Muscle cramp in Machado-Joseph disease: altered motor axonal excitability properties and mexiletine treatment

Prospective study of peripheral neuropathy in Machado-Joseph disease

Peripheral neuropathy (PN) has long been recognized in Machado-Joseph disease (MJD), but its natural history is an unsettled issue. Therefore, we prospectively assessed 40 with MJD for 13 months with nerve conduction (NC) studies and the revised total neuropathy score (TNSr) to study the progression of PN. There was no significant change in the TNSr score over the follow-up period. In contrast, the average sural sensory nerve action potential (SNAP) amplitude decreased significantly over the same interval from a mean of 13.2 muV to 9.8 muV (P < 0.001). There was an inverse correlation between the change in the sural SNAP amplitude and the length of the CAG triplet repeat expansion (r = 0.574, P < 0.001). The reduction in the mean sural SNAP amplitude also correlated with progression of ataxia. This indicates that PN progresses faster in individuals with larger (CAG)(n) expansions, and nerve conduction studies may be useful to study disease progression in MJD.

Defining the mechanisms that underlie cortical hyperexcitability in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis [ALS] is a rapidly progressive neurodegenerative disorder of motor neurons, heralded by the development of cortical hyperexcitability. Reduction of short interval intracortical inhibition [SICI] in ALS, a feature linked to the development of cortical hyperexcitability, may be mediated by degeneration of inhibitory circuits or alternatively activation of high threshold excitatory circuits. As such, determining the mechanisms of SICI reduction in ALS has clear diagnostic and therapeutic significance. Consequently, the present study utilized a novel threshold tracking paired-pulse paradigm to determine whether SICI reduction in ALS represented reduced inhibition or excessive excitation. Using a 90 mm circular coil, SICI was assessed at three different conditioning stimulus intensities: 40%, 70% and 90% of resting motor threshold [RMT]. Motor evoked potential responses were recorded over the abductor pollicis brevis muscle. Short interval intracortical inhibition was uniformly reduced across all three levels of conditioning intensities in ALS [40% RMT, ALS -0.6+/-0.7%, controls 2.0+/-0.6%, P<0.01; 70% RMT, ALS 0.6+/-2.7%, controls 12.8+/-2%, P<0.001; 90% RMT, ALS -15.9+/-1.3%, controls 2.2+/-4.1%, P<0.01]. In addition, the resting motor threshold was reduced, while the motor evoked potential amplitude was increased in ALS patients, in keeping with cortical hyperexcitability. These findings establish that SICI reduction in ALS represents degeneration of inhibitory cortical circuits, combined with excessive excitation of high threshold excitatory pathways. Neuroprotective strategies aimed at preserving the integrity of intracortical inhibitory circuits, in addition to antagonizing excitatory cortical circuits, may provide novel therapeutic targets in ALS.

Axonal ion channels from bench to bedside: a translational neuroscience perspective

Over recent decades, the development of specialised techniques such as patch clamping and site-directed mutagenesis have established the contribution of neuronal ion channel dysfunction to the pathophysiology of common neurological conditions including epilepsy, multiple sclerosis, spinal cord injury, peripheral neuropathy, episodic ataxia, amyotrophic lateral sclerosis and neuropathic pain. Recently, these insights from in vitro studies have been translated into the clinical realm. In keeping with this progress, novel clinical axonal excitability techniques have been developed to provide information related to the activity of a variety of ion channels, energy-dependent pumps and ion exchange processes activated during impulse conduction in peripheral axons. These non-invasive techniques have been extensively applied to the study of the biophysical properties of human peripheral nerves in vivo and have provided important insights into axonal ion channel function in health and disease. This review will provide a translational perspective, focusing on an overview of the investigational method, the clinical utility in assessing the biophysical basis of ectopic symptom generation in peripheral nerve disease and a review of the major findings of excitability studies in acquired and inherited neurological disease states.

Racing against the clock: recognizing, differentiating, diagnosing, and referring the amyotrophic lateral sclerosis patient

Recognition of the early symptoms and signs in amyotrophic lateral sclerosis, exclusion of alternative diagnoses, and referral to a tertiary center can have a significant positive impact on the lives of patients and their caregivers. This article provides the most current amyotrophic lateral sclerosis criteria, as well as helpful clinical clues to the diagnosis. An approach to laboratory testing, electrodiagnostic testing, and imaging to exclude diseases that mimic ALS also are discussed, as are atypical presentations that can confound timely diagnosis.

