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Comparison of Objective Measurement of Anal Sphincter Strength with Anal Sphincter Pressures and Levator Ani Function
Vol. 56, No.1 Printed in U.S.A.
GASTROENTEROLOGY
Copyright © 1969 by The Williams & Wilkins Co.
COMPARISON OF OBJECTIVE MEASUREMENT OF ANAL SPHINCTER STRENGTH WITH ANAL SPHINCTER PRESSURES AND LEVATOR ANI FUNCTION NICHOLAS
E.
DIAMANT, M .D ., AND LAURAN D . HARRIS, M.D.
Evans Memorial Department of Clinical Research, University Hospital, and Department of Medicine, Boston University School of Medicine, Boston University Medical Center, Boston, Massachusetts
The fidelity with which anal sphincter intraluminal pressure can assess the strength of that sphincter cannot be determined without a suitable objective measurement of sphincter strength. This study details such a measurement and compares it with sphincter pressure. Sphincter strength was defined as the force in grams required to pull a %-inch diameter firm rubber ball from the rectum, through the sphincter, to the outside. Comparison of this value with simultaneously obtained sphincter pressure showed an excellent correlation (r = 0.955, P < 0.001). Correlation was equally good for changes of strength in an individual and as a comparison of strength among individuals. It is emphasized that the device used to record sphincter pressure must incorporate a continuous infusion of fluid for these measurements to be valid. A minor modification of the apparatus used to measure sphincter strength also allowed levator ani function to be measured-inward motion of the anus and the strength of this inward motion. These two functions correlated well (r = 0.83, P < 0.001). Comparison of these values with sphincter strength, however, showed no correlation. It is concluded that no functional relationship exists between the levator ani and the anal sphincter. Recently, anal sphincter pressure recorded from a group of patients with fecal incontinence was compared to the pressure recorded from a group of normal subjects. 1 This study showed that a minor modification of the conventional recording system allowed complete separation of these two groups by the recorded sphincter pressure-all of the normal subjects had a higher pressure than all of the abnormal patients. While these data suggest the logical conclusion that the magnitude of recorded sphincter pressure Received April 13, 1968. Accepted June 26, 1968. Address requests for reprints to: Dr. Lauran D. Harris, University Hospital, 750 Harrison Avenue, Boston, Massachusetts 02118. This investigation was supported in part by Research Grant AM 11907-01 and Training Grant AM 05025-12 from the National Institute of Arthritis and Metabolic Diseases, National Institutes of Health, United States Public Health Service.
bears a direct relationship to sphincter strength, the accuracy of this conclusion cannot be tested without a measurement of sphincter competence more precise than simple clinical definitions of "continent" and "incontinent" or "normal" and "abnormal." For example, pressures recorded from the normal anal sphincter during "tightening" ranged from approximately 100 mg Hg to over 300 mm Hg. However, since the presence or absence of continence was the only available guide to sphincter strength, there was no apparent difference in sphincter strength despite this 3-fold difference in sphincter pressure. One must then assume either that pressures ranging from 100 mm Hg to 300 mm Hg can be recorded from sphincters of equal strength or that the 3-fold difference in sphincter pressure in fact indicates a 3-fold difference in sphincter strength that was masked by our
110
January 1969
ANAL SPHINCTER STRENGTH AND PRESSURE
111
crude clinical criterion of sphincter com- The same results were obtained with rates of 0.1 cm per sec and 0.5 cm per sec. A force petence. To explore the relationship between transducer (B) (Grass Model FT. 10) mounted sphincter pressure and strength, we on a low friction track was interposed in the have devised a simple objective meas- line to record this force. To measure levator ani function, the motorurement of anal sphincter strength and ized reel (C) was disconnected, and a transhave compared this measurement with ducer (A) designed to measure linear motion sphincter pressure. During the course of (Sanborn Model 585DT) was placed between this study, it was found that a minor modi- the ball and the force transducer (B). When fication of the procedure also allowed the subjects tightened their anal sphincters, measurement of levator ani function. any inward motion of the anus (presumably
Methods Fifty-one normal adults were studied without prior bowel preparation. Figure 1 diagrams the essential features of the apparatus used to measure sphincter strength. Subjects were studied while they were lying in the left lateral position with their buttocks placed firmly against a backboard having a central opening approximately 4 inches square. A firm rubber ball % inches in diameter was inserted into the rectum and attached by a line to a motorized reel. Except for 3 inches next to the ball, the line was made of light weight copper chain to minimize artifacts caused by stretching of the line. Sphincter strength was defined as the maximum force necessary for the reel to pull the ball at a constant rate of 0.25 cm per sec from the rectum, through the anal sphincter to the outside. This rate was chosen primarily for convenience.
