Correlation of the Position of the Anal Dimple with the Caudal Termination of the Striated Muscle Complex in Patients with Anorectal Malformations.

Background: In children with anorectal malformations (ARM), the vertical fibres of the striated muscle complex (SMC) are believed to be located within the limits of the anal dimple (AD).
Methods: Forty five cases of ARM underwent posterior sagittal anorectoplasty (PSARP), median age 8.5 months. During PSARP the anterior, posterior limits and midpoint of the AD and SMC were marked. The location of AD was correlated with SMC.
Results: A 'well developed' AD and SMC was seen in 80 % and 86.7 % patients respectively. The mean width of the AD and SMC was more in females than in males (20.96 vs. 18.98 mm and 14.24mm vs. 13.45mm respectively). In 36 cases (80 %), across the spectrum of ARM, the SMC was posterior in relation to the AD. In 7 cases (15.5 %) it correlated in position with the AD and in 2 cases (4.44 %), it was anterior to the AD. Conclusions: In the majority of cases AD and SMC were 'well developed' and the location of the SMC does not correlate with that of the AD. This has significant practical value in the important step of optimizing the placement of the rectum through the center of the SMC during repair. Copyright:

INTRODUCTION

In children with anorectal malformations (ARMs), precise placement of the neo-anus within the confines of the striated muscle complex (SMC) during reconstruction has important implications for future fecal continence.[12] The exact location and limits of the SMC can be ascertained during reconstructive surgery with muscle stimulation.[13] Children with ARM have a midline “anal dimple” (AD) or cutaneous areola along the line of the midline perineal raphe in males or the perineal groove in females.[13] This varies in development (well/moderately developed) and character (everted/inverted) among the different subtypes of ARM as well as with the gender.[4] It is generally thought that the vertical fibers of the SMC are located within the limits of the AD and extend from the skin to the levator ani muscle.[15] Okada et al.[6] believed that the AD represents the center of the external sphincter muscles, i.e., the SMC. Peña and Bischoff[7] stated that the AD represents the point in the perineum with the highest concentration of sphincter fibers. Similar findings have been reported by other authors based on computed tomography (CT) scanning[8] and on histologic studies.[9]

There is no earlier study available in the literature that correlates the position of the vertically oriented fibers of the SMC with the position of the AD. The present study was undertaken to see the correlation between the locations of the AD and the lower end of the SMC during definitive surgery for various ARMs as well as measure the width and assess the development of the AD and the SMC in the different subtypes of ARM.

MATERIALS AND METHODS

All patients with ARM managed from November 2015 to August 2017 were included in this prospective, observational cross-sectional study. Institutional Ethical Committee clearance was obtained and enrollment in the study was done after written informed parental consent. The demographic details, relevant clinical findings, results of investigations, diagnosis, and operative findings were recorded. Exclusion criteria included patients with distorted perineal anatomy making delineation of the anatomy difficult or those with a poorly developed or poorly delineated AD. As males with low ARM are treated by perineal anoplasty at our institution, they were necessarily excluded from the study.

Forty-five cases, including 20 (44.44%) males and 25 (55.56%) females, formed the study group. The age at the time of definitive surgery ranged from 2 months to 5 years. The median age was 8.5 months (interquartile range: 6.0, 25th percentile: 6.0, 75th percentile: 12.0), and the majority of patients (35/45, 77.8%) were <1-year age. The frequency of subtypes of ARM is shown in Table 1.

Table 1

Width of the anal dimple and striated muscle complex in various anorectal malformations (n=45)

Type of ARM (n)	Mean width of AD A-B (mm)±SD	Mean width of SMC A’- B’ (mm)±SD
Males
RBF (6)	18.6±1.63	15.16±2.99
RPF (7)	18±2.37	12.85±2.73
RVeF (2)	26±5.66	14.50±4.95
CPC with CVF (1)	20±0	10.0±00
ARM without fistula (4)	19.5±1.00	14.25±4.03
Females
PF (7)	17.7±4.49	13.0±4.80
VF (18)	22.16±5.62	14.72±3.32

RBF: Rectobulbar urethral fistula, RPF: Rectoprostatic urethral fistula, RVeF: Rectovesical fistula, CPC: Congenital pouch colon, CVF: Colovesical fistula, PF: Perineal fistula, VF: Vestibular fistula, ARM: Anorectal malformation, SMC: Striated muscle complex, AD: Anal dimple, SD: Standard deviation