Nerve membrane excitability testing

Routine motor nerve conduction studies measure latencies, conduction velocities and amplitudes of compound action potentials. These measurements can be very useful in defining the pathology, while they provide little insight into the underlying disease mechanisms. Increasingly, the technique of 'threshold tracking' is being used in research and clinical studies on large myelinated axons. Nerve excitability testing is a non-invasive approach in investigating the pathophysiology of peripheral nerve disorders, which determines the electrical properties of the nerve membrane at the site of stimulation. We have found evidence that in patients with critical illness polyneuropathy peripheral nerves are depolarized. The correlations with serum factors suggest that this membrane depolarization is related to endoneurial hyperkalemia and/or hypoxia. While other mechanisms of depolarization may well be involved, the degree to which potential-sensitive nerve excitability indices are related to serum potassium and bicarbonate suggests that other factors, independent of potassium and acid-base balance, are likely to be of relatively minor significance.

Changes in Na(+) channel expression and nodal persistent Na(+) currents associated with peripheral nerve regeneration in mice

Patients with peripheral neuropathy frequently suffer from positive sensory (pain and paresthesias) and motor (muscle cramping) symptoms even in the recovery phase of the disease. To investigate the pathophysiology of increased axonal excitability in peripheral nerve regeneration, we assessed the temporal and spatial expression of voltage-gated Na(+) channels as well as nodal persistent Na(+) currents in a mouse model of Wallerian degeneration. Crushed sciatic nerves of 8-week-old C57/BL6J male mice underwent complete Wallerian degeneration at 1 week. Two weeks after crush, there was a prominent increase in the number of Na(+) channel clusters per unit area, and binary or broad Na(+) channel clusters were frequently found. Excess Na(+) channel clusters were retained up to 20 weeks post-injury. Excitability testing using latent addition suggested that nodal persistent Na(+) currents markedly increased beginning at week 3, and remained through week 10. These results suggest that axonal regeneration is associated with persistently increased axonal excitability resulting from increases in the number and conductance of Na(+) channels.

Strength-duration properties in diabetic polyneuropathy

We sought to examine the changes in the strength-duration time constant (SDTC) of the median nerve in diabetic polyneuropathy. The SDTC is a measure of axonal excitability and depends on the biophysical properties of the axonal membrane. It may provide some information about Na(+) channel functioning. Forty-nine diabetic patients (30 men, 19 women; mean age, 54.6+/-9.4 years) and 15 age-matched healthy subjects (11 men, 4 women; mean age, 53.1+/-7.6 years) participated in the study. SDTC and rheobase values were 384.5+/-97.9 micros and 4.6+/-2.4 mA in patients and 313.8+/-46.7 micros and 6.0+/-2.6 mA in controls. The SDTC was found to be significantly different between the two groups (P=.009 for SDTC and P=.066 for rheobase). Fasting glucose and HbA1c levels of patients were 206.6+/-88.7 mg/dL and 8.8+/-2.4%, and these were not correlated with SDTC or rheobase (P<.05). We suggest that longer SDTC, indicating increased axonal excitability may develop in patients with polyneuropathy associated with uncontrolled diabetes.

Effects of botulinum toxin on strength-duration properties

Axonal excitability studies have been used in several diseases to investigate the underlying pathophysiology. The threshold tracking technique was developed to measure noninvasively several indices of axonal excitability, such as strength-duration properties. This study investigated the possible effects of botulinum toxin on strength-duration time constant (SDTC) in patients with the symptoms and signs of botulism. The clinical and electrophysiological findings of 13 patients who were admitted to the authors' clinic with botulism signs and symptoms were evaluated in a 5-day period after exposure to the toxin prospectively. After routine diagnostic electroneuromyographic examinations and electromyogram with repetitive nerve stimulation at 20-50 Hz, SDTC was studied. The results were compared with 13 age- and sex-matched healthy volunteers. The SDTCs were 381 +/- 60 micros and 471 +/- 84 micros in patients and controls, respectively. There was a statistical difference between the two groups (p = .003, Mann Whitney U test). These findings suggest a possible effect of botulinum toxin, known to be effective at neuromuscular junction, on Na(+)/K(+) pump activity, and Na(+) or K(+) conductance.

