Multifocal Motor Neuropathy
MOTOR NEUROPATHIES & LOWER MOTOR NEURON SYNDROMES part-2
Go to MMF Discussion page What is Multi Focal Neuropathy Cyclophosphamide is the only immunosuppressive medication that has been reported to induce long-term benefit in many patients (50% to 80%) with MMN. Unfortunately, its toxicity, especially the increased risk of neoplasia with high cumulative life-time doses (>75 g), requires a careful analysis of the risk:benefit ratio in each patient. Therapeutic regimens should utilize doses of cyclophosphamide that are high enough to reduce anti-GM1 antibody titers by 60%, or more. We originally used an initial dose of 3 g/M2 over 8 days followed by chronic oral medication (100-150 - mg/day for 6 to 12 months). More recent experience suggests that 6 monthly treatments with intravenous cyclophosphamide (1g/M2), each preceded by two plasma exchanges, is equally effective, has fewer adverse effects and utilizes a 50% to 70% lower cumulative dose of drug. This regimen produces a sustained reduction in titers of serum anti-GM1 antibodies in approximately 60% to 80% of patients. Most patients in whom antibody titers are reduced show functional benefit. Remission usually persists for 1-3 years; after which, antibody titers often rise and weakness recurs. Retreatment may then be necessary.
Deciding whether to treat patients with D-LMN syndromes without electrodiagnostic evidence of demyelination may be difficult. High titers of serum IgM anti-GM1 antibodies are a useful indicator that a D-LMN syndrome may be immune- mediated and treatable. Evidence of demyelination on sural nerve biopsy may also be helpful in this regard. Measurable improvement in strength after treatment with HIG may provide support for further immunotherapy, with agents such as cyclophosphamide or further periodic HIG infusions.
2. Differential Diagnosis of Motor NeuropathiesThe differential diagnosis of motor neuropathies includes motor neuron disorders, hereditary and acquired, on one hand, and demyelinating neuropathies, on the other.
Motor neuropathies and motor neuron disorders. Some motor neuropathies have been classified as ALS variants, with predominantly LMN signs and axonal changes on electrodiagnostic studies. Certain features can aid in the differentiation between motor neuropathies and ALS. Patients with motor neuropathies may have preserved reflexes in weak muscles, but overt spasticity and bulbar features are conspicuously lacking. This is in contrast to patients with ALS who often have prominent upper motor neuron and bulbar findings. The prolonged course that is often noted in patients with motor neuropathies also helps to differentiate their syndromes from typical ALS. Acquired motor neuropathies most often produce asymmetric weakness. This pattern is usually clinically distinct from the proximal symmetric weakness that characterizes most of the hereditary spinal muscular atrophies.
Several lower motor neuron syndromes have been described that are of uncertain etiology and could be disorders of the motor axon or cell body. A majority of patients with D-LMN syndromes have neither evidence of peripheral nerve demyelination nor serum anti-ganglioside antibodies. These patients tend to have more rapidly progressive weakness than is typical for the immune-mediated motor neuropathies. In contrast to typical ALS, many D-LMN patients never develop bulbar dysfunction. There are no reports of response to immunosuppressive treatment in D-LMN patients with neither demyelination nor serum autoantibodies. Some patients develop progressive asymmetric lower motor syndromes with predominant early weakness in proximal musculature (P-LMN syndromes). Characteristic features include late-age onset, male predominance (85%) and initial signs of weakness in the upper extremities (80%). Progression is slow. Weakness is often confined to one or two extremities for 3 to 5 years. Electrodiagnostic studies show only evidence of axonal loss. Some patients with P-LMN syndromes (30%) have selective serum antibody binding to GA1 ganglioside. However, there is no evidence that P-LMN syndromes respond to immunosuppressive treatment.
Monomelic amyotrophy, a syndrome that affects mainly young (15 to 25 years) males (80%), presents with weakness of the distal musculature of one upper extremity that progresses for 1 to 2 years and then remains stable. Occasional patients develop weakness in the opposite limb, mild sensory symptoms or tremor. Electrodiagnostic studies show denervation in the affected limb. There are no associated serum antibodies.
Rare patients with paraneoplastic LMN syndromes have been reported. The best described of these is a subacute motor neuronopathy associated with lymphomas, such as Hodgkin’s disease. Progressive, asymmetric weakness develops, most severely in the legs, at times when the neoplasm is in remission or during irradiation. The weakness is rarely severe, and often stabilizes or improves over a period of months to years. Pathological studies show a loss of motor neurons in the ventral horn of the spinal cord and some involvement of sensory tracts. LMN involvement has also been described as an occasional part of the paraneoplastic encephalomyelitis and sensory ganglionopathy syndromes that occur in association with anti-Hu antibodies. There is no clear evidence that there is an increased incidence of paraneoplastic "typical" ALS syndromes, with upper and lower motor neuron involvement. A few patients with ALS-like syndromes and neoplasms, including renal cell, lung and lymphoma, have been reported to improve or stabilize after treatment of the cancer.
Immune demyelinating Neuropathies. AlthoughMMN and CIDP are both demyelinating
neuropathies. MMN commonly presents with handor foot asymmetric weakness while in CIDP,
shoulder or hip symmetric weakness is a more common finding. The remitting and relapsing course that may occur in CIDP is uncommon in the motor neuropathies. Patients with MMN rarely have significant sensory symptoms while in CIDP, sensory signs are the rule. Electrophysiological testing may show conduction block in both conditions. However, other features of demyelination such as prolonged distal latencies and slowed conduction velocities are more prominent in CIDP. Abnormalities in sensory nerve conduction studies are usually seen in CIDP, but not in MMN, unless complicated by another disease process. The spinal fluid examination shows markedly increased protein concentration in the majority of cases of CIDP while this change is rare in patients with MMN. High titer anti-GM1 antibodies as well as more specific patterns of autoantibody reactivity (see above) are common in MMN. In CIDP anti-GM1 antibodies are unusual. Serum autoantibody binding to tubulin is more common. Finally, differences in the frequency of therapeutic response to prednisone and plasma exchange (common in CIDP, but rare in motor neuropathies) define a practical difference in the management of the two disorders.