Anal atresia

ICD 10- XVII Q42

Anal atresia refers to a congenital malformation in which the anus is missing or blocked. 

1:1.500 to 1:5.000 live newborns.¹
Male to female ratio 1.5:1.0.^{2, 3}
 

The anorectal and urogenital tract derive from the cloaca, the caudal part of the hindgut. This structure develops by the 4th week of intrauterine development.⁴ By the 6th week of development, the cloaca is divided by the urorectal septum into a separate ventral urogenital sinus and a dorsal hindgut.⁴ The anal membrane ruptures at the end of the 10th week.⁵ It has been hypothesised that incomplete development of the urorectal septum leads to the occurrence of anorectal malformations (ARMs). The spectrum of malformations is likely the result of the timing of arrest of the embryological development.⁴
  
 

The etiology remains unclear, but is assumed to be multifactorial. Both genetic and environmental factors have been identified in animal models.⁶ The involvement of genetic factors is also supported by the association with chromosomal abnormalities and reports of familial aggregation. Abnormalities in homeobox and sonic hedgehog-signalling pathways are thought to play a role.⁴
Maternal overweight and obesity, diabetes and paternal smoking also seem to be risk factors for the development of ARMs.^7,8

Anal atresia may be considered to be part of the wider spectrum of ARMs, in which the anus and the anal canal are always involved.⁹ The distal enteric tract may end blindly, or as a fistula into the perineum or the urinary or genital tract. In males, the fistula may be recto-vesical, recto-prostatic, recto-urethral or perineal and in females, recto-vaginal, recto-vestibular or perineal.¹⁰ The Wingspread classification divides ARMs into a low, intermediate and high type, depending on the location of the rectal pouch in relation to the puborectal sling. The Krickenbeck classification was developed more recently and is mainly based on the presence and type of fistula.¹¹ 

ARMs are often associated with other malformations, but the reported type and prevalence (20-70%) vary significantly among studies.¹²
The most common co-occurring abnormalities are urogenital and musculoskeletal abnormalities, followed by cardiovascular abnormalities, gastrointestinal abnormalities and abnormalities of the central nervous system.¹² Anal atresia can be part of the VACTERL association (≥3 of the following features: Vertebral anomalies, anal atresia, cardiac anomalies, tracheoesophageal fistula, renal and limb defect.)  Anal atresia is also associated with chromosomal abnormalities, such as trisomy 21 and a variety of syndromes, such as Townes-Brocks, Pallister-Hall and Currarino syndrome.^{12, 12}
 

Most cases are sporadic and have a low risk of recurrence among first degree relatives. The recurrence risk for siblings has been estimated to be 1%, but a recent study indicated a possibly higher recurrence risk, particularly for offspring of affected parents.¹⁴

The majority of cases are diagnosed after birth at the time of the initial physical examination, by failure to pass meconium within the first 48 hours of life, or by passing meconium through a fistula.¹⁵
Sonographic diagnosis can be challenging, but visualisation of the fetal perianal muscular complex (PAMC)  is possible. The PAMC can be visualised as a hypoechogenic circular rim with a central echogenic stripe in the second and third trimester.²⁰ The transverse view of the PAMC can be acquired by sliding the transducer cephalad from the tangential plane of the fetal perineum. After rotation, the long axis of the PAMC can be depicted as a hypoechoic tubular structure with a central echogenic stripe.¹⁶
One study demonstrated that between 23 and 34 weeks’ gestation the fetal PAMC could be identified in 90% -100% of low-risk pregnancies.¹⁶
In a high risk population, the absence of PAMC seemed a highly sensitive and specific marker for anorectal atresia.¹ Reference values of the anal sphincter diameter between 19 and 38 weeks’ gestation are available.¹⁶ With increasing experience and better resolution, earlier detection will probably be possible. Potential pitfalls are the presence of an ectopic anus or misdiagnosis of a perineal fistula.¹ Low anal atresia or imperforate anus might also be missed, because of the presence of a normally developed PAMC.
A distended large bowel (distal colon/rectum) can be a sign of an ARM, particularly in the presence of calcified intraluminal meconium. Reference values for the descending colon and rectum between 19 and 40 weeks’ gestation are available.¹⁷
When abnormalities such as sacral agenesis, hemivertebrae, caudal regression syndrome, genitourinary malformations or other abnormalities that are associated with anal atresia and other types of ARMs, visualisation of the PAMC is indicated.
In first trimester pregnancies, ARMs can be suspected in the presence of an intraabdominal cyst.^{5, 18}
 

In cases with dilatation of the fetal colon, meconium plug syndrome and Hirschsprung’s disease should be included in the differential diagnosis.