Uremic neuropathy: clinical features and new pathophysiological insights

Neuropathy is a common complication of end-stage kidney disease (ESKD), typically presenting as a distal symmetrical process with greater lower-limb than upper-limb involvement. The condition is of insidious onset, progressing over months. and has been estimated to be present in 60%-100% of patients on dialysis. Neuropathy generally only develops at glomerular filtration rates of less than 12 ml/min. The most frequent clinical features reflect large-fiber involvement, with paresthesias, reduction in deep tendon reflexes, impaired vibration sense, muscle wasting, and weakness. Nerve conduction studies demonstrate findings consistent with a generalized neuropathy of the axonal type. Patients may also develop autonomic features, with postural hypotension, impaired sweating, diarrhea, constipation, or impotence. The development of uremic neuropathy has been related previously to the retention of neurotoxic molecules in the middle molecular range, although this hypothesis lacked formal proof. Studies utilizing novel axonal excitability techniques have recently shed further light on the pathophysiology of this condition. Nerves of uremic patients have been shown to exist in a chronically depolarized state prior to dialysis, with subsequent improvement and normalization of resting membrane potential after dialysis. The degree of depolarization correlates with serum K(+), suggesting that chronic hyperkalemic depolarization plays an important role in the development of nerve dysfunction in ESKD. These recent findings suggest that maintenance of serum K(+) within normal limits between periods of dialysis, rather than simple avoidance of hyperkalemia, is likely to reduce the incidence and severity of uremic neuropathy.

Axonal excitability properties in hemifacial spasm

Hemifacial spasm (HFS) is characterized by involuntary, irregular contractions of muscles innervated by the facial nerve. Whether the facial nerve has a relative predisposition for ectopic activity has not been clarified. Nerve excitability techniques, which provide information about membrane potential and axonal ion channel function, were initially measured in 12 control subjects looking for biophysical differences that may predispose the facial nerve to generate ectopic activity. In a second series of studies, facial nerve excitability was assessed in nine HFS patients. In both series, stimulus-response behavior, threshold electrotonus, a current threshold relationship, and the recovery of excitability following supramaximal stimulation were recorded following stimulation of the facial nerve. When compared to normative data from nerves in the upper and lower limbs, there was a relative "fanning-in" of threshold electrotonus, reduced superexcitability, and increased subexcitability in facial nerve studies from control subjects (P < 0.05), consistent with relative axonal depolarization. These findings may underlie the propensity for the facial nerve to develop ectopic impulse activity in motor axons. In the HFS patient study, there were no significant differences in distal facial nerve excitability properties from the affected side in HFS patients when compared either to the unaffected side or to normative facial nerve data. It is concluded that the impulse generator underlying HFS must consequently be sited more proximally and does not cause a generalized disturbance of motor axon excitability.

Increased nodal persistent Na+ currents in human neuropathy and motor neuron disease estimated by latent addition

OBJECTIVE

To investigate the changes in nodal persistent Na(+) currents in human neuropathy and motor neuron disease. In human motor axons, approximately 1.0% of total Na(+) channels are active at rest, termed "persistent" Na(+) channels, and the conductance can be non-invasively estimated by the technique of latent addition in vivo.

METHODS

Latent addition was performed in median motor axons of 93 patients with axonal neuropathy (n=38), lower motor neuron disorder (LMND; n=19) or amyotrophic lateral sclerosis (ALS; n=36) and in 27 age-matched normal subjects. Brief hyperpolarizing conditioning current pulses were delivered, and threshold change at the conditioning-test interval of 0.2 ms was measured as an estimator of the magnitude of persistent Na(+) currents. Threshold electrotonus and supernormality were also measured as indicators of resting membrane potential.

RESULTS

Threshold changes at 0.2 ms were significantly greater in patients with neuropathy or LMND (p<0.05), and tended to be greater in ALS patients (p=0.075) than in normal controls. Threshold electrotonus and supernormality did not differ in each patient group and normal controls, suggesting that membrane potential is not altered in patients. In the recovery phase of axonal neuropathy, the threshold changes increased in parallel with an increase in amplitudes of compound muscle action potential.

