Dynamic change of proximal conduction in demyelinating neuropathies: A cervical magnetic stimulation combined with maximum voluntary contraction
Introduction
For the diagnosis of acquired demyelinating neuropathies (DN), such as chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) and multifocal motor neuropathy (MMN), motor nerve conduction studies (NCSs) for the distal hand muscles are commonly used to detect the site of conduction block or slowing. When the lesion site is located in the proximal segment, cervical root stimulation with a magnetic coil is also used. It effectively activates the deep structures with little discomfort (Chokroverty et al., 1991, Cros et al., 1990) and can be used to evaluate proximal motor conduction (Bischoff et al., 1993, Boyacıyan et al., 1996, Inaba et al., 2002, Maegaki et al., 1994, Takada and Ravnborg, 2000, Wöhrle et al., 1995).
It would, however, be difficult to test conduction abnormalities in DN patients with severely atrophied hand muscles due to secondary axonal degeneration, because of the low amplitude of compound muscle action potentials (CMAPs). NCSs using less atrophied proximal muscles would, then, be feasible rather than those of distal hand muscles (Cros et al., 1990, Epstein, 1993). NCSs of the musculocutaneous nerve have been investigated by electric stimulation applied at the axilla and Erb’s point (Trojaborg, 1976). However, this can cause severe discomfort in some subjects and will not detect lesion sites more proximal than Erb’s point.
According to previous reports (Cappelen-Smith et al., 2000, Kaji et al., 2000), patients with MMN and CIDP showed a peculiar fatigue phenomenon after sustained muscle contraction. The CMAPs of their distal hand muscles showed significant decreases of amplitude and increases of threshold after a maximum contraction for 60 s (Cappelen-Smith et al., 2000, Kaji et al., 2000). These studies indicate that hyperpolarization of the axonal membrane induced by activity causes a transient and activity-dependent conduction block, resulting in easy fatigability.
In this study, we examined conduction and also investigated the dynamic changes of CMAP parameters before and after sustained voluntary contraction in proximal muscles to detect occult demyelinating lesions in the nerves innervating the proximal muscle. We also compared the findings before and after intravenous immunoglobulin (IVIg) therapy.
Section snippets
Subjects
Seven CIDP patients (Four women and three men, age 35.4 ± 13.2 years (mean ± SD), range 19–60 years) and 5 MMN patients (one woman and four men, age 56.2 ± 7.8 years, range 48–64 years) participated in this study. Six MND patients (one woman and five men, age 60.0 ± 12.8 years, range 43–73 years) served as disease controls, and 12 healthy volunteers (three women and nine men, age 48.5 ± 15.9 years, range 26–72 years) served as normal controls. All patients but one (Patient 12 in Table 1) were recruited
Results
There were no significant differences in any of the CMAP parameters obtained by conventional NCSs (data not shown) and cervical magnetic stimulation, between the patients with CIDP and those with MMN (Table 2). These two groups were thus regarded as a single DN group. The MND patients were significantly older than the CIDP patients (P = 0.01), but the mean age of patients with DN (grouped CIDP and MMN patients together) (44.1 ± 15.2 years, range 19–64 years) was not significantly different from
Discussion
The present study demonstrated that cervical root magnetic stimulation combined with MVC maneuver could differentiate DN from MND and normal subjects. Motor threshold was increased and CMAP latencies were prolonged in most DN patients, and the MVC maneuver decreased CMAP amplitude selectively only in DN patients. Although the number of subjects was small and we tested the already proven patients, our preliminary findings suggested that the dynamic change of CMAP after MVC could predict
Conclusion
The nerves innervating the proximal muscles revealed conduction failure immediately after the MVC maneuver in patients with DN. The techniques used in this study are regarded as a dynamic conduction test in the time domain rather than in the space or nerve length domain, and is a possible method for differentiating treatable conditions from MND. This technique might detect occult lesions in the proximal site, if present, even though distal muscles are not available because of the severe atrophy
Acknowledgements
We thank Dr. Jun Kimura (University of Iowa, Iowa City, IA) for comments on CMAP change. We thank Dr. Nobuyuki Oka (South Kyoto Hospital, Japan) for assessing the antiganglioside antibody titers and sural nerve biopsy. We thank Drs. Akira Kuzuya and Hirofumi Yamashita for their help with some experiments.
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2013, Clinical NeurophysiologyCitation Excerpt :The focal activation site is explained by the concentration of induced currents into a small canal instead of penetrating the bony structures with a high resistance for electrical currents. Many papers have supported this assumption on the activation site and have demonstrated its clinical utility for cervical motor roots using a round coil (Chokroverty et al., 1991; Ugawa et al., 1990; Britton et al., 1990; Cros et al., 1990a; Schmid et al., 1990; Evans et al., 1990; Maegaki et al., 1994; Benecke, 1996; Boyaciyan et al., 1996; Öge et al., 1997; Inaba et al., 2002; Ugawa, 2004; Hitomi et al., 2007; Temuçin and Nurlu, 2011) and a figure-of-eight coil (Epstein et al., 1991; Mills et al., 1993), as well as for lumbosacral motor roots using a round coil (Chokroverty et al., 1989, 1993; Ugawa et al., 1990; Britton et al., 1990; Macdonell et al., 1992; Ertekin et al., 1994a; Benecke, 1996; Maccabee et al., 1996; Troni et al., 1996; Takada and Ravnborg, 2000; Ugawa, 2004; Souayah and Sander, 2006) and a figure-of-eight coil (Maccabee et al., 1996; Maegaki et al., 1997). Maccabee et al. (1991) verified the assumption that the activation site is the neuroforamina in vitro (Maccabee et al., 1991).
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