Muscle ischaemia in patients with orthostatic hypotension assessed by velocity recovery cycles

Muscle excitability testing

Conventional electrophysiological methods, i.e. nerve conduction studies and electromyography are suitable methods for the diagnosis of neuromuscular disorders, however, they provide limited information about muscle fibre membrane properties and underlying disease mechanisms. Muscle excitability testing is a technique that provides in vivo information about muscle fibre membrane properties such as membrane potential and ion channel function. Since the 1960s, various methodologies have been suggested to examine muscle membrane properties but technical difficulties have limited its use. In 2009, an automated, fast and simple application, the so-called multi-fibre muscle velocity recovery cycles (MVRC) has accelerated the use of muscle excitability testing. Later, frequency ramp and repetitive stimulation protocols have been developed. Though this method has been used mainly in research for revealing disease mechanisms across a broad range of neuromuscular disorders, it may have additional diagnostic uses; value has been shown particularly in muscle channelopathies. This review will provide a description of the state-of-the art of methodological and clinical studies for muscle excitability testing.

Muscle velocity recovery cycles in myopathy

OBJECTIVE

To understand the pathophysiology of myopathies by using muscle velocity recovery cycles (MVRC) and frequency ramp (RAMP) methodologies.

METHODS

42 patients with quantitative electromyography (qEMG) and biopsy or genetic verified myopathy and 42 healthy controls were examined with qEMG, MVRC and RAMP, all recorded from the anterior tibial muscle.

RESULTS

There were significant differences in the motor unit potential (MUP) duration, the early and late supernormalities of the MVRC and the RAMP latencies in myopathy patients compared to controls (p < 0.05 apart from muscle relatively refractory period (MRRP)). When dividing into subgroups, the above-mentioned changes in MVRC and RAMP parameters were increased for the patients with non-inflammatory myopathy, while there were no significant changes in the group of patients with inflammatory myopathy.

CONCLUSIONS

The MVRC and RAMP parameters can discriminate between healthy controls and myopathy patients, more significantly for non-inflammatory myopathy. MVRC differences with normal MRRP in myopathy differs from other conditions with membrane depolarisation.

SIGNIFICANCE

MVCR and RAMP may have a potential in understanding disease pathophysiology in myopathies. The pathogenesis in non-inflammatory myopathy does not seem to be caused by a depolarisation of the resting membrane potential but rather by the change in sodium channels of the muscle membrane.

Excitability properties of mouse and human skeletal muscle fibres compared by muscle velocity recovery cycles

Mouse models of skeletal muscle channelopathies are not phenocopies of human disease. In some cases (e.g. Myotonia Congenita) the phenotype is much more severe, whilst in others (e.g. Hypokalaemic periodic paralysis) rodent physiology is protective. This suggests a species' difference in muscle excitability properties. In humans these can be measured indirectly by the post-impulse changes in conduction velocity, using Muscle Velocity Recovery Cycles (MVRCs). We performed MVRCs in mice and compared their muscle excitability properties with humans. Mouse Tibialis Anterior MVRCs (n = 70) have only one phase of supernormality (increased conduction velocity), which is smaller in magnitude (p = 9 × 10-21), and shorter in duration (p = 3 × 10-24) than human (n = 26). This abbreviated supernormality is followed by a period of late subnormality (reduced velocity) in mice, which overlaps in time with the late supernormality seen in human MVRCs. The period of late subnormality suggests increased t-tubule Na+/K+-pump activity. The subnormal phase in mice was converted to supernormality by blocking ClC-1 chloride channels, suggesting relatively higher chloride conductance in skeletal muscle. Our findings help explain discrepancies in phenotype between mice and humans with skeletal muscle channelopathies and potentially other neuromuscular disorders. MVRCs are a valuable new tool to compare in vivo muscle membrane properties between species and will allow further dissection of the molecular mechanisms regulating muscle excitability.

Leg pain in neuropathic postural tachycardia syndrome is associated with altered muscle membrane properties

Purpose

In neuropathic postural tachycardia syndrome, peripheral sympathetic dysfunction leads to excessive venous blood pooling during orthostasis. Up to 84% of patients report leg pain and weakness in the upright position. To explore possible pathophysiological processes underlying these symptoms, the present study examined muscle excitability depending on body position in patients with neuropathic postural tachycardia syndrome and healthy subjects.

