A 72-year-old man with chronic kidney disease (eGFR 28 mL/min) and hypertension on aspirin 75 mg daily and amlodipine presents with sudden-onset lower abdominal pain, urgency to defecate, and passage of bright red blood per rectum mixed with stool. On examination, he is haemodynamically stable (BP 138/82 mmHg, HR 88/min). Digital rectal examination reveals no masses or haemorrhoids. Laboratory tests show Hb 10.2 g/dL (baseline 12 g/dL), normal coagulation profile, and normal platelets. Flexible sigmoidoscopy up to 60 cm shows normal mucosa with no source of bleeding. What is the most likely diagnosis?

Explanation

## Lower GI Bleeding: Ischaemic Colitis vs Other Causes ### Clinical Case Analysis **Key Point:** The combination of **sudden-onset lower abdominal pain**, urgency to defecate, bright red rectal bleeding, **advanced age**, **hypertension**, **CKD (vascular risk factors)**, and a **normal sigmoidoscopy up to 60 cm** is the classic presentation of **ischaemic colitis affecting the left colon** — specifically the watershed areas (splenic flexure and sigmoid colon). ### Why Ischaemic Colitis? 1. **Pain + bleeding triad:** Ischaemic colitis classically presents with sudden-onset crampy lower abdominal pain followed by urgency and passage of bright red blood mixed with stool. This triad is the hallmark (Harrison's Principles of Internal Medicine, 21e, Ch. 295). 2. **Vascular risk factors:** Age >70, hypertension, and CKD are all independent risk factors for mesenteric vascular insufficiency and ischaemic colitis. Aspirin use does not protect against ischaemic colitis. 3. **Normal sigmoidoscopy:** Ischaemic colitis most commonly affects the **splenic flexure** and **descending colon** — watershed zones of the inferior mesenteric artery (IMA). A sigmoidoscopy reaching only 60 cm may miss the splenic flexure (typically at 40–50 cm but variable), and mucosal changes may be patchy or proximal to the scope's reach. Importantly, the normal mucosa seen does NOT exclude ischaemic colitis at the splenic flexure or proximal descending colon. 4. **Haemodynamic stability:** Non-occlusive ischaemic colitis (the most common form) is typically self-limiting and haemodynamically stable, unlike acute mesenteric ischaemia. 5. **Anaemia:** A drop from Hb 12 to 10.2 g/dL is consistent with acute blood loss from ischaemic mucosal sloughing. ### Differential Diagnosis of Lower GI Bleeding | Feature | Ischaemic Colitis | Angiodysplasia | Diverticular Bleeding | Infectious Colitis | |---------|---|---|---|---| | **Pain** | YES — acute, crampy | Typically painless | Typically painless | Yes — diarrhoea-associated | | **Location** | Left colon, splenic flexure | Right colon (cecum) | Sigmoid/descending | Pancolonic | | **Sigmoidoscopy** | May be normal if proximal | Normal (right-sided) | Diverticula visible | Inflammation/pseudomembranes | | **Vascular risk** | Strong association | CKD association | No | No | | **Onset** | Sudden | Intermittent | Sudden, brisk | Gradual | | **Haemodynamics** | Usually stable (non-occlusive) | Stable | May be unstable | Stable | ### Why Not Angiodysplasia (Option D)? - Angiodysplasia is characteristically **painless**. This patient has prominent **lower abdominal pain and urgency**, which is atypical. - While CKD is associated with angiodysplasia, the acute painful presentation with urgency strongly favours ischaemia over angiodysplasia. - Angiodysplasia is a diagnosis of exclusion in obscure GI bleeding, not the primary diagnosis when a painful acute presentation exists. ### Why Not Diverticular Bleeding (Option A)? - Diverticular bleeding is typically **painless** and often brisk, causing haemodynamic instability. - Sigmoid diverticula would likely be visible on sigmoidoscopy; the normal sigmoidoscopy makes this less likely. ### Why Not Infectious Colitis — C. difficile (Option B)? - C. difficile colitis requires prior antibiotic exposure or hospitalisation (not mentioned). - Pseudomembranes would be visible on sigmoidoscopy. - The acute vascular presentation with pain in a high-risk patient is not consistent with infectious aetiology. **High-Yield:** Ischaemic colitis is the most common form of intestinal ischaemia and classically presents with sudden lower abdominal pain + rectal bleeding in elderly patients with vascular risk factors. It predominantly affects watershed zones (splenic flexure = Griffith's point; rectosigmoid junction = Sudeck's point) [Harrison 21e, Ch. 295; Sleisenger & Fordtran's GI Disease, 11e]. **Clinical Pearl:** A normal sigmoidoscopy does NOT exclude ischaemic colitis — the splenic flexure (the most common site) may be beyond or at the limit of a 60 cm sigmoidoscope. Colonoscopy or CT colonography is required for definitive diagnosis. ### Next Steps in Management 1. **CT abdomen with contrast** — to assess bowel wall thickening, "thumbprinting," and exclude perforation 2. **Colonoscopy** — to visualise the full left colon and confirm ischaemic changes (oedema, haemorrhagic mucosa, ulceration) 3. **Supportive care** — IV fluids, bowel rest, treat underlying vascular risk factors 4. **Avoid vasoconstrictors** — amlodipine is acceptable; avoid NSAIDs 5. **Monitor for complications** — perforation, stricture formation