When anal atresia is suspected, a thorough search for associated anomalies is recommended. 
If the PAMC cannot be visualised before 23 weeks of gestation in cases with anomalies that are known to be associated with anal atresia, a repeat scan in order to identify the PAMC can be considered. 
 

An advanced (targeted) anomaly scan should be performed. If anal atresia is suspected, invasive diagnostic testing should be offered. Counselling by a pediatric surgeon should be offered. Genetic consultation can be considered, especially in the presence of associated anomalies. Some parents may consider termination of pregnancy if this is a legally available option.  Delivery at a centre with neonatal intensive care and pediatric surgery facilities may be necessary.¹⁵
Postnatal screening for associated anomalies is recommended. The surgical approach depends on the type of ARM, but usually involves an anoplasty. A temporary colostomy is often necessary before surgical correction can take place.
 

In terms of survival, isolated anal atresia has a good prognosis. Depending on the type and number of associated anomalies, survival rates decrease. 
Follow-up among adults with ARMs shows higher prevalence of incontinence and constipation, compared to controls, particularly in patients with cloacal malformations, prostatic/vesical, rectobulbar, or vestibular fistulas.¹⁹ Impaired bowel function negatively affects sexual function and quality of life. The outcome in patients with perineal fistulas and anal stenosis is not very different from controls.¹⁹
 

1. Ochoa JH, Chiesa M, Vildoza RP, Wong AE, Sepulveda W. Evaluation of the perianal muscular complex in the prenatal diagnosis of anorectal atresia in a high-risk population. Ultrasound Obstet Gynecol. 2012;39:521-7 doi: 10.1002/uog.9083 [published Online First: 2011/07/06].

2. Endo M, Hayashi A, Ishihara M, et al. Analysis of 1,992 patients with anorectal malformations over the past two decades in Japan. Steering Committee of Japanese Study Group of Anorectal Anomalies. J Pediatr Surg. 1999;34:435-41 doi: S0022346899001980 [pii] [published Online First: 1999/04/22].

3. Zlotogora J, Abu-Dalu K, Lernau O, Sagi M, Voss R, Cohen T. Anorectal malformations and Down syndrome. Am J Med Genet. 1989;34:330-1 doi: 10.1002/ajmg.1320340307 [published Online First: 1989/11/01].

4. Gupta A, Bischoff A, Pena A, Runck LA, Guasch G. The great divide: septation and malformation of the cloaca, and its implications for surgeons. Pediatr Surg Int. 2014;30:1089-95 doi: 10.1007/s00383-014-3593-8 [published Online First: 2014/09/15].

5. Lam YH, Shek T, Tang MH. Sonographic features of anal atresia at 12 weeks. Ultrasound Obstet Gynecol. 2002;19:523-4 doi: 694 [pii] 10.1046/j.1469-0705.2002.00694.x [published Online First: 2002/05/02].

6. Kluth D. Embryology of anorectal malformations. Semin Pediatr Surg. 2010;19:201-8 doi: S1055-8586(10)00018-1 [pii]10.1053/j.sempedsurg.2010.03.005 [published Online First: 2010/07/09].

7. Svenningsson A, Gunnarsdottir A, Wester T. Maternal risk factors and perinatal characteristics of anorectal malformations. J Pediatr Surg. 2018; doi: S0022-3468(18)30264-1 [pii] 10.1016/j.jpedsurg.2018.04.021 [published Online First: 2018/06/17].

