Neurophysiological study of corticomotor pathways in restless legs syndrome

Defect of the Endogenous Inhibitory Pain System in Idiopathic Restless Legs Syndrome: A Laser Evoked Potentials Study

BACKGROUND

Restless legs syndrome (RLS) is a complex sensorimotor disorder. Symptoms worsen toward evening and at rest and are temporarily relieved by movement. Symptoms are perceived as painful in up to 45% of cases, and nociception system may be involved.

OBJECTIVES

To assess the descending diffuse noxious inhibitory control in RLS patients.

METHODS

Twenty-one RLS patients and twenty age and sex-matched healthy controls (HC) underwent a conditioned pain modulation protocol. Cutaneous heat stimuli were delivered via laser evoked potentials (LEPs) on the dorsum of the right hand (UL) and foot (LL). N2 and P2 latencies, N2/P2 amplitude and pain ratings (NRS) were recorded before (baseline), during, and after a heterotopic noxious conditioning stimulation (HNCS) application. The baseline/HNCS ratio was calculated for both UL and LL.

RESULTS

N2 and P2 latencies did not vary between groups at each condition and limbs. Both groups showed a physiological N2/P2 amplitude and NRS reduction during the HNCS condition in UL and LL in comparison to baseline and post conditions (all, P < 0.003). Between-groups comparisons revealed a significant lower amplitude reduction in RLS at the N2/P2 amplitude during the HNCS condition only for LL (RLS, 13.6 μV; HC, 10.1 μV; P = 0.004). Such result was confirmed by the significant difference at the ratio (RLS, 69%, HC, 52.5%; P = 0.038).

CONCLUSIONS

The lower physiological reduction during the HNCS condition at LL in RLS patients suggests a defect in the endogenous inhibitory pain system. Further studies should clarify the causal link of this finding, also investigating the circadian modulation of this paradigm. © 2023 International Parkinson and Movement Disorder Society.

Consensus guidelines on the construct validity of rodent models of restless legs syndrome

Our understanding of the causes and natural course of restless legs syndrome (RLS) is incomplete. The lack of objective diagnostic biomarkers remains a challenge for clinical research and for the development of valid animal models. As a task force of preclinical and clinical scientists, we have previously defined face validity parameters for rodent models of RLS. In this article, we establish new guidelines for the construct validity of RLS rodent models. To do so, we first determined and agreed on the risk, and triggering factors and pathophysiological mechanisms that influence RLS expressivity. We then selected 20 items considered to have sufficient support in the literature, which we grouped by sex and genetic factors, iron-related mechanisms, electrophysiological mechanisms, dopaminergic mechanisms, exposure to medications active in the central nervous system, and others. These factors and biological mechanisms were then translated into rodent bioequivalents deemed to be most appropriate for a rodent model of RLS. We also identified parameters by which to assess and quantify these bioequivalents. Investigating these factors, both individually and in combination, will help to identify their specific roles in the expression of rodent RLS-like phenotypes, which should provide significant translational implications for the diagnosis and treatment of RLS.

Contribution of transcranial magnetic stimulation in restless legs syndrome: pathophysiological insights and therapeutical approaches

Transcranial magnetic stimulation (TMS) may offer a reliable means to characterize significant pathophysiologic and neurochemical aspects of restless legs syndrome (RLS). Namely, TMS has revealed specific patterns of changes in cortical excitability and plasticity, in particular dysfunctional inhibitory mechanisms and sensorimotor integration, which are thought to be part of the pathophysiological mechanisms of RLS rather than reflect a non-specific consequence of sleep architecture alteration. If delivered repetitively, TMS is able to transiently modulate the neural activity of the stimulated and connected areas. Some studies have begun to therapeutically use repetitive TMS (rTMS) to improve sensory and motor disturbances in RLS. High-frequency rTMS applied over the primary motor cortex or the supplementary motor cortex, as well as low-frequency rTMS over the primary somatosensory cortex, seem to have transient beneficial effects. However, further studies with larger patient samples, repeated sessions, an optimized rTMS setup, and clinical follow-up are needed in order to corroborate preliminary results. Thus, we performed a systematic search of all the studies that have used TMS and rTMS techniques in patients with RLS.

Short-interval intracortical inhibition is decreased in restless legs syndrome across a range of severity

BACKGROUND

Decreased short-interval intracortical inhibition (SICI) to transcranial magnetic stimulation (TMS) of the primary motor cortex was described in subjects with restless legs syndrome/Willis-Ekbom disease (RLS/WED). It remained to be determined whether the magnitude of SICI decrease would be similar across levels of RLS/WED severity. Moreover, it was unknown whether, in addition to decreases in SICI, changes in cortical thickness or area could be detected in subjects with RLS/WED compared to controls. The objective of this study was to compare SICI, cortical thickness, and cortical area in subjects with idiopathic mild to moderate RLS/WED, severe to very severe RLS/WED, and controls.

METHODS

The severity of RLS/WED was assessed by the International Restless Legs Syndrome Severity Scale (IRLSS). SICI and 3T magnetic resonance imaging (MRI) data of subjects with RLS/WED and controls were compared. A receiver operating characteristic curve for SICI was designed for discrimination of participants with RLS/WED from controls. Cortical thickness and area were assessed by automated surface-based analysis.