Methods

In ten patients with neuropathic postural tachycardia syndrome and ten healthy subjects, muscle excitability measurements were performed repeatedly: in the supine position, during 10 min of head-up tilt and during 6 min thereafter. Additionally, lower leg circumference was measured and subjective leg pain levels were assessed.

Results

In patients with neuropathic postural tachycardia syndrome, muscle excitability was increased in the supine position, decreased progressively during tilt, continued to decrease after being returned to the supine position, and did not completely recover to baseline values after 6 min of supine rest. The reduction in muscle excitability during tilt was paralleled by an increase in lower leg circumference as well as leg pain levels. No such changes were observed in healthy subjects.

Conclusions

This study provides evidence for the occurrence of orthostatic changes in muscle excitability in patients with neuropathic postural tachycardia syndrome and that these may be associated with inadequate perfusion of the lower extremities. Insufficient perfusion as a consequence of blood stasis may cause misery perfusion of the muscles, which could explain the occurrence of orthostatic leg pain in neuropathic postural tachycardia syndrome.

A Dual Centre Study of Pain in Parkinson’s Disease and Its Relationship with Other Non-Motor Symptoms

Background: Pain is a disabling and often underestimated non-motor symptom (NMS) detrimentally affecting the quality of life of patients with Parkinson’s disease (PD). Objective: Here, we conducted a cross-sectional, observational international study on 167 patients with idiopathic PD in order to analyze the potential relationship between pain and other NMS. Methods: Subjects were assessed with the Unified Parkinson’s Disease Rating Scale (UPDRS) part III, Hoehn and Yahr (H&Y) stage, King’s Parkinson’s Disease Pain Scale (KPPS), Brief Pain Inventory (BPI), Non-Motor Symptoms Scale (NMSS), and Beck Depression Inventory (BDI). Spearman’s rank correlation coefficient, multiple regression and multiple index-based clustering algorithms were used for data analysis. Results: The prevalence of pain was 88.6%, was not correlated with age, motor severity (UPDRS part III) or disease duration, whereas a weak correlation with female gender and H&Y stage >2.5 was found. Multiple NMS correlated significantly with pain. Specifically, sleep disturbance had the strongest correlation with pain, followed by depression, gastrointestinal and cardiovascular disturbances. Further analyses showed that sleep and cardiovascular disturbance were independently associated with pain, and that these symptoms clustered together in a subset of PD patients. The relationship between pain, sleep and dysautonomia persisted independently from dopamine replacement therapy. Conclusion: Our study suggests that sleep disruption and cardiovascular disturbance are associated with pain in PD, and possibly identifies a specific subtype within PD patients with pain. Our data also indicate that sleep disruption, pain and dysautonomia may have a common pathophysiology, possibly involving non-dopaminergic pathways.

In vivo assessment of interictal sarcolemmal membrane properties in hypokalaemic and hyperkalaemic periodic paralysis

OBJECTIVE

Hypokalaemic periodic paralysis (HypoPP) is caused by mutations of Ca_v1.1, and Na_v1.4 which result in an aberrant gating pore current. Hyperkalaemic periodic paralysis (HyperPP) is due to a gain-of-function mutation of the main alpha pore of Na_v1.4. This study used muscle velocity recovery cycles (MVRCs) to investigate changes in interictal muscle membrane properties in vivo.

METHODS

MVRCs and responses to trains of stimuli were recorded in tibialis anterior and compared in patients with HyperPP(n = 7), HypoPP (n = 10), and normal controls (n = 26).

RESULTS

Muscle relative refractory period was increased, and early supernormality reduced in HypoPP, consistent with depolarisation of the interictal resting membrane potential. In HyperPP the mean supernormality and residual supernormality to multiple conditioning stimuli were increased, consistent with increased inward sodium current and delayed repolarisation, predisposing to spontaneous myotonic discharges.

CONCLUSIONS

The in vivo findings suggest the interictal resting membrane potential is depolarized in HypoPP, and mostly normal in HyperPP. The MVRC findings in HyperPP are consistent with presence of a window current, previously proposed on the basis of in vitro expression studies. Although clinically similar, HyperPP was electrophysiologically distinct from paramyotonia congenita.

SIGNIFICANCE

MVRCs provide important in vivo data that complements expression studies of ion channel mutations.

