Persistent and transient "conduction block" in motor neuron diseases

Electrodiagnostic profile of conduction slowing in amyotrophic lateral sclerosis

Objective

Since motor nerve conduction slowing can occur due to loss of large axons, we investigate the conduction slowing profile in amyotrophic lateral sclerosis (ALS) and identify the limits beyond which the diagnosis of exclusive axonal loss is unlikely.

Methods

First, using linear regression analysis, we established the range of motor conduction slowing in 76 chronic inflammatory demyelinating polyneuropathy (CIDP) patients. Demyelinating range confidence intervals were defined by assessing conduction velocity (CV), distal latency (DML), and F-wave latency (F) in relation to distal compound muscle action potential (CMAP) amplitude of median, ulnar, fibular, and tibial nerves. Results were subsequently validated in 38 additional CIDP patients. Then, the newly established demyelination confidence intervals were used to investigate the profile of conduction slowing in 95 ALS patients.

Results

CV slowing, prolonged DML, and abnormal F were observed in 22.2%, 19.6%, and 47.1% of the studied nerves respectively in ALS patients. When slowing occurred, it affected more than one segment of the motor nerve, suggesting that CMAP amplitude dependent conduction slowing caused by an exclusive loss of large axons is the main mechanism of slowing. No ALS patient had more than 2 nerves with CV slowing in the confidence interval defined by the regression equations or the American Academy of Neurology (AAN) research criteria for CIDP diagnosis.

Conclusions

The presence of more than two motor nerves with CV slowing in the demyelinating range defined by the regression analysis or AAN criteria in ALS patients suggests the contribution of acquired demyelination or other additional mechanisms exist in the electrodiagnostic profile of ALS.

Electrodiagnostic Assessment of Motor Neuron Disease

Motor neuron diseases involve degeneration of motor neurons in the brain (upper motor neurons), brain stem, and spinal cord (lower motor neurons). Symptoms vary depending on the degree of upper and lower neuron involvement, but progressive painless weakness is the predominant complaint. Motor neuron disease includes numerous specific disorders, including amyotrophic lateral sclerosis, spinal muscular atrophy, spinal bulbar muscular atrophy, and other inherited and acquired conditions. Abnormalities on nerve conduction studies, repetitive nerve stimulation, needle electromyography, and other electrodiagnostic techniques help to distinguish these disorders from each other, and from other disorders with progressive weakness.

Rapidly progressive amyotrophic lateral sclerosis initially masquerading as a demyelinating neuropathy

Rare cases of demyelinating neuropathy have been described in association with amyotrophic lateral sclerosis (ALS). We report two patients with typical ALS whose initial electroneuromyographic (ENMG) presentation could suggest the existence of a process of motor nerve fiber demyelination. However, subsequent ENMG examinations and the fatal course of the disease in a few months rather supported severe ongoing axonal degeneration at the origin of motor nerve conduction abnormalities. Repeated examinations could be required to distinguish between ENMG features of concomitant demyelinating neuropathy and rapidly progressive motor neuron loss in ALS.

Do A-waves help predict intravenous immunoglobulin response in multifocal motor neuropathy without block?

INTRODUCTION

Are there electrophysiological findings that predict response to intravenous immunoglobulin (IVIg) in patients with lower motor neuron (LMN) syndromes without multifocal conduction block (MCB)?

METHODS

We enrolled 9 patients with LMN syndromes without MCB to receive 18 weeks of IVIg therapy. Response was measured at weeks 2 and 18 using the Appel Amyotrophic Lateral Sclerosis (AALS) score (includes grip and pincer strength measures), ALS Functional Rating Scale (ALSFRS), and electrophysiological measures, including motor unit estimates (MUNEs).

RESULTS

No change occurred in AALS or ALSFRS scores posttreatment. Grip/pincer strength increased in 7 patients (P = 0.028) after initial treatment (responders); 2 showed no improvement (non-responders). No electrophysiological measure changed after treatment in either group but MUNEs trended higher (P = 0.055). "Abnormal A-waves" (complex, repetitive biphasic, or present in multiple nerves) occurred in pretreatment studies more often in responders (P = 0.028).

