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

iMAX: A new tool for assessment of motor axon excitability. A multicenter prospective study

OBJECTIVE

This study was undertaken to establish by a multicentric approach the reliability of a new technique evaluating motor axon excitability.

METHODS

The minimal threshold, the lowest stimulus intensity allowing a maximal response by 1 mA increments (iUP) and then by 0.1 mA adjustments (iMAX) were prospectively derived from three nerves (median, ulnar, fibular) in four university centers (Liège, Marseille, Fraiture, Nice). iMAX procedure was applied in 28 healthy volunteers (twice) and 32 patients with Charcot-Marie-Tooth (CMT1a), chronic inflammatory demyelinating polyneuropathy (CIDP), Guillain-Barré syndrome (SGB) or axonal neuropathy.

RESULTS

Healthy volunteers results were not significantly different between centers. Correlation coefficients between test and retest were moderate (> 0.5). Upper limits of normal were established using the 95th percentile. Comparison of volunteers and patient groups indicated significant increases in iMAX parameters especially for the CMT1a and CIDP groups. In CMT1a, iMAX abnormalities were homogeneous at the three stimulation sites, which was not the case for CIDP.

CONCLUSIONS

The iMAX procedure is reliable and allows the monitoring of motor axon excitability disorders.

SIGNIFICANCE

The iMAX technique should prove useful to monitor motor axonal excitability in routine clinical practice as it is a fast, non-invasive procedure, easily applicable without specific software or devices.

Long-term Effects of Intravenous Cyclophosphamide in Combination with Mesna Provided Intravenously and via Bladder Perfusion in a Patient with Severe Multifocal Motor Neuropathy

A 25-year-old woman presenting with progressive muscle weakness in the distal extremities in the absence of sensory involvement for 2 years was diagnosed with multifocal motor neuropathy (MMN). Her disease was difficult to manage with various immunosuppressants, and the muscle weakness eventually progressed to involve the respiratory muscles, necessitating mechanical ventilation. Intravenous cyclophosphamide (CY) dramatically improved her symptoms, and she has since maintained her ambulatory status for 18 years with intermittent CY therapy. Because the patient presented with hemorrhagic cystitis due to CY, we also implemented mesna administration by bladder perfusion. The administration of CY should therefore be considered in patients with severe MMN that is unresponsive to standard therapy.

Usefulness of cervical root magnetic stimulation in assessing proximal motor nerve conduction

OBJECTIVES

To evaluate the reliability and utility of cervical root magnetic stimulation in exploring proximal motor conduction.

METHODS

In 20 patients with demyelinating polyneuropathy (DPN), 20 patients with amyotrophic lateral sclerosis (ALS) and 25 healthy subjects, evoked compound muscle action potentials (CMAPs) were recorded from abductor digiti minimi muscle in response to electrical stimulation up to Erb's point and magnetic stimulation up to the cervical roots.

RESULTS

In all healthy and ALS subjects, magnetic root stimulation confirmed the absence of conduction abnormalities, including those in whom supramaximal responses at Erb's point were not achieved. In the DPN group, conduction block and/or temporal dispersion was revealed by magnetic root stimulation in 9 out of 20 patients (45%), 3 more than those detected at Erb's point.

CONCLUSIONS

Cervical root stimulation allowed clear distinction between motor neuronopathy and DPN. It is recommended as part of the routine evaluation of patients suspected of having DPN, especially when distal nerve studies are inconclusive.

Multifocal motor neuropathy

Multifocal motor neuropathy (MMN) was first described in 1988 as a purely motor neuropathy affecting multiple motor nerves. The diagnosis was based entirely on demonstrating electrophysiological evidence of a conduction block (CB) that selectively affected motor axons, with sparing of sensory axons even through the site of motor CB. Subsequently, a similar disorder was reported but with absence of demonstrable CB on routine nerve conduction studies and there is still some debate as to whether MMN without CB is related to MMN. MMN is thought to be an inflammatory neuropathy related to an immune attack on motor nerves. The conventional hypothesis is that the primary pathology is segmental demyelination, but recent research raises the possibility of a primary axonopathy. Anti-GM1 antibodies can be found in some patients but it is unclear whether these antibodies are pathogenic. Intravenous immunoglobulin is the mainstay of treatment but other immunosuppressive treatments can also be effective.