Orthostatic Hypotension and Falls in Older Adults: A Systematic Review and Meta-analysis

OBJECTIVES

Orthostatic hypotension is a potential risk factor for falls in older adults, but existing evidence on this relationship is inconclusive. This study addresses the association between orthostatic hypotension and falls.

DESIGN

Systematic review and meta-analysis of the cross-sectional and longitudinal studies assessing the association between orthostatic hypotension and falls, as preregistered in the PROSPERO database (CRD42017060134).

SETTING AND PARTICIPANTS

A literature search was performed on February 20, 2017, in MEDLINE (from 1946), PubMed (from 1966), and EMBASE (from 1947) using the terms orthostatic hypotension, postural hypotension, and falls. References of included studies were screened for other eligible studies. Study selection was performed independently by 2 reviewers using the following inclusion criteria: published in English; mean/median age of the population ≥65 years; blood pressure measurement before and after postural change; and assessment of the association of orthostatic hypotension with falls. The following studies were excluded: conference abstracts, case reports, reviews, and editorials. Data extraction was performed independently by 2 reviewers.

MEASURES

Unadjusted odds ratios of the association between orthostatic hypotension and falls were used for pooling using a random effects model. Studies were rated as high, moderate, or low quality using the Newcastle-Ottawa Scale.

RESULTS

Out of 5646 studies, 63 studies (51,800 individuals) were included in the systematic review and 50 studies (49,164 individuals) in the meta-analysis. Out of 63 studies, 39 were cross-sectional and 24 were longitudinal. Orthostatic hypotension was positively associated with falls (odds ratio 1.73, 95% confidence interval 1.50-1.99). The result was independent of study population, study design, study quality, orthostatic hypotension definition, and blood pressure measurement method.

CONCLUSIONS AND IMPLICATIONS

Orthostatic hypotension is significantly positively associated with falls in older adults, underpinning the clinical relevance to test for an orthostatic blood pressure drop and highlighting the need to investigate orthostatic hypotension treatment to potentially reduce falls.

Orthostatic hypotension and physical functioning in older adults: A systematic review and meta-analysis

BACKGROUND

Orthostatic hypotension (OH) may negatively affect physical functioning and aggravate morbidities, but existing evidence is contradictory.

METHODS

MEDLINE (from 1946), PubMed (from 1966) and EMBASE databases (from 1947) were systematically searched for studies on the association of OH and physical functioning in older adults, categorized as: balance, gait characteristics, walking speed, Timed Up and Go time, handgrip strength (HGS), physical frailty, exercise tolerance, physical activity, activities of daily living (ADL), and performance on the Hoehn and Yahr scale (HY) and Unified Parkinson's Disease Rating Scale (UPDRS). Study quality was assessed using the Newcastle Ottawa Scale.

RESULTS

Forty-two studies were included in the systematic review (29,421 individuals) and 29 studies in the meta-analyses (23,879 individuals). Sixteen out of 42 studies reported a significant association of OH with worse physical functioning. Meta-analysis showed a significant association of OH with impaired balance, ADL performance and HY/UPDRS III performance, but not with gait characteristics, mobility, walking speed, TUG, HGS, physical frailty, exercise tolerance, physical activity and UPDRS II performance.

CONCLUSIONS

OH was associated with impaired balance, ADL performance and HY/UPDRS III performance, but not with other physical functioning categories. The results suggest that OH interventions could potentially improve some aspects of physical functioning.

Early changes of muscle membrane properties in porcine faecal peritonitis

Introduction

Sepsis-induced myopathy and critical illness myopathy (CIM) are possible causes of muscle weakness in intensive care patients. They have been attributed to muscle membrane dysfunction. The aim of this study was to investigate membrane properties in the early stage of experimental sepsis by evaluating muscle excitability.

Methods

In total, 20 anaesthetized and mechanically ventilated pigs were randomized to either faecal peritonitis (n = 10) or to non-septic controls (n = 10). Resuscitation with fluids and vasoactive drugs was started 3 hours after peritonitis induction. Muscle membrane properties were investigated by measuring muscle velocity recovery cycles before induction of peritonitis as well as 6, 18 and 27 hours thereafter. Muscle relative refractory period (MRRP) and early supernormality (ESN) were assessed.

Results

Peritonitis lasting 27 hours was associated with an increase of MRRP by 28% from 2.38 ± 0.18 ms (mean ± SD) to 3.47 ± 1.79 ms (P <0.01) and a decrease of ESN by 31% from 9.64 ± 2.82% to 6.50 ± 2.64% (P <0.01). ESN reduction was already apparent 6 hours after induction of peritonitis. Values in controls did not show any significant alterations.

