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In multiple system atrophy (MSA) the motor neurons of sphincter muscles (Onuf's nucleus) degenerate earlier than those of limb muscles (Sunget al 1). Such lower motor neuron loss is, in principle, accompanied by muscle pathology, which may be readily assessed by EMG methods, of which there are several. Although there is no doubt that expertise may be accumulated with any of the various methods, data comparing different techniques are only beginning to appear. Giladet al (this issue, pp 596–99) report on results obtained by concentric needle EMG (CNEMG) single fibre EMG (SFEMG), and interference pattern (IP) analysis in a small group of patients with MSA.2 Their findings both confirm and contradict the commonly held views on sphincter EMG findings in MSA. As expected, some EMG abnormalities were found in all patients studied, but the reported changes in isolation (reduced recruitment pattern with a reduced number of active motor units during rest) are not specific for loss of lower motor neurons. Comparing patients with MSA and healthy controls, no significant differences in CNEMG and SFEMG parameters (which are known to correlate with reinnervation changes in muscle) were found. Thus, the “new” parameter may either be more sensitive in detecting early degeneration of Onuf's nucleus, or reflect upper motor neuron involvement. The failure of Gilad et al 2 to detect significant “classic” EMG abnormalities cannot be due to insensitivity of their CNEMG analysis (although the “multi-motor unit potential (multi-MUP) technique” has problems with detection of unstable complex MUPs, held to be typical of MSA) because of their confirmatory SFEMG findings. Thus, their results can actually be interpreted as showing that many patients with “possible” or “probable” MSA have no degeneration of Onuf's nucleus! However, practically all previous studies have demonstrated obvious (“classic”) EMG abnormalities! The dissenting voices in the literature do not question the presence of EMG abnormalities in MSA, but whether these are significantly different from those in other degenerative extrapyramidal diseases (the literature is ample and has recently been reviewed— see Vodušek3). The results of Gilad et al 2 thus question the value of “classic” sphincter EMG more radically (however, based on results of only 11 patients). The “majority opinion” of studies of a total of well over 500 patients with MSA still supports the “expert opinion” that the finding of pathological spontaneous activity and/or significant MUP abnormalities in the sphincter muscle favours—in the appropriate clinical setting—the diagnosis of MSA. The “appropriate setting” includes caveats that the patient with parkinsonism should be in the first 5 years of the disease course, and that he or she has no other cause for sphincter denervation; EMG does not differentiate between MSA and progressive supranuclear palsy. The absence of EMG abnormalities, however—and this was appreciated even before the article by Gilad et al 2—does not rule out MSA (see Vodušek3).
The techniques of EMG and the assessed parameters used up until now have varied, and thoroughly explored normative data from a large control group with a well standardised method of quantitative CNEMG have only recently been published—by Podnar et al,4 and now by Gilad et al 2. The method (“multi-MUP”) allows standardised quantitative assessment of a sizeable sample of motor unit potentials (MUPs) with little personal bias. With IP analysis there is even less personal bias, with Gilad et al 2 stressing the informative value of this method in patients with MSA. It seems that the above mentioned methods should be the logical choice in future studies,which should clarify the time course of sphincter motor neuron degeneration in MSA, preferably combining methods demonstrating functional and structural changes.
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