DISCUSSION

"Abnormal A-waves" may signal IVIg-responsive LMN syndromes even if conduction block is absent.

Multifocal motor neuropathy: current concepts and controversies

Multifocal motor neuropathy (MMN) is now a well-defined purely motor multineuropathy characterized by the presence of multifocal partial motor conduction blocks (CB), frequent association with anti-GM1 IgM antibodies, and usually a good response to high-dose intravenous immunoglobulin (IVIg) therapy. However, several issues remain to be clarified in the diagnosis, pathogenesis, and therapy of this condition including its nosological position and its relation to other chronic dysimmune neuropathies; the degree of CB necessary for the diagnosis of MMN; the existence of an axonal form of MMN; the pathophysiological basis of CB; the pathogenetic role of antiganglioside antibodies; the mechanism of action of IVIg treatments in MMN and the most effective regimen; and the treatment to be used in unresponsive patients. These issues are addressed in this review of the main clinical, electrophysiological, immunological, and therapeutic features of this neuropathy.

Multifocal motor neuropathy

Multifocal motor neuropathy (MMN) is an immune-mediated disorder characterised by slowly progressive, asymmetrical weakness of limbs without sensory loss. The clinical presentation of MMN mimics that of lower-motor-neuron disease, but in nerve-conduction studies of patients with MMN motor-conduction block has been found. By contrast with chronic inflammatory demyelinating polyneuropathy, treatment with prednisolone and plasma exchange is generally ineffective in MMN and even associated with clinical worsening in some patients. Of the immunosuppressants, cyclophosphamide has been reported as effective but only anecdotally. Various open trials and four placebo-controlled trials have shown that treatment with high-dose intravenous immunoglobulin leads to improvement of muscle strength in patients with MMN. Although clinical, pathological, imaging, immunological, and electrophysiological studies have improved our understanding of MMN over the past 15 years, further research is needed to elucidate pathogenetic disease mechanisms in the disorder.

Multifocal motor neuropathy: clinical and immunological features and response to IVIg in relation to the presence and degree of motor conduction block

OBJECTIVE

To determine whether patients with clinically typical multifocal motor neuropathy (MMN) with or without definite or probable conduction block (CB) differ in terms of clinical presentation, immunological findings, or response to treatment with intravenous immunoglobulin (IVIg).

METHODS

23 consecutive patients were studied with the typical clinical features of MMN, consisting of a progressive multineuropathic motor impairment with minimal or no sensory loss. In 14 patients, electrophysiological studies disclosed the presence of a definite or probable CB according to the criteria proposed by the American Association of Electrodiagnostic Medicine (AAEM) in at least one motor nerve. Six patients had possible CB, defined as a degree of CB 10% less than that required by the AAEM for probable CB, while no CB was detected in three patients.

RESULTS

Patients with possible CB did not differ from those with a definite or probable CB in terms of age at disease onset (mean 38.8 v 38.2 years, respectively), distribution and severity of limb weakness, clinical impairment (mean Rankin score 2.2 in both), and frequency of antiganglioside antibodies (33% v 29%). Patients with possible CB had a longer mean disease duration (9 v 5.9 years, p < 0.05) and a less frequent consistent response to IVIg (67% v 86%) than those with a definite or probable CB. Patients without a detectable CB had a similar frequency of antiganglioside antibodies (33%) but had a longer disease duration (20.3 years), greater impairment (Rankin score 2.7), and more frequent signs of axonal degeneration (41% of examined motor nerves) than patients with CB (13-15%, p < 0.005). Only one patient without detectable CB (33%) consistently improved with IVIg.

CONCLUSIONS

Patients with possible CB were clinically and immunologically indistinguishable from those with definite or probable CB, albeit with a slightly less frequent response to IVIg. This finding suggests that failure to fulfil AAEM criteria for CB in patients with otherwise clinically typical MMN should not preclude this diagnosis and consequently a treatment trial with IVIg. Whether the longer duration and greater severity of the disease and more frequent axonal impairment in patients without detectable CB than in those with CB explain their lower response to IVIg remains to be established.

