From brain to spinal cord: neuromodulation by direct current stimulation and its promising effects as a treatment option for restless legs syndrome

Neuromodulation is a fast-growing field of mostly non-invasive therapies, which includes spinal cord stimulation (SCS), transcranial direct current stimulation (tDCS), vagal nerve stimulation (VNS), peripheral nerve stimulation, transcranial magnetic stimulation (TMS) and transcutaneous spinal direct current stimulation (tsDCS). This narrative review offers an overview of the therapy options, especially of tDCS and tsDCS for chronic pain and spinal cord injury. Finally, we discuss the potential of tsDCS in Restless Legs Syndrome as a promising non-invasive, alternative therapy to medication therapy.

Case report: An OSAS patient with comorbid RLS/PLMS - a closer look at the causes of PAP intolerance

Since 2017, hypoglossal nerve stimulation has been included in the S3-guidelines on restorative sleep/sleep disorders as an alternative treatment for patients with obstructive sleep related breathing disorders who cannot tolerate conventional PAP-therapy. Under certain conditions, some of these patients have the option to have a tongue pacemaker implanted during a surgical procedure to regain a restful night's sleep. However, in some cases it does not solve the problem. In this case report, we present a patient who continued to have restless sleep despite implantation of a hypoglossus nerve stimulator. We provide a closer look at the underlying causes of PAP intolerance and emphasize the importance of a combined pneumological and neurological approach to sleep medicine in sleep-specific therapy evaluation.

Identification of novel risk loci for restless legs syndrome in genome-wide association studies in individuals of European ancestry: a meta-analysis

BACKGROUND

Restless legs syndrome is a prevalent chronic neurological disorder with potentially severe mental and physical health consequences. Clearer understanding of the underlying pathophysiology is needed to improve treatment options. We did a meta-analysis of genome-wide association studies (GWASs) to identify potential molecular targets.

METHODS

In the discovery stage, we combined three GWAS datasets (EU-RLS GENE, INTERVAL, and 23andMe) with diagnosis data collected from 2003 to 2017, in face-to-face interviews or via questionnaires, and involving 15 126 cases and 95 725 controls of European ancestry. We identified common variants by fixed-effect inverse-variance meta-analysis. Significant genome-wide signals (p≤5 × 10-8) were tested for replication in an independent GWAS of 30 770 cases and 286 913 controls, followed by a joint analysis of the discovery and replication stages. We did gene annotation, pathway, and gene-set-enrichment analyses and studied the genetic correlations between restless legs syndrome and traits of interest.

FINDINGS

We identified and replicated 13 new risk loci for restless legs syndrome and confirmed the previously identified six risk loci. MEIS1 was confirmed as the strongest genetic risk factor for restless legs syndrome (odds ratio 1·92, 95% CI 1·85-1·99). Gene prioritisation, enrichment, and genetic correlation analyses showed that identified pathways were related to neurodevelopment and highlighted genes linked to axon guidance (associated with SEMA6D), synapse formation (NTNG1), and neuronal specification (HOXB cluster family and MYT1).

INTERPRETATION

Identification of new candidate genes and associated pathways will inform future functional research. Advances in understanding of the molecular mechanisms that underlie restless legs syndrome could lead to new treatment options. We focused on common variants; thus, additional studies are needed to dissect the roles of rare and structural variations.

FUNDING

Deutsche Forschungsgemeinschaft, Helmholtz Zentrum München-Deutsches Forschungszentrum für Gesundheit und Umwelt, National Research Institutions, NHS Blood and Transplant, National Institute for Health Research, British Heart Foundation, European Commission, European Research Council, National Institutes of Health, National Institute of Neurological Disorders and Stroke, NIH Research Cambridge Biomedical Research Centre, and UK Medical Research Council.

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.

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.

Targeted resequencing and systematic in vivo functional testing identifies rare variants in MEIS1 as significant contributors to restless legs syndrome