8. Zwink N, Jenetzky E, Brenner H. Parental risk factors and anorectal malformations: systematic review and meta-analysis. Orphanet J Rare Dis. 2011;6:25 doi: 1750-1172-6-25 [pii] 10.1186/1750-1172-6-25 [published Online First: 2011/05/19].

9. Jonker JE, Trzpis M, Broens PMA. Underdiagnosis of Mild Congenital Anorectal Malformations. J Pediatr. 2017;186:101-4 e1 doi: S0022-3476(17)30466-3 [pii]10.1016/j.jpeds.2017.03.054 [published Online First: 2017/05/02].

10. Alamo L, Meyrat BJ, Meuwly JY, Meuli RA, Gudinchet F. Anorectal Malformations: Finding the Pathway out of the Labyrinth. Radiographics. 2013;33:491-512 doi: 33/2/491 [pii] 10.1148/rg.332125046 [published Online First: 2013/03/13].

11. Holschneider A, Hutson J, Pena A, et al. Preliminary report on the International Conference for the Development of Standards for the Treatment of Anorectal Malformations. J Pediatr Surg. 2005;40:1521-6 doi: S0022-3468(05)00645-7 [pii]10.1016/j.jpedsurg.2005.08.002 [published Online First: 2005/10/18].

12. Stoll C, Alembik Y, Dott B, Roth MP. Associated malformations in patients with anorectal anomalies. Eur J Med Genet. 2007;50:281-90 doi: S1769-7212(07)00049-3 [pii]10.1016/j.ejmg.2007.04.002 [published Online First: 2007/06/19].

13. Torres R, Levitt MA, Tovilla JM, Rodriguez G, Pena A. Anorectal malformations and Down's syndrome. J Pediatr Surg. 1998;33:194-7 doi: S0022-3468(98)90430-4 [pii] [published Online First: 1998/03/14].

14. Dworschak GC, Zwink N, Schmiedeke E, et al. Epidemiologic analysis of families with isolated anorectal malformations suggests high prevalence of autosomal dominant inheritance. Orphanet J Rare Dis. 2017;12:180 doi: 10.1186/s13023-017-0729-710.1186/s13023-017-0729-7 [pii] [published Online First: 2017/12/15].

15. Brantberg A, Blaas HG, Haugen SE, Isaksen CV, Eik-Nes SH. Imperforate anus: A relatively common anomaly rarely diagnosed prenatally. Ultrasound Obstet Gynecol. 2006;28:904-10 doi: 10.1002/uog.3862 [published Online First: 2006/11/09].

16. Moon MH, Cho JY, Kim JH, Min JY, Yang JH, Kim MY. In-utero development of the fetal anal sphincter. Ultrasound Obstet Gynecol. 2010;35:556-9 doi: 10.1002/uog.7598 [published Online First: 2010/02/26].

17. Zalel Y, Perlitz Y, Gamzu R, Peleg D, Ben-Ami M. In-utero development of the fetal colon and rectum: sonographic evaluation. Ultrasound Obstet Gynecol. 2003;21:161-4 doi: 10.1002/uog.26 [published Online First: 2003/02/26].

18. Khalil A, Cooke PC, Mantovani E, Bhide A, Papageorghiou AT, Thilaganathan B. Outcome of first-trimester fetal abdominal cysts: cohort study and review of the literature. Ultrasound Obstet Gynecol. 2014;43:413-9 doi: 10.1002/uog.12552 [published Online First: 2013/07/06].

19. Danielson J, Karlbom U, Graf W, Wester T. Outcome in adults with anorectal malformations in relation to modern classification - Which patients do we need to follow beyond childhood? J Pediatr Surg. 2017;52:463-8 doi: S0022-3468(16)30563-2 [pii]10.1016/j.jpedsurg.2016.10.051 [published Online First: 2016/11/30].

20. The Prenatal Ultrasonographic Visualization of Imperforate Anus in Monoamniotic Twins, Edwin R. Guzman, MD, Angela Ranzini, MD, Debra Day-Salvatore, MD, PhD, Barry Weinberger, MD, Nitsana Spigland, MD, Anthony Vintzileos, MD J Ultrasound Med 14:547-551, 1995 

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