CONCLUSIONS

Persistent Na(+) currents appear to increase commonly in disorders involving lower motor neurons, possibly associated with axonal regeneration or collateral sprouting or changes in Na(+) channel gating.

SIGNIFICANCE

The increased axonal excitability could partly be responsible for positive motor symptoms such as muscle cramping frequently seen in lower motor neuron disorders.

Effects of sex and age on strength–duration properties

OBJECTIVE

To evaluate the possible effects of sex and age on strength-duration time constant (SDTC).

METHODS

The SDTC of 126 healthy volunteers was measured following stimulation of right median nerve at the wrist. Variations in values were evaluated according to sex and age.

RESULTS

The SDTC was 438.6+/-114.5 micros in women and 396.2+/-90.3 in men (P=.023). In men, as age increased, so did SDTC. However, this was not true in women. Comparing the values of women and men, aged below 40, demonstrated a difference in excitability, confined to younger patients.

CONCLUSIONS

As SDTC depends on the biophysical properties of the axonal membrane and can provide some information about Na(+) channel function, these data raise the possibility of a difference in Na(+) channel function between men and women and a difference in the conductance with age.

SIGNIFICANCE

The age- and sex-related differences shown in this study suggest a possible biochemical or hormonal influence on axonal excitability.

Nerve excitability testing and its clinical application to neuromuscular diseases

Non-invasive nerve excitability testing measures the membrane polarization, ion channel function and paranodal/internodal condition of peripheral nerves. This technique has been recently used for various neuromuscular disorders, such as pure motor conduction block in multifocal motor neuropathy, conduction block in carpal tunnel syndrome and Na(+) channel function disorders in diabetic neuropathy, to shed light on their pathophysiology. Here, we review the basics of ion channel functions and membrane properties that influence nerve excitability, the basic principles of nerve excitability testing and the reported findings in various disorders.

Axonal excitability properties in amyotrophic lateral sclerosis

OBJECTIVE

To investigate axolemmal ion channel function in patients diagnosed with sporadic amyotrophic lateral sclerosis (ALS).

METHODS

A recently described threshold tracking protocol was implemented to measure multiple indices of axonal excitability in 26 ALS patients by stimulating the median motor nerve at the wrist. The excitability indices studied included: stimulus-response curve (SR); strength-duration time constant (tauSD); current/threshold relationship; threshold electrotonus to a 100 ms polarizing current; and recovery curves to a supramaximal stimulus.

RESULTS

Compound muscle action potential (CMAP) amplitudes were significantly reduced in ALS patients (ALS, 2.84+/-1.17 mV; controls, 8.27+/-1.09 mV, P<0.0005) and the SR curves for both 0.2 and 1 ms pulse widths were shifted in a hyperpolarized direction. Threshold electrotonus revealed a greater threshold change to both depolarizing and hyperpolarizing conditioning stimuli, similar to the 'fanned out' appearance that occurs with membrane hyperpolarization. The tauSD was significantly increased in ALS patients (ALS, 0.50+/-0.03 ms; controls, 0.42+/-0.02 ms, P<0.05). The recovery cycle of excitability following a conditioning supramaximal stimulus revealed increased superexcitability in ALS patients (ALS, 29.63+/-1.25%; controls, 25.11+/-1.01%, P<0.01).

CONCLUSIONS

Threshold tracking studies revealed changes indicative of widespread dysfunction in axonal ion channel conduction, including increased persistent Na+ channel conduction, and abnormalities of fast paranodal K+ and internodal slow K+ channel function, in ALS patients.

SIGNIFICANCE

An increase in persistent Na+ conductances coupled with reduction in K+ currents would predispose axons of ALS patients to generation of fasciculations and cramps. Axonal excitability studies may provide insight into mechanisms responsible for motor neuron loss in ALS.

Nodal persistent Na+ currents in human diabetic nerves estimated by the technique of latent addition

OBJECTIVE

To investigate the effects of hyperglycemia on persistent Na+ currents in human diabetic nerves, eliminating the factors of passive membrane properties as a factor. Previous studies show that strength-duration time constant of a nerve is shortened under hyperglycemia, suggesting reduced axonal persistent Na+ currents. However, the time constant is also affected by changes in passive membrane properties. Latent addition using computerized threshold tracking is a new method that can separately evaluate Na+ currents and passive membrane properties.

