Volume 31, Issue 6, December 2022, 151226Author links open overlay panelAbstract

The etiology of anorectal malformations (ARM) is complicated because of the spectrum of anomalies seen clinically, many of which are isolated and seemingly unique. Recent studies suggest that ARMs result from abnormal cloacal membrane (CM) development that contributes to disrupt normal local muscle and nerve development. If CM maldevelopment is severe, the rectal pouch lies above the pelvic floor, resulting in asymmetric and/or deviated musculature, so a midline incision is likely to cause trauma or be detrimental. Autonomic nerve plexuses can be associated with a fistula tract in ARMs and are also at risk for damage during surgery and contribute to genitourinary complications. Understanding the anatomy and development of the perineum is crucial for treating the broad spectrum of anomalies associated with ARMs and may assist in predicting/managing other related morbidity.

Introduction

ARMs include various anatomic anomalies ranging from anal stenosis or anorectal agenesis, with or without a fistula, to more complex cloacal malformation (Fig. 1 (a-e)).1 Despite a long history of embryologic research, the development of the anorectum remains somewhat controversial. It is commonly believed that the urorectal septum (URS) divides the embryonic cloaca into a ventral urogenital sinus and separate dorsal hindgut,2, 3, 4, 5, 6 and the spectrum of anomalies seen clinically likely results from the timing of some kind of developmental arrest.7 However, the mechanism of cloacal septation has been debated with the emphasis shifting to the cloacal membrane (CM) playing a more important role during development of the anorectum8, 9, 10 and the mechanism for changes in embryonic anorectal morphology regarded as being the result of embryonic cell differentiation, cell proliferation, and apoptosis.11,12

Section snippetsEtiology of ARMs

The definitive cause of ARMs is unknown, but one third of ARMs are isolated and the remainder associated with other congenital urogenital, cardiovascular, skeletomuscular and gastrointestinal tract anomalies.13,14 Thus, the etiology of ARMs is considered to be multifactorial15,16 with environmental and genetic components. Exposure of the fetus to fever, diabetes, excessive weight gain, tobacco smoke and caffeine, vitamin A deficiency, and asthma medications have been reported in relation to

Normal development of the anorectum

Historically, the anorectum has been described as developing from the cloaca, and by the 6th week, the URS divides the embryonic cloaca into urogenital and anorectal compartments, and the URS fuses with the cloacal membrane.2, 3, 4, 5, 6 However, recent embryologic studies have not been able to confirm URS/CM fusion. Instead, the cloaca appears to remain undivided until the CM ruptures.8,10,31 Normal anorectal and genitourinary development also appears to depend on the dorsal and ventral CM,10

Abnormal development of the anorectum

The traditional view that abnormal septation related to embryologic developmental arrest causes ARM7 has been replaced by a process involving the CM. Thus, abnormal CM development causes ARM anomalies. Some specific ARMs will be discussed as examples.

Anatomic consequences of ARMs

ARMs disrupt local muscle and nerve development.37, 38, 39, 40 Classification of ARMs as low or high is common and largely historical, referring to the position of the rectal pouch in relation to the levator ani (LA) or the pelvic floor. In low type ARMs with mild CM maldevelopment, the rectal pouch is located below the pelvic floor which means the pouch has less impact on the pelvic floor, but in high type ARMs with severe CM maldevelopment, it is located above the pelvic floor and as a

Surgery based on anatomy and embryology

The most commonly used operative procedures for treating ARMs are the posterior sagittal anorectoplasty (PSARP) procedure described by Pena et al.,42 and the laparoscopic or laparotomic anorectoplasty (LAARP) procedure described by Georgeson et al.43 In ARMs, the pelvic floor is abnormal, especially in high type ARMs with severe CM maldevelopment, and this influences postoperative outcome. PSARP requires incision of pelvic floor muscles in the midline to perform but is valid only when the

Conclusion

While understanding the embryology and development of ARM anomalies is beneficial and assists in planning surgery, thorough inspection and examination are mandatory in all ARM patients. While diagnostic imaging is crucial and indispensable for determining internal anatomic relations in the perineum and pelvic floor, external genitourinary/gastrointestinal tract relations must be assessed exhaustively. In particular, check inside the vulvae and check inside the anus.

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