Varied parameters and utility of the anal position index: a systematic review and meta-analysis

The anal position index (API) was described in 1984 by Reisner et al. Since then, it has been measured in different ethnic populations and its utility in various medical conditions have been explored. We aimed to review the literature regarding the various values reported analytically.

A Pubmed Search was carried out with the terms Anal Position Index. There were 158 articles hit by the search. The description of the API was uniform in all studies, described as ratio of anus-fourchette distance in girls and anus-scrotum distance in boys to the distance between coccyx and fourchette/scrotum. 18 relevant studies were included describing the values in different ethnic groups and describing its utility. One study was excluded from statistics due to different landmark for measurement. The studies were grouped into 4. Group A:B:C:D comprised of Newborns:Infants:All age groups:Constipated children. We carried out meta-analysis to estimate effect size (mean difference) using STATA software version 16.0.

The different ethnic populations in which API were measured, and were Indian (2), Turkish (3), Israel (2), Taiwanese, Thai, Iranian, Spanish, Italian, and Mexican. The API was higher by 0.129 in boys. The mean API in males was 0.53:0.54:0.48:0.52 in Group A:B:C:D with an overall mean (SD) API of 0.51 (0.04). The mean API in females was 0.40:0.40:0.38:0.37 in Group A:B:C:D with an overall mean (SD) API of 0.40(0.03). Two studies were done in mice and showed the relation of API to intrauterine exposure to androgens. In children, API was mostly used to diagnose an anterior ectopic anus and see correlation with constipation. The abnormal values varied from less than 0.30-0.34 in girls and less than 0.41-0.46 in boys. The effect size by regions showed that the estimated effect size for all the regions were within 95% Confidence limits of overall estimate (0.13: 95% CI: 0.13-0.14). Therefore, it can be inferred that there was no significant ethnic variation in the study parameter. The anal position in relation to genitalia was measured in nulliparous women, menopause women, and women with levator deficiency. It was affected in vulvovaginal atrophy. Dichlorodiphenyldichloroethylene exposure during the first trimester of pregnancy was reported to alter the anal position in male infants.

The anteriorly positioned anus has been associated with constipation. API been measured in various ethnic populations with insignificant variations. The API is significantly (p < 0.05) higher in males. The presence of constipation does not seem to alter API. One should adopt a single method for measurement. API should not be considered at the sole indication for any surgical intervention.