1. Introduction
Multiple Sclerosis (MS) is a chronic neurological disease that affects the Central Nervous System (CNS), typically manifesting between the ages of 20 and 40 [
1]. Less commonly, MS begins before the age of 18, accounting for approximately 3–5% of cases; this form is referred to as Pediatric-Onset Multiple Sclerosis (POMS) [
2]. The exact pathophysiology of MS remains unclear, but it is widely believed to involve an autoimmune response triggered by environmental factors in genetically predisposed individuals. This response leads to inflammatory infiltration in the CNS via the blood-brain barrier, resulting in demyelination, gliosis, and neuronal damage [
2]. In pediatric patients, the most common manifestation of MS is Relapsing Remitting MS (RRMS), characterized by episodes of clinical relapses with CNS symptoms alternating with phases of remission. Compared to adult-onset MS, POMS tends to have a more inflammatory and active disease course, with a higher relapse rate, particularly in the initial years following diagnosis [
2]. Despite variability in clinical manifestations, the most frequent symptoms of POMS include motor deficits (30%), sensory disturbances (15–30%), brainstem symptoms (e.g., diplopia, facial sensory symptoms, facial weakness) (25%), optic neuritis (10–22%), and ataxia (5–15%) [
2]. POMS is also commonly associated with fatigue (20–43% [
3]), depression (17–29% [
4]), non-motor MS symptoms (31% [
5]), and sleep disturbances (42–65% [
6] in adults, with no defined prevalence in pediatric patients [
7]). All these factors significantly impact patients’ quality of life [
4]. POMS patients report a lower quality of life compared to the general population, particularly in areas related to school, social engagement, and emotional well-being [
4,
8].
Sleep disorders (SDs) are a significant concern in MS, impacting both physical and mental health, especially in chronic conditions. While SDs are well-recognized in adults with MS, their prevalence in POMS remains less well-established [
6,
9]. However, the impact of sleep on quality of life, fatigue, and physical activity is already acknowledged in this population [
7]. Additionally, sleep disturbances can negatively affect mental health, leading to mood swings, behavioral changes, and social difficulties. This suggests that sleep patterns should be carefully examined to improve the well-being of POMS patients [
7,
10]. Common SDs in POMS include insomnia, sleep-related movement disorders, sleep-related breathing disorders, and circadian rhythm disorders [
9]. Considering the high prevalence of bladder dysfunctions in POMS, we should consider that nocturia, defined as the need to wake up to urinate during the night, could affect sleep quality, although no specific data are currently available on the pediatric population [
11,
12].
RLS, categorized among sleep-related movement disorders [
13], is characterized by an urge to move the legs, worsening in the evening, often accompanied by unpleasant sensations, and relieved by movement or walking. Diagnostic criteria, established by the International Restless Legs Syndrome Study Group (IRLSSG) [
14] and adapted for pediatric patients [
15] (
Table 1), include five key criteria.
RLS significantly impacts sleep quality, daily functioning, and mood [
16], but remains underdiagnosed, affecting 5–13% of the population in Europe and North America [
16,
17]. It can be primary or secondary, often linked to iron deficiency, pregnancy, renal failure, and neurological disorders such as MS and Parkinson’s disease [
18]. Though once thought to affect only middle-aged and older adults, RLS is now recognized in children, with a prevalence of 2–4% [
17,
18]. Risk factors include Periodic Limb Movement Disorder and a family history of RLS [
19]. The incidence of RLS in MS patients ranges between 13.5% and 65% [
20], much higher than in the general population, and correlates with longer disease duration and greater disability [
20,
21]. Only one study has specifically investigated RLS in POMS, showing a similar increased prevalence and its association with higher disability levels [
22].
Additionally, the IRLSSG recently identified Restless Sleep Disorder (RSD), characterized by sleep disruption and frequent nocturnal body movements involving large muscle groups, causing daytime impairment [
23]. Unlike RLS, RSD requires polysomnography for diagnosis (
Table 2).
