Wai, Clifford Y. MD1; Rahn, David D. MD1; White, Amanda B. MD1; Word, R Ann MD1,2
Anal sphincter lacerations sustained during vaginal delivery occur in 0.7–19.3% of women.1–3 In addition to spontaneous lacerations, episiotomies are another source of potential injury to the anal sphincter complex. Despite mounting evidence challenging their usefulness, in addition to recent recommendations against their liberal use, episiotomies are often still performed for various reasons, ranging from nonreassuring fetal heart pattern, operative vaginal delivery, and shoulder dystocia to protection of the perineum.4
The rate of postpartum anal incontinence ranges from 6–24%.5–8 Although the cause is multifactorial, mechanical disruption of the anal sphincter complex that occurs during vaginal delivery is believed to be a major cause of anal incontinence in women.8–10 Perineal laceration, especially when it involves the anal sphincter, was found to be an important risk factor for postpartum fecal incontinence.11 In agreement with Crawford et al,9 who described a sixfold increase in flatal incontinence with a third- or fourth-degree perineal laceration, Nygaard et al12 demonstrated bothersome flatal incontinence in women that sustained disruption of the anal sphincter during delivery. Sphincter defects are associated with flatal incontinence in 43% and 25% in women 3 and 6 months postpartum, respectively.13,14 Furthermore, 30–50% of women with a third- or fourth-degree perineal laceration during delivery have anal incontinence, even several months after child birth.6,15,16 Clearly, these studies, together with others, indicate that direct injury of the anal sphincter musculature is associated with anal incontinence.
However, the exact mechanism of postpartum anal incontinence and its natural history is unclear. Further, the integrity and physiology of the external anal sphincter as it changes over time after initial injury has not been determined. Specifically, why some women improve with time after vaginal delivery while others worsen remains unknown. Understanding the complex interaction of vaginal delivery with anal sphincter damage and how physiologic function changes over time may begin to clarify the mechanisms of injury, assist in patient counseling and expectations, as well as help guide intrapartum and postpartum practices.
Previously, we determined that significant compromise of anal sphincter function occurred 3 weeks after anal sphincter transection/repair with and without vaginal distention.17 However, the time course of injury and potential recovery or deterioration was not addressed. The objective of this study was to estimate the relative contributions of prolonged vaginal distention and anal sphincter laceration (with repair) on physiologic function of the external anal sphincter as a function of time in an animal model.
MATERIALS AND METHODS
All procedures were approved by the Institutional Animal Care and Use Committee at the University of Texas Southwestern Medical Center. After anesthesia with ketamine (40 mg/kg), acepromazine (0.2 mg/kg), and xylazine (10 mg/kg), for each of three time points (3 days, 3 months, and 6 months), young (2–3 months) virginal female rats (200–300 g) were randomly assigned to one of four treatment groups using a random number table: sham operation, vaginal balloon distention, proctoepisiotomy with repair, or combined vaginal balloon distention and proctoepisiotomy/repair (Fig. 1). To control for the potential effects of suture and repair, a superficial vaginal incision (not involving the anal sphincter or rectum) was performed in all rats (including sham). The superficial incision was closed with 4-0 braided polyglactin suture (Vicryl, Ethicon, Inc, Piscataway, NJ). With the exception of the 3-day time point, all operative procedures were conducted in the same time frame. The 3-day time point was conducted at the end of the series. Data from animals at the 3-week time point were reported previously.17
For sham-operated animals, in addition to a superficial vaginal incision, a deflated 5-mL, 12-F Foley catheter (Bardex I. C., C. R. Bard, Inc., Covington, GA) (with the tip cut off approximately 2 cm to ensure the balloon portion of the catheter was positioned in the vaginal canal) was placed in the vagina of sham-operated controls for 1 hour. For the proctoepisiotomy with repair group, a 7-mm incision was made with dissecting scissors through the anal sphincter complex. This incision extended beyond the superior margins of the external anal sphincter, which is approximately 3–4 mm in longitudinal length. The rectal mucosa was reapproximated with single interrupted stitches (1 mm apart) of 5-0 braided polyglactin suture (Vicryl, Ethicon). A second layer of single interrupted stitches (1 mm apart) of the same suture and caliber was used as a reinforcing layer. The external anal sphincter was reapproximated with two single interrupted stitches of 5-0 braided polyglactin suture. For simulated prolonged vaginal delivery, a modification of a previously described animal model was used.18 The 12-F Foley catheter was inserted into the vagina and secured in place with a purse-string suture of 4-0 braided polyglactin, with special care made to avoid the vicinity of the anal sphincter. The Foley balloon was inflated with 2.0 mL of water and left in place for 1 hour. A 50-g weight was attached to the Foley catheter and suspended over the edge of the operating table. For combination balloon distention and proctoepisiotomy repair, rats underwent vaginal distention as described above. After deflation of the catheter, a proctoepisiotomy was performed and repaired as described.