RESULTS

SICI was significantly reduced in patients with mild to moderate and severe to very severe RLS/WED, compared to controls (one-way analysis of variance: F = 9.62, p < 0.001). Receiver operating characteristic curve analysis predicted RLS/WED when SICI was above 35% (area under the curve = 0.79, 95% CI 0.67-0.91, p < 0.001). Analyses of the whole brain and of regions of interest did not reveal differences in gray matter thickness or area between controls and subjects with RLS/WED.

CONCLUSION

SICI is an accurate cortical biomarker that can support the diagnosis of RLS/WED even in subjects with mild symptoms, but cortical thickness and area were not useful for discriminating subjects with this condition from controls.

Impaired Sensorimotor Integration in Restless Legs Syndrome

Objective: Restless legs syndrome (RLS) is a complicated sensorimotor syndrome that may be linked to changes in sensorimotor integration. The mechanism of such changes is unclear. The aim of this study was to investigate sensorimotor integration in patients with RLS through transcranial magnetic stimulation-motor evoked potentials (TMS-MEPs) preceded by peripheral electric stimulation.

Methods: Fourteen RLS patients and 12 healthy, age-matched controls were investigated. The clinical severity of RLS was evaluated based on the International Criteria of the International Restless Legs Syndrome Study Group (IRLSSG) severity scores. The tibial and median H-reflexes and the resting motor threshold (RMT) of the abductor pollicis brevis (APB) were tested in all 26 subjects. The RMT of the tibialis anterior (TA) was tested in 8 patients and 7 controls. All 26 subjects underwent measurement of unconditioned MEPs of the APB. Electric pulses were applied to the right median nerve, followed by TMS pulses over the left motor cortex at interstimulus intervals (ISIs) of 20, 25, 30, 50, 100, 150, and 200 ms. Unconditioned MEPs of the TA were measured in 8 patients and 7 controls. Electric pulses were applied to the right peroneal nerve, followed by TMS pulses over the left motor cortex at ISIs of 30, 35, 45, 60, 100, and 200 ms. The degree of modulation of MEPs by electric stimulation was expressed as the ratio of the conditioned MEP amplitude to the unconditioned MEP amplitude. Ratios <1 indicated inhibition, and ratios >1 indicated facilitation.

Results: No significant differences in RMT or H-reflex latencies or amplitudes were found between RLS patients and controls. A significant increase in unconditioned MEP amplitudes of the TA was observed in patients compared to controls (p = 0.03). Long-latency afferent inhibition (LAI) of the median nerve in RLS patients was decreased significantly at ISIs of 150 (p = 0.000) and 200 ms (p = 0.004). Upon peroneal nerve stimulation, no significant difference was observed between the two groups at any ISI.

Conclusions: Our results suggest increased motor cortical excitability of the legs and disturbed sensorimotor integration in RLS patients; this disturbance might originate at the cortical level.

Abnormal Sleep Delta Rhythm and Interregional Phase Synchrony in Patients with Restless Legs Syndrome and Their Reversal by Dopamine Agonist Treatment

Background and Purpose

The purpose of this study was to characterize abnormal cortical activity during sleep in restless legs syndrome (RLS) patients and to determine the effects of treatment with a dopamine agonist. Based on whole-brain electroencephalograms, we attempted to verify alterations in the functional network as well as the spectral power of neural activities during sleep in RLS patients and to determine whether the changes are reversed by treatment with pramipexole.

Methods

Twelve drug-naïve RLS patients participated in the study. Overnight polysomnography was performed before and after treatment: the first recording was made immediately prior to administering the first dose of pramipexole, and the second recording was made 12–16 weeks after commencing pramipexole administration. Sixteen age-matched healthy participants served as a control group. The spectral power and interregional phase synchrony were analyzed in 30-s epochs. The functional characteristics of the cortical network were quantified using graph-theory measures.

Results

The delta-band power was significantly increased and the small-world network characteristics in the delta band were disrupted in RLS patients compared to the healthy controls. These abnormalities were successfully treated by dopaminergic medication. The delta-band power was significantly correlated with the RLS severity score in the RLS patients prior to treatment.

Conclusions

Our findings suggest that the spectral and functional network characteristics of neural activities during sleep become abnormal in RLS patients, and these abnormalities can be successfully treated by a dopamine agonist.

The duration of the cortical silent period is not abnormal in Restless Legs Syndrome/Willis-Ekbom Disease

OBJECTIVE

To compare the duration of the cortical silent period (CSP) measured in a hand muscle in subjects with primary Restless Legs Syndrome (RLS/WED) and controls, using four different methods of analysis.

METHODS

The CSP to transcranial magnetic stimulation of the dominant motor cortex was assessed in the abductor digiti minimi of 33 subjects with RLS/WED and 24 controls. CSP duration was measured by an automated and three visually-guided methods.

RESULTS

There were significant differences between absolute values of CSP duration according to the method of analysis. However, irrespectively of the method used for CSP assessment, no differences were found between measurements performed in subjects with RLS/WED and subjects from the control group.

CONCLUSIONS

Absolute values of CSP durations analyzed by different methods should not be directly compared, because significantly different results can be obtained from the same data set.