FIG. 1. Diagram of apparatus. (1) Measurement of anal sphincter strength. The rubber ball in the rectum to the left of the illustration was attached by a line to a motorized reel (C). To record the force developed as the reel pulled the ball from the rectum through the anal sphincter, the force transducer (B) , mounted on a low friction track, was interposed in the line. (2) Measurement of levator ani {unction . The motorized reel (C) was disconnected and the transducer designed to measure linear motion (A) was placed between the ball in the rectum and the force transducer (B) . With the force transducer moving freely on its track, inward movement could be measured by the linear motion transducer. With the force transducer locked in position, the strength of the potential inward movement was recorded by the force transducer.
due to contraction of the levator ani muscles) also moved the ball inward, and this movement was recorded by the linear motion transducer. The force transducer was allowed to move freely on its track. The strength of this inward movement or levator ani strength was recorded by locking the force transducer in position. Then, when the subjects tightened their anal sphincters, the locked force transducer prevented inward motion and instead measured the strength of this potential movement. Sphincter pressure was simultaneously recorded by a single polyvinyl tube placed in the anal canal. This pressure recording tube, 2.5 mm in outside diameter, had a side opening approximately 1.2 mm in diameter and was continuously infused with water at a rate of 7 /lliters per sec. Pressure was transmitted to a Sanborn Transducer (267A) leveled with the anal orifice as the zero reference point. Outputs from all of the transducers were graphed on a direct-writing recorder.
Results Sphincter strength, i.e., the maximum force in grams required to pull the ball from the rectum to the outside, was compared with the sphincter pressure recorded at the same time. Figure 2 shows all of the 111 observations obtained from 51 subjects during maximum voluntary contraction of their anal sphincters. Good correlation (r = 0.74, P < 0.001) between sphincter pressure and strength was found. To see if this same pressure-strength relationship existed with changes in sphincter strength in an individual, studies were also performed during less than maximal tightening. Figure 3 illustrates several such observations in an individual subject. As he maintained sphincter contraction at various levels, pressure and
112
Vol. 56, No . 1
DIAMANT AND HARRIS
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FiG. 2. Comparison of the simultaneously determined anal sphincter pressure (mm Hg) and sphincter strength (grams) during maximum voluntary contraction of the anal sphincter. The points represent all of the 111 observations obtained from 51 normal male subjects.
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FiG. 3. The relation of anal sphincter pressure to changes of sphincter strength in an individual. The observations were made as the subject voluntarily maintained sphincter contraction at various levels.
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January 1969
strength changed in a linear fashion. Further observations of this type were made in 27 subjects. Figure 4 shows these additional values (open circles) superimposed on the values obtained during maximum voluntary contraction (closed circles). Sphincter pressure seemed to be an excellent index of sphincter strength (r = 0.955, P < 0.001) over the range of pressures measured from as low as 60 mm Hg to over 300 mm Hg. The rubber ball % inches in diameter was chosen for its convenient size and firm consistency. Since the high friction surface might have influenced our results, studies were repeated using a ball the same size but made of Teflon and therefore with a smooth, low friction surface. Identical results were obtained using either ball, 100 mm Hg sphincter pressure being equivalent to approximately 600 g of sphincter strength. Obviously,
this pressure-strength relationship is quantitative only for a %-inch diameter ball and would not be expected to be the same if balls of different size were used to determine sphincter strength. To see if the pressure-strength relationship was also linear for a different sized ball, a Teflon ball V2 inch in diameter was used in 68 studies on 29 subjects. As expected, the force necessary to pull this smaller ball from the rectum to the outside was less than that required for the larger ball, approximately 300 g per 100 mm Hg sphincter pressure as contrasted with approximately 600 g per 100 mm Hg sphincter pressure. Greater variation in sphincter strength measurements was found when the V2-inch ball was used, but correlation of these results with simultaneously obtained sphincter pressure . was still reasonably good (r = 0.562, P < 0.001). Soon after beginning these studies, it
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FIG. 4. The relation of anal sphincter pressure to sphincter strength combining all of the observations made during maximum voluntary contraction of the anal sphincter (.) and during less than maximal tightening(O).