Rats were killed with pentobarbital (50 mg/kg intraperitoneally) 3 days, 3 months, or 6 months (Fig. 1) after sham operation, vaginal balloon distention, proctoepisiotomy with repair, or combined vaginal balloon distention and proctoepisiotomy/repair, similar to a previous study where a group of animals were studied at 3 weeks after surgery.17 The anal sphincter complex was dissected and removed by making a circumferential incision approximately 5 mm from the external anal orifice. Gentle traction of the perianal skin was used to facilitate sharp dissection of the anal complex from the connective tissue in the ischiorectal fossa laterally and the vagina anteriorly. The anal complex was removed intact after transecting the rectum 1.5–2 cm cephalad from the external anal orifice. Thereafter, a microscope was used to dissect the anal complex free of perianal fat and most of the perianal skin. To preserve integrity of the external anal sphincter, however, perianal tissue that was immediately adjacent to the sphincter was not teased away. The striated muscle of the external anal sphincter was identified, and the lower rectum was transected 1–2 mm above the sphincter. Immediately after dissection, tissues were mounted as a ring between two stainless steel wires in water-jacketed baths for assessment of contractile function as previously described.19 Within 30–60 minutes after mounting the tissue, muscles were stretched to optimal length for force development (ie, 1.2–1.8 g resting tension). Optimal resting tension was maintained throughout the course of the experiment. Tissue dissection and experimental protocols were performed in physiological salt solution of the following composition: NaCl (120.5 mmol/L), KCl (4.8 mmol/L), MgSO4 (1.2 mmol/L), NaH2PO4 (1.2 mmol/L), NaHCO3 (20.4 mmol/L), CaCl2 (1.6 mmol/L), D-glucose (10 mmol/L), and pyruvate (1 mmol/L), pH of 7.4. The solution was gassed with 95% oxygen and 5% carbon dioxide.
For electrical stimulation, platinum wire electrodes were mounted parallel to the suspended tissues. Electrodes were connected to an isolated pulse stimulator and a Grass stimulator (Grass Model S48, Grass Instrument Co., Quincy, MA) through a current-boosting amplifier (Bipolar Power Supply Amplifier, Model 6826A, Agilent Technologies, Santa Clara, CA). Stimulation of each tissue was also controlled by driver and signal-conditioning/amplifier units (Grass DC driver amplifiers 7DAE or 7DAF, Grass low-level DC Amplifiers 7PIF). Analog force signals were captured with a multichannel analog to digital computer interface (National Instruments PCI-6032E, Austin, TX). Data were acquired at 100 Hz through the computerized interface. Data were collected before, during, and after initiation of field stimulation. Stimulation patterns were controlled by a computer program coupled to the voltage stimulator source using a computer interface (National Instruments PC-DIO-24 Digital I/O Board controlling a specially designed circuit). A central computerized controller delivered precise stimulation duration and frequencies. Voltage and pulse duration (in msec) was determined manually through settings on the stimulator unit. Voltage settings, duration and frequency were varied to obtain force-frequency and force-voltage response curves. Peak force production (defined as the maximal force produced during a contraction) was determined.
