In vivo assessment of muscle membrane properties in myotonic dystrophy

Introduction: Myotonia in myotonic dystrophy types 1 (DM1) and 2 (DM2) is generally attributed to reduced chloride‐channel conductance. We used muscle velocity recovery cycles (MVRCs) to investigate muscle membrane properties in DM1 and DM2, using comparisons with myotonia congenita (MC). Methods: MVRCs and responses to repetitive stimulation were compared between patients with DM1 (n = 18), DM2 (n = 5), MC (n = 18), and normal controls (n = 20). Results: Both DM1 and DM2 showed enhanced late supernormality after multiple conditioning stimuli, indicating delayed repolarization as in MC. Contrary to MC, however, DM1 showed reduced early supernormality after multiple conditioning stimuli, and weak DM1 patients also showed abnormally slow latency recovery after repetitive stimulation. Conclusions: These findings support the presence of impaired chloride conductance in both DM1 and DM2. The early supernormality changes indicate that sodium currents were reduced in DM1, whereas the weakness‐associated slow recovery after repetitive stimulation may provide an indication of reduced Na⁺/K⁺‐ATPase activation. Muscle Nerve 54 : 249–257, 2016     

Sarcolemmal excitability in myotonic dystrophy: Assessment through surface EMG

A motor point stimulation protocol was carried out on the tibialis anterior of myotonic dystrophy (MyD) patients. The surface myoelectric signal was monitored to record average rectified value (ARV), median frequency of power spectrum (MDF), and conduction velocity (CV) parameters. The ARV curve showed a decreasing trend that reveals a reduction in the M-wave amplitude during stimulation. MDF presented a significant decrement in the first seconds of sustained contraction, probably caused by abnormal lengthening of the depolarization zone. CV was significantly lower in patients, suggesting reduced mean fiber size. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21:543–546, 1998.     

Axonal function and activity-dependent excitability changes in myotonic dystrophy

To investigate peripheral nerve function and its potential contribution to symptoms of weakness in myotonic dystrophy type 1 (MD), nerve excitability was assessed in 12 MD patients. Compound muscle action potentials (CMAPs) were recorded at rest from abductor pollicis brevis (APB) following stimulation of the median nerve. Stimulus-response behavior, threshold electrotonus, a current-threshold relationship, and recovery cycles were successfully recorded in each patient. Compared with controls, there was significant reduction in CMAP amplitude in MD patients. This was accompanied by reduction in depolarizing threshold electrotonus and an increase in refractoriness and in the duration of the relative refractory period. To determine whether alteration in axonal resting membrane potential was a factor underlying these changes, axonal excitability was assessed following maximal contraction of APB for 60 seconds. Following contraction, there was reduction in CMAP amplitude for a submaximal stimulus (by 51.5+/-11.8%) and an increase in super-excitability (of 22.2+/-12.0%), consistent with activity-dependent hyperpolarization, with a greater increase in threshold for MD patients compared to controls (MD group, 22.3+/-5.1%; controls, 11.7+/-2.1%; P<0.04) and prolonged recovery to baseline. The present study has established that greater activity-dependent changes in excitability may be induced in MD patients by maximal voluntary contraction when compared to controls. The excitability changes and prolonged recovery of threshold following contraction are likely to contribute to symptoms of fatigue and weakness in MD patients.     

Abnormal M-wave responses during exercise in myotonic muscular dystrophy: A Na+-K+ pump defect?

Maximum M-waves (muscle compound action potentials) were studied in the abductor pollicis brevis (APB), extensor digitorum brevis (EDB), and tibialis anterior (TA) muscles of 12 patients with myotonic muscular dystrophy (MMD) and in the same number of control subjects, matched for age and sex. The peak-to-peak amplitudes and voltage-time areas of the responses were measured at rest, between 40 maximum voluntary contractions (each lasting 3 s) and also during a 2-min recovery period. In 34 of the 36 control muscles the M-waves potentiated during the period of intermittent voluntary contractions. In the MMD patients, however, the M waves exhibited initial declines in 25 of 30 muscles. In the APB and EDB muscles the normalized mean values for the smallest M-waves, recorded during the 350 s total observation periods, differed significantly between the 2 groups of subjects. It is suggested that the sarcolemmal Na(+)-K+ pump has a raised threshold for activation in MMD patients.     

