Diverticulosis and diverticulitis
While most patients are asymptomatic, diverticulosis can cause bleeding in 5 to 15% of patients and is most likely from the (right/left) colon in Western countries.
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 62-year-old man comes to the primary care clinic because of pain in his lower left abdominal quadrant, painless hematochezia, and constipation for the past few weeks. Medical history shows that he has suffered a myocardial infarction two years ago and has been taking aspirin daily since. His doctor orders a colonoscopy and notices small sacs bulging out around the large intestine. Which of the following is the most likely explanation for this patient’s physical findings?
Usually we talk about these caves or pouches in the context of the large intestine, so it’d be a colonic diverticula, but it can also happen in the small intestine as well as other hollow structures in the body.
The walls of the large intestine are made up of several layers, starting with the inner layer, the mucosa, then the submucosa, then the muscle layer, and finally serosa.
Sometimes these little out-pouches include all the layers, from mucosa to serosa, and these are true diverticula, and sometimes only the mucosa and submucosa poke through the muscle layer, and these would be called pseudo or false diverticula, where the muscle layer isn’t included and the mucosa and submucosa are covered only by serosa.
It’s thought that the formation of colonic diverticula is a result of high pressures that literally push the walls such that they bubble out and form these pouches.
Now remember that the large intestine has this smooth muscle layer, right? And it’s able to contract using that muscle layer, just like any other muscle we contract, except that we can’t consciously control these because it’s smooth muscle rather than skeletal muscle.
This smooth muscle contraction accomplishes a couple things, like mixing ingested food and moving digested food toward the end of the line.
Whenever it contracts, though, higher pressures are generated inside the lumen, since it’s sort of like being squeezed and compressing the air inside, and normally you’d imagine that the higher pressure would be equally felt throughout the lumen, right?
Well, it’s thought that the contractions in patients with diverticula are exaggerated or abnormal in some way, which causes an unequal distribution of pressure with some specific areas having really high pressures during contractions.
It’s not quite clear exactly what leads to these abnormal contractions, but some studies suggest it may have to do with the smooth muscle itself.
Now let’s take a step back and think about something called Laplace’s law which says that the pressure on the wall of a cylinder is proportional to the inverse of its diameter, so as diameter decreases, pressure increases.
In other words, a really small cylinder is going to feel a lot more pressure on its walls than a really large cylinder.
Similarly, since the sigmoid colon is the smallest portion of the colon in diameter, it’s subject to relatively high pressures according the Laplace’s law, basically the spot in the large intestine where the walls get pushed on the hardest.
The sigmoid colon is therefore where most diverticula form.
Additionally, diverticula tend to form in the spots where the blood vessels that supply the intestinal walls traverses the muscle layer, this spot is slightly weaker, making it more likely for diverticula to form.