A low-fiber colon strains, the wall herniates into little pouches, one of them plugs and microperforates, and the pain lands in the left lower corner. Learn the sigmoid setup, the CT picture, the Hinchey stages, and the line between antibiotics, a drain, and the knife.
The Setup
Pouches Born From Pressure
Everything starts with a high-pressure colon pushing its lining out through weak spots. Walk that chain once and the disease, the location, and the complications all line up.
A 61-year-old with a lifelong low-fiber diet has two days of steady left lower abdominal pain, a temperature of 100.8 F, no appetite, and a small change in his bowel habits. His belly is tender in the left lower quadrant without a rigid, board-like wall.
What single mechanism best explains the whole picture?
Start with the wall. A diverticulumAn outpouching of the colon wall. In the colon these are false diverticula: only the mucosa and submucosa herniate, not the full muscle layer. is a pouch where the inner lining pushes out through a gap in the muscular wall. Those gaps are not random: they sit exactly where the vasa recta (the small arteries) pierce the wall to feed the mucosa. A low-fiber diet means small, hard stool → the colon squeezes harder to move it → intraluminal pressure climbs → the lining balloons out through those arterial gaps. Because only the mucosa and submucosa herniate, these are false diverticula.
Why the sigmoid, why the left. By the law of Laplace, wall tension rises as the tube narrows, and the sigmoid colon is the narrowest segment, so it sees the highest pressure and grows the most pouches. The sigmoid lives in the left lower quadrant, which is why diverticulitis hurts on the lower left and is nicknamed left-sided appendicitis.
Two words that get confused. Flip each card to keep them straight.
DiverticulosisTap to flip
The pouches, quietJust the presence of diverticula, usually silent and found incidentally on colonoscopy. Very common with age. Its one dangerous trick is painless lower GI bleeding when a pouch erodes the vessel at its neck. It does not mean infection.
DiverticulitisTap to flip
The pouch, inflamedA pouch obstructs (often by a hard stool pellet), pressure rises inside it, the wall thins and microperforates, and the surrounding tissue inflames. This brings pain, fever, and a high white count. Diverticulitis itself rarely bleeds.
The risk factorsTap to flip
What raises pressureA low-fiber, Western diet is the classic driver, plus older age, obesity, smoking, and physical inactivity. Right-sided diverticula are more common in East Asian populations and can mimic appendicitis. Nuts and seeds are no longer thought to cause attacks.
The pouch herniates exactly where the vasa recta pierce the muscle. That same vessel is why diverticulosis bleeds, and the thin pouch wall is why diverticulitis microperforates.
Clinical Images
CT: sigmoid diverticulitis, small abscess · tap
CT: perforation, free gas · tap
Endoscopy: diverticular openings · tap
Gross: pouches along the colon · tap
At the Bedside
The Story and the Complications
Uncomplicated diverticulitis has a quiet, steady story. The board points hide in the complications: abscess, fistula, obstruction, perforation, and the bleed that comes from the other twin.
The classic presentation. Steady left lower quadrant pain over a day or two, a low-grade fever, loss of appetite, and a mild rise in white cells. Bowel habits shift either way, constipation or loose stool, and some patients feel an urge to pass stool from rectal irritation. A localized tender spot is typical; a rigid, board-like belly means free perforation and a surgical emergency.
Know the complications cold. Tap each to reveal. These are where the questions live.
Abscess
Tap to reveal
The microperforation walls itself off into a pocket of pus. Suspect it with a swinging fever and a higher white count. A small one (under about 3 cm) often resolves on antibiotics; a larger one needs a drain placed through the skin.
Colovesical fistula
Tap to reveal
The most common diverticular fistula. The inflamed sigmoid tunnels into the bladder, giving air in the urine (pneumaturia), stool in the urine (fecaluria), and recurrent polymicrobial urinary infections. More common in men, because in women the uterus sits between colon and bladder.
Obstruction
Tap to reveal
Repeated attacks scar the sigmoid into a narrow stricture. This can mimic an obstructing colon cancer, which is exactly why a colonoscopy is done after recovery to tell the two apart.
Free perforation
Tap to reveal
The pouch ruptures into the open abdomen, spilling pus (purulent) or stool (feculent) and causing diffuse peritonitis with a rigid belly and free air on CT. This is Hinchey III or IV and goes to the operating room.
Diverticular bleed
Tap to reveal
This comes from diverticulosis, not diverticulitis. A pouch erodes the vasa recta at its neck and gives brisk, painless hematochezia. It is the most common cause of acute lower GI bleeding in adults and usually stops on its own.
From the Attending
Hold the two twins apart and half these questions answer themselves. Diverticulosis bleeds and does not hurt. Diverticulitis hurts and does not bleed. If a stem gives painless large-volume rectal bleeding, you are in diverticulosis. If it gives left lower quadrant pain with fever, you are in diverticulitis. Same pouches, opposite stories.