According to the type of ARM, including both sexes, the most common abnormality was vestibular fistula (VF) (n = 18, 40%), followed by perineal fistula (PF) and rectoprostatic urethral fistula (RPF) (n = 7, 15.55%) each, rectobulbar urethral fistula (RBF) (n = 6, 13.3%), rectovesical fistula (RVeF) (n = 2, 4.4%), High anorectal malformation (HARM) without fistula (n = 4, 8.9%), and congenital pouch colon (CPC) with colovesical fistula (CVF) (n = 1, 2.2%). All cases of PF were female while all four cases of HARM without fistula were male [Table 1].

Observations during posterior sagittal anorectoplasty

Posterior sagittal anorectoplasty (PSARP) was performed in the prone jackknife position.[12] The AD, based on subjective assessment, was classified as either “well developed” or “moderately developed.” The anterior (point A) and posterior (point B) limits of the AD were marked with indelible ink and the midpoint of this distance was taken as center of the AD (point C) [Figure 1a].

Figure 1

(a and b) A 6-month-old boy with rectoprostatic urethral fistula showing (a) well-developed anal dimple, A-B -15 mm, and (b) well-developed posteriorly placed striated muscle complex, A’-B’ 10 mm, C- C’ 4 mm

During PSARP, points A’ and B’ were marked at the anterior and posterior limits, and point C’ at the midpoint of the SMC at the level of merging of the vertical fibers of the SMC with the subcutaneous parasagittal fibers [Figures 1b-4b]. These markings were made by visual assessment aided by the use of muscle stimulation (Dr. Bajpai's muscle stimulator, AIIMS, New Delhi) to identify the anterior and posterior limits of caudocephalad contraction of the vertical fibers of the SMC. After complete midline division of the sphincteric musculature, measurements were taken using Vernier calipers and the SMC was also subjectively classified as “well/moderately developed” on the basis of density of fibers and its overall development.

Figure 4

(a and b) A 7-month-old girl with vestibular fistula showing (a) well-developed anal dimple, A-B 20 mm, and (b) moderately developed anteriorly placed striated muscle complex (A’- B’ 12 mm), C- C’ 15 mm

Statistical analysis

Statistical analysis was performed using Chi-square or Fisher's exact test for categorical data. Continuous variables were compared by using a Student's t-test. Statistical analysis was performed using a SPSS Statistics for Windows, version 11.0 (SPSS Inc., Chicago, IL., USA), and the differences were considered significant if P < 0.05.

RESULTS

Thirty-six patients (80%) had a “well-developed” AD [Figure 1a, 3a, 4a] while nine patients (20%) had a “moderately developed” AD [Figure 2a]. On gender-wise analysis, 17/20 males (85%) and 19/25 girls (76%) had a “well-developed” AD. The three males with a “moderately developed” AD included RBF (n = 1), RVeF (n = 1), and the patient with CPC and a CVF. Two of 7 girls with a PF (28.5%) and 4/18 girls with a VF (22.2%) had a “moderately developed” AD. On applying Chi-square test, comparing the type of ARM with the characteristics of the AD, P = 0.104 was not statistically significant.

Figure 3

(a and b) A 10-month-old boy with rectoprostatic urethral fistula showing (a) well-developed anal dimple, A-B 15 mm, and (b) well-developed centrally placed striated muscle complex (A’- B’ 10 mm), C- C’ 0 mm

Figure 2

(a and b) A 7-month-old male with rectobulbar urethral fistula showing (a) moderately developed anal dimple, A-B 20 mm, and (b) well-developed posteriorly placed striated muscle complex (A’- B’ 18 mm), C- C’ 5 mm

In all cases, during PSARP, the site of maximal cephalocaudal contraction of the fibers of the SMC correlated with the visible vertically oriented fibers of the muscle complex. Overall, 39/45 (86.7%) had a “well-developed” SMC while 6 patients (13.3%) had a “moderately developed” SMC. On gender-wise analysis, a “well-developed” SMC was slightly more common in males (18/20, 90%) as compared to females (21/25 patients, 84%). A “moderately developed” SMC was seen in one patient each with RVeF and CPC in males, and two patients each with a PF and a VF in females. The overall mean width of the AD and SMC was more in females than in males (21.36 ± 5.87 mm vs. 18.98 ± 3.43 mm and 14.24 ± 3.76 mm vs. 13.45 ± 3.10 mm, respectively), the differences not being statistically significant (P = 0.104 and 0.102, respectively). The mean width of the SMC in patients with a well-developed SMC was 14.00 ± 3.54 mm as compared to 13.16 ± 3.12 mm in those with a “moderately developed” SMC (P = 0.59). The width of the AD and the SMC (mean ± standard deviation) in the various subtypes of ARM is shown in Table 1.