DIAMANT AND HARRIS
114
became apparent that as the subjects tightened their anal sphincters, the anus moved inward. The recorded extent of this inward movement during maximum tightening of the anal sphincter-presumably due to contraction of the levator ani muscles-was remarkably constant in each individual, varying by less than ± 10%. As figure 5 shows, however, there was a wide range among individuals. No relationship was apparent between the extent of this inward movement and the height, body build, or general muscular development of the individuals. The strength of this inward movement (or levator ani strength) also showed wide variation from subject to subject, ranging from under 200 g to over 1000 g in the
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FIG. 5. Extent of the inward movement of the anus that occurred as the anal sphincter was tightened. Each point represents the maximum value recorded for a single subject.
Vol. 56, No.1
19 subjects studied. Figure 6 shows that the two measurements of levator ani function-extent of the inward movement and strength of the inward movementcorrelate well (r = 0.83, P < 0.001). Thus, either measurement would appear to be a suitable index of levator ani function. Discussion It seems clear that anal sphincter pressure measurements are an accurate indication of that sphincter's ability to contain a solid bolus. A difference in sphincter strength was associated with a proportional difference in sphincter pressure not only in an individual but also when comparison was made among individuals. It should be emphasized that sphincter pressures shown in this study were obtained while the recording tubes were continuously infused with water. Sphincter pressures obtained by either the conventional uninfused system or with small fluid-filled balloons show no correlation with pressures recorded by the continuously infused system. J - 3 Sphincter pressures obtained with these systems do not seem to be suitable for use as an index of sphincter strength. The range of sphincter pressures shown in figure 4 (60 mm Hg to over 300 mm
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FIG. 6. Comparison of the two indices of levator ani muscle function, the extent of the inward movement and strength of this inward movement (levator ani strength) .
January 1969
115
ANAL SPHINCTER STRENGTH AND PRESSURE
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FIG. 7. The relation of levator ani function (levator ani strength) to anal sphincter strength.
Hg) was obtained while the subjects tightened their anal sphincters with varying degrees of voluntary effort. Values below 60 mm Hg presumably represent the contribution of resting or involuntary sphincter tone, and thus sphincter pressure, which is always present. Therefore, sphincter pressures below 60 mm Hg could not be recorded. Are the human levator ani muscles anatomically distinct from the external anal sphincter, or do the levator ani muscles form the external anal sphincter? This basic and seemingly straightforward question is surprisingly hard to answer. The external sphincter has been described as being formed by and therefore indistinguishable from the levator ani,4-6 and also as being completely separate and distinct from the levator ani. 7 Between these two extremes are descriptions of varying degrees of intermingling of otherwise distinct muscle groups.8-12 From meticulous dissection of many examples, the author of an excellent recent treatise on the subject takes the latter view. 13
It is hardly surprising that opinion about the functional relationships of levator ani and anal sphincter also varies. 4- 12 , 14-17 Chiefly responsible for this confusion has been the lack of a quantitative measurement of these functions. Our study allows us to quantitate both a function of the muscles concerned with lifting the anus (or levator ani function) and the strength of the anal sphincter. Therefore, it would also seem to allow a meaningful comparison of these two functions (fig. 7). Regardless of the anatomical relationships of levator ani and anal sphincter, there appears to be no functional relationship between the two. REFERENCES 1. Harris, L. D., C . S. Winans, and C. E. Pope, II. 1966. Determination of yield pressures: a method for measuring anal sphincter competence. Gastroenterology 50: 754-760. 2. Winans, C. S., and L. D. Harris. 1967. Quantitation of lower esophageal sphincter competence. Gastroenterology 52: 773-778. 3. Pope, C. E., II. 1967. A dynamic test of sphincter strength: its application to the lower esophageal sphincter. Gastroenterology 52: 779-786.