Activity-dependent conduction failure: molecular insights

Weakness and fatigue are commonly encountered symptoms in neurological disorders and significantly impair quality of life. In the case of motor axons, conduction block contributes to weakness and fatigue and may be associated with aberrant nerve activity including fasciculations and cramp. These symptoms result from dysfunction of the constituent channels and pumps of the axonal membrane. In critically conducting axons, impulse conduction can be impaired by the effects of activity or by other mechanisms that produce a significant shift in membrane potential. Conduction failure may be accentuated or relieved by maneuvers that manipulate the time course of the driving current, including the administration of agents that interfere with Na(+) channel function. In patients with inflammatory neuropathies, normal activity may be sufficient to precipitate conduction failure at sites of impaired function in multifocal motor neuropathy (MMN) and chronic inflammatory demyelinating polyneuropathy (CIDP). From a clinical perspective, these features are not assessed adequately by conventional neurophysiological techniques. As weakness and fatigue may only develop following activity or exertion, it is useful to assess the effects of impulse trains to determine the extent of conduction failure and the resulting symptoms in neurological patients. These techniques and the physiological mechanisms underlying the development of activity-dependent hyperpolarization will be critically appraised in this review, with a focus on demyelinating neuropathies, MMN and the neurodegenerative disease, and amyotrophic lateral sclerosis (ALS).

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.

Pathophysiological heterogeneity of conduction blocks in multifocal motor neuropathy

The pathophysiological mechanisms responsible for conduction block in multifocal motor neuropathy (MMN) are still unclear. To clarify the physiological abnormalities at the site of the block, we tested the effects induced by polarizing direct currents on motor conduction along forearm nerves in 25 normal nerves (13 subjects), and at the site of conduction block in six nerves (five patients) with MMN. In healthy controls, whereas nerve depolarization failed to change the conditioned compound muscle action potential (CMAP), hyperpolarization elicited a significant, charge-dependent, decrease in the conditioned CMAP size. Hyperpolarization with 4 mC elicited CMAPs that were 86.76 +/- 5.22% (mean +/- SEM) of the control unconditioned response (P < 0.05). Analysis of individual MMN nerves showed that polarizing currents elicited markedly heterogeneous effects: depending on the nerve tested, depolarization or hyperpolarization in most cases significantly improved conduction along motor fibres across the conduction block. In three MMN nerves, pathophysiological abnormalities were consistent with a hyperpolarizing block, in two with a depolarizing block, and in one with a mixed block. Our observations indicate that the pathophysiological abnormalities at the site of conduction block in MMN may arise from depolarization or hyperpolarization, probably depending on the course of disease.

Intravenous immunoglobulin for multifocal motor neuropathy

BACKGROUND

Multifocal motor neuropathy is a rare, probably immune mediated disorder characterised by slowly progressive, asymmetric, distal weakness of one or more limbs with no objective loss of sensation. It may cause prolonged periods of disability. The treatment options for multifocal motor neuropathy are sparse. Patients with multifocal motor neuropathy do not usually respond to steroids or plasma exchange, and may even worsen with these treatments. Many uncontrolled studies have suggested a beneficial effect of intravenous immunoglobulin.

OBJECTIVES

To review systematically the evidence from randomised controlled trials concerning the efficacy and safety of intravenous immunoglobulin in multifocal motor neuropathy.

