The position of the anal dimple in newborns and infants with anorectal malformations and its correlation with the normal anal position

Background/purpose

The anal position index (API) defines the normal anal position as the ratio of fourchette–anal distance to fourchette–coccyx distance for females and the scrotum–anal distance to scrotum‐coccyx distance for males. In this study, measurement of the API in newborns and infants with anorectal malformations (ARM), using the center of the midline anal dimple (AD) to represent the center of the proposed neoanus, was performed to assess whether or not the AD was located in a significantly abnormal position as correlated with the normal anal position.

Methods

The cases comprised 65 consecutive newborn and infants with ARM, divided into 2 age-based groups (Group A: 1st day to 1 month; Group B: 1–12 months), without sacral or significant perineal abnormalities. Controls included an equal number of age and gender matched patients admitted for other conditions. The characteristics of the AD (‘well developed’ or ‘moderately developed’) as well as those of the midline perineal raphé in males and the perineal groove in females were also recorded.

Results

Combining both age groups, the mean API ± SD was 0.41 ± 0.012 in male cases and 0.53 ± 0.07 in male controls (p value 0.003). The corresponding values for female cases and controls were 0.31 ± 0.09 and 0.36 ± 0.07 respectively (p = 0.040). In male newborns, the API in cases was significantly lower than the API in controls (p < 0.001). Combining both males and female newborns, the differences between the API in cases and controls was also statistically significant (p < 0.001). In older boys as well as in girls from both age groups, apart from cases of persistent cloaca, the API was lower in cases than in controls, especially in boys, although the difference was not statistically significant. On comparing the API in individual male ARM with that in male controls, cases of rectobulbar urethral fistula (RBUF) and rectovesical fistula had a statistically significant lower API (p < 0.001). In the 4 cases of persistent cloaca, the mean API (0.40 ± 0.1) was higher than the mean API in female controls and the AD was well developed with a well-delineated, narrow perineal groove. The majority (53/65; 81.53%) of patients had a ‘well developed’ AD. Twelve patients (18.47%) had a ‘moderately developed’ AD including 10 females and 2 males. There were 15 patients with a depressed AD (anal fossette); of these 12 (80%) were females. All boys had a well delineated perineal raphé in contrast to 10 girls (29.4% of total female ARM) who had a poorly delineated midline perineal groove.

Conclusions

During definitive surgery for ARM, if the AD is taken as the site of the proposed neoanus, the neoanal position is likely to be anterior to the normal anal position in both males and females and especially so in males. Overall, girls with ARM appear to be more likely to have a relatively poorly developed and/ or depressed AD and a poorly delineated perineal groove.

Type of study

Clinical Research.

Level of evidence

Level III.