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481 Strength-Duration Testing of the External Anal Sphincter in Females with Anorectal Dysfunction [2002년 1월 DCR] 2011-11-17 1853
 
P. M. Mills, M.Sc., G. L. Hosker, M.Phil., E. S. Kiff, M.D., F.R.C.S., A. R. B. Smith, M.D., F.R.C.O.G.
From the Manchester School of Physiotherapy, Manchester Royal Infirmary, Manchester, United kingdom, The Warrell Unit, Saint Mary’s Hospital, Manchester, United Kingdom, and the Department of Surgery, University Hospital of South Manchester, Wythenshawe, Manchester, United Kingdom
PURPOSE: The strength-duration test has been suggested as a means of assessing external anal sphincter function. This study was designed to investigate this claim by comparing the strength-duration test with established measures of external anal sphincter function.
METHODS: Forty-nine females undergoing diagnostic anorectal testing (manometry, rectal sensation, electromyogram, pudendal nerve terminal motor latency, and endoanal ultrasound) also had the
strength-duration test performed (which was repeated for each patient after a short rest period). RESULTS: The strength-duration test was repeatable. Statistically significant correlations were found between this test at pulse durations of 3 ms, 1 ms, and 0.3 ms with electromyographic activity of the external anal sphincter and with pressure in the anal canal during voluntary contraction. Significant correlations were found for durations of 100 ms, 30 ms, 10 ms, and 3 ms with the pudendal nerve terminal motor latency on the right and for the 3 ms and 0.3 ms durations with latency on the left. There were no correlations between the strength-duration test and resting pressure in the anal canal.
CONCLUSION: The strength-duration test significantly correlates with the established measures of external anal sphincter function and its innervation. Therefore, this simple test appears to provide a simple measure of external anal sphincter denervation.
In a recent study Monk et al. (1998) reconsidered the strength-duration test. This is a measure of
muscle denervation and involves placing an electrode on the skin overlying the muscle. The current intensity required to elicit a twitch from the muscle using progressively shorter pulse durations of an applied electric stimulus is recorded. In denervated muscle, a greater current intensity is required to evoke a muscle response. Monk et al. used this simple, minimally invasive test to assess the external anal sphincter in females with normal bowel control and in those who had fecal incontinence. They found that the test could discriminate between the two groups with 77 percent sensitivity and 84 percent specificity. When combined with manometry data, the sensitivity increased to 95 percent and the specificity to 100 percent. The authors concluded that the strength-duration test may be useful in assessing external anal sphincter function, but had no direct evidence to support this statement.
Therefore, the aim of this study was to see whether there are relationships between the strength-duration test and other tests related to external anal sphincter function. It was decided that a heterogeneous group of females would be most suited for this purpose because it would provide the greatest possible range of values with which correlations could be investigated.
DISCUSSION
The results show that significant correlations exist between the strength-duration test and other measurements relating to the external anal sphincter. The positive correlation between the strength-duration test and nerve latency is as expected. When the nerve supply to a muscle is damaged, a greater current intensity is required to elicit a muscle twitch with the strength-duration test and the latency of nerve conduction is increased.
The correlations between the strength-duration test and both the pressure in the anal canal on voluntary
contraction and the EMG activity of the sphincter are negative as expected. Where muscle weakness exists, a greater current intensity is required to elicit a muscle twitch whereas anal pressure and EMG activity on contraction are reduced.
There were no correlations found between the strength-duration test and the maximum resting pressure
in the anal canal but this was expected because the resting tone is predominantly caused by internal
anal sphincter activity.
In most instances, the correlations are found at the shorter pulse durations (as can be seen in Table 3).
This is as would be expected because the strengthduration test is more discriminating at the shorter
durations (as can been seen in Fig. 1). However, it can also be seen from Table 3 that no correlations were found at the shortest pulse duration of 0.1 ms although these would have been expected. This may be because 18 patients failed to exhibit a muscle twitch at the maximum output of the machine at this duration. This limited the data available for correlation analysis at 0.1 ms and the reduced sample size may explain why no significant correlations were obtained. This decreased excitability to the shortest stimulus may indicate that these patients have more sphincter denervation than the rest of the group. This requires further study but it is not possible with the present equipment. However, it is interesting to note that these 18 females have significantly poorer sphincter function on routine anorectal physiology studies than the rest of the group.
