Anal canal duplication (ACD) is the rarest malformation of the distal alimentary tract, usually defined as “cases with an additional blind anal orifice, sagittally positioned, exhibiting the same histological features as normal anal canal” (1). The pathology criteria were set by Ochiai et al. (2) in 2002, as follows: the presence of (1) squamous epithelial cells in the caudal end, (2) transitional epithelium in the cranial end, and (3) smooth muscle cells in the lesion wall. There were only about 110 cases described in the literature, with a female-to-male ratio of 15:1 (3–8). Patients usually present with a second opening in the midline, at 6 o’clock behind the true anus (9, 10). Rare cases such as 5 o’clock position orifice or anal canal triplication were also reported (7, 11). Infant patients were often asymptomatic. Some patients, mostly adults and children older than 5 years, complained of constipation, mucous drainage, or perineal abscess. Some patients were misdiagnosed with complex anal fistula and suffered from abscesses being incised and drained many times (6, 12–15). Anorectal malformation, anal duplication cyst, malrotation, sacrococcygeal teratoma, myelomeningocele, tethered cord, congenital heart defects, cleft lip/palate, and genitourinary anomalies were found in approximately 25% of the cases, with presacral mass accounting for 50% of the total (3, 9). Complete excision and simple mucosectomy have been demonstrated with satisfactory outcomes (16).

We describe two additional infant cases: case 1 presented with refractory constipation, and case 2 was asymptomatic. A barium enema of case 1 revealed anorectal stenosis and mega-rectosigmoid. Two patients underwent surgery, and the pathology confirmed the diagnosis.

Anal canal duplication is a rare malformation with poorly understood etiology and less defined clinical management criteria. Patients are always asymptomatic or just with local symptoms around the perianal region. Differential diagnosis should be carefully evaluated between ACD with perianal fistula and perianal abscess. Comorbidities were found in 25% of the patients, of which presacral masses were the most common, accounting for 50%, and anorectal malformations (ARMs) account for 5% (3, 5). We report a case associated with anorectal stenosis, accompanying secondary mega-rectosigmoid, which has never been described before.

Early diagnosis is crucial but easy to miss in neonates (17). Our patient was suspected of having ACD and anorectal stenosis 8 months after birth; mega-rectosigmoid most likely occurred due to delayed diagnosis and treatment. The diagnosis should be made when a second perineal opening appears behind the normal anus at 6 o’clock. The diagnosis of concomitant malformations is also important because of their direct impact on the surgical strategy. Auxiliary examinations such as fistulography and MRI are useful tools for ACD diagnosis. Contrast enema can describe the diameter and length of the duplication and, more importantly, the communication with the rectum. MRI can inform the relationship between ACD and adjacent soft tissue like the anal sphincter and striated muscle. A key role of MRI is to find other abnormalities such as presacral mass, intrasacral meningocele, or duplicated rectum. Methylene blue detection can be performed to check whether there is communication between ACD and the anus.

The etiology and the reason for the female incidence dominance remain poorly understood due to the complexity of embryonic development. Interestingly, male patients make up a larger proportion of patients with an imperforate anus (18). Previous studies believe that the formation of ACD is closely related to the process of urorectal septum development; however, a further investigation should be performed (19). Direct observation of embryonic development is admissible, but the difficulty in creating ACD animal models is a problem.

The prognosis for surgery is favorable, but the necessity is controversial (16). Akova et al. proposed that asymptomatic patients who do not undergo surgery might remain free of symptoms (5). This could be due to the small number of follow-up cases (only 2) and the short average follow-up period (3.5 ± 1.0 years). The statistical data of 110 cases reported in the literature show that patients who develop symptoms are mostly adults or children older than 5 years (3, 6, 9, 10). Moreover, it has been proposed that the possibility of developing symptoms and complications increases with age, patients will develop abdominal pain, perianal pain, constipation, and other atypical symptoms, and the situation will deteriorate as they become older (6, 12, 14). This may relate to an increase in stool volume as the patient ages. It is also unknown whether the condition has an impact on mental development. Furthermore, research indicated that ACD patients might be predisposed to colloid carcinoma (20). All cases we described underwent surgery, and the follow-ups were all uneventful. Given the excellent outcomes of surgery, we believe that surgery is the best option to treat ACD before it progresses into bothersome symptoms and reduces the risk of developing colloid cancer.

The clinical features of ACD, the anatomic relationship with the anal sphincter, and its associated malformations are important determinants of the surgical approach (21). For cases associated with ARMs and presacral mass, the posterior sagittal approach is recommended (22). In our case, the ACD was located in the rectum and no presacral malformations were associated. For complete resection of the stenotic rectum and to reduce peripheral neuromuscular injury, we performed a transanal pull-through procedure for the rectum with full thickness in the range of the sphincter, which showed good outcomes.

ACD is a rare malformation with a lack of well-defined clinical care guidelines and a poorly understood etiology. Early clinical suspicion of ACD and association elimination are crucial. Contrast enema and MRI are useful auxiliary examinations for ruling out presacral masses and ARMs. For ACD associated with anorectal stenosis, a transanal pull-through procedure of the rectum with full thickness is recommended with good results.