Restless legs syndrome (RLS) is a common neurologic condition characterized by nocturnal dysesthesias and an urge to move, affecting the legs. RLS is a complex trait, for which genome-wide association studies (GWASs) have identified common susceptibility alleles of modest (OR 1.2-1.7) risk at six genomic loci. Among these, variants in MEIS1 have emerged as the largest risk factors for RLS, suggesting that perturbations in this transcription factor might be causally related to RLS susceptibility. To establish this causality, direction of effect, and total genetic burden of MEIS1, we interrogated 188 case subjects and 182 control subjects for rare alleles not captured by previous GWASs, followed by genotyping of ∼3,000 case subjects and 3,000 control subjects, and concluded with systematic functionalization of all discovered variants using a previously established in vivo model of neurogenesis. We observed a significant excess of rare MEIS1 variants in individuals with RLS. Subsequent assessment of all nonsynonymous variants by in vivo complementation revealed an excess of loss-of-function alleles in individuals with RLS. Strikingly, these alleles compromised the function of the canonical MEIS1 splice isoform but were irrelevant to an isoform known to utilize an alternative 3' sequence. Our data link MEIS1 loss of function to the etiopathology of RLS, highlight how combined sequencing and systematic functional annotation of rare variation at GWAS loci can detect risk burden, and offer a plausible explanation for the specificity of phenotypic expressivity of loss-of-function alleles at a locus broadly necessary for neurogenesis and neurodevelopment.

Blood cis-eQTL analysis fails to identify novel association signals among sub-threshold candidates from genome-wide association studies in restless legs syndrome

Restless legs syndrome (RLS) is a common neurologic disorder characterized by nightly dysesthesias affecting the legs primarily during periods of rest and relieved by movement. RLS is a complex genetic disease and susceptibility factors in six genomic regions have been identified by means of genome-wide association studies (GWAS). For some complex genetic traits, expression quantitative trait loci (eQTLs) are enriched among trait-associated single nucleotide polymorphisms (SNPs). With the aim of identifying new genetic susceptibility factors for RLS, we assessed the 332 best-associated SNPs from the genome-wide phase of the to date largest RLS GWAS for cis-eQTL effects in peripheral blood from individuals of European descent. In 740 individuals belonging to the KORA general population cohort, 52 cis-eQTLs with p_nominal<10⁻³ were identified, while in 976 individuals belonging to the SHIP-TREND general population study 53 cis-eQTLs with p_nominal<10⁻³ were present. 23 of these cis-eQTLs overlapped between the two cohorts. Subsequently, the twelve of the 23 cis-eQTL SNPs, which were not located at an already published RLS-associated locus, were tested for association in 2449 RLS cases and 1462 controls. The top SNP, located in the DET1 gene, was nominally significant (p<0.05) but did not withstand correction for multiple testing (p = 0.42). Although a similar approach has been used successfully with regard to other complex diseases, we were unable to identify new genetic susceptibility factor for RLS by adding this novel level of functional assessment to RLS GWAS data.

Differentiation of typical and atypical Parkinson syndromes by quantitative MR imaging

BACKGROUND AND PURPOSE

The differential diagnosis of Parkinson syndromes remains a major challenge. Quantitative MR imaging can aid in this classification, but it is unclear which of the proposed techniques is best suited for this task. We, therefore, conducted a head-to-head study with different quantitative MR imaging measurements in patients with IPS, MSA-type Parkinson, PSP, and healthy elderly controls.

MATERIALS AND METHODS

Thirty-one patients and 13 controls underwent a comprehensive quantitative MR imaging protocol including R2*-, R2- and R1-mapping, magnetization transfer, and DTI with manual region-of-interest measurements in basal ganglia regions. Group differences were assessed with a post hoc ANOVA with a Bonferroni error correction and an ROC.

RESULTS

The best separation of MSA from IPS in patients and controls could be achieved with R2*-mapping in the PU, with an ROC AUC of ≤0.96, resulting in a sensitivity of 77.8% (with a specificity 100%). MD was increased in patients with PSP compared with controls and to a lesser extent compared with those with IPS and MSA in the SN.

CONCLUSIONS

Among the applied quantitative MR imaging methods, R2*-mapping seems to have the best predictive power to separate patients with MSA from those with IPS, and DTI for identifying PSP.

Individual voxel-based subtype prediction can differentiate progressive supranuclear palsy from idiopathic Parkinson syndrome and healthy controls

Voxel-based morphometry (VBM) shows a differentiated pattern in patients with atypical Parkinson syndrome but so far has had little impact in individual cases. It is desirable to translate VBM findings into clinical practice and individual classification. To this end, we examined whether a support vector machine (SVM) can provide useful accuracies for the differential diagnosis. We acquired a volumetric 3D T1-weighted MRI of 21 patients with idiopathic Parkinson syndrome (IPS), 11 multiple systems atrophy (MSA-P) and 10 progressive supranuclear palsy (PSP), and 22 healthy controls. Images were segmented, normalized, and compared at group level with SPM8 in a classical VBM design. Next, a SVM analysis was performed on an individual basis with leave-one-out cross-validation. VBM showed a strong white matter loss in the mesencephalon of patients with PSP, a putaminal grey matter loss in MSA, and a cerebellar grey matter loss in patients with PSP compared with IPS. The SVM allowed for an individual classification in PSP versus IPS with up to 96.8% accuracy with 90% sensitivity and 100% specificity. In MSA versus IPS, an accuracy of 71.9% was achieved; sensitivity, however, was low with 36.4%. Patients with IPS could not be differentiated from controls. In summary, a voxel-based SVM analysis allows for a reliable classification of individual cases in PSP that can be directly clinically useful. For patients with MSA and IPS, further developments like quantitative MRI are needed.