Multifocal motor neuropathy

Multifocal motor neuropathy (MMN) is a recently identified peripheral nerve disorder characterized by progressive, predominantly distal, asymmetric limb weakness mostly affecting upper limbs, minimal or no sensory impairment, and by the presence on nerve conduction studies of multifocal persistent partial conduction blocks on motor but not sensory nerves. The etiopathogenesis of MMN is not known, but there is some evidence, based mostly on the clinical improvement after immunological therapies, that the disease has an immunological basis. Antibodies, mostly IgM, to the gangliosides GM1, and though less frequently, GM2 and GD1a, are frequently detected in patients' sera, helping in the diagnosis of this disease. Even if there is some experimental evidence that these antibodies may be pathogenic in vitro, their role in the neuropathy remains to be established. Patients with MMN do not usually respond to steroids or plasma exchange, which may occasionally worsen the symptoms, while the efficacy of cyclophosphamide is limited by its relevant side effects. More than 80% of MMN patients rapidly improve with high dose intravenous immunoglobulin therapy (IVIg). The effect of this therapy is, however, transient and improvement has to be maintained with periodic infusions. A positive response to interferon-beta has been recently reported in a minority of patients, some of whom were resistant to IVIg. Even if many progresses have been made on the diagnosis and therapy of MMN, there are still several issues on the nosological position, etiopathogenesis and long-term treatment of this neuropathy that need to be clarified.

Electrodiagnostic studies in amyotrophic lateral sclerosis and other motor neuron disorders

The clinical electrodiagnostic medicine (EDX) consultant asked to assess patients with suspected amyotrophic lateral sclerosis (ALS) has a number of responsibilities. Among the most important is to provide a clinical assessment in conjunction with the EDX study. The seriousness of the diagnoses and their enormous personal and economic impact require a high-quality EDX study based on a thorough knowledge of and experience with motor neuron diseases (MNDs) and related disorders. Clinical evaluation will help determine which of the EDX tools available to the EDX consultant should be applied in individual patients. Although electromyography (EMG) and nerve conduction study are the most valuable, each of the following may be helpful in the assessment of selected patients based on their clinical findings: repetitive nerve stimulation, motor unit number estimate, single-fiber EMG, somatosensory evoked potential, autonomic function test, and polysomnography. The pertinent literature on these is reviewed in this monograph. The selection and application of these EDX tools depend on a thorough knowledge of the MNDs and related disorders.

Sensitivity and specificity of different conduction block criteria

OBJECTIVES

To resolve the discrepancy between conduction block criteria derived from healthy controls and stricter criteria suggested by computer simulation of interphase cancellation through altered motor units.

METHODS

An EMG database provided control nerves from patients with amyotrophic lateral sclerosis (ALS) or neural muscular atrophy (CMT1) (disease controls) and from subjects without neuromuscular diseases (healthy controls). We estimated normal limits from the healthy controls (criterion A) and from the pooled sample of healthy and disease controls (criterion B). We compared their sensitivity with that of an arbitrary limit of 0.5 (criterion C) in acute (AIDP) and chronic inflammatory demyelinating neuropathy (CIDP) and in multifocal motor neuropathy (MMNP). Specificity was assessed in ALS and CMT1.

RESULTS

Limits estimated from healthy controls (criterion A: amplitude ratio of <0.7 in median and peroneal nerves and <0.8 in the ulnar nerve) gave false positive results in 17.3% of the ALS nerves. High scatter of the amplitude ratio of the nerves with distal response amplitudes below 1 mV required amplitude-dependent limits (0.36 for distal responses below 1 mV, 0.56 between 1 and 2 mV, and between 0.67 and 0.73 for higher response amplitudes) for criterion B. It was false positive in 4.3% of the ALS nerves and in 28.3% of the CMT1 nerves. A limit of 0.5 for nerves with distal responses above 1 mV and a limit of 0.36 for smaller responses (criterion D) avoided false positive results in ALS without further impairing sensitivity per patient in MMNP. Sensitivity in AIDP was 34.9% for criterion A, 19.5% for criterion B, and 10.2% for criterion D. Amplitude ratios were more sensitive than area ratios in CIDP and MMNP, but less specific in CMT1.