SIGNIFICANCE

The CSP assessed from a hand muscle is unlikely to be a biomarker of primary RLS/WED. Our results highlight the importance of standardizing the definition of CSP onset and offset, as well as of describing in detail the methodology chosen to record and measure CSP duration, in order to enable comparisons between studies.

F-Wave Duration as a Specific and Sensitive Tool for the Diagnosis of Restless Legs Syndrome/Willis-Ekbom Disease

STUDY OBJECTIVES

Restless legs syndrome, also known as Willis-Ekbom disease (RLS/WED), is a frequent condition, though its pathophysiology is not completely understood. The diagnosis of RLS/WED relies on clinical criteria, and the only instrumental tool, the suggested immobilization test, may lead to equivocal results. Recently, neurophysiological parameters related to F-wave duration have been proposed as a diagnostic aid. The aim of this study is to assess and compare the diagnostic values of these parameters in diagnosis of RLS/WED.

METHODS

Fifteen women affected by primary RLS/WED and 17 age- and sex- matched healthy subjects. A complete electroneurographic evaluation, including nerve conduction studies (NCS), cutaneous silent period (CSP), and F-wave parameters, namely amplitude, F-wave duration (FWD), and the ratio between FWD and duration of the corresponding compound muscle action potential (FWD/CMAPD).

RESULTS

No subject showed alterations of the NCS. However, FWD and FWD/CMAPD of both upper and lower limbs were significantly longer in patients than controls. Tibial FWD/CMAPD best discriminated RLS/WED patients from controls. A cutoff of 2.06 yielded a sensitivity of 69.2%, a specificity of 94.1%, a positive predictive power of 90%, and a negative predictive power of 80% (area under the curve = 0.817; 95% confidence interval = 0.674-0.959). The combination of ulnar or tibial FWD/CMAPD increases the sensitivity (85.7%) while slightly decreasing the specificity (87.5%, positive predictive value: 85.7%, negative predictive value: 87.5%).

CONCLUSIONS

Lower limb FWD/CMAPD ratio may represent a supportive diagnostic tool, especially in cases of evening lower leg discomfort of unclear interpretation.

Central and peripheral nervous system excitability in restless legs syndrome

Neurophysiological techniques have been applied in restless legs syndrome (RLS) to obtain direct and indirect measures of central and peripheral nervous system excitability, as well as to probe different neurotransmission pathways. Data converge on the hypothesis that, from a pure electrophysiological perspective, RLS should be regarded as a complex sensorimotor disorder in which cortical, subcortical, spinal cord, and peripheral nerve generators are all involved in a network disorder, resulting in an enhanced excitability and/or decreased inhibition. Although the spinal component may have dominated in neurophysiological assessment, possibly because of better accessibility compared to the brainstem or cerebral components of a hypothetical dysfunction of the diencephalic A11 area, multiple mechanisms, such as reduced central inhibition and abnormal peripheral nerve function, contribute to the pathogenesis of RLS similarly to some chronic pain conditions. Dopamine transmission dysfunction, either primary or triggered by low iron and ferritin concentrations, may also bridge the gap between RLS and chronic pain entities. Further support of disturbed central and peripheral excitability in RLS is provided by the effectiveness of nonpharmacological tools, such as repetitive transcranial magnetic stimulation and transcutaneous spinal direct current stimulation, in transiently modulating neural excitability, thereby extending the therapeutic repertoire. Understanding the complex interaction of central and peripheral neuronal circuits in generating the symptoms of RLS is mandatory for a better refinement of its therapeutic support.

REM sleep behavior disorder and periodic leg movements during sleep in ALS

OBJECTIVE

To assess sleep characteristics and the occurrence of abnormal muscle activity during sleep, such as REM sleep without atonia (RSWA), REM sleep behavior disorder (RBD), and periodic leg movements during sleep (PLMS), in patients with amyotrophic lateral sclerosis (ALS).

METHODS

A total of 41 patients with ALS and 26 healthy subjects were submitted to clinical interview and overnight video-polysomnography.

RESULTS

A total of 22 patients with ALS (53.6%) reported poor sleep quality. Polysomnographic studies showed that patients with ALS had reduced total sleep time, increased wakefulness after sleep onset, shortened REM and slow-wave sleep, and decreased sleep efficiency, compared to controls. Polysomnographic abnormalities were not different in patients reporting good or poor sleep and were not correlated to clinical and demographic variables. The PLMS index was significantly higher in patients with ALS than in healthy subjects, and 22 patients (53.6%) showed a PLMS index > 15/h, vs 4 (15.4%) controls (P < 0.001). Finally, two patients with ALS (4.9%) had RBD, and two more patients presented RSWA (4.9%), whereas no controls showed abnormalities of REM sleep.

CONCLUSION

Patients with ALS frequently present abnormalities of sleep that can be documented both at the clinical interview and at the polysomnographic evaluation, including insomnia, fragmented sleep, and increased PLMS. Moreover, abnormalities of REM sleep can be found in some of these patients.

