MOTOR NEUROPATHIES AND LOWER MOTOR NEURON SYNDROMES&Return to Ist page of MMN) Patients with distal lower motor neuron (D-LMN) syndromes have, Anti-GM1 Antibodies definition, motor axon loss but no conduction block. However, they are clinically similar to MMN patients, with asymmetric, slowly progressive weakness that most commonly begins in the arms. In many patients with D-LMN syndromes electrophysiologic studies provide no definitive help in distinguishing immune-mediated motor neuropathy from untreatable motor neuron disease. However, temporal dispersion of compound muscle action potentials, F-wave abnormalities and other changes suggestive of demyelination occur in some patients with D-LMN syndromes. Evidence of mild segmental demyelination on nerve biopsy, while not diagnostic, may also provide a clue that a D-LMN syndrome is immune-mediated.
High titer serum IgM anti-GM1 antibodies occur frequently in MMN (80% to 90% of cases) and in some patients with
neuropathy syndromes. High titers of IgM anti-GM1 antibodies (above the normal range) are rare in most other disorders (< 1%) such as ALS and peripheral neuropathies. Anti-GM1 antibodies may be found in acute motor neuropathies without evidence of demyelination
(35% to 40%).
High titers (>1:1000) of selective serum IgG binding to GM1 ganglioside have strong specificity for chronic D-LMN syndromes and acute axonal motor neuropathies.
High titers of IgM binding to asialo-GM1 may also occur in MMN.
Overall, the finding of high-titer serum antibodies to Co-GM1 or NP-9:
1. provides strong independent support for the specific diagnosis of immune-mediated MMN and D-LMN syndromes
2. excludes other neurologic disorders such as polyneuropathies and ALS, and
3. indicates that immune-modulating treatment may be beneficial.
The sensitivity of antibody (IgM vs Co-GM1 and NP-9) testing for MMN is now 85% to 90%. In general, gM anti-GM1 antibodies should be detected in:
1. at least 80% of patients with multifocal motor neuropathy, and
2. less than 1% of patients with typical amyotrophic lateral sclerosis.
D. Pathogenic Mechanisms
Immunization of rabbits with GM1 may result in the production of a neuropathy with similarities to MMN. Conduction block has been induced experimentally by the intraneural injection of serum immunoglobulin from patients with motor neuropathy and anti-GM1 antibodies. Anti-GM1 antibodies can alter K+ and Na+ currents in myelinated axons. Specific anatomical patterns of binding of some anti-GM1 antibodies to peripheral nerve, spinal cord, and motor neurons are consistent with clinical and electrophysiologic findings in MMN. Clinically, titers of anti-GM1 antibodies decline before improvement in strength after cyclophosphamide treatment. Finally, there is more GM1 ganglioside in myelin from motor nerves than from sensory nerves. This could render motor fibers more susceptible to attack by anti-GM1 autoantibodies and explain their selective involvement in MMN and D-LMN syndromes. E. Treatment with cyclophosphamide or human immune globulin (HIG) can produce useful functional improvement in patients with MMN.
Improvement in strength after treatment with IVIG (for example 1 g/kg/day x 2 days) is common (50% to 70% of cases), but the length of benefit is variable, lasting from 2 weeks to 6 or more months. The dose and frequency of subsequent treatments is based on individual patient response.
1. The period of maximum improvement after IVIG treatment should be monitored. Subsequent treatments should be given just before a relapse is expected. The minimum effective dose of VI can be determined by sequentially reducing the subsequent HIG doses by 10% until a level is found that produces somewhat less benefit (length of time, or degree, of improved strength). The minimum dose that produced an optimal improvement is then used for long-term therapy.
2. Lack of improved strength after one, or at most two, treatments (total 2 to 3 g/kg each) should be considered a treatment failure. No further HIG should be used.
Although the side effects of IVIG are usually benign, its great expense mandates objective documentation of any benefit, including quantitative muscle testing and functional assessment, to justify continued use. Further studies are required to document whether HIG treats the underlying pathogenic process in MMN or produces symptomatic benefit while allowing the underlying immune process to progress. Corticosteroid (Prednisone or Solumedrol) treatment is rarely helpful in MMN and may often exacerbate weakness.
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