Force training induces changes in human muscle membrane properties

Introduction: Human muscle membrane properties can be assessed in vivo by recording muscle velocity recovery cycles (MVRCs). This study was undertaken to study the effect of muscle force training on MVRC parameters. Methods: MVRCs with 1 to 5 conditioning stimuli were recorded from brachioradialis muscle before and after 2 weeks of muscle force training in 12 healthy subjects. The effects of training on relative refractory period and early and late supernormality were quantified. Results: Force training induced a reduction of relative refractory period (P < 0.0001), while early supernormality was increased (P < 0.02) and peaked earlier (P < 0.01). Late supernormality and the increases in late supernormality due to 2 and 5 conditioning stimuli remained unchanged. Conclusions: Muscle force training leads to hyperpolarization of the resting muscle membrane potential, probably caused by an increase in the number of sodium pump sites. Muscle Nerve 54 : 144–146, 2016     

Velocity recovery cycles of human muscle action potentials and their sensitivity to ischemia

This study was undertaken to test whether recovery cycle measurements can provide useful information about the membrane potential of human muscle fibers. Multifiber responses to direct muscle stimulation through needle electrodes were recorded from the brachioradialis of healthy volunteers, and the latency changes measured as conditioning stimuli were applied at interstimulus intervals of 2–1000 ms. In all subjects, the relative refractory period (RRP), which lasted 3.27 ± 0.45 ms (mean ± SD, n = 12), was followed by a phase of supernormality, in which the velocity increased by 9.3 ± 3.4% at 6.1 ± 1.3 ms, and recovered over 1 s. A broad hump of additional supernormality was seen at around 100 ms. Extra conditioning stimuli had little effect on the early supernormality but increased the later component. The two phases of supernormality resembled early and late afterpotentials, attributable respectively to the passive decay of membrane charge and potassium accumulation in the t‐tubules. Five minutes of ischemia progressively prolonged the RRP and reduced supernormality, confirming that these parameters are sensitive to membrane depolarization. Velocity recovery cycles may provide useful information about altered muscle membrane potential and t‐tubule function in muscle disease. Muscle Nerve, 2008     

Temperature dependency of human muscle velocity recovery cycles

Introduction: Velocity recovery cycles (VRCs) of human muscle action potentials have been proposed as a new technique for studying muscle membrane function. This study was undertaken to determine the temperature dependency of VRC parameters. Methods: VRCs with 1 conditioning stimulus were recorded repeatedly from brachioradialis muscle during cooling, and intramuscular temperature was recorded. VRCs in 6 normal volunteers were measured at 1°C intervals from 34°C to 28°C. The effects of temperature on relative refractory period and early and late supernormality were quantified. Results: Muscle VRC curves shift progressively upward and to the right with cooler temperatures. The most sensitive parameter to temperature changes was relative refractory period, whereas early supernormality was rather insensitive. Late supernormality was unaffected by temperature changes. Conclusions: Knowledge of temperature‐induced changes of muscle VRC parameters is essential for future studies. Thus, the results of this study provide a firm basis for clinical studies with this technique. Muscle Nerve 46: 264–266, 2012     

Axonal hyperpolarization associated with acute hypokalemia: multiple excitability measurements as indicators of the membrane potential of human axons

Multiple nerve excitability measurements have been proposed for clinical testing of nerve function, and an important determinant of excitability is membrane potential. We report a patient with acquired hypokalemic paralysis in whom multiple excitability indices (stimulus-response curve, strength-duration properties, threshold electrotonus, recovery cycle) were measured during and after an acute hypokalemic attack (serum K(+) level, 2.1 mEq/L and 4.5 mEq/L, respectively). During hypokalemia, there was a shift of the stimulus-response curve to the right, a decrease in strength-duration time constant, a "fanning-out" of responses during threshold electrotonus, a reduction in relative refractory period, and an increase in superexcitability; all of these indicate axonal hyperpolarization, presumably due to the K(+) equilibrium potential being more negative. These indices returned to normal 20 h later, associated with normalization of the serum K(+) level. These results demonstrate that the changes associated with hypokalemic paralysis are not confined to muscle and that axons undergo hyperpolarization in vivo. Multiple excitability measurements can be used as a tool to identify changes in membrane potential of human axons.     

Effects of length changes on Na+ current amplitude and excitability near and far from the end-plate

Na⁺ current (I_Na), membrane capacitance (C_m), action potential (AP) properties, and cable properties were studied on the end-plate (E), the end-plate border (EB), and extrajunctional (EJ) membrane of rat fast twitch muscle fibers. I_Na normalized to C_m, which is proportional to the density of Na⁺ channels, was the same on the E and the EB and smallest on EJ membrane. The AP threshold was lower and rate of rise of the AP was larger at the EB compared with EJ membrane. On the E and the EB, C_m and I_Na did not change in response to changes in fiber length. On EJ membrane, I_Na, C_m, and membrane cable, properties changed in a manner consistent with folding and unfolding of the sarcolemma during length changes. The stiffness of the E membrane may add mechanical stability to the neuromuscular junction so that the electrical properties of the end-plate do not change with fiber length. The higher density of Na⁺ channels near the end-plate increases the safety factor for neuromuscular transmission by lowering the AP threshold. © 1996 John Wiley & Sons, Inc.     