Transcranial magnetic stimulation for evaluation of motor cortical excitability in restless legs syndrome/Willis-Ekbom disease

There is no consensus about mechanisms underlying restless legs syndrome (RLS), also known as Willis-Ekbom disease (WED). Cortical excitability may be abnormal in RLS. Transcranial magnetic stimulation (TMS) can provide insight about cortical excitability. We reviewed studies about measures of excitability to TMS in RLS. Original studies published between January 1999 and January 2015 were searched in PubMed, Scopus, and Web of Science databases. Inclusion criteria were as follows: original studies involving primary RLS in patients from both sexes and ages between 18 and 85 years; TMS protocols clearly described; and they were written in English, in peer-reviewed journals. Fifteen manuscripts were identified. TMS protocols were heterogeneous across studies. Resting motor threshold, active motor threshold, and amplitudes of motor-evoked potentials were typically reported to be normal in RLS. A reduction in short-interval intracortical inhibition (SICI) was the most consistent finding, whereas conflicting results were described in regard to short-interval intracortical facilitation and the contralateral silent period. Decreased SICI can be reversed by treatment with dopaminergic agonists. Plasticity in the motor cortex and sensorimotor integration may be disrupted. TMS may become a useful biomarker of responsiveness to drug treatment in RLS. The field can benefit from increases in homogeneity and sizes of samples, as well as from decrease in methodological variability across studies.

Distinctive patterns of cortical excitability to transcranial magnetic stimulation in obstructive sleep apnea syndrome, restless legs syndrome, insomnia, and sleep deprivation

Altered responses to transcranial magnetic stimulation (TMS) in obstructive sleep apnea syndrome (OSAS), restless legs syndrome (RLS), insomnia, and sleep-deprived healthy subjects have been reported. We have reviewed the relevant literature in order to identify eventual distinctive electrocortical profiles based on single and paired-pulse TMS, sensorimotor modulation, plasticity-related and repetitive TMS measures. Although obtained from heterogeneous studies, the detected changes might be the result of the different pathophysiological substrates underlying OSAS, RLS, insomnia and sleep deprivation rather than reflect the general effect of non-specific sleep loss and instability. OSAS tends to exhibit an increased motor cortex inhibition, which is reduced in RLS; intracortical excitability seems to be in favor of an "activating" profile in chronic insomnia and in sleep-deprived healthy individuals. Abnormal plasticity-related TMS phenomena have been demonstrated in OSAS and RLS. This review provides a perspective of TMS techniques by further understanding the role of neurotransmission pathways and plastic remodeling of neuronal networks involved in common sleep disorders. TMS might be considered a valuable tool in the assessment of sleep disorders, the evaluation of the effect of therapy and the design of non-pharmacological approaches.

Direct comparison of cortical excitability to transcranial magnetic stimulation in obstructive sleep apnea syndrome and restless legs syndrome

OBJECTIVE

Changes to transcranial magnetic stimulation (TMS) have been reported in obstructive sleep apnea syndrome (OSAS) and restless legs syndrome (RLS), although no direct comparison study is available. The aim of this new investigation is to assess and compare cortical excitability of OSAS and RLS patients using the same methodology and under the same experimental conditions.

METHODS

Fourteen patients with OSAS and 12 with RLS were compared to 14 age-matched controls. All patients were untreated and had a severe degree of disease. Resting motor threshold (rMT), cortical silent period (CSP) and motor evoked potentials MEPs, as well as intracortical inhibition (ICI) and facilitation at interstimulus interval (ISI) of 3 and 10 ms, respectively, were explored from the right first dorsal interosseous muscle, during wakefulness.

RESULTS

rMT was higher in OSAS than in RLS and controls. CSP was shorter in RLS only when compared to apneic patients, whereas it was similar between OSAS and controls. OSAS subjects exhibited slightly prolonged central motor conductivity, whereas MEP amplitude was smaller in both patient groups. The ICI ratio at ISI of 3 ms was decreased in RLS patients only.

CONCLUSIONS

Distinct changes of responses at TMS were found, probably connected with the different neurophysiological substrates underlying OSAS and RLS and could not be interpreted as a mere reflection of the effects of sleep architecture alteration. TMS can be considered an additional tool for the understanding of clinical and pathophysiological aspects of sleep disorders, and possibly for the evaluation of the effect of therapy.

Pharmacological treatment of sleep disorders and its relationship with neuroplasticity

Sleep and wakefulness are regulated by complex brain circuits located in the brain stem, thalamus, subthalamus, hypothalamus, basal forebrain, and cerebral cortex. Wakefulness and NREM and REM sleep are modulated by the interactions between neurotransmitters that promote arousal and neurotransmitters that promote sleep. Various lines of evidence suggest that sleep disorders may negatively affect neuronal plasticity and cognitive function. Pharmacological treatments may alleviate these effects but may also have adverse side effects by themselves. This chapter discusses the relationship between sleep disorders, pharmacological treatments, and brain plasticity, including the treatment of insomnia, hypersomnias such as narcolepsy, restless legs syndrome (RLS), obstructive sleep apnea (OSA), and parasomnias.