CONCLUSIONS

Limits derived from healthy controls are unspecific in chronic neuromuscular diseases and in nerves with low response amplitudes. Criterion D should be used if motor unit restructuring or conduction delays have to be taken into account. Criterion A may be applicable in early AIDP if the distal response amplitude is above 1 mV.

Nerve conduction studies in amyotrophic lateral sclerosis

We studied 137 ulnar nerves and abductor digiti minimi (ADM) muscles in 70 patients with amyotrophic lateral sclerosis (ALS), and correlated the results with ADM strength graded on the Medical Research Council (MRC) scale, to address the potential value of a standardized neurophysiological assessment of this nerve-muscle system. The ulnar nerves of 35 normal subjects matched for age, gender, and height served as controls. Reduced compound muscle action potential (CMAP) amplitude and area in the ADM muscle recordings correlated strongly with weakness. Distal motor latency, proximal conduction time, and F-wave frequency were abnormal with minimally detectable weakness. In weaker ADM muscles, conduction velocities and F-wave latencies were also abnormal. Conduction block was never observed and sensory potentials were normal. An "ALS neurophysiological index" was derived from these ulnar nerve studies and consisted of the expression: (CMAP amplitude/DML) x F frequency -, where F frequency was expressed as the number of F responses recorded in 20 trials. This index was strongly correlated with ADM weakness (r = 0.74, P < 0.001). Neurophysiological studies restricted to a single nerve-muscle system, the ulnar nerve/ADM, appear potentially useful in objectively assessing change in ALS.

Central motor conduction time in patients with multifocal motor conduction block

The finding of conduction block (CB) within short consecutive segments along a motor nerve is a key feature of multifocal motor neuropathy (MMN). Despite their different pathogenesis, this may be the only clinical difference between some cases of MMN and the pure spinal muscular atrophy form of motor neuron disease (MND). In 12 patients with distal atrophy and fasciculations and electrophysiological evidence of CBs in the upper limbs, we measured the peripheral and central motor conduction times (PMCT and CMCT) to hand muscles. We reasoned that patients with MMN should show an abnormally prolonged PMCT with normal CMCT, whereas an increased CMCT would suggest MND. All patients had delayed F-wave latency and increased PMCT. Three patients had increased CMCT. Follow-up showed little clinical and electrophysiological change in 7 of the 9 patients with normal CMCT, and a progressive motor deficit leading ultimately to death in 1 of the 3 patients with increased CMCT. This patient's electrophysiological follow-up showed a significant decrement of the compound motor action potential to both proximal and distal stimulation points, with disappearance of earlier CBs. Autopsy revealed loss of anterior horn cells and axons of the ventral root, and degeneration of large myelinated fibers. We conclude that determining the CMCT may help in differentiating MND from MMN. Persistence of a stable clinical picture over a span of at least 1 year and lack of electrophysiological signs of involvement of upper motor neurons should both be required before establishing the diagnosis of MMN even with electrophysiological evidence of CB.

Root stimulation improves the detection of acquired demyelinating polyneuropathies

Thirty-one patients with a presentation compatible with an acquired demyelinating polyradiculoneuropathy were prospectively evaluated for proximal conduction block using root stimulation (RS) of cervical and/or lumbar roots. These same techniques were applied to 78 controls (40 pathological). Proximal conduction block was noted in 20/31 cases and none of the controls. Only 7 of 31 cases met published demyelinating criteria. Intravenous immune globulin therapy was completed by 21/31 patients and 20/21 responded. RS is superior to published electrophysiologic criteria for identifying demyelinating polyneuropathies and predicting who will respond to treatment. RS should be performed in all patients suspected of having an acquired demyelinating polyneuropathy when traditional nerve conduction studies criteria are nondiagnostic. RS reliably predicts those likely to respond to immunosuppressive therapy.