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4. Courtney, H. 1950. Anatomy of the pelvic dia-
phragm and anorectal musculature as related to sphincter preservation in anorectal surgery. Amer. J . Surg. 79: 155-173. 5. Goligher, J. C., A. G. Leacock, and J .-J. Brossy. 1955. The surgical anatomy of the anal canal. Brit. J. Surg. 43: 51- 61. 6. Hollinshead, W. H. 1956. The rectum and anal canal, p. 693-738. In Anatomy for surgeons, Vol. 2. Paul B. Hoeber, Inc. , New York. 7. Grant, J . C. B., and C. G. Smith. 1953. Musculature of the pelvic outlet, p. 528-543. In J. P. Schaeffer [ed.), Morris' human anatomy, Ed. 11. Blakiston Division, McGraw-Hill Book Company, New York. 8. Lockhart, R. D. 1964. Muscles and fasciae of the abdominal wall, perineum, and pelvis, p. 301-315. In G. J. Romanes [ed.), Cunningham's textbook of anatomy, Ed. 10. Oxford University Press, New York. 9. Gray, H . 1954. Anatomy of the human body, Ed. 26. Lea and Febiger, Philadelphia, 1480 pp. 10. Milligan, E. T . C., and C. N. Morgan. 1934. Sur-
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gical anatomy of the anal canal. Lancet 2: 1150. 11. Shackelford, R. T . 1955. The anorectal tract, p. 1635-1973. In Surgery of the alimentary
tract. W. B. Saunders Company, Philadelphia. 12. Smith, W. C. 1923. The levator ani muscle: its
structure in man, and its comparative relationships. Anat. Rec. 26: 175-203. 13. Wilson, P . M. 1967. Anchoring mechanisms of the ano-rectal region. S. Afr. Med . J. 41: 11271132,1138-1143. 14. Berglas, B. , and I. C. Rubin. 1953. Study of the
supportive structures of the uterus by levator myography. Surg. Gynec. Obstet. 97: 677692. 15. Hiatt, R. B., and T. V. Santulli. 1962. Important
factors influencing the treatment of imperforate anus. Dis. Colon Rectum 5: 110-114. 16. Kiesewetter, W. B. 1966. Imperforate anus: the role and results of the sacro-abdomino-perineal operation. Ann. Surg. 164: 655-661. 17. Kottmeier, P. K. 1966. A physiological approach to the problem of anal incontinence through use of the levator ani as a sling. Surgery 60: 1262-1266.
GASTROENTEROLOGY
Copyright © 1969 by The Williams & Wilkins Co.
COMPARISON OF OBJECTIVE MEASUREMENT OF ANAL SPHINCTER STRENGTH WITH ANAL SPHINCTER PRESSURES AND LEVATOR ANI FUNCTION NICHOLAS
E.
DIAMANT, M .D ., AND LAURAN D . HARRIS, M.D.
Evans Memorial Department of Clinical Research, University Hospital, and Department of Medicine, Boston University School of Medicine, Boston University Medical Center, Boston, Massachusetts
The fidelity with which anal sphincter intraluminal pressure can assess the strength of that sphincter cannot be determined without a suitable objective measurement of sphincter strength. This study details such a measurement and compares it with sphincter pressure. Sphincter strength was defined as the force in grams required to pull a %-inch diameter firm rubber ball from the rectum, through the sphincter, to the outside. Comparison of this value with simultaneously obtained sphincter pressure showed an excellent correlation (r = 0.955, P < 0.001). Correlation was equally good for changes of strength in an individual and as a comparison of strength among individuals. It is emphasized that the device used to record sphincter pressure must incorporate a continuous infusion of fluid for these measurements to be valid. A minor modification of the apparatus used to measure sphincter strength also allowed levator ani function to be measured-inward motion of the anus and the strength of this inward motion. These two functions correlated well (r = 0.83, P < 0.001). Comparison of these values with sphincter strength, however, showed no correlation. It is concluded that no functional relationship exists between the levator ani and the anal sphincter. Recently, anal sphincter pressure recorded from a group of patients with fecal incontinence was compared to the pressure recorded from a group of normal subjects. 1 This study showed that a minor modification of the conventional recording system allowed complete separation of these two groups by the recorded sphincter pressure-all of the normal subjects had a higher pressure than all of the abnormal patients. While these data suggest the logical conclusion that the magnitude of recorded sphincter pressure Received April 13, 1968. Accepted June 26, 1968. Address requests for reprints to: Dr. Lauran D. Harris, University Hospital, 750 Harrison Avenue, Boston, Massachusetts 02118. This investigation was supported in part by Research Grant AM 11907-01 and Training Grant AM 05025-12 from the National Institute of Arthritis and Metabolic Diseases, National Institutes of Health, United States Public Health Service.
