Statistical methods

Electromyographic parameters by age were analyzed using a polynomial trend curve to capture the parameter change by age. The normative presentation was followed by a case-control study. The controls were drawn from the normative cohort and matched with the study patients by age and sex. The means were then compared by t test.

Results

The quantitative EMG data at rest and during recruitment of the normal control group are summarised in table 1. There was no significant effect of sex on any parameter (table 2). In the group of 67 women, four were nulliparous and 63 of varying parity (mean vaginal delivery 2.4 (SD 1.0)).1-5 On regression analysis, parity did not significantly affect the MUP and IP parameters. The control study population was divided into age groups according to their natural distribution (table 3). The results are reported in quintiles of age. The parameter of age did not significantly affect the MUP data.

Comparing the quantitative EMG values for the sphincter of patients with MSA with the matched controls, there was no statistically significant difference between the groups in amplitudes, durations, areas, polyphasicity, and fibre densities. A significant difference was found only in the parameters of the recruitment pattern. The activities (as defined above) of all patients were significantly decreased. The envelope amplitudes were decreased, as well as NSS (fig 1). Another parameter that was significantly reduced in the patients was the number of active motor units during rest (table 4). No fibrillation potentials were detected in any patient. The late components found were calculated within the duration of the MUPs.

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Figure 1

Interference pattern data and number of MUPs at rest in 11 patients with MSA versus 22 peer matched controls. (A) Mean envelope with removed outliers (μV). (B) Percentage activity (percentage of time with EMG activity). (C) number of short segments (NSS). (D) Number of MUPs at rest. For further details see text.

Despite non-significant mean differences in MUP size between the patients and matched controls while considering individual MUPs, there were three patients with MSA who showed more than 10% of MUPs with duration above 10 ms (outliers). No outliers were seen in the control group.

Discussion

In the present study we examined the external anal sphincter muscle in normal subjects and patients with MSA. There was no difference in the various EMG parameters between sexes, nor was there any effect of parity or age on MUP/IP analysis. These results resemble those of Podnar et al.16 17 19 On the other hand, the mean values of MUP parameters (amplitude duration area) were slightly greater than the values in one of these studies, but the IP parameters were similar to their data.17 The difference could emanate from a higher activation during analysis, recruiting larger units, although we sampled MUPs at a level of 3–5 MUPs activation, similarly to their technique.

Comparing patients with MSA with controls, the most obvious parameter for distinction between the two groups was the number of firing MUPs at rest and recruitment pattern data—that is, the activity, NSS, and envelope during effort. These values were found to be abnormal in every patient in respect to mean values of the control group, although it is a relatively small group of patients with MSA.

Some studies suggested that either the detection of spontaneous activity,4 polyphasicity above 30%,5 or MUP duration above 10 ms,3 are indicators of motor neuron degeneration in Onuf's nucleus. By contrast, our results show no significant differences in these parameters between the patients with MSA and controls. Employing the technique used by us, which detects 3–5 MUPs/insertion site, rise time is not taken into account, and hence units that are not very close to the needle electrode are recorded as well. This multi-MUP analysis was found to be practical and useful for obtaining normative data, being precise and easily performed, as noted by Podnar and Vodusštek.11However, it does not explain the difference between previous results and ours. If we consider the presence of more than 10% outliers (duration exceeding 10 ms) which was not detected in any of the controls, then three of 11 patients with MSA would be considered as having some neurogenic changes. Therefore, in addition to our other findings, some “classic EMG abnormalities” were also present in certain patients.

The most important finding in our study was the poor recruitment values, which seem to be characteristic of the pathological process in patients with MSA. To the best of our knowledge, recruitment has not been systematically investigated previously. The reduced number of firing units at rest and decreased activity parameters with effort may reflect the few remaining motor cells in Onuf's nucleus. The non-detection of large MUPs and the non-increased fibre density in our patients imply that consequential reinnervation has not occurred.

Another plausible explanation for the poor recruitment with no weighty reinnervation process may be upper motor neuron involvement. In limb muscles, reduced recruitment without rapidly firing units may ensue either from voluntary inactivity or from upper motor neuron affection. Because voluntary inactivity in our patients seems highly unlikely, and the nature of the firing units during rest is not influenced by voluntary activation, upper motor neuron involvement seems reasonable. In fact, upper neuron involvement in patients with MSA has already been described. Urodynamic studies showed urethral sphincter hyperactivity16 18 suggesting this mechanism. However, upper motor neuron control of the bladder and anal sphincter is not well delineated and its involvement and role in MSA is not adequately known.

In a recent study of anal sphincter EMG10 comparing patients with Parkinson's disease and MSA, no significant difference in the MUP parameters and in the presence of spontaneous activity between the groups was found. Thus, MUP analysis does not seem to have an important value in distinguishing between MSA and other extrapyramidal diseases. Recruitment was not investigated in patients with Parkinson's disease and other extrapyramidal syndromes. Thus, it is not clear whether studying recruitment is helpful in detecting patients with MSA among other extrapyramidal diseases.

In conclusion, external anal sphincter EMG may detect abnormality in patients with MSA. The reduced interference activity and reduced number of MUPs at rest are the main EMG abnormalities in MSA. It is not clear whether these EMG findings are specific for MSA and their diagnostic value should be further studied, particularly in comparison with Parkinson's disease. Our normative data obtained by a rapid practical technique from a large control group may serve as a basis for other EMG studies of the anal sphincter.