Confirming It
CT First, Scope Later, Hinchey to Stage
The diagnosis runs on contrast CT, the colonoscopy waits, and the Hinchey number tells you how aggressive to be.
Labs and the look-alikes. Expect a modest leukocytosis and a bumped CRP. In a reproductive-age woman, a pregnancy test still comes first to keep ectopic and ovarian causes on the table. The differential on the left includes ischemic colitis, a perforated colon cancer, ureteral stone, and inflammatory bowel disease.
The imaging algorithm. Three forks. Try each step before you reveal it.
A stable adult with left lower quadrant pain and a low fever. What is the best first imaging study?
Contrast CT is the test of choice: it shows wall thickening, inflamed pericolic fat, the diverticula, and any abscess or free air, and it stages severity. Suspected acute diverticulitis? CT abdomen and pelvis with contrast.
CT shows a contained 5 cm pericolic abscess. The patient is stable. Best next step?
A sizable abscess (roughly over 3 to 4 cm) is drained through the skin under imaging, with intravenous antibiotics, which usually avoids an emergency operation. A small abscess can resolve on antibiotics alone. Larger abscess: drain it percutaneously, do not cut yet.
A few weeks after a first episode resolves, the patient has never had a colon cancer screen. What now?
A colon cancer can hide behind, or even cause, what looks like diverticulitis. After the inflammation cools (about 6 to 8 weeks), a colonoscopy confirms there is no malignancy. Scope after recovery, never during the acute attack.
The Hinchey stages. This is how surgeons grade a perforated diverticulitis and decide who needs the operating room.
Hinchey stage
What it means
General approach
I
Small confined pericolic abscess or phlegmon
Antibiotics, drain if larger
II
Larger pelvic or distant walled-off abscess
Percutaneous drainage plus antibiotics
III
Purulent peritonitis (pus spread in the abdomen)
Surgery
IV
Feculent peritonitis (stool spilled in the abdomen)
Surgery, Hartmann procedure
Board Trap
Do not reach for a colonoscopy or a barium enema during an acute attack. Pushing air or contrast into an inflamed, microperforated sigmoid can blow a contained leak into a free perforation. CT first, always. Save the scope for weeks later to rule out cancer.
The Plan
Antibiotics, Drain, or Hartmann
Match the treatment to the severity. The whole game is sorting uncomplicated from complicated and knowing when to physically remove the source.
Uncomplicated diverticulitis. No abscess, no perforation, no obstruction. A reliable patient who can keep fluids down is treated as an outpatient with a clear liquid diet advanced as tolerated, and in many cases oral antibiotics covering gut bacteria (gram-negatives and anaerobes), such as a fluoroquinolone plus metronidazole or amoxicillin with clavulanate. Selected mild cases can even be watched without antibiotics, but for clinical practice the default for symptomatic uncomplicated disease is bowel rest plus antibiotics. Admit for intravenous antibiotics if the patient cannot tolerate oral intake, has significant comorbidity, or fails outpatient therapy.
Complicated diverticulitis. An abscess, perforation, obstruction, or fistula changes the plan. A larger abscess gets percutaneous drainage plus intravenous antibiotics. Free perforation with purulent or feculent peritonitis (Hinchey III or IV) or hemodynamic instability goes to the operating room for a Hartmann procedure: resect the diseased sigmoid, bring up an end colostomy, and close off the rectal stump, with reconnection planned for later.
Elective resection. Recurrent attacks, a stricture causing obstruction, or a fistula (such as colovesical) are reasons for a planned, elective sigmoid resection once everything is calm. Immunosuppressed patients are managed more aggressively because they wall off infection poorly.
From the Attending
The decision tree is short if you anchor on one question: is this complicated or not? Uncomplicated gets bowel rest and antibiotics. An abscess gets a drain. Spilled pus or stool in the belly gets a Hartmann. And every first-time diverticulitis earns a colonoscopy weeks later to be sure a cancer is not wearing the disease as a mask.
Board Trap
A man with recurrent urinary infections that grow several different gut bacteria, plus air bubbling out when he urinates, has a colovesical fistula from diverticulitis until proven otherwise. Confirm with CT (air in the bladder with no recent catheter), then plan an elective sigmoid resection with closure of the fistula. Do not keep treating it as simple cystitis.
Prove It
Board Walkthrough
Seven original clinical vignettes, 5 dealt per round, answer choices shuffled, never repeating within a round. Tap a wrong answer first to see why it almost works, then read the glowing clues.
Medically reviewed by Kaitlyn Cocuzzo, MD and Fatima Ali, DO · Last reviewed June 2026
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