bears a direct relationship to sphincter strength, the accuracy of this conclusion cannot be tested without a measurement of sphincter competence more precise than simple clinical definitions of "continent" and "incontinent" or "normal" and "abnormal." For example, pressures recorded from the normal anal sphincter during "tightening" ranged from approximately 100 mg Hg to over 300 mm Hg. However, since the presence or absence of continence was the only available guide to sphincter strength, there was no apparent difference in sphincter strength despite this 3-fold difference in sphincter pressure. One must then assume either that pressures ranging from 100 mm Hg to 300 mm Hg can be recorded from sphincters of equal strength or that the 3-fold difference in sphincter pressure in fact indicates a 3-fold difference in sphincter strength that was masked by our
110
January 1969
ANAL SPHINCTER STRENGTH AND PRESSURE
111
crude clinical criterion of sphincter com- The same results were obtained with rates of 0.1 cm per sec and 0.5 cm per sec. A force petence. To explore the relationship between transducer (B) (Grass Model FT. 10) mounted sphincter pressure and strength, we on a low friction track was interposed in the have devised a simple objective meas- line to record this force. To measure levator ani function, the motorurement of anal sphincter strength and ized reel (C) was disconnected, and a transhave compared this measurement with ducer (A) designed to measure linear motion sphincter pressure. During the course of (Sanborn Model 585DT) was placed between this study, it was found that a minor modi- the ball and the force transducer (B). When fication of the procedure also allowed the subjects tightened their anal sphincters, measurement of levator ani function. any inward motion of the anus (presumably
Methods Fifty-one normal adults were studied without prior bowel preparation. Figure 1 diagrams the essential features of the apparatus used to measure sphincter strength. Subjects were studied while they were lying in the left lateral position with their buttocks placed firmly against a backboard having a central opening approximately 4 inches square. A firm rubber ball % inches in diameter was inserted into the rectum and attached by a line to a motorized reel. Except for 3 inches next to the ball, the line was made of light weight copper chain to minimize artifacts caused by stretching of the line. Sphincter strength was defined as the maximum force necessary for the reel to pull the ball at a constant rate of 0.25 cm per sec from the rectum, through the anal sphincter to the outside. This rate was chosen primarily for convenience.
FIG. 1. Diagram of apparatus. (1) Measurement of anal sphincter strength. The rubber ball in the rectum to the left of the illustration was attached by a line to a motorized reel (C). To record the force developed as the reel pulled the ball from the rectum through the anal sphincter, the force transducer (B) , mounted on a low friction track, was interposed in the line. (2) Measurement of levator ani {unction . The motorized reel (C) was disconnected and the transducer designed to measure linear motion (A) was placed between the ball in the rectum and the force transducer (B) . With the force transducer moving freely on its track, inward movement could be measured by the linear motion transducer. With the force transducer locked in position, the strength of the potential inward movement was recorded by the force transducer.
due to contraction of the levator ani muscles) also moved the ball inward, and this movement was recorded by the linear motion transducer. The force transducer was allowed to move freely on its track. The strength of this inward movement or levator ani strength was recorded by locking the force transducer in position. Then, when the subjects tightened their anal sphincters, the locked force transducer prevented inward motion and instead measured the strength of this potential movement. Sphincter pressure was simultaneously recorded by a single polyvinyl tube placed in the anal canal. This pressure recording tube, 2.5 mm in outside diameter, had a side opening approximately 1.2 mm in diameter and was continuously infused with water at a rate of 7 /lliters per sec. Pressure was transmitted to a Sanborn Transducer (267A) leveled with the anal orifice as the zero reference point. Outputs from all of the transducers were graphed on a direct-writing recorder.
Results Sphincter strength, i.e., the maximum force in grams required to pull the ball from the rectum to the outside, was compared with the sphincter pressure recorded at the same time. Figure 2 shows all of the 111 observations obtained from 51 subjects during maximum voluntary contraction of their anal sphincters. Good correlation (r = 0.74, P < 0.001) between sphincter pressure and strength was found. To see if this same pressure-strength relationship existed with changes in sphincter strength in an individual, studies were also performed during less than maximal tightening. Figure 3 illustrates several such observations in an individual subject. As he maintained sphincter contraction at various levels, pressure and
112
Vol. 56, No . 1
DIAMANT AND HARRIS
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FiG. 2. Comparison of the simultaneously determined anal sphincter pressure (mm Hg) and sphincter strength (grams) during maximum voluntary contraction of the anal sphincter. The points represent all of the 111 observations obtained from 51 normal male subjects.
