Anatomy, Physiology, and Evaluation of the Anorectal Disorders

Published on Feb 21, 2022

Learn from an industry-leading doctor how to do a step-by-step evaluation of the anorectum and evaluate defecatory disorders.

"Everybody poops" is a common phrase. However, not many realize just how complex and coordinated successful defecation is. And, when things go awry, you need to have the latest evaluation and diagnostic tests available to best help your patients.

Watch this short video from an industry-leading expert, Dr. Abhitabh Patil. In the video you will learn:

  • The anatomy and physiology of the anorectum and the complex process of defecation

  • How to perform a digital rectal examination

  • The different diagnostic tests that are used to evaluate defecatory disorders

  • A simple algorithm for patients who present with a suspected defecation disorder

Anatomy, Physiology and Evaluation of Anorectal Disorders

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Video Transcript

Dr. Abhitabh Patil: “Next, we'll review anatomy and physiology of the anorectum. In this slide, you can see how the anorectum is organized. It's a very complex group of muscles, but I'd like to highlight some of the important muscles involved in defecation. At the top, you have the puborectalis muscle. In the anorectal canal, you have the internal anal sphincter and the external anal sphincter. Above the internal anal sphincter, typically we have the hemorrhoidal plexus, which we call the internal hemorrhoids, and below the dentate line, we have the external hemorrhoids.

In the resting state, the puborectalis muscle is contracted. This changes the angle from the rectum and the sigmoid colon to make an S shape. This itself holds the stool inside the rectal vault. During a defecation, this process is reversed, which will be demonstrated in the next couple of slides.

Patients also have a reflex called the rectoanal inhibitory reflex. When the rectum is distended, a nerve impulse is sent to the spinal cord, which is then returned to relax the internal anal sphincter. This is called the rectoanal inhibitory reflex. This also ends up straightening the muscles and allowing for defecation from the sigmoid colon and rectum out of the anus, or expulsion.

Let's go over evaluation of the anorectum. The digital rectal exam is probably the most important component of the physical examination. Start by inserting the finger into the anal canal. What you want to palpate for is the resting tone of the anus. You should feel a slight squeeze around the finger. You should be also able to palpate any tenderness that may be a sign of an anal abscess or perianal abscess.

The next thing we do is ask the patient to squeeze. Here, what we're feeling for is a contraction of the external anal sphincter against your finger. Next, we ask the patient to squeeze and hold as if they're holding a bowel movement. This is where we can palpate the puborectalis muscle. We should feel the muscle contracting and making an S shape.

Next, we ask the patient to bear down like they're having a bowel movement. This allows us to see if they're able to reverse all of these muscular contractions. During this process, you should feel a give inside the rectal canal. The puborectalis muscle should relax. The sigmoid colon should straighten, and you should feel relaxation on the internal anal sphincter as well as the external anal sphincter. This allows expulsion of stool, and you should be able to feel your finger slipping out the anal canal.

Let's go over some of the diagnostic tests that are used to evaluate defecatory disorders. First most common is a colonoscopy. This is very helpful for excluding underlying colorectal cancer or other mechanical causes. Anoscopy can be used to evaluate the anal rectum. Anorectal manometry, which we'll demonstrate in-depth, as well as balloon expulsion testing, can check for other defecatory disorders related to contraction or squeeze. Colon transit studies can be helpful for evaluating slow transit constipation. MRI pelvis can evaluate for structural or anatomic disorders as well as defecography.

This is a simple algorithm for patients who present with a suspected defecation disorder. Start off by, of course, performing a history, a physical examination, including a thorough rectal exam. But if you suspect that there's a defecation disorder or an expulsion disorder, start by performing an anorectal manometry with a balloon expulsion test. If the test is normal, there is no defecation disorder present. If the anorectal manometry or balloon expulsion test is abnormal, then consider performing further testing. If it's indeterminate, consider defecography, and if the defecography is normal, then there's no defecation disorder. If it's abnormal, then pursue the abnormal result. If the anorectal manometry and the balloon expulsion test are abnormal, then you have a defecation disorder, and this should be pursued.

Anorectal manometry is used to assess the internal anal sphincter and external anal sphincter as well as rectal sensations and expulsion patterns. A pressure-sensitive catheter is placed into the anorectum to measure resting and squeeze pressures of the anal canal. In the next segment, we'll demonstrate how to perform anorectal manometry with balloon expulsion.”

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