METHODS

Latent addition was used to estimate nodal Na+ currents in median motor axons of 83 diabetic patients. Brief hyperpolarizing conditioning current pulses were delivered, and threshold changes at the conditioning-test interval of 0.2 ms were measured as an indicator of nodal persistent Na+ currents. Seventeen patients were examined before and after insulin treatment.

RESULTS

There was an inverse linear relationship between hemoglobin A1c levels and threshold changes at 0.2 ms (P=0.02); the higher hemoglobin A1c levels were associated with smaller threshold changes. After insulin treatment, there was a significant improvement in nerve conduction velocities associated with greater threshold changes at 0.2 ms (P=0.03), suggesting an increase in persistent Na+ currents. The fast component of latent addition, an indicator of passive membrane properties, was not affected by the state of glycemic control.

CONCLUSIONS

Hyperglycemia could suppress nodal persistent Na+ currents, presumably because of reduced trans-axonal Na+ gradient or impaired Na+ channels, and this can be rapidly restored by glycemic control.

SIGNIFICANCE

Reduced nodal Na+ currents may partly contribute to the pathophysiology of human diabetic neuropathy.

Stiff man-like syndrome and generalized myokymia in spinocerebellar ataxia type 3

We describe the novel association of spinocerebellar ataxia type 3/Machado-Joseph disease (SCA3/MJD) phenotype combining classical clinical presentation and semeiology mimicking stiff man syndrome (SMS). The studied pedigree comprises seven affected members in three generations. Their clinical picture consisted of cerebellar ataxia, pyramidal signs, facial myokymia, and ophthalmoplegia. The proband was a 39-year-old man in whom such a clinical picture, 5 years after onset at age 29, evolved to severe SMS and widespread myokymia. Electrophysiological study revealed continuous muscle activity in proximal limb muscles. Molecular study demonstrated the MJD gene mutation in all four examined patients with 73 to 76 CAG repeats in the expanded allele. We conclude that an excess of motor unit activity including stiff man-like syndrome and widespread myokymia may be an integral part of the SCA3 clinical spectrum.

Altered axonal excitability properties in amyotrophic lateral sclerosis: impaired potassium channel function related to disease stage

Fasciculations are a characteristic feature of amyotrophic lateral sclerosis (ALS), and can arise proximally or distally in the motor neuron, indicating a widespread disturbance in membrane excitability. Previous studies of axonal excitability properties (i.e. threshold electrotonus, strength-duration time constant) have suggested respectively that change in potassium or sodium channels may be involved. To reinvestigate these changes and explore their correlation with disease stage, multiple axonal excitability properties (threshold electrotonus, strength-duration time constant, recovery cycle and current-threshold relationship) were measured for the median nerve at the wrist in 58 ALS patients, and compared with 25 age-matched controls. In ALS, there were greater changes in depolarizing threshold electrotonus (i.e. less accommodation) (P < 0.001) and greater supernormality in the recovery cycles (P < 0.001). These abnormalities were more prominent in patients with moderately reduced CMAP (1-5 mV). Modelling the excitability changes in this group supported the hypothesis that axonal potassium conductances are reduced, resulting in increased supernormality despite membrane depolarization. The tendency for strength-duration time constant to be prolonged in ALS was only significant for patients with normal CMAP amplitude (>5 mV). Patients with severely reduced CMAP (<1 mV) alone showed reduced threshold changes to hyperpolarizing current. These results suggest a changing pattern of abnormal membrane properties with disease progression. First, persistent Na+ conductance increases, possibly associated with collateral sprouting, and then K(+) conductances decline. Both changes cause axonal hyperexcitability, and may contribute to the generation of fasciculations. These serial changes in axonal properties could provide insights into the pathophysiology of ALS, and implications for future therapeutic options.