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SPHINCTER STRENGTH (GRAMS)
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FiG. 3. The relation of anal sphincter pressure to changes of sphincter strength in an individual. The observations were made as the subject voluntarily maintained sphincter contraction at various levels.
113
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January 1969
strength changed in a linear fashion. Further observations of this type were made in 27 subjects. Figure 4 shows these additional values (open circles) superimposed on the values obtained during maximum voluntary contraction (closed circles). Sphincter pressure seemed to be an excellent index of sphincter strength (r = 0.955, P < 0.001) over the range of pressures measured from as low as 60 mm Hg to over 300 mm Hg. The rubber ball % inches in diameter was chosen for its convenient size and firm consistency. Since the high friction surface might have influenced our results, studies were repeated using a ball the same size but made of Teflon and therefore with a smooth, low friction surface. Identical results were obtained using either ball, 100 mm Hg sphincter pressure being equivalent to approximately 600 g of sphincter strength. Obviously,
this pressure-strength relationship is quantitative only for a %-inch diameter ball and would not be expected to be the same if balls of different size were used to determine sphincter strength. To see if the pressure-strength relationship was also linear for a different sized ball, a Teflon ball V2 inch in diameter was used in 68 studies on 29 subjects. As expected, the force necessary to pull this smaller ball from the rectum to the outside was less than that required for the larger ball, approximately 300 g per 100 mm Hg sphincter pressure as contrasted with approximately 600 g per 100 mm Hg sphincter pressure. Greater variation in sphincter strength measurements was found when the V2-inch ball was used, but correlation of these results with simultaneously obtained sphincter pressure . was still reasonably good (r = 0.562, P < 0.001). Soon after beginning these studies, it
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FIG. 4. The relation of anal sphincter pressure to sphincter strength combining all of the observations made during maximum voluntary contraction of the anal sphincter (.) and during less than maximal tightening(O).
DIAMANT AND HARRIS
114
became apparent that as the subjects tightened their anal sphincters, the anus moved inward. The recorded extent of this inward movement during maximum tightening of the anal sphincter-presumably due to contraction of the levator ani muscles-was remarkably constant in each individual, varying by less than ± 10%. As figure 5 shows, however, there was a wide range among individuals. No relationship was apparent between the extent of this inward movement and the height, body build, or general muscular development of the individuals. The strength of this inward movement (or levator ani strength) also showed wide variation from subject to subject, ranging from under 200 g to over 1000 g in the
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FIG. 5. Extent of the inward movement of the anus that occurred as the anal sphincter was tightened. Each point represents the maximum value recorded for a single subject.
Vol. 56, No.1
19 subjects studied. Figure 6 shows that the two measurements of levator ani function-extent of the inward movement and strength of the inward movementcorrelate well (r = 0.83, P < 0.001). Thus, either measurement would appear to be a suitable index of levator ani function. Discussion It seems clear that anal sphincter pressure measurements are an accurate indication of that sphincter's ability to contain a solid bolus. A difference in sphincter strength was associated with a proportional difference in sphincter pressure not only in an individual but also when comparison was made among individuals. It should be emphasized that sphincter pressures shown in this study were obtained while the recording tubes were continuously infused with water. Sphincter pressures obtained by either the conventional uninfused system or with small fluid-filled balloons show no correlation with pressures recorded by the continuously infused system. J - 3 Sphincter pressures obtained with these systems do not seem to be suitable for use as an index of sphincter strength. The range of sphincter pressures shown in figure 4 (60 mm Hg to over 300 mm
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FIG. 6. Comparison of the two indices of levator ani muscle function, the extent of the inward movement and strength of this inward movement (levator ani strength) .
January 1969
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ANAL SPHINCTER STRENGTH AND PRESSURE
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SPHINCTER STRENGTH (GRAMS)
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FIG. 7. The relation of levator ani function (levator ani strength) to anal sphincter strength.