To determine twitch tension, tetanic force generation, and fatigue, experiments were conducted at 30°C. Twitch tension was determined after stimulation with one square 0.4 msec pulse of 50 V. For tetanic force generation, the muscle was allowed to recover and then force-frequency curves were determined by stimulation at 10–120 Hz, 50 V, 0.4 msec pulse duration, for 300 msec. After each frequency, muscles were stimulated with maximal stimulation of 50 V, 150 Hz. Muscles were allowed to recover for a minimum of 120 sec before the next increase in frequency. Forces obtained at each frequency were compared with forces obtained with maximal stimulation immediately thereafter. The ratio of single twitch tension to maximal tetanic tension was determined for each sphincter. Fatigue was determined by maximally stimulating the muscle for 30 sec at 50 V, 150 Hz. Force generation at 30 sec was expressed relative to initial maximal force generation. Maximal responses to electrical field stimulation were then determined at 37°C.
Atropine resistance of field-stimulated contractions was determined at 37°C by stimulation of the external anal sphincter tissues 20 minutes after treatment with atropine (10−6 mmol/L). After atropine-independent contractions were determined, tetrodotoxin (5×10−7 mmol/L) was added, and after 10 minutes, a final stimulation was conducted to assess nerve-independent contractile force.
Statistical comparisons between groups were conducted by analysis of variance. The Student-Neuman-Keuls pair-wise multiple comparison test procedure was used to assess differences between means after statistically significant analysis of variance findings. P≤.05 was considered significant. Data from previous experiments were used to predict the force-generating capacity of sphincters from sham-operated animals. We assumed that sham surgery would not alter maximal force-generating capacity of the external anal sphincter over time and anticipated a 60% initial decrease of maximal force production that may return to baseline levels. To examine differences in time profiles for the different experimental conditions (significance level of .05 and 80% power), a sample size of eight animals in each group for each of the time points was determined (significance level of .05 and 80% power, PROC GLMPOWER, SAS 9.1, SAS Institute, Cary, NC). To allow for unequal cell sizes due to the nature of possible loss of animals, we targeted 11 animals per group (132 animals). In some groups, the number of animals was 13 to optimize use of all muscle baths when it was necessary to repeat experiments due to failure of software to record data for two experiments.
Twitch tension of the external anal sphincter was evaluated at 3 days, 3 weeks, 3 months, and 6 months after each treatment (Fig. 2). Prolonged vaginal distention did not result in significant impairment of twitch tension at any time point. In contrast, after 3 days, maximal twitch contraction was impaired significantly in both the sphincter transection/repair (ASL) groups with or without vaginal distention compared with sham-operated animals. Twitch tension remained impaired in the ASL groups at 3 weeks but recovered to sham values by 3 months after injury (Fig. 2). This recovery in twitch tension was maintained at 6 months after injury.
The maximal tetanic force generation was evaluated for each treatment group (Fig. 3A). After prolonged vaginal distention, maximal force production during tetanic force contraction was reduced after 3 days (P<.01). Recovery of tetanic force generation (approximately 20% increase), however, was evident as early as 3 weeks after injury and remained stable at 3 and 6 months. In contrast, after proctoepisiotomy with repair (ASL) and combined injury (VD+ASL), maximal tetanic force generation was decreased dramatically 3 days after injury, and recovery was not observed until 3 months after injury (Fig. 3A). At 3 months, there was no statistically significant difference in maximal tetanic force contraction in sphincters from sham-operated animals and those treated with balloon distention of the vagina, anal sphincter transection with repair, or combination injury.