Transcranial magnetic stimulation and sleep disorders: pathophysiologic insights

The neural mechanisms underlying the development of the most common intrinsic sleep disorders are not completely known. Therefore, there is a great need for noninvasive tools which can be used to better understand the pathophysiology of these diseases. Transcranial magnetic stimulation (TMS) offers a method to noninvasively investigate the functional integrity of the motor cortex and its corticospinal projections in neurologic and psychiatric diseases. To date, TMS studies have revealed cortical and corticospinal dysfunction in several sleep disorders, with cortical hyperexcitability being a characteristic feature in some disorders (i.e., the restless legs syndrome) and cortical hypoexcitability being a well-established finding in others (i.e., obstructive sleep apnea syndrome narcolepsy). Several research groups also have applied TMS to evaluate the effects of pharmacologic agents, such as dopaminergic agent or wake-promoting substances. Our review will focus on the mechanisms underlying the generation of abnormal TMS measures in the different types of sleep disorders, the contribution of TMS in enhancing the understanding of their pathophysiology, and the potential diagnostic utility of TMS techniques. We also briefly discussed the possible future implications for improving therapeutic approaches.

Recurrent CSPs after Transcranial Magnetic Stimulation of Motor Cortex in Restless Legs Syndrome

Aims. The aim of this study was to investigate the motor control and central silent period (CSP) in restless legs syndrome (RLS). Methods. Transcranial magnetic stimulation was focused on the dominant and nondominant hemispheric areas of motor cortex in six subjects with RLS and six controls. The responses were recorded on the contralateral abductor digiti minimi (ADM) and tibialis anterior (TA) muscles with intramuscular needle electrodes. Results. No significant differences were found in the motor conduction or central motor conduction time, in the latency, or in the duration of the CSPs between or within the groups, but multiple CSPs were observed in both groups. The number of the CSPs was significantly higher in both ADMs and in the dominant TA (P ≤ 0.01) in the RLS group compared to the controls. Conclusion. Descending motor pathways functioned correctly in both groups. The occurrence of the recurrent CSPs predominantly in the RLS group could be a sign of a change of function in the inhibitory control system. Further research is needed to clarify the role of the intramuscular recording technique and especially the role of the subcortical generators in the feedback regulation of the central nervous system in RLS.

Cırcadian changes in cortical excitability in restless legs syndrome

Various investigations have revealed a widespread and somewhat controversial pattern of cerebral, cerebellar and brainstem involvement in the pathophysiology of restless legs syndrome (RLS). However, several studies which investigated functional or structural aspects indicated cortical involvement in RLS. In this study, we aimed to analyze circadian changes of cortical excitability in idiopathic RLS patients by means of transcranial magnetic stimulation (TMS). Eleven idiopathic RLS patients and eight healthy age and sex matched subjects were investigated using single-pulse TMS and motor nerve conduction studies during early afternoon when there were no symptoms and late at night (22:00-23:00) when the symptoms reappeared. Central motor conduction time, latencies and amplitudes of scalp and cervical motor evoked potentials, resting and active motor thresholds, and cortical silent period were measured. Measured parameters were similar between RLS patients and healthy subjects during the daytime. At night, cortical silent periods tended to shorten, and motor thresholds tended to decrease in the RLS group, whereas in controls they tended to increase. At night, active motor-threshold measurements were significantly lower in the RLS group (28.5 ± 6.2% vs 40.4 ± 8.4%, p=0.006). Therefore, we propose that in patients with RLS, conduction along the motor corticospinal axons is normal, with the possible loss of subcortical inhibition at nighttime.

Restless legs syndrome and conditions associated with metabolic dysregulation, sympathoadrenal dysfunction, and cardiovascular disease risk: a systematic review

Restless legs syndrome (RLS) is a distressing sleep and sensorimotor disorder that affects a large percentage of adults in the western industrialized world and is associated with profound reductions in quality of life. However, the etiology of RLS remains incompletely understood. Enhanced understanding regarding both the antecedents and sequelae of RLS could shed new light on the pathogenesis of RLS. Evidence from an emerging body of literature suggests associations between RLS and diabetes, hypertension, obesity, and related conditions linked to sympathetic activation and metabolic dysregulation, raising the possibility that these factors may likewise play a significant role in the development and progression of RLS, and could help explain the recently documented associations between RLS and subsequent cardiovascular disease. However, the relation between RLS and these chronic conditions has received relatively little attention to date, although potential implications for the pathogenesis and treatment of RLS could be considerable. In this paper, we systematically review the recently published literature regarding the association of RLS to cardiovascular disease and related risk factors characterized by sympathoadrenal and metabolic dysregulation, discuss potential underlying mechanisms, and outline some possible directions for future research.

Is disturbed intracortical excitability a stable trait of chronic insomnia? A study using transcranial magnetic stimulation before and after multimodal sleep therapy

BACKGROUND

Chronic insomnia is a poorly understood disorder. Risk factors for developing chronic insomnia are largely unknown, yet disturbances in brain indexes of arousal seem to accompany the disorder. We here investigate whether insomnia patients and control participants differ with respect to brain responses to direct stimulation, i.e., cortical excitability. Transcranial magnetic stimulation (TMS) offers a method to directly investigate the excitability level of the human cerebral cortex in psychiatric and neurological disease.

METHODS

We investigated cortical excitability in 16 insomnia patients and 14 carefully matched control participants using absolute and relative amplitudes of motor evoked potentials in response to single- and paired-pulse stimulation using TMS.

RESULTS

Nonmedicated insomnia patients showed, first, an exaggerated absolute response to both suprathreshold single- and paired-pulse stimulation compared with control participants and second, a reduced relative response to paired-pulse stimulation at long interpulse intervals (i.e., a reduced intracortical facilitation). The abnormal excitability persisted despite sleep therapy that effectively improved sleep quality as well as behavioral and neuroimaging indexes of brain function.