Velocity recovery cycles of human muscle action potentials: Repeatability and variability

Objective

Velocity recovery cycles (VRCs) of human muscle action potentials have been proposed as a new technique for assessing muscle membrane function in myopathies. This study was undertaken to determine the variability and repeatability of VRC measures such as supernormality, to help guide future clinical use of the method.

Methods

To assess repeatability, VRCs with one and two conditioning stimuli were recorded from brachioradialis muscle by direct muscle stimulation in 20 normal volunteers, and the measurements repeated 1 week later. To further assess variability and dependence on electrode separation, age and sex, recordings from an additional 20 normal volunteers were added.

Results

There was a high intraclass correlation between repeated recordings of early supernormality, indicating excellent reliability of this VRC measure. However, relative refractory period had a smaller coefficient of repeatability in relation to the changes previously described during ischemia. We found no evidence that any of the excitability measures depended on electrode separation, conduction time or apparent velocity. There were also no significant differences between the recordings from men and women, or between the recordings from older (mean 44.9 y) and younger (26.5 y) subjects.

Conclusions

VRC measures are sufficiently consistent to be suitable for comparing muscle membrane function both within subjects and between groups. Early supernormality measurements benefit most from within subject comparisons.

Significance

These normative data sets provide a firm basis for planning clinical studies.     

Value of short exercise and short exercise with cooling tests in the diagnosis of myotonic dystrophies (DM1 AND DM2)

Introduction: Standard electromyography (EMG) is useful in the diagnosis of myotonic dystrophy type 1 (DM1) and type 2 (DM2), but it does not differentiate between them. The aim of this study was to estimate the utility of the short exercise test (SET) and short exercise test with cooling (SETC) in differentiating between DM1 and DM2. Methods: SET and SETC were performed in 32 patients with DM1 (mean age 35.8 ± 12.7 years) and 28 patients with DM2 (mean age 44.5 ± 12.5 years). Results: We observed a significant decline in compound motor action potential (CMAP) amplitude in DM1 with both SET and SETC immediately after effort. In DM2, there was no marked change in CMAP amplitude with either SET or SETC. Conclusions: SET and SETC may serve as useful tools for clinical differentiation between DM1 and DM2, and they may be used as a guide for molecular testing. Muscle Nerve 49 : 277–283, 2014     

Muscle phenotype in patients with myotonic dystrophy type 1

Introduction: The pathogenesis of muscle involvement in patients with myotonic dystrophy type 1 (DM1) is not well understood. In this study, we characterized the muscle phenotype in patients with confirmed DM1. Methods: In 38 patients, muscle strength was tested by hand‐held dynamometry. Myotonia was evaluated by a handgrip test and by analyzing the decrement of the compound muscle action potential. Muscle biopsies were assessed for morphological changes and Na⁺‐K⁺ pump content. Results: Muscle strength correlated with a decline in Na⁺‐K⁺ pump content (r = 0.60, P < 0.001) and with CTG expansion. CTG expansion did not correlate with severity of myotonia, proximal histopathological changes, or Na⁺‐K⁺ pump content. Histopathologically, we found few centrally placed nuclei (range 0.2–6.9%). Conclusions: The main findings of this study are that muscle weakness correlated inversely with CTG expansion and that central nuclei are not a prominent feature of proximal muscles in DM1. Muscle Nerve 47:409‐415, 2013     

Excitability differences in lower-limb motor axons during and after ischemia

Neuropathic diseases typically begin distally and spread proximally. Irrespective of the etiology, pathological investigations often indicate changes consistent with ischemia. In the present study, threshold tracking was used to investigate length-dependent differences in ischemic susceptibility of lower-limb axons in 6 healthy volunteers, with ischemia induced by a sphygmomanometer cuff inflated to 200 mm Hg and maintained for 13 minutes. Following stimulation of the peroneal nerve at the fibula neck, compound muscle action potentials were recorded proximally from tibialis anterior (TA) and distally from extensor digitorum brevis (EDB). During ischemia, excitability changes were consistent with nerve depolarization, with a greater reduction in threshold in EDB than TA. This reduction in threshold was associated with an increase in refractoriness, decrease in superexcitability, and prolongation of strength-duration time constant, consistent with axonal depolarization. With release of ischemia, reversal of these changes was associated with an increase in threshold, greater in EDB than TA, indicating axonal hyperpolarization. The rate of recovery of threshold was similar proximally and distally, arguing against a gradient in Na(+)/K(+) pump function along the peroneal nerve. The greater changes in threshold in EDB during and after ischemia suggest an increased susceptibility of more distal axons to ischemia and are likely to contribute to the length-dependent development of neuropathy.     