Hg) was obtained while the subjects tightened their anal sphincters with varying degrees of voluntary effort. Values below 60 mm Hg presumably represent the contribution of resting or involuntary sphincter tone, and thus sphincter pressure, which is always present. Therefore, sphincter pressures below 60 mm Hg could not be recorded. Are the human levator ani muscles anatomically distinct from the external anal sphincter, or do the levator ani muscles form the external anal sphincter? This basic and seemingly straightforward question is surprisingly hard to answer. The external sphincter has been described as being formed by and therefore indistinguishable from the levator ani,4-6 and also as being completely separate and distinct from the levator ani. 7 Between these two extremes are descriptions of varying degrees of intermingling of otherwise distinct muscle groups.8-12 From meticulous dissection of many examples, the author of an excellent recent treatise on the subject takes the latter view. 13
It is hardly surprising that opinion about the functional relationships of levator ani and anal sphincter also varies. 4- 12 , 14-17 Chiefly responsible for this confusion has been the lack of a quantitative measurement of these functions. Our study allows us to quantitate both a function of the muscles concerned with lifting the anus (or levator ani function) and the strength of the anal sphincter. Therefore, it would also seem to allow a meaningful comparison of these two functions (fig. 7). Regardless of the anatomical relationships of levator ani and anal sphincter, there appears to be no functional relationship between the two. REFERENCES 1. Harris, L. D., C . S. Winans, and C. E. Pope, II. 1966. Determination of yield pressures: a method for measuring anal sphincter competence. Gastroenterology 50: 754-760. 2. Winans, C. S., and L. D. Harris. 1967. Quantitation of lower esophageal sphincter competence. Gastroenterology 52: 773-778. 3. Pope, C. E., II. 1967. A dynamic test of sphincter strength: its application to the lower esophageal sphincter. Gastroenterology 52: 779-786.
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4. Courtney, H. 1950. Anatomy of the pelvic dia-
phragm and anorectal musculature as related to sphincter preservation in anorectal surgery. Amer. J . Surg. 79: 155-173. 5. Goligher, J. C., A. G. Leacock, and J .-J. Brossy. 1955. The surgical anatomy of the anal canal. Brit. J. Surg. 43: 51- 61. 6. Hollinshead, W. H. 1956. The rectum and anal canal, p. 693-738. In Anatomy for surgeons, Vol. 2. Paul B. Hoeber, Inc. , New York. 7. Grant, J . C. B., and C. G. Smith. 1953. Musculature of the pelvic outlet, p. 528-543. In J. P. Schaeffer [ed.), Morris' human anatomy, Ed. 11. Blakiston Division, McGraw-Hill Book Company, New York. 8. Lockhart, R. D. 1964. Muscles and fasciae of the abdominal wall, perineum, and pelvis, p. 301-315. In G. J. Romanes [ed.), Cunningham's textbook of anatomy, Ed. 10. Oxford University Press, New York. 9. Gray, H . 1954. Anatomy of the human body, Ed. 26. Lea and Febiger, Philadelphia, 1480 pp. 10. Milligan, E. T . C., and C. N. Morgan. 1934. Sur-
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gical anatomy of the anal canal. Lancet 2: 1150. 11. Shackelford, R. T . 1955. The anorectal tract, p. 1635-1973. In Surgery of the alimentary
tract. W. B. Saunders Company, Philadelphia. 12. Smith, W. C. 1923. The levator ani muscle: its
structure in man, and its comparative relationships. Anat. Rec. 26: 175-203. 13. Wilson, P . M. 1967. Anchoring mechanisms of the ano-rectal region. S. Afr. Med . J. 41: 11271132,1138-1143. 14. Berglas, B. , and I. C. Rubin. 1953. Study of the
supportive structures of the uterus by levator myography. Surg. Gynec. Obstet. 97: 677692. 15. Hiatt, R. B., and T. V. Santulli. 1962. Important
factors influencing the treatment of imperforate anus. Dis. Colon Rectum 5: 110-114. 16. Kiesewetter, W. B. 1966. Imperforate anus: the role and results of the sacro-abdomino-perineal operation. Ann. Surg. 164: 655-661. 17. Kottmeier, P. K. 1966. A physiological approach to the problem of anal incontinence through use of the levator ani as a sling. Surgery 60: 1262-1266.