Predicting IVIG Response in Multifocal Motor Neuropathy
Abstract & Commentary
Source: Van den Berg-Vos RM, et al. Ann Neurol 2000;48: 919-926.
Multifocal motor neuropathy with conduction block (MMNCB) is a progressive, peripheral, purely motor, multifocal, demyelinating neuropathy resulting in motor weakness, muscle atrophy, and impaired deep tendon reflexes. It responds to intravenous immunoglobulin (IVIG), though not to prednisone or plasmapheresis, but treatment is costly and response is not entirely predictable.
Between 1996 and 1999, 37 patients presenting with a lower motor neuron syndrome and electrodiagnostic evidence of conduction block or demyelination were prospectively studied. By design, none had bulbar signs or symptoms, upper motor neuron signs, sensory deficit on examination, or sensory nerve conduction abnormalities, in keeping with a diagnosis of MMNCB. All underwent extensive electrophysiologic studies, blood work, and IVIG treatment (0.4 g/kg for 5 days) to determine which factors positively predicted response to IVIG in MMNCB. Response was defined as a 50% strength improvement in two or more muscle groups with no muscle group weakening by 25%, as graded by hand-held dynamometry and medical research council (MRC) scale. A total of eight muscle groups were defined, encompassing the proximal and distal segment of each limb. Fisher’s exact test, the Mann-Whitney U test, stepwise forward logistic regression, and X2 testing were used for statistical analysis.
Clinically positive response to IVIG was significantly associated with younger age at onset (mean age 35.1 ± 9.1 years vs 46 ± 16.3 years), and fewer affected limb regions (3.3 vs 5.1). In the serum, lower creatine kinase levels (< 180 U/L) and elevated anti-GM1 antibodies were positive predictors. Electrophysiologically, definite conduction block (> 50% area reduction over long nerve segments or > 30% amplitude reduction over 2.5 cm segment) and higher distal mean motor amplitude (mean, 7.1 mV vs 5.5 mV) on nerve conduction studies were significantly associated with positive response. Patient gender, disease duration, cerebrospinal fluid protein concentration, and abnormal brachial plexus magnetic resonance imaging (MRI), including nerve swelling or increased signal intensity, were not predictive of response. Response to IVIG in MMNCB may be predicted based on these criteria.
Commentary
In a multicenter efficacy study, 16 patients with MMNCB were randomized into a double-blind, placebo-controlled, crossover trial of IVIG (0.4 g/kg for 5 days; Neurology 2000;55:1256-1262). Five patients failed to improve but the remainder had dramatic (n = 9), moderate, or mild (n = 1 each) improvement. In this study, perhaps due to its small size, only younger age predicted positive response, whereas number of affected limbs, abnormal nerve territories, and anti-GM1 titer did not correlate. IVIG is the treatment of choice for surgery.
Accurate prediction of response to MMNCB must await larger series. For now, despite the expense, patients must be given the benefit of the doubt, and therapy initiated. Continuance thereof will depend on clinical response. — Michael Rubin
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