A similar effect was seen when electrical field stimulation was used to evaluate the maximal force-generating capacity of the external anal sphincter. Each sphincter was stimulated for 5 sec at optimal parameters, and maximal contractile force was quantified (Fig. 3B, Table 1). Three days after injury, maximal force generation in response to electrical field stimulation was impaired significantly with each treatment group compared with sham-operated animals. Impairment was most severe after sphincter transection plus vaginal distention (Fig. 3B). Whereas maximal force generating ability recovered rapidly in the prolonged vaginal distention group, sphincter force generation remained impaired in the ASL groups at 3 weeks, with full recovery noted at 3 months (Fig. 3B, Table 1). At 6 months, this recovery was maintained and there was no statistical difference in contractile force generation in sphincters between sham-operated animals and any of the treatment groups.
The external anal sphincter complex was stimulated at 50V, 150 Hz for 30 seconds and fatigue was expressed as a percentage of force generation at 30 seconds relative to initial maximal force generation (Fig. 4). The external anal sphincter was more fatigable 3 days after ASL (22±3%, P<.012) and VD+ASL (22±3%, P<.017) compared with sham-operated animals (36±2%). However, the fatigability of balloon vaginal distended animals was not significantly different from shams (P=.23). By 3 weeks, fatigability of the sphincter transection/repair and the combination injury repair groups returned to baseline and were not significantly different from sham-operated controls. This recovery was maintained at 6 months.
To further define responses to electrical field stimulation, force-frequency and force-voltage response curves were determined in the external anal sphincters from all animals at each time point (Fig. 5). Frequency- and voltage-induced force generation in all three treatment groups were statistically significantly different from control animals at 3 days after treatment (Fig. 5A and 5C), with the combined vaginal distention and sphincter transection/repair group demonstrating the greatest amount of impairment (P<.001). Field-stimulated contractions in response to low frequency remained impaired in the anal sphincter laceration/repair group 6 months after treatment (Fig. 5B). Interestingly, 6 months after treatment, force generation in response to low frequency and voltage was increased in animals with vaginal distention. By 6 months after treatment, force generation at near maximal stimulation of the vaginal distention group and at all voltages and frequencies of the ASL and VD+ASL groups were similar to those of sham-operated animals (Fig. 5D).
Two studies were conducted to assess the possibility of denervation injury of the sphincter after laceration with or without vaginal distention. Previous studies suggest that the twitch to tetanic tension ratio is increased in denervated slow-twitch muscle. Although twitch tension and tetanic forces were compromised after sphincter laceration and repair at a certain time point, both measurements were decreased proportionately. Thus, the twitch tension/tetanic tension ratio was not significantly different in sphincters from sham and the other treatment groups at any of the time points (Fig. 6). These data suggest that denervation was an unlikely cause of the compromise in sphincter contractility. To examine the effect of proctoepisiotomy and vaginal distention on cholinergic nerve-mediated contractions, maximal force of contraction was measured before and after treatment with the nonspecific muscarinic receptor antagonist, atropine. At 3 days, 3 weeks, 3 months, and 6 months, field stimulated contractions were inhibited approximately 20–30% by atropine in sphincters from sham operated controls (Table 1). Incubation with atropine resulted in similar inhibition of contraction in sphincters from the other treatment groups at each time point (Table 1). Tetrodotoxin resulted in almost complete abolishment of field-stimulated contractions in all treatment groups at all time points (Table 1). These results suggest that the proportion of cholinergic nerves innervating the external anal sphincter is not affected by vaginal distention, proctoepisiotomy repair, nor combined injury. Stated differently, compromise of external anal sphincter function after proctoepisiotomy repair resulted in proportionate decreases in cholinergic and non–cholinergic-mediated contractions, and that specific nerve fibers were not affected by sphincter injury.
Overall, the main findings of this study are that both vaginal distention and anal sphincter transection/repair result in significant changes in external anal sphincter function. The extent of compromise and the time to recovery, however, are significantly different. In vaginal distention, impaired contractile function seems to be transient, not as severe, and recovers more quickly. In anal sphincter laceration, compromise seems to be more severe and the rate of recovery more prolonged.
