Acute motor axonal neuropathy and multifocal motor neuropathy: More in common than not

Clinical and Electrophysiological Features of Chronic Motor Axonal Neuropathy

PURPOSE

To determine the clinical and electrophysiological characteristics of chronic motor axonal neuropathy (CMAN) and identify the associated similarities and differences between CMAN, acute motor axonal neuropathy (AMAN), and motor neuropathy secondary to amyotrophic lateral sclerosis.

METHODS

The study described clinical and electrophysiological features of five patients with CMAN and compared with 20 AMAN patients, 42 amyotrophic lateral sclerosis patients and 41 healthy controls. To compare the distribution of different nerve involvement in the same limb, split ratio was introduced. Split ratio of upper limb = amplitude of compound muscle action potential abductor pollicis brevis (APB)/amplitude of compound muscle action potential abductor digiti minimi, and split ratio of lower limb = amplitude of compound muscle action potential extensor digitorum brevis/amplitude of compound muscle action potential abductor hallucis.

RESULTS

Chronic motor axonal neuropathy patients manifested lower motor neuron syndrome with positive IgG anti-monosialoganglioside antibodies and good outcome. The CMAN patients shared similar clinical manifestation with AMAN patients except for disease course and higher Medical Research Council scores. Compared with healthy controls, the split ratio of lower limb was higher in both CMAN and AMAN, despite comparable split ratio of upper limb. There was significant difference between CMAN group and amyotrophic lateral sclerosis group in nerve involvement presented as split hand and split leg signs in amyotrophic lateral sclerosis and reverse split leg sign in CMAN.

CONCLUSIONS

Chronic motor axonal neuropathy associated with monosialoganglioside might be a "mild" AMAN with chronic onset by similar clinical and electrophysiological features. There was a unique pattern of nerve involvement presenting as reverse split leg sign in both CMAN and AMAN.

Excitability of motor and sensory axons in multifocal motor neuropathy

OBJECTIVE

To assess excitability differences between motor and sensory axons of affected nerves in patients with multifocal motor neuropathy (MMN).

METHODS

We performed motor and sensory excitability tests in affected median nerves of 20 MMN patients and in 20 age-matched normal subjects. CMAPs were recorded from the thenar and SNAPs from the 3rd digit. Clinical tests included assessment of muscle strength, two-point discrimination and joint position.

RESULTS

All MMN patients had weakness of the thenar muscle and normal sensory tests. Motor excitability testing in MMN showed an increased threshold for a 50% CMAP, increased rheobase, decreased stimulus-response slope, fanning-out of threshold electrotonus, decreased resting I/V slope, shortened refractory period, and more pronounced superexcitability. Sensory excitability testing in MMN revealed decreased accommodation half-time and S2-accommodation and less pronounced subexcitability. Mathematical modeling indicated increased Barrett-Barrett conductance for motor fibers and increase in internodal fast potassium conductance for sensory fibers.

CONCLUSIONS

Excitability findings in MMN suggest myelin sheath or paranodal seal involvement in motor fibers and, possibly, paranodal detachment in sensory fibers.

SIGNIFICANCE

Excitability properties of affected nerves in MMN differ between motor and sensory nerve fibers.

Complement activity is associated with disease severity in multifocal motor neuropathy

Objective:

To investigate whether high innate activity of the classical and lectin pathways of complement is associated with multifocal motor neuropathy (MMN) and whether levels of innate complement activity or the potential of anti-GM1 antibodies to activate the complement system correlate with disease severity.

Methods:

We performed a case-control study including 79 patients with MMN and 79 matched healthy controls. Muscle weakness was documented with Medical Research Council scale sum score and axonal loss with nerve conduction studies. Activity of the classical and lectin pathways of complement was assessed by ELISA. We also determined serum mannose-binding lectin (MBL) concentrations and polymorphisms in the MBL gene (MBL2) and quantified complement-activating properties of anti-GM1 IgM antibodies by ELISA.

Results:

Activity of the classical and lectin pathways, MBL2 genotypes, and serum MBL concentrations did not differ between patients and controls. Complement activation by anti-GM1 IgM antibodies was exclusively mediated through the classical pathway and correlated with antibody titers (p < 0.001). Logistic regression analysis showed that both high innate activity of the classical pathway of complement and high complement-activating capacity of anti-GM1 IgM antibodies were significantly associated with more severe muscle weakness and axonal loss.

Conclusion:

High innate activity of the classical pathway of complement and efficient complement-activating properties of anti-GM1 IgM antibodies are determinants of disease severity in patients with MMN. These findings underline the importance of anti-GM1 antibody–mediated complement activation in the pathogenesis and clinical course of MMN.

The pathogenesis of multifocal motor neuropathy and an update on current management options

Multifocal motor neuropathy (MMN) is a rare and disabling disease. Several experimental studies and clinical data are strongly suggestive of an immune-mediated pathogenesis, although underlying mechanisms in MMN seem to be very specific, mainly because of the presence of IgM anti-GM1 serum antibodies and the dramatic response to intravenous immunoglobulins (IVIg). The origin of antiganglioside antibodies and the way in which they act at the molecular level remain unclear. Several studies have demonstrated the key role of complement activation in the underlying mechanisms of MMN, as well as in animal models of acute motor axonal neuropathy (AMAN). Deposition of the membrane attack complex may disrupt the architecture of the nodes of Ranvier and paranodal areas, causing local disruption of nodal sodium-channel clusters. In patients with MMN, muscle weakness is the consequence of conduction blocks (CB), which leads to secondary axonal degeneration, consequently the aim of the treatment is to reverse CB at early stages of the disease. High-dose immunoglobulin is to date the only therapy which has proven efficacy in MMN patients in providing transient improvement of muscle strength, but long-term follow-up studies show a progressive motor decline. Therefore, other therapies are needed to improve the conduction nerve properties in long-term design. The reduction of complement activation and more generally the gain in paranodal stabilization could be directions for future therapeutic strategies.