CONCLUSIONS

The results suggest that a subtly disturbed intracortical excitability characterizes patients with chronic insomnia: a relatively reduced intracortical facilitation in the context of a globally increased absolute excitability. The findings do not resemble TMS findings after sleep deprivation or in sleep apnea and thus seem specific to insomnia. They may offer diagnostic value and implications for assessment of risk to develop this common and disabling disorder.

Changes of cortical excitability after dopaminergic treatment in restless legs syndrome

OBJECTIVE

Dopaminergic pathways are most likely involved in the pathophysiology of restless legs syndrome (RLS). In previous investigations, an alteration of cortical excitability was suggested to be related to a dopaminergic dysfunction in RLS. The purpose of our study was to compare practice-dependent plasticity in RLS patients before and after a month of dopaminergic treatment.

METHODS

Single-pulse transcranial magnetic stimulation (TMS) was used to define motor evoked potential (MEP) amplitude, motor threshold, and silent period (SP) as well. Subjects performed three exercise blocks (bimanual motor task). MEP amplitude, registered immediately after each exercise block and after a rest period, was compared to baseline. The time course of intra-cortical inhibition was tested using paired-pulse TMS at short inter-stimulus intervals. For the single-pulse TMS procedures, we enrolled 12 patients affected by primary RLS and 12 normal subjects. For the paired-pulse TMS procedures, only six patients underwent the examination. RLS patients underwent the examination in both pre- and post-dopaminergic treatment conditions.

RESULTS

In RLS patients MEP amplitude increased after the rest period only in the post-treatment condition, showing a delayed facilitation. After exercise, MEP amplitude increased, but not enough to be significant, showing a positive trend but not a clear-cut post-exercise facilitation. In the pre-treatment condition instead, MEP amplitude did not change either after rest period or after exercise. RLS patients showed a marked increase of the central motor inhibition, assessed by using paired-pulse TMS at short inter-stimulus intervals after pramipexole treatment. On the contrary, the duration of the SP did not change compared to the pre-treatment condition.

CONCLUSIONS

In RLS patients after dopaminergic treatment, the main finding was the changing of MEP amplitude after rest following a motor task. Since dopaminergic treatment can reverse delayed facilitation in RLS, we hypothesized that cortical plasticity related to dopaminergic systems may play a crucial role in RLS pathophysiology.

Cortical involvement in the sensory and motor symptoms of primary restless legs syndrome

BACKGROUND

Restless legs syndrome (RLS) is characterized by closely interrelated motor and sensory disorders. Two types of involuntary movement can be observed: periodic leg movements during wakefulness (PLMW) and periodic leg movements during sleep (PLMS). Basal ganglia dysfunction in primary RLS has often been suggested. However, clinical observations raise the hypothesis of sensorimotor cortical involvement in RLS symptoms. Here, we explored cortical function via movement-related beta and mu rhythm reactivity.

METHODS

Twelve patients with idiopathic, primary RLS were investigated and compared with 10 healthy subjects. In the patient group, we analyzed event-related beta and mu (de)synchronization (ERD/S) for PLMS and PLMW during a suggested immobilization test (SIT). An ERD/S analysis was also performed in patients and controls during self-paced right ankle dorsal flexion at 8:30 PM (i.e., the symptomatic period for patients) and 8:30 AM (the asymptomatic period).

RESULTS

Before PLMS, there was no ERD. Intense ERS was recorded after PLMS. As with voluntary movement, cortical ERD was always observed before PLMW. After PLMW, ERS had a diffuse scalp distribution. Furthermore, the ERS and ERD amplitudes and durations for voluntary movement were greater during the symptomatic period than during the asymptomatic period and in comparison with healthy controls, who presented an evening decrease in these parameters. Patients and controls had similar ERD and ERS patterns in the morning.

CONCLUSION

On the basis of a rhythm reactivity study, we conclude that the symptoms of RLS are related to cortical sensorimotor dysfunction.

Pathophysiology of restless legs syndrome: evidence for iron involvement

Neuroimaging, analysis of cerebrospinal fluid, and studies on postmortem tissue are generating data that support the concept that iron availability to the brain is a contributory process to, if not a cause of, restless legs syndrome. These data are reviewed and related to the dopaminergic system because of the use of dopamine agents in treating restless legs syndrome.

Update of the pathophysiology of the restless-legs-syndrome

The Restless Legs Syndrome (RLS) is a heterogeneous disease. Symptomatic or secondary forms encompass iron deficiency, uremia, pregnancy, polyneuropathy, and other causes. The so-called idiopathic RLS syndrome preferentially affects patients with a younger onset before the age of 30. Here we summarize pathophysiological results along the anatomical route, beginning at the cortex and followed by the basal ganglia, thalamus, A11 neurones, substantia nigra, brainstem nuclei, and spinal cord. Genetic risk variants for RLS have recently been identified in two genes, one of them the homeobox gene MEIS1, known to be involved in embryonic development and variants in a second locus containing the genes encoding mitogen-activated protein kinase MAP2K5, and the transcription factor LBXCOR1. A third one, the BTBD9 gene with unknown function encodes a BTB(POZ) domain. Accordingly, new concepts on pathophysiology have to bridge conventional knowledge with possible consequences deriving from these findings. Furthermore, this may create a framework to help understand why dopamine, opioid, and some anticonvulsant therapies are effective in RLS patients.