Conclusions

Muscle membrane abnormalities consistent with membrane depolarization and/or sodium channel inactivation occurred within 6 hours of peritonitis induction. This indicates that changes that have been described in established sepsis-induced myopathy and/or CIM start early in the course of sepsis. Muscle excitability testing facilitates evaluation of the time course of these changes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13054-014-0484-2) contains supplementary material, which is available to authorized users.

Chloride channels in myotonia congenita assessed by velocity recovery cycles

INTRODUCTION

Myotonia congenita (MC) is caused by congenital defects in the muscle chloride channel CLC-1. This study used muscle velocity recovery cycles (MVRCs) to investigate how membrane function is affected.

METHODS

MVRCs and responses to repetitive stimulation were compared between 18 patients with genetically confirmed MC (13 recessive, 7 dominant) and 30 age-matched, normal controls.

RESULTS

MC patients exhibited increased early supernormality, but this was prevented by treatment with sodium channel blockers. After multiple conditioning stimuli, late supernormality was enhanced in all MC patients, indicating delayed repolarization. These abnormalities were similar between the MC subtypes, but recessive patients showed a greater drop in amplitude during repetitive stimulation.

CONCLUSIONS

MVRCs indicate that chloride conductance only becomes important when muscle fibers are depolarized. The differential responses to repetitive stimulation suggest that, in dominant MC, the affected chloride channels are activated by strong depolarization, consistent with a positive shift of the CLC-1 activation curve.

Transient loss of consciousness and syncope

Syncope describes a sudden and brief transient loss of consciousness (TLOC) with postural failure due to cerebral global hypoperfusion. The term TLOC is used when the cause is either unrelated to cerebral hypoperfusion or is unknown. The most common causes of syncopal TLOC include: (1) cardiogenic syncope (cardiac arrhythmias, structural cardiac diseases, others); (2) orthostatic hypotension (due to drugs, hypovolemia, primary or secondary autonomic failure, others); (3) neurally mediated syncope (cardioinhibitory, vasodepressor, and mixed forms). Rarely neurologic disorders (such as epilepsy, transient ischemic attacks, and the subclavian steal syndrome) can lead to cerebal hypoperfusion and syncope. Nonsyncopal TLOC may be due to neurologic (epilepsy, sleep attacks, and other states with fluctuating vigilance), medical (hypoglycemia, drugs), psychiatric, or post-traumatic disorders. Basic diagnostic workup of TLOC includes a thorough history and physical examination, and a 12-lead electrocardiogram (ECG). Blood testing, electroencephalogram (EEG), magnetic resonance imaging (MRI) of the brain, echocardiography, head-up tilt test, carotid sinus massage, Holter monitoring, and loop recorders should be obtained only in specific contexts. Management strategies involve pharmacologic and nonpharmacologic interventions, and cardiac pacing.

Coat-hanger ache in orthostatic hypotension

Background

Questionnaires administered to orthostatic hypotension (OH) patients reveal frequent occurrence of coat-hanger ache (CHA), but laboratory-based precipitation of CHA during head-up tilt (HUT) has not been investigated. This study compared the frequency and clinical aspects of CHA in the same group of OH patients during daily activities versus during HUT.

Methods

Retrospective IRB-approved review of prospectively collected data on 22 dysautonomic patients. Heart rate response to deep breathing, Valsalva manoeuver, HUT and thermoregulatory sweat test evaluated cardiovagal, adrenergic and sudomotor functions. Occurrence and clinical features of CHA during daily activities and during HUT were recorded. Data were analysed with descriptive statistics.

Results

All patients demonstrated severe adrenergic (OH), cardiovagal and sudomotor impairment. Of 22 patients, 13 (59%) reported CHA within 3–5 minutes of standing or after 10 minutes to 2 hours of sitting that was relieved within 5–20 minutes of recumbency. During HUT, 4 of 13 (18%) patients developed CHA. Clinical features varied.

Conclusions

CHA was reported by 59% of OH patients during daily activities and by only 18% during HUT. The clinical characteristics of CHA episodes displayed inter-individual and intra-individual variability. These findings militate against direct association between OH and CHA and suggest a complex pathophysiology.