Multifocal motor neuropathy: clinical and electrophysiological findings

Multifocal motor neuropathy (MMN) can be differentiated from motor neuron disease by electrophysiological evidence of conduction block. To increase the probability of recording conduction block, we studied the whole nerve length including proximal segments in 84 patients with pure motor syndromes, using a special stimulation technique. In 8 patients, the diagnosis of MMN was confirmed by electrophysiological evidence of conduction block or temporal dispersion. The typical clinical picture of MMN with chronic progressive, asymmetrical, marked distal weakness was observed in our patients. Electrophysiological routine tests of distal nerves were usually normal except in nerve segments with conduction block. In 4 patients, conduction block could be recorded only in proximal nerve segments or spinal roots. All patients showed rapid improvement of clinical features and parallel reduction of conduction block during or after high-dose intravenous immunoglobulin (ivIG) therapy, supporting the diagnosis of an immune-mediated neuropathy. Three of them are now in remission without any therapy, whereas 5 still receive a regular ivIG course every 2-12 weeks as long-term treatment. In all patients with pure or predominantly motor syndromes and normal findings in electrophysiological routine tests of distal nerve segments, there should be proximal conduction block studies to avoid overlooking a treatable disorder such as MMN.

Persistent increased threshold of electrical stimulation selective to motor nerve in multifocal motor neuropathy

In multifocal motor neuropathy (MMN) the threshold of electrical stimulation showed a persistent, marked increase for the motor nerve which decreased after treatment with intravenous immunoglobulin or oral cyclophosphamide; whereas, the threshold was normal for the sensory nerve. This discrepancy of the thresholds for motor and sensory nerves indicates that the increased threshold for motor nerve is not caused by change in perineural capacitance, such as subperi- and endoneural edema or perineural thickening. Inching studies showed that the site of the elevated motor nerve threshold was closely associated with conduction slowing and block. For the cause of the increased threshold, therefore, we suppose the presence of a factor which interferes with reorganization of the nodal property in the remyelinative process or which directly blocks sodium channels where the blood-nerve barrier is impaired in MMN.

Pathological findings in a patient with amyotrophic lateral sclerosis and multifocal motor neuropathy with conduction block

We studied a 53-year-old woman with progressive weakness of the left arm, gradually spreading to the other limbs. Neurological examination revealed a motor neuron syndrome with paresis, fasciculations and atrophy. Electrophysiological studies showed multiple motor conduction blocks. The anti-GM1 IgM titer was elevated. The patient was thought to have a multifocal motor neuropathy. Despite intravenous cyclophosphamide treatment, however, she died with respiratory insufficiency. On postmortem examination, the brachial plexus showed patches of demyelination underlying different areas of motor conduction block. The spinal cord, however, revealed severe neuronal loss in the ventral horn and axonal loss in the corticospinal tract, indicative of amyotrophic lateral sclerosis. Demyelination of peripheral nerves could have been responsible for the other conduction blocks in this patient. The prominent degeneration of motor neurons, however, must also have played a role in the clinical picture. Some patients with the syndrome of a multifocal motor neuropathy may have MND rather than, or in addition to, a demyelinating peripheral motor neuropathy.

Multifocal motor neuropathy with conduction block: a study of 24 patients

Twenty four patients with pure motor neuropathy are reported. The chronic motor involvement associated with fasciculations and cramps, mainly in the arms, led, in most patients, to an initial diagnosis of motor neuron disease. In some patients (nine of 24), there was no appreciable muscle atrophy. Tendon reflexes were often absent or weak. The finding of persistent multifocal conduction block confined to motor nerve fibres raises questions about the nature and the importance of this syndrome. Segmental reduction of motor conduction velocity occurred at the site of the block, but significant slowing of motor nerve conduction was not found outside this site. The response to intravenous IVIg treatment seems to be correlated with the absence of amyotrophy. Patients with little or no amyotrophy had an initial and sustained response to IVIg, and did not develop amyotrophy during the follow up study. They could be considered to have a variant of chronic inflammatory demyelinating polyneuropathy. Patients with pronounced amyotrophy independent of the disease duration did not respond as well to IVIg treatment, suggesting the existence of a distinct entity. Among the patients treated about two thirds who had an initial good response to IVIg had high or significant antiganglioside GM1 (anti-GM1) antibody titres, but there was no correlation between the high titres before treatment and long lasting response to IVIg treatment.