Transcranial direct current stimulation of the motor cortex induces distinct changes in thermal and mechanical sensory percepts

OBJECTIVE

The aim of this single-blinded, complete crossover study was to evaluate the effects of tDCS on thermal and mechanical perception, as assessed by quantitative sensory testing (QST).

METHODS

QST was performed upon the radial part of both hands of eight healthy subjects (3 female, 5 male, 25-41years of age). These subjects were examined before and after cathodal, anodal or sham tDCS, applied in a random order. TDCS was administered for 15min at a 1mA current intensity, with the active electrode placed over the left primary motor cortex and the reference electrode above the right orbit.

RESULTS

After cathodal tDCS, cold detection thresholds (CDT), mechanical detection thresholds (MDT), and mechanical pain thresholds (MPT) significantly increased in the contralateral hand, when compared to the baseline condition.

CONCLUSIONS

Cathodal tDCS temporarily reduced the sensitivity to A-fiber mediated somatosensory inputs.

SIGNIFICANCE

Impairment of these somatosensory percepts suggests a short-term suppression of lemniscal or suprathalamic sensory pathways following motor cortex stimulation by cathodal tDCS.

Classifying risk factors for dyskinesia in Parkinson's disease

BACKGROUND

Currently there is no classification of risk factors applicable to an individual patient with Parkinson's disease for the development of dyskinesia.

METHODS

We conducted literature search to identify and classifying risk factors into groups - (a) intrinsic vs extrinsic and (b) modifiable vs non-modifiable.

RESULTS

Younger age, young age of onset and severity of PD are major intrinsic non-modifiable risk factors for dyskinesia, female gender is another factor but not independent of other factors. Genetic expression and plasticity may determine pre-disposition to age of onset of PD and dyskinesia, these are currently non-modifiable factors arising due to an interaction of intrinsic and extrinsic factors. Lower initial body weight and weight loss during the course of the disease increase the risk of dyskinesia. Levodopa dose per kilogram body weight is a more significant risk factor than absolute levodopa dose. Early use of longer acting non-levodopa (i.e. dopamine agonists) medications delays the onset of dyskinesia. Interaction between body weight, levodopa dose and mode and duration of drug delivery is a significant modifiable factor.

CONCLUSION

Dyskinesia in PD arises as a consequence of the interaction of intrinsic versus extrinsic and modifiable versus non-modifiable factors. Identification and manipulation of modifiable factors for an individual patient may reduce the risk and burden of dyskinesia. Adjustment of levodopa dose according to body weight during the course of the disease seems to be a significant modifiable risk factor for dyskinesia.

Effect of thermode application pressure on thermal threshold detection

Studies using quantitative sensory testing (QST) often present incongruent results due to intra- and intersubject as well as interobserver variability which limit widespread use of the technique. Eliminating or reducing the factors responsible for this variability is of great interest, as it increases reliability and reproducibility of QST. Thermal sensory threshold determination is a crucial part of QST. It was previously suggested that the pressure of the thermode on the skin could influence measurements. To verify this, we developed a new thermode with a built-in pressure sensor. Thresholds obtained with this thermode were compared to those obtained with a commercially available thermotesting device (Medoc TSA-II). Heat detection and heat pain detection thresholds were higher, and cold detection thresholds were lower when measured with our thermode than they were with the Medoc thermode. Cold pain detection thresholds did not differ between the thermodes. Analysis of the heat transfer capacity of the thermodes indicated that the material of the skin contact surface of the thermode may play a role in these shifts in threshold values. Altering the thermode pressure on the skin did not affect the thermal thresholds. Furthermore, the intrasubject variability of the measurements (minimal-to-maximal range of measured threshold values in individual subjects) was also not influenced by the pressure with which the thermode was attached to the skin. Our results suggest that the pressure with which the thermode is attached to the skin does not significantly affect the intra- and intersubject reproducibility of the thermal sensory threshold measurements.