Altered motor nerve excitability in end-stage kidney disease

Although multiple toxins have been implicated in the development of uraemic neuropathy, no causative agent has been identified. In the present study, the excitability properties of lower limb motor nerves in patients with end-stage kidney disease treated with haemodialysis were measured before, during and after a standard 5 h haemodialysis session, in an attempt to explore the pathophysiology of uraemic neuropathy. Compound muscle action potentials were recorded from tibialis anterior and extensor digitorum brevis, following stimulation of the common peroneal nerve in 14 patients. Measures of excitability were assessed in relation to changes in serum levels of potential neurotoxins, including potassium, calcium, urea, uric acid, parathyroid hormone and beta-2-microglobulin. Before dialysis, measures of nerve excitability were significantly abnormal in the patient group for axons innervating tibialis anterior and extensor digitorum brevis, consistent with axonal depolarization: refractoriness was increased and superexcitability and depolarizing threshold electrotonus were reduced. Pre-dialysis excitability abnormalities were strongly correlated with serum K+. Correlation was also noted between the severity of symptoms and excitability abnormalities. Haemodialysis normalized the majority of nerve excitability parameters. In conclusion, lower limb motor axons in uraemic patients are depolarized before dialysis. The correlation between serum K+ and excitability measures indicates that hyperkalaemia is primarily responsible for uraemic depolarization, and a likely contributing factor to the development of neuropathy.

Clinical and molecular events in patients with Machado-Joseph disease under lamotrigine therapy

OBJECTIVE

Machado-Joseph disease (MJD)/spinocerebellar ataxia type 3 is an autosomal dominant spinocerebellar degeneration, for which there is no effective treatment.

PATIENTS AND METHODS

This study involved the clinical response of lamotrigine (LTG) on six MJD patients with early truncal ataxia and the effect of LTG on the alteration of ataxin-3 expression in the transformed MJD lymphoblastoid cells.

RESULT

LTG medication was found, on the basis of single leg standing test tandem gait index, to effectively improve gait balance, but did not prove to be effective in the withdrawal period. In Western blot analysis of ataxin-3 in MJD lymphoblastoid cells, extracellular application of LTG, while leaving the normal level of ataxin-3 intact, decreased the expression of mutant ataxin-3 in a dose-related manner.

CONCLUSION

Our results indicated that LTG may have significant benefits in relief of gait disturbance in MJD patients with early ataxia, and may be related to the decreased expression of mutant ataxin-3.

Axonal potassium conductance and glycemic control in human diabetic nerves

OBJECTIVE

To investigate the effects of hyperglycemia on axonal excitability and potassium conductance in human diabetic nerves.

METHODS

Threshold tracking was used to measure excitability indices, which depend on potassium channels (supernormality, late subnormality, threshold electrotonus, and a current/threshold relationship) in median motor axons of 96 diabetic patients. The effects of hyperglycemia on these indices were analyzed.

RESULTS

Among diabetic patients, higher serum hemoglobin A1c (HbA1c) levels were significantly associated with greater supernormality (P = 0.04) and smaller late subnormality (P = 0.02), suggestive of reduced nodal/paranodal potassium currents under hyperglycemia. Threshold electrotonus and current/threshold relationships did not correlate with HbA1c levels, but partly related with nerve conduction slowing.

CONCLUSIONS

Hyperglycemia could reduce nodal potassium conductances, possibly due to reduced membranous potassium gradient or suppression of potassium channels. In contrast, internodal potassium conductances may be determined by both metabolic factors and structural changes such as exposure of internodal channels by demyelination.

SIGNIFICANCE

Measurements of the excitability indices could provide new insights into nodal and internodal axonal membrane properties in human diabetic neuropathy, whereas multiple factors can affect especially internodal properties.

Altered nerve excitability properties in established diabetic neuropathy

The underlying cause of diabetic neuropathy remains unclear, although pathological studies have suggested an ischaemic basis related to microangiopathy, possibly mediated through effects on the energy-dependent Na+/K+ pump. To investigate the pathophysiology of diabetic neuropathy, axonal excitability techniques were undertaken in 20 diabetic patients with neuropathy severity graded through a combination of quantitative sensory testing (QST) using a vibratory stimulus, assessment of symptom severity using the Total Neuropathy Symptom Score (T-NSS) and measurement of glycosylated haemoglobin as a marker of disease control. To assess axonal excitability, compound muscle action potentials were recorded at rest from abductor pollicis brevis following stimulation of the median nerve, and stimulus-response behaviour, threshold electrotonus, a current-threshold relationship and the recovery of excitability were recorded in each patient. All patients had established neuropathy, with abnormalities of T-NSS present in all patients and QST abnormalities present in 65%. Compared with controls, diabetic neuropathy patients had significant reduction in maximal CMAP amplitude (P < 0.0005), accompanied by a 'fanning in' of threshold electrotonus. In addition, the strength-duration time constant was decreased in diabetic neuropathy patients and recovery cycles were altered with reductions in refractoriness, the duration of the relative refractory period, superexcitability and subexcitability. It is proposed that while the changes in threshold electrotonus with supportive findings in the current-threshold relationship are consistent with axonal depolarization, possibly mediated by a decrease in Na+/K+ pump activity, the alterations in the recovery cycle of excitability could be explained on the basis of a smaller action potential, reflecting a limitation on the nodal driving current imposed by a reduction in Na+ conductances.