All the correlations are weak but this is to be expected given that each test measures a different physiological aspect of skeletal muscle. The strength-duration test measures the response of the muscle-nerve complex of the external sphincter itself whereas the latency measurements also include the terminal pathway along the pudendal nerve (from the ischial spine) to the muscle. Both these tests measure a nonvolitional aspect of the muscle response to an applied electric stimulus and, as such, are truly objective, whereas EMG of the external anal sphincter and manometry on voluntary contraction, also have to rely on the ability of the patient to recruit the motor units on command.
It can be seen in Table 3 that there was a statistically significant correlation between the strength-duration test at the 0.3 ms duration and the volume at which a “call to stool” was experienced on rectal distention. This was an unexpected finding because the lower end of the gut is supplied by the autonomic and not the somatic nervous system. It is likely that this finding is spurious because there were no other significant correlations found between the strength-duration test and the other manifestations of rectal sensation. However, it is possible that the same mechanism which injures the pudendal nerve may also injure the other nerves in the pelvis.
Interestingly, the 14 patients who had a disrupted external sphincter, showed a consistent trend to need
a greater current intensity to obtain a muscle response with each pulse duration of the strength-duration test. Although this only reached statistical significance at the 10-ms pulse duration, presumably because of the small number of patients in this group, the consistency of this trend suggests there is decreased excitability (i.e., weakness) of the disrupted external anal sphincter muscle.
Although it is not directly relevant to this correlation study, Figure 2 is of some interest. It shows the
mean strength-duration curves for the three main symptomatic categories. These curves appear to be
distinct from each other but there were no statistically significant differences between them. However, there were small numbers of patients in each of the categories and a larger sample size may reveal important differences in these subgroups.
In summary, this study shows that the strengthduration test provides information relating to the condition
of the external anal sphincter as suggested by Monk et al.
The strength-duration test has several advantages over other electric tests of the external anal sphincter.
It is minimally invasive compared with nerve latency assessment and needle EMG. All except one patient in this study found the test acceptable. The test, which can be performed by any healthcare professional, is easy to learn and it requires only two to three supervised tests to become competent. This compares favorably with nerve latency assessment and needle EMG that require longer training to achieve specialist skills. The equipment is portable and can be more easily used in any hospital setting compared with the other electrophysiological tests. The capital outlay is a quarter of that required for nerve latency assessment and needle EMG and does not involve the recurrent cost of disposables required for the other tests.
The main disadvantage of the strength-duration test is that two people are required to carry it out; one to
hold the electrode in place and observe the muscle twitch, the other to operate the stimulator and record
the results. Although the test relies on the ability of a person to observe a minimal muscle twitch, Monk et al. found good interobserver and intraobserver reliability (the former being confirmed in Table 2 of this
study).
The strength-duration test, which measures a different aspect of the muscle-nerve complex of the
external anal sphincter than the other electrophysiological tests, may be helpful in further understanding
the cause, nature, and extent of external sphincter weakness. It may also have a role as one of the
routine anorectal diagnostic procedures.
This test may have more sensitivity in assessing the degree of motor innervation of the external anal
sphincter than can be provided by the measurement of PNTML. Measurements of latency are inherently an insensitive indicator of nerve damage because, even when the nerve has undergone considerable trauma, it is possible for some fast conducting neurones to remain, thus preserving normal latencies. Indeed, it is interesting to note that many of the 18 patients who had the most abnormal strength-duration curves (i.e., those who showed no response at 55 mA and 0.1 ms)had poorer sphincter function and normal nerve latencies.
However, before the strength-duration test can be advocated for research or clinical purposes, it needs
further refinement and investigation. It would be desirable to modify the test so one operator can perform
it rather than the two presently used. Increased objectivity could be achieved if the equipment was able
to detect the onset of a muscle twitch rather than relying on observation. Increased current output of
the equipment would enable further investigation of those patients who fail to respond at the shortest
pulse duration. The effect of applying the stimulus to sites other than the skin overlying the sphincter in the 3 o’clock position also needs to be investigated.