From a previous study of external anal sphincter physiology in the rat at 3 weeks, vaginal distention alone did not appreciably affect neurophysiologic function of the external anal sphincter.17 However, anal sphincter transection with repair regardless of concomitant vaginal distention resulted in compromised physiologic function of the external anal sphincter. This previous study was conducted at a single time point and did not take into account possible transient effects of vaginal distention on external anal sphincter function. In the current investigation, significant impairment of the various characteristics of striated muscle physiology occurred at 3 days after vaginal distention alone and with anal sphincter transection/repair with and without preceding vaginal distention. Whereas decreased contractile force generated in response to field stimulation recovered rapidly in the vaginal distention–only group, impaired contractile force generation was persistent in both ASL and VD+ASL groups at 3 weeks. Thus, damage imparted by either anal sphincter laceration or combined VD+ASL was more pronounced and appeared to last longer than vaginal distention alone. Nevertheless, sphincter function was restored by 3 months in all treatment groups. The results suggest that prolonged vaginal distention results in a more transient, less severe injury compared with sphincter transection.
The rapid recovery of the sphincter from animals with vaginal distention alone may be due to several factors. First, the mechanism of injury during vaginal distention is probably different from that of surgical transection and may involve hypoxia or acidosis of the sphincter. Second, the relative severity of a mechanical disruption of the external anal sphincter is predicted to recover slower than less traumatized muscle.
The rat external anal sphincter is composed of both slow- and fast-twitch fibers. In general, after denervation, the proportion of fast-twitch fibers increases.20 This results in increased fatigability and in increased twitch-tetany ratios, both hallmarks of denervated striated muscle. Since fatigability and twitch tension to tetanic tension ratios were not significantly different from sham-operated animals, we conclude that impaired external anal sphincter function after vaginal distention or laceration with repair, at least in the rat, is probably not due to denervation. Interestingly, although fatigue of the external anal sphincter was unaffected 3 days after vaginal distention, it was significantly impaired in both the ASL and VD+ASL groups. Recovery of fatigability in these two treatment groups occurred fairly rapidly, such that by 3 weeks after initial exposure, fatigability normalized. This also suggests that, of all the characteristics of external anal sphincter function, fatigability seems either to recover first or to be one of the more resistant characteristics to insult.
It is interesting to consider the mechanisms of healing of a transected sphincter. Skeletal muscle fibers are fully differentiated and do not proliferate in response to injury. Different phases of healing occur after muscular injury and are all interrelated and time-dependent. Initially, muscle degeneration and inflammation occurs. Activation, recruitment, and differentiation of satellite cells at the time of injury secrete growth factors and cytokines, which aid in regenerative and reparative processes. Finally, scar formation occurs as a result of fibroblast in growth and ultimately determines the strength of repair.21 This fibrosis provides an “anchor-point” or fixed surface for skeletal muscle to attach and contract against. In women, any of these mechanisms may fail, leading to persistent compromise of external anal sphincter function and anal incontinence after sphincter disruption. Because the main cause of impairment of sphincter physiology seems to be mechanical disruption and not denervation, perhaps more focus should be placed not only on ideal repair but also on defining an optimal environment for healing in the immediate postpartum period. It is important to recognize that the surgical procedures in this study were conducted under optimal experimental conditions and may differ in several ways from the environment encountered in the clinical setting. For instance, transection of the anal sphincter was a clean surgical incision rather than a true laceration. Also, in addition to ideal lighting and exposure of the operative site, meticulous repair was facilitated with the assistance of a dissecting microscope.
In conclusion, data from this investigation indicate that vaginal distention, anal sphincter transection with repair, and combination vaginal distention/laceration (with repair) result in compromise of external anal sphincter function of different severity and rates of recovery. These data suggest that although injury from vaginal distention or anal sphincter transection with repair occurs early on, recovery of function of the external anal sphincter eventually occurs with time.
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