Position of striated muscle complex in relation to the anal dimple

In 36 cases (80%), the SMC was posterior in relation to the position of the AD [Figures 1b and 2b], whereas in 7 cases (15.5%), it correlated in position with the AD [Figure 3b], and in 2 cases, both with VF (4.44%), it was anterior to the AD [Figure 4b and Table 2].

Table 2

Location of striated muscle complex in relation to the anal dimple in the various anorectal malformations (n=45)

Type of ARM (n)	Location of SMC with respect to AD, n (%); deviation of SMC from AD (C-C’) (mean±SD mm)
	Anterior	Superimposed	Posterior
Males
RBF (6)	0	0	6 (100); 4.1±0.75
RPF (7)	0	3 (42.8)	4 (57.2); 8.5±8.49
RVeF (2)	0	0	2 (100); 12±8.49
CPC with CVF (1)	0	0	1 (100); 5±0
ARM without fistula (4)	0	2 (50)	2 (50); 7.5±3.54
Females
PF (7)	0	1 (14.3)	6 (85.7); 5.5±1.97
VF (18)	2 (11.11) 15, 20 mm	1 (5.5)	15 (83.3); 6±1.96

RBF: Rectobulbar urethral fistula, RPF: Rectoprostatic urethral fistula, RVeF: Rectovesical fistula, CPC: Congenital pouch colon, CVF: Colovesical fistula, PF: Perineal fistula, VF: Vestibular fistula, SMC: Striated muscle complex, ARM: Anorectal malformation, AD: Anal dimple, SD: Standard deviation

The mean displacement of the center of the AD with the center of SMC (C-C’ distance) was 4.8 ± 3.4 mm posterior, with the mean posterior deviation being more in females (5.4 ± 2.5 mm) than in males (4.36 ± 4.13 mm). On comparing the C-C’ distance among the various subtypes of ARM, it was found that maximum posterior deviation was seen in cases of RVeF, followed by RPF and ARM without fistula [Table 2]. In the two girls with VF and anterior displacement of the SMC, the C-C’ distance was quite high at 15 mm and 20 mm, respectively.

Associated anomalies included hypospadias with a stricture of the urethra in a patient of HARM without fistula while another patient of HARM without fistula had a coccygeal pit with unilateral dysplastic kidney. A unilateral dysplastic kidney was also seen in another case of VF. None of the associated anomalies affected the primary management of ARM significantly.

DISCUSSION

In this study, the majority of patients (36/45, 80%), both males and females, had a “well-developed” AD, only 9 patients (20%) having a “moderately developed” AD. This is similar to the findings of an earlier study of the characteristics and position of the AD in patients with ARM which found that the majority (53/65, 81.53%) of patients had a “well-developed” AD.[4] As in the previous study,[4] a “well-developed” AD was somewhat more common in males than in females.

In a report of CT scanning, following rectoplasty, in 18 patients with ARM, Ong et al.[10] stated that sufficient criteria of “normal” development of the SMC were not available and graded SMC development simply as “adequate” or “deficient.” In our study, we had subjectively classified the SMC as “well developed” or “moderately developed.” In our study, only two boys, one with a RVeF and another with CPC, had a “moderately developed” SMC. All seven patients with a RPF had a “well-developed” AD and SMC, and this is somewhat surprising as this subtype of ARM is often associated with a poorer development of the perineum and the sphincteric musculature.[15] However, it should be remembered that patients with a poorly developed AD were necessarily excluded from the study.

