A practical definition of conduction block in IvIg responsive multifocal motor neuropathy

• Published in: Journal of neurology, neurosurgery and psychiatry neurosurgery and psychiatry, 2005-09, Vol.76 (9), p.1264-1268
• Main Authors: Ghosh, A, Busby, M, Kennett, R, Mills, K, Donaghy, M
• Format: Article
• Language: English
• Subjects: Action Potentials; Adult; Biological and medical sciences; Case-Control Studies; CMAP; compound muscle action potential; conduction block; Cranial nerves. Spinal roots. Peripheral nerves. Autonomic nervous system. Gustation. Olfaction; Diagnosis, Differential; Diseases of striated muscles. Neuromuscular diseases; distal latencies; Elbow; Electrophysiology; Female; Humans; Immunoglobulins, Intravenous - therapeutic use; intravenous immunoglobulin; IvIg; Male; Medical sciences; Middle Aged; MMN; Motor Neuron Disease - diagnosis; Motor Neuron Disease - drug therapy; Motor Neuron Disease - immunology; multifocal motor neuropathy; Nervous system (semeiology, syndromes); Neural Conduction - physiology; Neurology; Patients; Studies; temporal dispersion
• ISSN: 0022-3050; 1468-330X
• DOI: 10.1136/jnnp.2004.047688

Background: Multifocal motor neuropathy with conduction block (MMN) can be mistaken for motor neurone disease or other lower motor neurone syndromes, but is treatable with intravenous immunoglobulin (IvIg). Formal electrophysiological criteria for conduction block (CB) are so stringent that substantial numbers of patients may miss out on appropriate treatment. Methods: Electrophysiological data were collected from 10 healthy volunteers and compared to data from 10 patients who satisfied the clinical criteria for MMN and who responded to IvIg. This produced a definition of CB in MMN patients which was compared with existing definitions to assess “miss rates”. Results: Mean values for compound muscle action potential area, amplitude, and duration were calculated in normal subjects. Results beyond 3 SD of their respective means were considered abnormal. Using these criteria, CB in the context of MMN was defined as a reduction in negative peak area >23% along a distal nerve segment or >29% across a proximal segment; or a reduction in amplitude >32% across a distal segment or >33% across a proximal segment. All IvIg responsive patients had at least one nerve segment showing such CB. Employing some criteria from the literature would have denied treatment to over 30% of responsive patients. Conclusion: In the clinical setting of suspected MMN, less stringent criteria for CB can improve the diagnosis of this treatable disorder. Exclusions on grounds of temporal dispersion may be over-restrictive. A little over one third of CBs occur proximally.