haustra

**Definition/anatomy:** Haustra (singular: haustrum) are outpouchings of the large bowel wall separated by semilunar folds (plicae semilunares). They form because the three longitudinal smooth muscle bands (taeniae coli: mesocolic, omental, free) are shorter than the colon, which gathers the wall into sacculations. The taeniae are formed from the outer longitudinal muscle layer, which is incomplete in the colon (it is concentrated into the three bands). Other colonic features that distinguish it from small bowel: appendices epiploicae (fat-filled peritoneal pouches) and a larger caliber. The rectum has no haustra or taeniae, because the taeniae merge into a continuous longitudinal layer.

**Function:** Haustral contractions ("haustral shuttling") are slow, segmenting movements that mix stool and expose it to the mucosa. This maximizes absorption of water and electrolytes (Na⁺ in, K⁺ out) and short-chain fatty acids. Mass movements (high-amplitude propagated contractions), often triggered by the gastrocolic reflex, move stool toward the rectum.

**Imaging:** - Normal: haustra are visible on plain film and barium enema. They are incomplete folds that do not span the full colon width. This distinguishes them from the valvulae conniventes of the small bowel (complete, circumferential folds). - **Loss of haustra ("lead pipe" or "stovepipe" colon):** chronic ulcerative colitis. Causes are repeated inflammation, mucosal and muscular atrophy or fibrosis, and a shortened, rigid colon. Also seen in long-standing Crohn colitis, and rarely in cathartic colon (chronic laxative abuse). - **Toxic megacolon:** colonic dilation (>6 cm) with loss of haustral markings plus systemic toxicity. Causes are UC, Crohn, and C. difficile colitis. Risk of perforation. - **Thumbprinting:** thickened, edematous haustral folds seen in ischemic colitis, infectious colitis (including C. difficile), and others. - **Sigmoid volvulus ("coffee bean"):** the loop lacks haustra. Cecal volvulus is also described.

**Related high-yield associations:** - **Colonic diverticula** (false or pseudodiverticula) form where the vasa recta penetrate the muscular wall, typically between the taeniae, from high intraluminal pressure and low-fiber diets. They are most common in the sigmoid colon. - **Hirschsprung disease:** the aganglionic segment is narrow, with a dilated proximal colon. - **Pseudo-obstruction (Ogilvie syndrome):** colonic dilation with preserved haustra and no mechanical obstruction.

**Pearls:** Haustra = colon (with taeniae coli and appendices epiploicae). Valvulae conniventes = small bowel (jejunum most prominent). "Lead pipe" colon = chronic UC.