Fibromyalgia and Overlapping Disorders: The Unifying Concept of Central Sensitivity Syndromes

OBJECTIVES

To discuss fibromyalgia syndrome (FMS) and overlapping conditions, eg, irritable bowel syndrome, headaches, and chronic fatigue syndrome, within the concept of central sensitivity syndromes (CSS).

METHODS

A critical overview of the literature and incorporation of the author's own views.

RESULTS

The concept of CSS seems viable. It is based on mutual associations among the CSS conditions as well as the evidence for central sensitization (CS) among several CSS members. However, such evidence is weak or not available in other members at this time, requiring further studies. The biology of CSS is based on neuroendocrine aberrations, including CS, that interact with psychosocial factors to cause a number of symptoms.

CONCLUSIONS

CSS is an important new concept that embraces the biopsychosocial model of disease. Further critical studies are warranted to fully test this concept. However, it seems to have important significance for new directions for research and patient care involving physician and patient education. Each patient, irrespective of diagnosis, should be treated as an individual considering both the biological and psychosocial contributions to his or her symptoms and suffering.

Pathophysiological concepts of restless legs syndrome

Pathophysiological concepts of restless legs syndrome (RLS) are based mainly on neuroimaging and on neurophysiological data. Furthermore treatment effects contribute essentially to the present understanding of the disease, unless the genetic progress expected in the near future will clarify substantially open issues. The concept agreed on assumes a dysfunction of the dopaminergic system, possibly on the level of striatal and/or spinal dopamine receptors, and the A11 neuron group localized in the hypothalamus as an integrated part of the system. These neurons modulate spinal excitability, alterations of which in turn affect sensory processing predominantly of leg afferents in brain stem structures. Neurophysiologically excitability alterations can be measured by a variety of methods such as determination of pain thresholds, H-reflex testing, and quantitative sensory testing.

Possible joint origin of restless leg syndrome (RLS) and migraine

Sleep disorders have been described in migraine patients. Among sleep disorders RLS has been reported in up to one-third of migraineurs. Adverse effects of anti migraine therapy by dopamine antagonists can not fully explain this association. Therefore we present the hypothesis that RLS and migraine may have a joint origin. The hypothesis is supported by: (1) the same genetic origin for migraine without aura and RLS in single Italian family on chromosome 14q21; this gene codes survival motor neuron-interacting protein 1 (SIP1) which can play role in both diseases. (2) Correlation of both RLS and migraine with fibromyalgia. (3) Alteration of cortical excitability in both migraine and RLS.

Cabergoline reverses cortical hyperexcitability in patients with restless legs syndrome

OBJECTIVE

To reverse the profile of abnormal intracortical excitability in patients with restless legs syndrome (RLS) by administering the dopaminergic agonist cabergoline.

METHODS

The effects of this drug on motor cortex excitability were examined with a range of transcranial magnetic stimulation (TMS) protocols before and after administration of cabergoline over a period of 4 weeks in 14 patients with RLS and in 15 healthy volunteers. Measures of cortical excitability included central motor conduction time; resting and active motor threshold to TMS; duration of the cortical silent period; short latency intracortical inhibition (SICI) and intracortical facilitation using a paired-pulse TMS technique.

RESULTS

Short latency intracortical inhibition was significantly reduced in RLS patients compared with the controls and this abnormal profile was reversed by treatment with cabergoline; the other TMS parameters did not differ significantly from the controls and remained unaffected after treatment with cabergoline. Cabergoline had no effect on cortical excitability of the normal subjects.

CONCLUSIONS

As dopaminergic drugs are known to increase SICI, our findings suggest that RLS may be caused by a central nervous system dopaminergic dysfunction. This study demonstrates that the cortical hyperexcitability of RLS is reversed by cabergoline, and provides physiological evidence that this dopamine agonist may be a potentially efficacious option for the treatment of RLS.

[Neurophysiological and neuroimaging studies for restless legs syndrome and periodic leg movement disorder]

The restless legs syndrome (RLS) is a clinical diagnosis based on the four essential criteria defined by the International Restless Legs Syndrome Study Group (IRLSSG). An idiopathic form can be separated from a symptomatic form. Neurophysiological studies have investigated the pathophysiology of the idiopathic RLS or have been used to exclude a symptomatic cause, in particular polyneuropathy. So far cortical excitability changes, corticomotor, somatosensory and auditory pathways, spinal cord excitability, B-wave rhythm and cycling alternating pattern, as well as reflex mechanisms have been investigated by electroencephalography, evoked potentials, Bereitschaftspotentials, nerve conduction and thermal threshold measurements, electromyography, transcranial Doppler sonography, measurements of the spinal flexor reflex as well as neuroimaging techniques. The etiology of the RLS cannot be revealed by these methods, neurophysiological studies in RLS are, however, useful for a better understanding of the pathophysiology and for exclusion of a polyneuropathy or other symptomatic causes. In addition to neurophysiological investigations, small fiber neuropathy, which seems to be a more common finding in RLS patients than expected to date, may need biopsy for confirmation. This review will focus on investigations of the different systems involved with diverse neurophysiological methods.