Homocysteine in restless legs syndrome

BACKGROUND AND PURPOSE

Total plasma homocysteine (tHcy) may be a risk factor for vascular diseases and is associated with renal failure or deficiency of vitamin B12 or folate. Recently, elevated tHcy concentrations were observed in patients with Parkinson's disease (PD), particularly those under levodopa treatment. Our objective was to determine whether changes in tHcy are also found in patients with restless legs syndrome (RLS) in relation to levodopa treatment and whether folate and vitamins B6 and B12 play a role in RLS.

METHODS

In a total of 228 subjects, tHcy and B vitamin status (vitamins B6 and B12, folate) were studied: 97 patients with idiopathic RLS (40 under levodopa therapy), 39 with PD (25 under levodopa therapy), and 92 healthy controls adjusted for age and gender.

RESULTS

No significant differences were observed in tHcy levels between RLS patients and controls or between the RLS groups without treatment or with levodopa or dopamine agonist treatment. Mean tHcy was significantly higher in PD patients (13.8 micromol/l) than in either RLS patients (11.7 micromol/l) or controls (11.0 micromol/l; p<0.001). There was an inverse association between tHcy and vitamin B12 in each group.

CONCLUSIONS

RLS and, in particular, levodopa treatment in RLS are not associated with hyperhomocysteinemia. Elevated tHcy could, however, be confirmed in PD patients.

Growth hormone response to low-dose apomorphine in restless legs syndrome

INTRODUCTION

Low-dose apomorphine challenge has been shown to cause a rise in growth hormone (GH) in patients with Parkinson's disease (PD). This was interpreted as an increased postsynaptic sensitivity of hypothalamic dopamine receptors in the course of a generalized degeneration of dopaminergic neurons. The dopaminergic system in the restless legs syndrome (RLS) has been assumed to play a role in its pathophysiology. It is therefore the aim of this study to determine whether the GH response to subcutaneously applied low-dose apomorphine is generally altered in patients with RLS as compared to healthy controls.

PATIENTS AND METHODS

We examined 40 patients with idiopathic RLS as well as 20 age- and sex-matched healthy control subjects by means of the low-dose apomorphine test. GH was analyzed at baseline, as well as 45 and 60 min after subcutaneous low-dose apomorphine injection in the morning.

RESULTS

Forty RLS patients (58.3+/-11.9 years, 32 females) with a mean RLS severity scale score of 23.9+/-6.6 (range 10-37) were examined. GH was not significantly increased 45 and 60 min after injection (p=0.397) (2.44+/-2.35 ng/ml at baseline versus 2.71+/-2.29 ng/ml after 45 min and 2.18+/-1.83 ng/ml after 60 min). The results were independent of pre-treatment with levodopa. Age, sex, duration, and severity of the disease did not show a covariate effect with GH levels. There was no difference compared with healthy controls.

CONCLUSIONS

RLS patients did not show an increase in GH after stimulation with low-dose apomorphine. Lack of sensitivity alteration of extrastriatal hypothalamic dopamine receptors suggests that RLS is not a general dopaminergic degenerative disease or might only show circadian alterations.

Genome-wide association study of restless legs syndrome identifies common variants in three genomic regions

Restless legs syndrome (RLS) is a frequent neurological disorder characterized by an imperative urge to move the legs during night, unpleasant sensation in the lower limbs, disturbed sleep and increased cardiovascular morbidity. In a genome-wide association study we found highly significant associations between RLS and intronic variants in the homeobox gene MEIS1, the BTBD9 gene encoding a BTB(POZ) domain as well as variants in a third locus containing the genes encoding mitogen-activated protein kinase MAP2K5 and the transcription factor LBXCOR1 on chromosomes 2p, 6p and 15q, respectively. Two independent replications confirmed these association signals. Each genetic variant was associated with a more than 50% increase in risk for RLS, with the combined allelic variants conferring more than half of the risk. MEIS1 has been implicated in limb development, raising the possibility that RLS has components of a developmental disorder.

Pregabalin in restless legs syndrome with and without neuropathic pain

BACKGROUND

Restless legs syndrome (RLS) is a common neurological disorder complicated in many patients by augmentation to dopaminergic therapy or comorbidities such as neuropathic pain.

AIMS

To explore the effectiveness of pregabalin in RLS in a pragmatic clinical setting.

METHODS

After observing improvement of restless legs symptoms in seven patients treated with pregabalin for neuropathic pain, we extended the clinical observation to a total of 16 patients with secondary RLS, in most of them due to neuropathy, and to three patients with idiopathic RLS.

