Anorectal manometry tests the function of your bowels. You might be a good candidate for anorectal manometry if you are constipated, have fecal urgency (a constant feeling like you have to poop, but you don’t actually have to), or fecal incontinence (the medical term for leaking poop).
To understand anorectal manometry, you need a good understanding of your gastrointestinal, or digestive, system. We’ll start by breaking down your digestive tract before diving into the details of the testing.
If you’d rather watch than read this content, check out this video from my youtube channel:
Your Digestive System: An Overview
Let’s follow a piece of food from your mouth to your anus as a review of your digestive tract.
- You chew food in your mouth.
- You swallow the food, and it travels down your esophagus.
- Your esophagus empties into your stomach.
- Your stomach mixes and chops the food. Food stays in your stomach for around 2-4 hours.
- The food then enters your small intestines. Here, the majority of digestion and absorption of nutrients occurs. Food stays in your small intestines for 4-6 hours.
- The food is now a thick liquid. That thick liquid enters your large intestines, where it will it will stay for 24-72 hours. Your large intestines are incredible! They receive between 1-1.5 liters of thick liquid a day, and absorb all but 100-150mL.
- That last 100-150mL of thick liquid, which is now considered waste, or stool, or feces, or poop, then enters your rectum.
- Finally, the waste passes from your rectum to your anus and exits your body.
Your digestive system breaks down into two parts: an upper portion, and a lower portion. The upper portion consists of your mouth, esophagus, stomach, and small intestines. The lower portion consists of your large intestines, rectum, and anus. Anorectal manometry focuses on the lower portion. With that in mind, let’s make sure functions of the lower portion make sense!
Your Lower Digestive System: Basic Review
Your large intestines, also known as your colon, consist of three parts: an ascending, transverse, and descending colon. The descending colon eventually becomes your rectum, which eventually becomes your anus.
Most of your large intestines live close to the front of your abdomen. Your descending colon takes a turn towards your spine, and follows the curvature of your sacrum. It officially becomes your rectum about half way down your sacrum (S3-S5 are considered your rectum for those that are curious). Then, when your sacrum ends and your tailbone begins, your rectum becomes your anal canal.
Let’s focus strictly on rectum and anus. Your anal canal is about 3-6 cm (or 1-2 inches) long. Above it, your rectum is about 12 cm (or 4-5 inches) long. There are two sphincters within your anal canal: your external anal sphincter, and your internal anal sphincter. Your external anal sphincter lives close to bottom of the canal, near your anal opening. It is made of skeletal muscle, and you have some voluntary control over it. Your internal anal sphincter lives above it, near the top of your anal canal. It is made of smooth muscle, and you do not have voluntary control of it (much like you do not have voluntary control over your intestines).
On top of your internal anal sphincter we find my favorite group of muscles—your pelvic floor! There is a pelvic floor muscle, called your puborectalis, that lives right where your anus becomes your rectum. You have voluntary control over this muscle, and it affects the angle that stool travels from your rectum to your anus—your anorectal angle—as you defecate.
Let’s interject a fun and important fact about your anal canal! There is a line about half way into your anal canal where the way that your brain and tissue communicate changes. In other words, below the line—towards your feet—your anal canal can feel pain, touch, and temperature. Above the line—towards your head—your anal canal and rectum can only feel stretch. This is why external hemorrhoids that you can visibly see both bleed and hurt, and internal hemorrhoids that you cannot see bleed, but do not hurt.
Now that you have an idea of what your lower digestive tract looks like and where it lives, let’s talk about how you poop!
Your Lower Digestive System: How Do You Poop?
To understand the full mechanism of defecation, or pooping, we need to return to food entering the stomach.
- Food enters your stomach. This triggers the gastrocolic reflex.
- The gastrocolic reflex initiates peristalsis in your large intestines. Think about a snake eating a mouse. The snake’s body rhythmically contracts to digest the mouse—your intestines do the same thing! That rhythmic contraction—or peristalsis—propels stool towards your rectum.
- Stool enters your rectum, causing it to stretch. Remember our fun fact from earlier? The only way your rectal tissues communicate with your brain is through stretch! So, stool stretches your rectum and lets your brain know that something is there.