The width of the SMC, as assessed during PSARP, has not been described earlier in the literature. Pringle et al.[11] performed magnetic resonance imaging in untreated patients with ARM and found that the SMC was extremely narrow, being 3–5 mm in thickness from side to side and 1–2 cm in anteroposterior extent (width as measured by us). In our study, the mean width of the SMC was 13.45 mm in males and 14.24 mm in females, and these findings are similar to those reported by Pringle et al.[11] There was no significant difference in the width of the SMC among the various subtypes of ARM although the patient with CPC had a significantly lower width of the SMC at 10 mm. A limitation in the interpretation of these findings is that the age of the patients at the time of PSARP varied greatly, ranging from 2 months to 60 months, and in addition, the number of cases of each subtype of ARM was not high.

The most important finding of the study was that in as many as 36/45 patients (80%), the cephalocaudal band of fibers constituting the SMC was placed posterior to the position of the AD. This posterior location of the SMC was present in the majority of cases across the spectrum of subtypes of ARM. Overall, in only 7/45 patients (15.6%), the SMC was located within the limits of the AD while 2 patients with a VF had the SMC located anterior to the AD. These findings are extremely significant as they are contrary to the general acceptance that the SMC is located within the center of the AD.[156789] The clinical significance of these findings would appear to relate mainly to the procedure of laparoscopy-assisted anorectoplasty (LAARP),[1213] as during open surgery for ARM (PSARP), the SMC is displayed during the dissection and care is taken to place the terminal bowel within its center. During LAARP, a short 1–2-cm incision is made over the AD at the site of maximal contraction and caudocephalad elevation of the perineum by transcutaneous electrostimulation.[1213] This visual impression can necessarily have a significant error in interpretation. It can, therefore, be suggested that a long incision over the mid-perineum carried to a depth of around 2–3 cm would be better to correctly identify the bundle of SMC, both in its caudocephalad direction and in the center of its thickness for correct placement of the trocar through its center into the pelvis. This is important because, as stressed by Pringle et al.,[11] despite the narrow width and short extent of the SMC, it has a very important role to play in postoperative fecal continence. Okada et al.[6] have also stressed on the importance of bringing the rectum through the center of the vertical fibers of the SMC.

In an earlier study from our center, Patel et al.[4] studied the position of the AD in newborns and infants with ARM, as defined by the anal position index (API), and correlated it with the normal anal position in controls. The API is a quantitative measurement to define the normal anal position by measuring the ratio of fourchette–anus distance to fourchette–coccyx distance for females and of scrotum–anus distance to scrotum–coccyx distance for males.[14] Bar-Maor and Eitan[15] reported the API or anogenital index as 0.39 ± 0.09 in normal girls and 0.56 ± 0.20 in normal boys. In their study, Patel et al.[4] considered the midpoint of the AD to represent the anal position in patients with ARM and found that the position of the AD was significantly anterior to the normal anal position as shown by lower API values, the difference being statistically significant in newborn males (P = 0.001). In older boys as well as in newborn and older girls, apart from cases of persistent cloaca, the API was lower in patients with ARM than in controls, especially in boys, although the difference was not statistically significant. The authors concluded that during definitive surgery, if the AD is taken as the site of the proposed neo-anus, the neo-anal position is likely to be anterior to the normal anal position in both males and females and especially so in males.[4] The findings of the present study, that in as many as 36 cases (80%) the SMC was posterior in relation to the position of the AD, would suggest that in most instances the position of the cutaneous insertion of the SMC is closer to the normal anal position.

Certain lacunae in the study deserve attention. The sample size was relatively small at 45 patients so that the number of patients in each subtype of ARM was not high. There was no patient with persistent cloaca in the study group and the nature of the study, necessitating the AD to be at least “moderately developed,” eliminated several patients of ARM from the study. The age of the patients varied from 2 months to 60 months so that linear measurements such as the width of the AD or the SMC could not be compared among the various subtypes of ARM. In conclusion, however, our finding that the position of the SMC did not correlate with that of the AD in as many as 38/45 patients (84.44%) is contrary to the prevalent belief that the mass of the SMC is located directly cephalad to the AD. These findings also have significant practical value in optimizing the placement of the rectum through the center of the SMC during PSARP and especially during LAARP. Another important finding of the study is that the degree of development and of the SMC did not correlate with the type of ARM. However, it is recommended that a further study incorporating all subtypes of ARM and with a larger number of patients within each subtype would help in arriving at more definitive conclusions regarding the findings of our study and their relevance in the practical management of ARM.

CONCLUSIONS

In majority of patients anal dimple and SMC were well developed, the location of the SMC does not correlate with that of AD across the spectrum of subtypes of anorectal malformations.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.