The effects of mexiletine on excitability properties of human median motor axons

OBJECTIVE

To investigate the effects of mexiletine, an analog of lidocaine, on excitability of human axons in vivo.

METHODS

Threshold tracking was used to measure multiple excitability indices (strength-duration time constant, rheobase, refractoriness, supernormality, and threshold electrotonus) in median motor axons of 20 patients with neuropathic pain or muscle cramping, before and 3 months after treatment with oral 300 mg mexiletine per day.

RESULTS

After treatment, there was a reduction in pain/muscle cramps, associated with decreased strength-duration time constants (P=0.01), increased rheobasic currents (P=0.06), and lower refractoriness (P=0.02), all of which were consistent with reduced nodal Na+ currents. Supernormality and threshold electrotonus did not change significantly. The changes in strength-duration properties suggest a decrease in persistent Na+ conductance. The lowered refractoriness after treatment might result from reduced transient Na+ currents, but the lack of change in supernormality and threshold electrotonus was not consistent with this hypothesis.

CONCLUSIONS

Oral mexiletine in a dosage of 300 mg daily suppresses persistent Na+ currents in human motor axons.

SIGNIFICANCE

Measurements of the excitability indices can be used for non-invasive assessment and monitoring of the effects of mexiletine in patients with neuropathic pain or muscle cramps.

Strength-duration properties and glycemic control in human diabetic motor nerves

OBJECTIVE

To investigate the influences of hyperglycemia on axonal excitability in human diabetic nerves. Hyperglycemia results in decreased Na+-K+ pump function, presumably leading to intra-axonal Na+ accumulation and thereby, reduced Na+ currents.

METHODS

The strength-duration time constant (tau(SD)), which partly depends on persistent Na+ conductance active at the resting membrane potential, was measured in median motor axons of 79 diabetic patients. The relationship of tau(SD) with the state of glycemic control (hemoglobin A1c [HbA1c] levels) was analyzed.

RESULTS

The mean tau(SD) was longer for diabetic patients than for normal controls, but the difference was not significant. Among diabetic patients, the subgroup of patients with good glycemic control (HbA1c<7%) had significantly longer tau(SD) than the patient group with poor control (HbA1c>9%; P=0.04). The mean tau(SD) was longest at the HbA1c level of 5-6%, gradually decreasing and reaching a plateau around the HbA1c level of 9%. There was an inverse relationship between HbA1c levels and tau(SD), when the HbA1c levels ranged from 5 to 9% (P=0.04).

CONCLUSIONS

In diabetic nerves, tau(SD) is generally longer than normal, but hyperglycemia is associated with paradoxically shortened tau(SD), because of a decrease in axonal persistent Na+ conductance, possibly related to reduced membranous Na+ gradient, tissue acidosis, or other metabolic factors.

SIGNIFICANCE

Measurements of tau(SD) could provide a new insight into changes in ionic conductance in human diabetic nerves.