- Then, another reflex takes over. This reflex is called your sampling response. Here, your internal anal sphincter relaxes, and your external sphincter contracts, which allows your anus to “sample” your stool. Is it liquid, gas, or solid? Think about the other end of your digestive tract—your mouth! You know if you are going to burp or vomit, right? Your anus does the same thing!
- Now, you are aware that something is in your rectum, and you know if it is solid, liquid, or gas. You likely know from personal experience that you don’t have to act this urge to defecate. You can delay the urge with a different reflex, called your rectoanal inhibitory reflex (sometimes called RAIR). Essentially, if you voluntarily hold your external anal sphincter closed for about 45 seconds to a minute, this will cause your internal anal sphincter to contract and pull the sample back into your rectum, and the urge to defecate will subside.
- If you instead decide to act on the urge, your external anal sphincter will remain contracted until you find the appropriate place to defecate.
- You sit down or squat, which increases the anorectal angle we talked about earlier.
- Your pelvic floor muscles relax, which further increases the anorectal angle. (Recall puborectalis and the anorectal angle from earlier!)
- Your external anal sphincter relaxes.
- Pending on how long you’ve delayed the urge, you may need to generate some intra-abdominal pressure (pushing/bearing down) to begin defecating.
Another fun fact—if you delay the urge, you need a little bit of increased intra-abdominal pressure (pushing) to begin pooping, but peristalsis should take over, and you shouldn’t have to push for very long. If you didn’t delay the urge, peristalsis pushes your poop for you and you shouldn’t have to use much intra-abdominal pressure at all!
So, your gastrointestinal system is pretty wild. Knowing how it works and what is normal makes anorectal manometry make SO much more sense.
Anorectal Manometry: What Does It Test?
Anorectal manometry consists of a catheter with a balloon on the end which is inserted rectally.
The testing will look at:
- The function of your anal sphincters by assessing the different pressures in your anus and rectum. There are norms for the resting pressure (around 30-40mmHg) in your anus, and that pressure correlates with internal anal sphincter function. Then, if you voluntarily squeeze (or activate your external anal sphincter), you should produce around 150% of your resting pressure. This correlates with your external anal sphincter function.
- Your anorectal reflexes. Remember the RAIR (or rectoanal inhibitory reflex) that we talked about earlier? Can you voluntarily contract and hold your external anal sphincter for 45-60 seconds and successfully suppress the urge to poop? If you can’t, could that be why you’re leaking feces? Maybe! There is another reflex, called the rectoanal contractile reflex, which essentially means when there is a sudden, big increase your intra-abdominal pressure (also known as a cough or sneeze), do the right muscles contract so that you don’t leak feces?
- Rectal sensation. Remember your rectum and brain only communicate via stretch! There are established norms for the volume that should fill your rectum to indicate that first sensation (20-35mL), give you an urge to poop (120-150mL), and reach your maximum tolerance for stretch (200-300mL).
- That maximum tolerance for stretch also measures rectal compliance—how well do your rectal tissues move? Do they move too much, and that’s why your constipated? Do they not move enough, and that’s why you feel like you have to poop all of the time? Maybe!
- Finally, anorectal manometry will assess what your muscles do when you poop, or expel a water-filled balloon. Ideally, as we talked about, your pelvic floor will relax, your external anal sphincter will relax, and you might need a little bit of intra-abdominal pressure (or pushing) to defecation. However, could you generate enough pressure but contract those muscles instead of relax? Sure, and that could cause constipation.
Is anorectal manometry bizarre? Sure. But you can make sense of it if you break it down! If you had anorectal manometry testing, you may be a good candidate for pelvic physical therapy. Maybe you are leaking feces because your external anal sphincter is weak, we can retrain that! Perhaps your pelvic floor muscles contract instead of relax when you are trying to have a bowel movement and that’s why you’re constipated… we can retrain that! If the reason that you feel like you have to poop all of the time is because your rectum is hypersensitive and you get an urge to defecate at 50mL instead of 150mL… we can retrain that!
Colorectal is a subspecialty within pelvic floor physical therapy. If you do pursue pelvic therapy, make sure to ask the office if there is a specific therapist that focuses more on bowel dysfunction. Find a pelvic provider in your area and get yourself evaluated to see if pelvic physical therapy is right for you!