In many cases, when someone approaches me with issues of "constipation", what they actually tend to be describing is an issue with defecation. Often the issue is that they are finding it difficult to get the poop out — hence the term "obstructed". In reality, the situation is one where the force generated to push the poop out is not being directed in alignment with the anal outlet.

Obstructed defecation is a type of constipation, and it is essentially a biomechanical problem. So management and treatment of the condition needs to approach it from that perspective, considering the forces and vectors involved.

What are the symptoms of obstructed defecation?

Patients that come to see me for this condition usually find that they have to strain a lot to push the poop out. And in many cases, they find that they are exerting a lot of force when straining but hardly any poop comes out.

Then eventually, after a long period of sitting on the toilet and repeated attempts at straining, they manage to get some out. And even then, there is often a sensation of incomplete emptying and a sense of dissatisfaction. This tends to be something that happens regularly — most days, if not every day.

Is obstructed defecation caused by a blockage?

Sometimes, patients have strictures or narrowings at the anus or lower rectum. But often, the issue is what we term "functional" rather than "mechanical" — this is used in the medical context rather than the engineering one.

Even though this is "functional" in the medical sense (meaning it is not caused by structural blockage), the way we think about and treat it is distinctly biomechanical — concerned with forces, angles, and the structures that direct them.

How does a normal bowel movement work?

Whilst the wave-like contractions of the colon are responsible for the movement of digested food it receives from the small intestine to the rectum, the actual pushing out of the poop through the outlet is dependent on the mass downward force that occurs with abdominal contraction and bracing — known as the Valsalva manoeuvre.

The Valsalva manoeuvre affords us the ability to generate quite substantial force. The greater force is necessary when we consider that defecation involves the expulsion of a solid object through a narrow (though stretchable) sphincter. However, the force generated is somewhat "diffuse", in that it is simply being directed downwards. And this is where the pelvic floor helps: it acts as a funnel to channel that force towards the anal opening. When the pelvic floor is weak, that ability to direct the force is affected.

Also, the tube in which the poop is passing across just before it comes out — the rectum — needs to be sufficiently stable so that the poop can flow smoothly down to the opening. The rectosigmoid colon and upper rectum are intra-peritoneal, meaning that they can flop around a bit. So ideally, we need some degree of fixation to help direct the force towards the anal opening.

What causes obstructed defecation?

The reason that more strain and force doesn't get the poop out is because, oftentimes, the force is directed at an angle to the outlet. Multiple factors can cause this:

• A relatively weak pelvic floor that is not adequately suspending and holding the lower rectum steady and at the correct angle.

• The laxity of the rectum is due to repeated straining, which, instead, results in an intussusception (so-called "internal prolapse") — where part of the rectal wall gets pulled inwards, a bit like a sock being partly turned inside-out.

• A weakened portion of the rectum (usually at the front part that borders on the vagina, in women) that causes the stool to get trapped in a divot there. This bulge at the front of the rectum is known as a rectocoele.

Does the shape of my stool matter?

Yes, quite a lot. The stool consistency and fragmentation (smaller chunks vs one solid lump) play a role in absorbing the downward force from the Valsalva manoeuvre, and also in being able to direct or disperse the force.

The analogy I use is that if the poop is pellet-like, then the pellets act a bit like billiard balls — especially if the rectum is too stretchy. Whereas a banana-shaped poop will likely flow down smoother, as the force will direct it downwards along the long axis.

A more contiguous solid stool (i.e. a banana-shaped stool) will be easier to push out, as the force does not dissipate as much when it presses against the top part of the stool. This is in contrast to a softer or pellet-like stool that can allow the forces to spread to the sides — reducing the effective force directed towards the anus.

How is obstructed defecation diagnosed?

As the management of the condition involves attempting to correct the mechanics of the pelvic floor and the process of defecation, the assessment and work-up will go into what happens during defecation.

After eliciting a history of straining, I would want to know where the patient feels the force pressing against, and whether there are any positional manoeuvres attempted to change the angle of the rectum and the direction of the force being applied.

I would next address the issue of stool shape and consistency. For this, I usually reference the Bristol Stool Chart and use the images and descriptions to have an objective assessment of stool consistency.

During the physical examination, I would be looking for signs of pelvic floor weakness — specifically, what is known as perineal descent. I would also be doing an internal examination to assess the laxity of the rectal wall and, in women, to see if any laxity has resulted in a rectocoele.

What is a dynamic MRI defecography?

A dynamic MRI defecography is an MRI scan with the patient needing to strain to push out a bit of gel that we insert into the rectum to mimic the act of pooping. As with many biomechanical assessments, sometimes a video of the movement helps — and this is essentially what the scan provides.

The scan allows us to see exactly how the muscles are contracting, and the coordination of the muscles and the pelvic floor, during a simulated defecation. I would arrange this in some situations, rather than for every patient.

How is obstructed defecation treated?

Treatment approaches the condition from the biomechanical perspective, considering the forces and vectors involved. Some of this involves strengthening the pelvic floor — such as specific exercises under the guidance of a trained physiotherapist. Surgical approaches work to help hitch up the rectum and pelvic floor.