Oxaliplatin-induced neurotoxicity and the development of neuropathy

The pathophysiology of oxaliplatin-induced neurotoxicity remains unclear, although in vitro studies suggest involvement of voltage-gated Na+ channels. In the present study, clinical assessment was combined with nerve conduction studies (NCS) and nerve excitability studies in 16 patients after completion of oxaliplatin therapy. Chronic neuropathic symptoms persisted in 50% of patients. NCS confirmed abnormalities in symptomatic patients: sensory potentials were significantly low, whereas motor studies remained essentially normal. At 12-month follow-up of symptomatic patients, positive sensory symptoms improved but NCS abnormalities persisted. Cumulative oxaliplatin dose was a predictor of neuropathy, and long-term effects appeared to be minimized by low single-infusion dosages. Nerve excitability measures in symptomatic patients established that axons were of high threshold. Refractoriness was significantly greater in patients (symptomatic group, 56.3 +/- 24.9%; entire patient group, 46.3 +/- 12.5%; controls, 27.1 +/- 1.9%; P < 0.05). Thus, although positive sensory symptoms of oxaliplatin-induced neuropathy improved, negative sensory symptoms and abnormalities of sensory nerve conduction persisted. Differences in nerve excitability measures, particularly refractoriness, support in vitro studies indicating involvement of voltage-gated transient Na+-channel dysfunction in the development of oxaliplatin-induced neurotoxicity.

Non-movement disorder heralds symptoms of Machado-Joseph disease years before ataxia

We describe three patients with the Machado-Joseph disease (MJD) genetic abnormality who had non-movement disorder neurological symptoms or signs that preceded the gait ataxia by several years. This implies that some clinical manifestations other than ataxia may be considered part of the herald symptoms of MJD, especially in the setting of a positive family history.

Autosomal dominant cerebellar ataxias: clinical features, genetics, and pathogenesis

Autosomal dominant cerebellar ataxias are hereditary neurodegenerative disorders that are known as spinocerebellar ataxias (SCA) in genetic nomenclature. In the pregenomic era, ataxias were some of the most poorly understood neurological disorders; the unravelling of their molecular basis enabled precise diagnosis in vivo and explained many clinical phenomena such as anticipation and variable phenotypes even within one family. However, the discovery of many ataxia genes and loci in the past decade threatens to cause more confusion than optimism among clinicians. Therefore, the provision of guidance for genetic testing according to clinical findings and frequencies of SCA subtypes in different ethnic groups is a major challenge. The identification of ataxia genes raises hope that essential pathogenetic mechanisms causing SCA will become more and more apparent. Elucidation of the pathogenesis of SCA hopefully will enable the development of rational therapies for this group of disorders, which currently can only be treated symptomatically.

Nerve excitability properties in lower-limb motor axons: Evidence for a length-dependent gradient

In this study, nerve excitability protocols were adapted for lower-limb recordings in 25 healthy subjects to enable comparison of excitability parameters between proximal and distal recording sites of the same nerve and between different nerves. Excitability parameters (stimulus-response curves, strength-duration properties, threshold electrotonus, a current-threshold relationship, and the recovery cycle) were recorded from tibialis anterior, extensor digitorum brevis, and abductor hallucis. Excitability recordings were technically possible from each site, and normative values were established for lower-limb nerves. In this process, inter- and intranerve differences in excitability properties were demonstrated: stimulus intensity and rheobase were reduced in recordings from proximal sites; the relative refractory period and late subexcitability were increased; superexcitability was reduced; and a relative "fanning-in" occurred for threshold electrotonus curves recorded from proximal sites. Such a length-dependent gradient in nerve excitability may underlie the greater tendency for ectopic activity to arise from the proximal segments of motor axons and may contribute to the length-dependent involvement of motor axons in the development of peripheral neuropathy.

Excitability properties of human median axons measured at the motor point

Threshold tracking was used to measure excitability indices (strength-duration properties, threshold electrotonus, and the current-threshold relationship) at the motor point of the abductor pollicis brevis, and the results were compared with those of the median nerve at the wrist. Using an accelerometer placed at the thumb tip, movement-related potentials were recorded as target responses. When stimulating at the same site, excitability measurements were no different, and their variability no greater, when the target responses were movements rather than muscle action potentials. Motor point stimulation resulted in significantly shorter strength-duration time-constant and higher rheobase than wrist stimulation. In addition, the technique of latent addition showed that a slow component was much smaller at the motor point than at the wrist. In threshold electrotonus, threshold changes in response to depolarizing and hyperpolarizing conditioning currents were significantly smaller at the motor point than at the wrist. The differences in strength-duration time-constant and latent addition suggest that persistent Na(+) current at the resting potential is smaller at the motor point. The differences in threshold electrotonus may depend in part on altered fiber geometry but suggest that inward and possibly outward rectification are increased distally. Motor point excitability testing may provide new insights into the pathophysiology of the nerve terminals in a variety of peripheral neuropathies and motor neuron disorders.