SEARCH STRATEGY

We used the search strategy of the Cochrane Neuromuscular Disease Review Group to search the Disease Group register (searched September 2003), MEDLINE (January 1990 to September 2003), EMBASE (January 1990 to September 2003) and ISI (January 1990 to September 2003) databases for randomised controlled trials.

SELECTION CRITERIA

Randomised controlled studies examining the effects of any dose of intravenous immunoglobulin versus placebo in patients with definite or probable multifocal motor neuropathy. Outcome measures had to include one of the following: disability, strength, or conduction block. Studies which reported the frequency of adverse effects were used to assess safety.

DATA COLLECTION AND ANALYSIS

Two authors reviewed literature searches to identify potentially relevant trials, scored their quality and extracted data independently. For dichotomous data, we calculated relative risks, and for continuous data, effect sizes and weighted pooled effect sizes. Statistical uncertainty was expressed with 95% confidence intervals.

MAIN RESULTS

Four randomised controlled trials including a total of 34 patients were suitable for this systematic review. Strength improved in 78% of patients treated with intravenous immunoglobulin and only 4% of placebo-treated patients. Disability improved in 39% of patients after intravenous immunoglobulin treatment and in 11% after placebo (statistically not significantly different). Mild, transient side effects were reported in 71% of intravenous immunoglobulin treated patients. Serious side effects were not encountered.

AUTHORS' CONCLUSIONS

Limited evidence from randomised controlled trials shows that intravenous immunoglobulin has a beneficial effect on strength. There was a non-significant trend towards improvement in disability. More research is needed to discover whether intravenous immunoglobulin improves disability and is cost-effective.

Electrophysiological evaluation of conduction in the most proximal motor root segment

Root conduction time (RCT), defined as the time difference between M-wave latency by cervical magnetic stimulation (CMS) and the total peripheral motor conduction time calculated from the shortest F-wave latency, was investigated in patients with inflammatory demyelinating neuropathies (IDP) and amyotrophic lateral sclerosis (ALS). The minimal threshold for CMS also was studied. In the IDP patients, conduction in the proximal motor root segment was considered abnormal in 52% by the RCT and in 47% by the minimal threshold for CMS, whereas both were normal in 85% of the ALS patients. These findings suggest that the RCT and minimal threshold for CMS might be additional parameters for evaluating motor nerve conduction in IDP.

Evidence for axonal membrane hyperpolarization in multifocal motor neuropathy with conduction block

Multiple nerve excitability measurements were used to investigate axonal membrane properties of patients diagnosed with multifocal motor neuropathy (MMN). Six patients were selected, all with evidence of distal focal motor conduction block involving the median nerve in the forearm. In all patients, the median nerve was stimulated at the wrist, just distal to the site of block, and the resulting compound muscle action potentials were recorded from abductor pollicis brevis. Stimulus-response behaviour, the strength--duration time constant, threshold electrotonus to 100 ms polarizing currents, a current-threshold relationship and the recovery of excitability following supramaximal activation were recorded using a protocol described recently. When compared with control values, patients demonstrated significantly greater superexcitability, a 'fanning out' of threshold electrotonus recordings, and a significant change in the slope of the current--threshold relationship. These abnormalities in axonal membrane excitability parameters closely resembled those in normal axons hyperpolarized following release from ischaemia. To test for axonal hyperpolarization, DC depolarizing currents were applied to the nerves of three patients, and all the excitability parameters were normalized by depolarization. Attempts to trace excitability measures proximally towards the site of block were unsuccessful, as the nerve became inexcitable in all cases. It is suggested that the distal hyperpolarization is probably linked to focal depolarization and that the clinical features of MMN are consistent with a depolarizing/hyperpolarizing lesion.

Proximal motor conduction evaluated by transcranial magnetic stimulation in acquired inflammatory demyelinating neuropathies

OBJECTIVES

To evaluate conduction abnormalities in the proximal motor nerve in patients with acquired inflammatory demyelinating neuropathies by transcranial magnetic stimulation (TMS).