The restless legs syndrome

The restless legs syndrome (RLS) is one of the commonest neurological sensorimotor disorders at least in the Western countries and is often associated with periodic limb movements (PLM) during sleep leading to severe insomnia. However, it remains largely underdiagnosed and its underlying pathogenesis is presently unknown. Women are more affected than men and early-onset disease is associated with familial cases. A genetic origin has been suggested but the mode of inheritance is unknown. Secondary causes of RLS may share a common underlying pathophysiology implicating iron deficiency or misuse. The excellent response to dopaminegic drugs points to a central role of dopamine in the pathophysiology of RLS. Iron may also represent a primary factor in the development of RLS, as suggested by recent pathological and brain imaging studies. However, the way dopamine and iron, and probably other compounds, interact to generate the circadian pattern in the occurrence of RLS and PLM symptoms remains unknown. The same is also the case for the level of interaction of the two compounds within the central nervous system (CNS). Recent electrophysiological and animals studies suggest that complex spinal mechanisms are involved in the generation of RLS and PLM symptomatology. Dopamine modulation of spinal reflexes through dopamine D3 receptors was recently highlighted in animal models. The present review suggests that RLS is a complex disorder that may result from a complex dysfunction of interacting neuronal networks at one or several levels of the CNS and involving numerous neurotransmitter systems.

Abnormal H-reflexes in periodic limb movement disorder; impact on understanding the pathophysiology of the disorder

OBJECTIVE

To get more insight in the pathophysiological basis of periodic limb movement disorder (PLMD) with or without restless legs syndrome (RLS), we investigated whether these patients have spontaneous changes in H-reflexes or show altered reflex patterns after (external) inhibition or excitation of the relevant spinal segment.

METHODS

The ratio of the peak-to-peak values of the maximal soleus H-reflex and the maximal direct muscle potential (H/M ratio), H-reflex recruitment curves, vibratory inhibition and recovery curves of the soleus H-reflex in double stimulus experiments were measured in 9 PLMD patients and 11 controls.

RESULTS

In comparison to controls the vibratory inhibition, predominantly reflecting pre-synaptic inhibitory action, was depressed in PLMD patients. The soleus H-reflex recovery curves showed increased late facilitation and depressed late inhibition, both reflecting diminished inhibition due to post-synaptic central activity.

CONCLUSIONS

Our data indicate diminished inhibition at spinal level in PLMD patients. This is probably due to altered function of the descending spinal tracts, peripheral influence or changes at the inter-neural circuitry at spinal level itself, or combinations of these 3 possibilities.

SIGNIFICANCE

The results of this study give further insight in the pathophysiology of PLMD and RLS by stressing the importance of diminished central inhibition.

Evaluation of periodic leg movements and associated transcranial magnetic stimulation parameters in restless legs syndrome

Restless legs syndrome (RLS), a sensorimotor disorder characterized by unpleasant sensations commonly localized in the legs, is frequently associated with periodic limb movements (PLMs) during sleep. We investigated the role of transcranial magnetic stimulation (TMS) and cortical silent period (CSP) duration as diagnostic and monitoring tools in 20 patients with primary RLS before and after 1 month of treatment and also studied 15 normal age- and gender-matched subjects. Polysomnographic assessment was undertaken and the PLM index determined in 17 of the 20 patients. We also studied the correlation between sleep efficiency index and CSP duration because of the increasing severity of the sleep disturbance and PLMs in patients with RLS. Our results demonstrate that the duration of the CSP was reduced in patients with RLS, and that dopaminergic treatment normalized this duration. There was no correlation between the PLM index and CSP duration. It may be speculated that PLMs and the CSP are due to different inhibitory mechanisms and they may be used separately as diagnostic and monitoring tools in patients with primary RLS.

Influence of sleep stage and wakefulness on spectral EEG activity and heart rate variations around periodic leg movements

OBJECTIVE

Typical changes in spectral electroencephalographic (EEG) activity and heart rate (HR) have been described in periodic leg movements (PLM) associated with or without microarousals (MA). We aimed to determine the effects of sleep stage and wakefulness on these responses to ascertain whether a common pattern of EEG and HR activation takes place.

METHODS

The time course of EEG spectral activity and HR variability associated with PLM was analysed in 13 patients during light NREM sleep, rapid-eye-movement (REM) sleep and wakefulness. The same analysis was also conducted for PLM without MA occurring in stage 2.

RESULTS

A significant EEG and electrocardiogram (ECG) activation was found associated with PLM during sleep, but not during wakefulness. While in light NREM sleep, an increase in delta and theta bands was detected before the PLM onset, in REM sleep the EEG activation occurred simultaneously with the PLM onset. Moreover, during stage 1 and REM sleep, alpha and fast frequencies tended to remain sustained after the PLM onset. In contrast, during wakefulness, a small and not significant increase in cerebral activity was present, starting at the PLM onset and persisting in the post-movement period. A typical pattern of cardiac response was present during NREM and REM sleep, the autonomic activation being lesser and prolonged during wakefulness.

CONCLUSIONS

We conclude that the EEG and HR responses to PLM differ between sleep stages and wakefulness with lesser changes found during wakefulness.

SIGNIFICANCE

These findings suggest that specific sleep state-dependent mechanisms may underlie the occurrence of PLM.