In much of general surgery, we correct the anatomical problem, which often involves removing a growth or other abnormality and then reconstructing anatomy as much as possible. In many respects, surgery for obstructed defecation is more akin to orthopaedic surgery, in that we are trying to correct a functional problem, and so we need to consider the mechanics and the forces involved. The difference is trying to do this with soft tissue rather than hard bone.

Is there a diet for obstructed defecation?

There is probably no one diet that will work for everyone. I start by flagging the target — a more contiguous, banana-shaped stool — and then talk a bit about how to achieve it.

The reality is that stool consistency is influenced by fibre; by water in the colon (which is related to water intake to some extent, but also to the osmotic pressure from the digested food substances, and hence the food that we eat); and by the microbiome and its interaction with the food that reaches the colon.

So instead, I tend to explain that stool consistency is a bit like baking bread. The consistency of the dough is influenced by the amount of flour and water — but also by salt and/or yeast. If you add more flour, you get lumpier and drier dough. Too much water and you get mushy dough.

Will using a footstool help?

In some cases, yes — simple manoeuvres such as using a foot stool to raise the knees might help. But it really depends on the exact issue from a biomechanical perspective.

If the issue is a pelvic floor weakness that affects its ability to brace against the downward force, this is less likely to be resolved with positional modifications. Similarly for when the issue is a rectocoele.

Can pelvic floor physiotherapy help with obstructed defecation?

Yes. The muscles involved in defecation work through a coordinated contraction and relaxation in a specific sequence, and specific exercises under the guidance of a trained physiotherapist can help to train this.

This is not that different from other movements that involve multiple muscle groups, such as in sports. The main difficulty is that the muscles and movement tend to happen internally, in a location that is not visible, and hence require good body awareness.

Sometimes, as in the case of Kegel-type exercises, the patient can be asked to imagine stopping their urine flow, for instance. This is, however, only one type of pelvic floor exercise. While this is a good starting point, training the other movements and contractions has less obvious cues. This is where specialised training and expertise from a physiotherapist who has expertise in this area are useful.

When is surgery needed for obstructed defecation?

In many cases, I would lean towards physiotherapy first. Surgery is an option when dynamic, muscular components have been improved as much as possible, and the main factor still causing difficulties with defecation is related to persistent anatomical misalignments.

Rectal prolapse is the exception. There is a group of patients that might need surgery as the initial step: individuals who have difficulty with defecation caused by a lax rectum that results in rectal prolapse or rectal intussusception (so-called "internal prolapse"). For this group of patients, physiotherapy won't really resolve the prolapse, so surgery might be necessary first — and then they are likely to require some physiotherapy to help with pelvic floor strengthening afterwards.

Even with surgery, defecation is still a predominantly dynamic process. If there is significant muscle weakness or dyssynergia (poor coordination between the muscles involved), the improvement in defecation may not be that great. In some cases, where the predominant issue is coordination rather than a misdirection of force from rectal prolapse or pelvic floor descent due to weakness, physiotherapy may be more useful.

What is a ventral mesh rectopexy?

Ventral mesh rectopexy is the main surgical repair for obstructed defecation. It involves anchoring a mesh to the pelvic floor and the lower portion of the front surface of the rectum, and tethering the other end to the sacrum.

This acts sort of like the guy line of a tent, and holds the pelvic floor and muscles in position during defecation. It acts as an anchor but also as a tension line, and essentially pulls the pelvic floor up a little as well.

In addition, the broad surface area of the mesh placed against the rectum also acts to buttress and strengthen that portion of the rectum, and so helps to limit the rectocoele from stretching. As the mesh is essentially not elastic, its function is to act as a limiter of sorts — limiting the extent of pelvic descent and rectal prolapse, as well as preventing a rectocoele from bulging too much.

Eventually, the mesh gets covered in fibrous scar tissue, which further strengthens the repair.

How is a ventral mesh rectopexy performed?

Ventral mesh rectopexy is performed using minimally invasive techniques. The procedure involves placing a mesh over the front of the rectum, and securing one end of the mesh to the pelvic floor and lower rectum and the other end to the sacrum with sutures.

This is done through several small incisions on the abdomen, which allow long laparoscopic instruments to be used to free up the space in the abdominal cavity and place the mesh accordingly. Because the pelvis is a narrow space, we often do laparoscopic surgery with the aid of a surgical robot, as this gives us access to instruments that have a better range of movement in a limited space.

How long is the hospital stay after ventral mesh rectopexy?

Due to the smaller incisions, most patients only need to stay in hospital overnight for monitoring, and are often fit to be discharged the day after the surgery.

When should I see a doctor about straining?

Although constipation symptoms are common, if you find that you are having to strain a lot to open your bowels, this tends to indicate that there might be an issue with defecation mechanics.

So if this happens frequently, it might be worth seeing a doctor to assess if there is an issue that might benefit from physiotherapy or even surgery — and where a rectal prolapse is the cause, surgery may be the first step.