METHODS

TMS intensity and background voluntary contraction (BVC) to evoke maximal size of motor evoked potential (MEP) in hand muscle were investigated in 24 normal subjects. Effect of experimentally induced conduction block by injecting local anesthetics in the peripheral nerve on MEP size was also studied in two normal subjects. In 22 patients with inflammatory demyelinating neuropathies, maximal MEPs were recorded in the deteriorating and recovery stages of the illness.

RESULTS

In normal subjects, the MEP became maximal with 30-50% of maximal BVC and at more than 80% the maximal stimulator output of the 2.0 T circular coil. The change in MEP size well reflected the degree of conduction block induced by local anesthetics. Findings for patients suggested conduction abnormalities proximal to axilla in 9 patients, and that the abnormal reduction of Erb CMAP was the result of submaximal stimulation, not true conduction block, in 3 patients. The increase in MEP/wrist CMAP ratio was better correlated with improvement in muscle strength than with change in the axilla or Erb CMAP/wrist CMAP ratio.

CONCLUSIONS

Problems such as conduction abnormalities in the motor tract of the central nervous system could not fully be excluded, but we consider that maximal MEP size can be used to predict proximal motor nerve conduction abnormalities.

Chronic demyelinating hypertrophic brachial plexus neuropathy

A patient with unilateral, painless, chronic progressive upper limb sensorimotor deficit showed electrophysiological evidence of a focal demyelinating neuropathy with almost complete conduction block across the brachial plexus. Magnetic resonance imaging disclosed marked brachial plexus hypertrophy. Intravenous immunoglobulin led to fast and complete recovery, maintained by intermittent perfusions. Hypertrophic brachial plexus neuropathy can be a presentation of focal chronic inflammatory demyelinating polyradiculoneuropathy. Objective and quantitative assessment of hand function is useful to evaluate treatment results and to optimize treatment regimens.

Temporal effects of isometric contraction maneuvers on threshold sural amplitude

OBJECTIVE

To investigate the effect of isometric biceps brachii contraction and neck flexion on the time course of threshold sural amplitude.

DESIGN

Twelve healthy subjects, who were asked to lie supine on an examination bench, performed 1 min of muscle contraction. The sural sensory nerve action potential was recorded before, immediately after, and at 2-min intervals after muscle contraction. The preexercise level of stimulus intensity remained unchanged for sural readings throughout the entire course of the experiment.

RESULTS

The temporal changes in sensory nerve action potential amplitudes for both maneuvers were similar (P = 0.9734, two-way interaction). The mean sural amplitude after neck flexion increased from 6.0 +/- 2.9 microV (SD) to 10.6 +/- 6.6 microV (SD) 10 min after contraction. Similarly, mean sural amplitude increased from 6.5 +/- 1.8 microV (SD) to 14.5 +/- 9.7 microV (SD) 8 min after biceps brachii contraction. Statistical analysis performed using repeated measures with post hoc least significant difference showed a significant temporal effect in the two groups (P = 0.04).

CONCLUSION

The temporal responses of threshold sural amplitudes after isometric biceps brachii contraction and central reinforcement neck flexion maneuvers are nearly identical with regard to increase in the amplitude.

Chronic relapsing brachial plexus neuropathy with persistent conduction block

Idiopathic brachial plexus neuropathy (BPN) is an immune-mediated disorder characterized by an acute onset of painful weakness in one or both upper extremities. The course is usually monophasic with gradual improvement over months; however, occasionally BPN can recur. Electrophysiologic studies suggest the pathogenesis is primarily axonal in the majority of cases. We describe an unusual case of BPN in which the patient had a chronic and relapsing course of painless weakness associated with conduction blocks and other electrophysiologic features of demyelination across the brachial plexus. The patient improved following treatment with intravenous immunoglobulin. The neuropathy falls within the spectrum of chronic inflammatory demyelinating polyneuropathy and multifocal motor neuropathy.