RESULTS

Three patients discontinued pregabalin because of side effects (rash, fatigue, loss of efficacy). The other 16 patients self-rated a satisfactory or good alleviation of RLS symptoms and maintained pregabalin, five with add-on medication, on a mean daily dose of 305 mg (standard deviation, 185 mg), and with a mean duration of 217 (standard deviation, 183) days.

CONCLUSION

These data propose pregabalin as a new option in the treatment of secondary RLS for patients with neuropathic pain, which should be further investigated with randomized, placebo-controlled trials.

Body weight in patients with Parkinson's disease

There is some evidence suggesting that Parkinson's disease (PD) patients exhibit lower body weight when compared to age-matched healthy subjects. Low body mass index (BMI) is correlated with low bone mineral density, both of which are major risk factors for hip fractures. Possible determinants of weight loss in PD patients include hyposmia, impaired hand-mouth coordination, difficulty chewing, dysphagia, intestinal hypomotility, depression, decreased reward processing of dopaminergic mesolimbic regions, nausea, and anorexia as the side effects of medication, and increased energy requirements due to muscular rigidity and involuntary movements. It is unclear whether PD patients in general, or only a subgroup of those affected, definitely show lower BMI in the advanced stages of the disease. We therefore recommend that the body weight of PD patients be monitored monthly as the disease progresses, and that a patient's nutrition should be supplemented with sufficient amounts of vitamin D and calcium to reduce the risk of hip fractures and strengthen bone density. Because meal times may coincide with unpredictable off periods associated with akinesia and impaired hand-mouth coordination, PD patients also need flexible food schedules that accommodate the associated symptoms of this disease.

[Restless legs syndrome: therapeutic possibilities in the medical practice]

The diagnosis of restless legs syndrome is made on the basis of the clinical symptoms and, if applicable, complemented with a polysomnography. This is followed by neurophysiological examinations and laboratory diagnostics and permits a differentiation between idiopathic and secondary RLS. The therapy depends upon the form and severity of the disease. For secondary RLS, the primary disease must be treated or the symptom-inducing medication must be discontinued. An idiopathic RLS is treated with drugs. Primarily, L-dopa/benserazide and dopaminergic agonists are used, but opioids and anticonvulsants are also successful.

The selective glucocorticoid receptor antagonist ORG 34116 decreases immobility time in the forced swim test and affects cAMP-responsive element-binding protein phosphorylation in rat brain

Glucocorticoid receptor (GR) antagonists can block the retention of the immobility response in the forced swimming test. Recently, we showed that forced swimming evokes a distinct spatiotemporal pattern of cAMP-responsive element-binding protein (CREB) phosphorylation in the dentate gyrus (DG) and neocortex. In the present study, we found that chronic treatment of rats with the selective GR antagonist ORG 34116 decreased the immobility time in the forced swim test, increased baseline levels of phosphorylated CREB (P-CREB) in the DG and neocortex and affected the forced swimming-induced changes in P-CREB levels in a time- and site-specific manner. Overall, we observed that, in control rats, forced swimming evoked increases in P-CREB levels in the DG and neocortex, whereas in ORG 34116-treated animals a major dephosphorylation of P-CREB was observed. These observations underscore an important role of GRs in the control of the phosphorylation state of CREB which seems to be of significance for the immobility response in the forced swim test and extend the molecular mechanism of action of GRs in the brain.

Effect of chronic administration of selective glucocorticoid receptor antagonists on the rat hypothalamic-pituitary-adrenocortical axis

The effects of the selective glucocorticoid receptor (GR) antagonists ORG 34850, ORG 34116, and ORG 34517 on the rat hypothalamic-pituitary-adrenocortical (HPA) system were investigated. To assess the potency of the compounds to occupy GR in the brain and pituitary, we applied a single acute subcutaneous (s.c.) injection (10 mg/kg). ORG 34517 was most potent to occupy GR in the anterior pituitary and distinct brain areas, whereas all compounds were unable to occupy mineralocorticoid receptor (MR). Chronic administration of ORG 34850, ORG 34116, and ORG 34517 (20 mg/kg/day) for 1, 3, and 5 weeks resulted only in minor changes in brain GR levels. However, profound increases of hippocampal MR were observed virtually at all time points. Treatment with ORG 34850 and ORG 34116 elicited episodic increases in HPA axis activity, whereas ORG 34517 did not cause any changes in HPA activity. Thus, the GR antagonists exert distinct effects on the HPA axis, which may be pertinent for the proposed antidepressant activity of these compounds.