevalence followed by studies having limited the assessment to the 4 minimal criteria described by the International Restless Legs Syndrome Study Group. In this last case, prevalence of RLS in the adult general population ranges between 5% and 8.8%. However, at least half of these cases have infrequent and/or mild RLS.
2) Sex of the individuals: In most of studies, RLS is about twice higher in women than in men.
3) Age: As illustrated in , prevalence of RLS increases with age in North America and Europe. However, the same trend is not observed in Asian countries.
4) Race: Data on race are still too fragmentary to reach definite conclusions but from what we know to date Asian countries appear to have much lower prevalence of RLS.
5) Co-morbid conditions. Insomnia symptoms, excessive sleepiness and depressive and/or anxiety symptoms have been consistently associated with RLS. Several medical conditions have been less consistently associated with RLS, e.g.; cardiovascular diseases, hypertension, diabetes, obesity and pain. The effects of medications on RLS symptoms have not been investigated in epidemiological studies.
RLS is common in the general population but a full appreciation of its severity and impact on daily life remains unclear. The earliest epidemiological studies using IRLSSG criteria are based on the 1995 criteria, while the most recent have used the 2003 revised criteria, which makes direct comparison of these studies difficult. There is also problem of participation rates, as in at least 10 studies where the percentage is below 60%. The overwhelming majority of prevalence rates reported in epidemiological studies of RLS are based solely on the presence of one symptom or a constellation of symptoms (four minimal criteria proposed by the IRLSSG group) that might occur only infrequently, regardless of the severity of the symptoms. While this may be defensible from an epidemiological point of view; for the physicians, it raises the question of “When does one need to treat these people?”. Two studies (19
) have examined more closely who are the individuals with RLS who seek medical advice or help for their symptoms. Treatment seeking is closely associated with the severity of symptoms: nearly all of those with severe or very severe RLS symptoms consulted while this proportion decreased to half in those with moderate symptoms and to 10% with those with mild symptoms (27
Epidemiological studies conducted in clinical settings were more rigorous in the attribution of RLS diagnosis. Most of them required a frequency of symptoms of at least two times per week accompanied by moderate to severe distress and the diagnosis confirmed by the treating physician. Such studies yield prevalence estimates about five times lower than when prevalence is based solely on RLS minimal criteria. As the International RLS Study Group has pointed out, the minimal criteria were not intended to confirm the presence of RLS but to indicate the possibility of the syndrome. Minimal RLS criteria without applying frequency and severity yield a false positive rate around 50% (88
). Consequently, using only the four minimal symptoms to determine the prevalence of RLS in the general population can cause an inflation of the prevalence of RLS. It is especially true when studies seek only yes/no answers or presence/absence of criteria. Several conditions can produce symptoms similar to RLS, for example, leg cramps or arthritic pain in lower limbs. It is therefore important that epidemiological studies include some questions that allow excluding individuals for whom, leg symptoms can be attributed to other disorders. Additionally, quantification of the frequency and severity of the symptoms will allow eliminating a sizable number of individuals for whom a RLS diagnosis is not warranted.
It should be kept in mind that prevalence observed in primary care practices cannot be applied to the general population: 1) even when comparable age groups were used, the mean age of subjects in primary care practice samples was higher than the one of general population samples and 2) only a portion of the general population consults at least once a year a primary care practitioner and mostly because the individuals have some health problem concerns: as it was shown, individuals of the general population with RLS are more likely to have health problems than non-RLS people and, therefore, have greater needs to consult a physician.
Another consequence of using cases with infrequent RLS in general population-based studies is that it can mask the presence of some risk factors. This is probably one of the reasons why there are so many conflicting results when it comes to identifying medical conditions associated with RLS.
- Prevalence of RLS decreases as defining criteria increase in strictness in European and North American general adult populations.
- When a differential diagnostic approach is taken, prevalence estimates are 1.9–4.6% of European and North American general adult populations.
- European and North American populations demonstrate higher RLS prevalences compared to Asian populations.
- European and North American populations demonstrate an age-increase in RLS prevalence, while Asian populations do not.
- RLS prevalance in women is approximately double that of men across all populations and ages.
- Future epidemiological studies should focus on the general population and include systematic assessment using diagnostic criteria including frequency and severity of symptoms.
- Additional epidemiological studies outside of Europe and North America are needed to clarify apparent population differences in RLS prevalence.
- Studies should seek to determine the conditions associated with RLS, particularly severe RLS, as well as its functional consequences.