While its prevalence in the general population is unknown, RSD has been found in 7.7% of children with sleep concerns [
24]. The pathogenesis of RSD is not well defined, though it may share some risk factors with RLS, such as iron deficiency [
25].
This study aims to assess the prevalence and characteristics of sleep disorders, particularly RLS, in our cohort of patients with POMS. We also aimed to describe their demographic, clinical, neuroradiological, and laboratory features, and to evaluate a possible correlation with quality of life.
4. Discussion
Our study suggests a higher proportion of SDs in POMS patients (61%) compared to the general pediatric population (25%), based on the existing epidemiological data [
32]. While data on the prevalence of SDs in POMS is limited, our findings align with studies on adult-onset MS, where SDs affect 42–65% of patients, approximately three times higher than in healthy individuals [
6].
Previous research on SDs in POMS has yielded mixed results. For example, Zafar et al. [
7] found no significant difference in sleep quality between POMS patients and healthy controls. Similarly, Stephens et al. [
33], using actigraphy, reported no differences in sleep parameters such as sleep time, sleep efficiency, number of awakenings, number of minutes awake after sleep onset, sleep latency, or time in bed between these groups. Conversely, Jaeggi et al. [
34] identified a higher risk of sleep-disordered breathing in youth with MS compared to controls. These discrepancies likely stem from differing methodologies and small sample sizes, emphasizing the need for larger studies with objective assessment tools.
Regarding RLS, we found a prevalence of 22.7% in the POMS population, consistent with previous reports of a higher prevalence of RLS in POMS compared to the general pediatric population (2–4%) [
19,
22]. Nearly all patients with RLS reported moderate-to-severe symptom severity and poor sleep quality, with a significant correlation between these factors. Furthermore, in our study, RLS patients were significantly older and had higher disability scores, in line with the findings of Yalcinkaya et al. [
22].
Although the average EDSS score in both groups is clinically almost non-contributory, it is important to consider this result in the context that pediatric patients are less likely to exhibit overt clinical impairment, showing better recovery post-relapse compared to adults and accumulating less disability in the early years of the disease [
35]. This is reflected in our sample, where most patients demonstrated either no clinical or only mild symptoms in a single functional domain. Moreover, it is well-established that incomplete recovery following the first relapse and the presence of clinical signs in the early years of the disease are negative prognostic factors for long-term disability accumulation [
35,
36]. Therefore, exploring this aspect longitudinally would be of great interest.
The high proportion of RLS in patients with MS, coupled with the association between greater disability and RLS in MS patients and the lower occurrence of familial RLS in these patients (10% versus 60% in idiopathic RLS [
18]), could suggest that RLS in MS may be a secondary form linked to the underlying disease. Several studies support this pathogenetic hypothesis, based on RLS’s connection to lesions in motor control pathways, particularly the pyramidal system and spinal cord [
37,
38]. In fact, the pathophysiology of RLS in MS is thought to involve disruption of dopaminergic circuits, which may be further exacerbated by iron deficiency [
38]. A key area of interest is the A11 diencephalo-spinal pathway, which is commonly affected by MS lesions [
39]. Dopaminergic A11 hypothalamic cells, the major source of dopamine in the spinal cord, project widely throughout it and modulate pain and locomotor networks [
38]. This hypothesis is reinforced by animal studies showing sleep-related limb movements in rats with spinal cord lesions [
40] and increased locomotor activity following damage to the A11 pathway [
41]. Furthermore, the A11 nucleus receives input from the suprachiasmatic nucleus, which regulates circadian rhythms, potentially explaining the fluctuation of RLS symptoms [
41].