Multiple measures of axonal excitability in peripheral sensory nerves: an in vivo rat model

Excitability measurements on human motor and sensory nerves have provided new insights into axonal membrane changes in peripheral nerve disorders. The aim of this study was to establish an in vivo rat preparation suitable for threshold tracking of sensory nerve action potentials (SNAPs) to model clinical sensory nerve excitability studies. In Sprague-Dawley rats anesthetized with ketamine and xylazine, current stimuli were applied to the base of the tail and SNAPs recorded from distal needle electrodes. Multiple excitability data were obtained as previously described for human nerves and compared to recordings from the motor tail axons and to sensory recordings from human median and ulnar nerves. The pattern of excitability changes in rats was broadly similar to that in humans, although some parameters differed significantly. Individual recordings were stable for at least 3 h. These data show that the rat tail enables excitability properties of sensory as well as motor axons to be studied experimentally, e.g., in models of nerve disease and during pharmacological interventions.     

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. Muscle Nerve 49 : 845–857, 2014     

Whole‐body high‐field MRI shows no skeletal muscle degeneration in young patients with recessive myotonia congenita

Kornblum C, Lutterbey GG, Czermin B, Reimann J, von Kleist‐Retzow J‐C, Jurkat‐Rott K, Wattjes MP. Whole‐body high‐field MRI shows no skeletal muscle degeneration in young patients with recessive myotonia congenita.
Acta Neurol Scand: 2010: 121: 131–135.
© 2009 The Authors Journal compilation © 2009 Blackwell Munksgaard. Background – Muscle magnetic resonance imaging (MRI) is the most sensitive method in the detection of dystrophic and non‐dystrophic abnormalities within striated muscles. We hypothesized that in severe myotonia congenita type Becker muscle stiffness, prolonged transient weakness and muscle hypertrophy might finally result in morphologic skeletal muscle alterations reflected by MRI signal changes. Aim of the study – To assess dystrophic and/or non‐dystrophic alterations such as fatty or connective tissue replacement and muscle edema in patients with severe recessive myotonia congenita. Methods – We studied three seriously affected patients with myotonia congenita type Becker using multisequence whole‐body high‐field MRI. All patients had molecular genetic testing of the muscle chloride channel gene (CLCN1). Results – Molecular genetic analyses demonstrated recessive CLCN1 mutations in all patients. Two related patients were compound heterozygous for two novel CLCN1 mutations, Q160H and L657P. None of the patients showed skeletal muscle signal changes indicative of fatty muscle degeneration or edema. Two patients showed muscle bulk hypertrophy of thighs and calves in line with the clinical appearance. Conclusions – We conclude that (i) chloride channel dysfunction alone does not result in skeletal muscle morphologic changes even in advanced stages of myotonia congenita, and (ii) MRI skeletal muscle alterations in myotonic dystrophy must be clear consequences of the dystrophic disease process.     

Ethanol reduces excitability in a subgroup of primary sensory neurons by activation of BK(Ca) channels.

Ethanol effects on the central nervous system have been well investigated and described in recent years; modulations, by ethanol, of several ligand-gated and voltage-gated ion channels have been found. In this paper, we describe a shortening of action potential duration (APD) by ethanol in approximately equal to 40% of small diameter neurons in rat dorsal root ganglia (DRG). In these neurons, designated as group A neurons, we observed an ethanol-induced increase in whole-cell outward-current. As iberiotoxin, a specific blocker of large-conductance calcium-activated K+ channels (BK(Ca) channels), blocks the effects of ethanol, we investigated the interaction between these channels and ethanol in outside-out patches. Open probability of BK(Ca) channels was increased 2-6 x depending on the concentration (40-80 mM approximately equal to 2-4 per thousand v/v) of ethanol. Functional consequences were a prolongation of the refractory period, which was reversible after addition of iberiotoxin, and reduced firing frequency during ethanol application. In contrast, another type of neuron (group B) showed a prolonged APD during application of ethanol which was irreversible in most cases. In 90% of cases, neurons of group A showed a positive staining for isolectin B4 (I-B4), a marker for nociceptive neurons. We suggest that the activation of BK(Ca) channels induced by clinically relevant concentrations of ethanol, the resulting modulations of APD and refractory period of DRG neurons, might contribute to clinically well-known ethanol-induced analgesia and paresthesia.