Iron deficiency is another key element in RLS pathogenesis, as it plays a crucial role in dopaminergic regulation. Low brain iron stores are commonly observed in RLS patients, and ferritin serum levels below 50 ng/mL have been identified as pathogenic in secondary forms related to iron deficiency [
42]. The role of iron in RLS in MS remains controversial. Some studies have found increased iron deposits in brain tissues, while others report that serum ferritin levels remain normal in patients with stable MS but are elevated in those with chronically active disease, due to the inflammatory activation of ferritin as an acute-phase protein [
20,
43]. In our study, we found lower ferritin levels in the RLS group compared to the non-RLS group, though the difference did not reach statistical significance. Interestingly, a large portion of patients in the RLS group had ferritin levels below 50 ng/mL, and could be considered in the context of a potential link between iron deficiency and RLS in MS.
While studies in adult MS have found associations between the number or location of MRI lesions and RLS, our study did not show such a relationship [
37,
39]. This discrepancy between clinical and radiological findings is common for various symptoms in pediatric MS and may be due to more effective remyelination and lesion reversibility in younger patients [
44].
It is also possible that neuroinflammation plays a more significant role in pediatric RLS compared to neurodegeneration [
44]. OCB, IgG, and pleocytosis are well-established inflammatory markers in MS, correlating with disease severity and prognosis [
45,
46,
47]. However, no studies have investigated neuroinflammatory status through these CSF markers in patients with both MS and RLS, despite evidence suggesting that alterations in other systems, such as the endogenous opioid and melanocortin systems, play a role in RLS pathogenesis [
48]. Our study, in line with that of Yalcinkaya et al., did not find a statistically significant increase in CSF neuroinflammatory markers (OCB, IgG, and pleocytosis) in the RLS+ group. This suggests that in our sample, the inflammatory aggressiveness at disease onset may not be a decisive factor in the development of RLS. Unfortunately, we were not able to analyze CSF features closer to the time of the interview or include other markers employed to monitor disease severity and progression, such as neurofilament light chain. These findings highlight the need to broaden the analysis to identify more specific inflammatory biomarkers for RLS in MS.
In our sample, patients with RLS were significantly older compared to the group without RLS. Age is a known risk factor for RLS in both children and adults [
18,
19]. However, diagnosing RLS in younger children can be challenging due to difficulties in symptom description and incomplete reports from parents. Using age-appropriate tools, such as visual aids, can improve symptom description in younger children [
49].
Differentiating RLS from RSD is an emerging challenge. Half of our patients with RLS also met the clinical criteria for RSD, suggesting a potential overlap in the pathogenesis of these conditions, possibly involving iron deficiency and disrupted dopaminergic pathways [
23]. This finding warrants further investigation, given the limited literature on the topic.
This is the second study demonstrating the increased proportion of RLS in POMS patients compared to epidemiological data about the healthy pediatric population and its correlation with disability status.
Despite these interesting findings, our study has some limitations. We did not include a control group in our study, and we compared our results with the currently available epidemiological data, which could limit the validity and generalizability of our results, together with the relatively small sample size, affecting the ability to detect statistically significant differences in some analyses, such as correlations between RLS and blood iron profile, and not allowing the use of multivariate analyses, which would risk producing unreliable estimates. Additionally, the cross-sectional design does not allow for the determination of causal relationships between sleep disorders or RLS and other clinical symptoms or radiological parameters.
Data collection on sleep primarily relied on self-reported questionnaires, which may introduce response bias, particularly in pediatric patients who might struggle to accurately describe their symptoms. Moreover, we used two different sleep assessment tools, based on age validation, to ensure diagnostic accuracy; while this approach may introduce some heterogeneity that could affect data comparability, we tried to minimize this issue by reproducing the analyses within age-defined subgroups. The lack of objective tools, such as polysomnography or actigraphy, to assess sleep disorders is another limitation that could have reduced the accuracy of diagnosing certain conditions.
Finally, the variability in diagnostic criteria for RLS and other sleep disorders, combined with the challenge of distinguishing specific RLS symptoms from other pediatric conditions such as RSD, may have impacted diagnostic accuracy. Future studies should include larger samples, longitudinal methodologies, and objective diagnostic tools to enhance the accuracy and clinical relevance of these findings and to perform multivariable modeling to allow the identification of robust predictors of sleep outcomes.