What is the appropriate management and diagnosis for a 41-year-old male (y/o/m) with rectal bleeding following recent acute gastroenteritis?

Differential Diagnosis: 41-Year-Old Male with Post-Gastroenteritis Rectal Bleeding

In a 41-year-old male with rectal bleeding following recent acute gastroenteritis, the most likely diagnoses include post-infectious colitis with persistent mucosal ulceration, ischemic colitis triggered by dehydration/hypoperfusion during the acute illness, infectious colitis with ongoing inflammation (particularly if bacterial pathogens like Shigella, Campylobacter, or E. coli O157:H7 were involved), inflammatory bowel disease (IBD) unmasked by the infectious trigger, or less commonly, acute hemorrhagic rectal ulcer (AHRU) if the patient has risk factors.

Primary Differential Considerations

Post-Infectious Colitis with Mucosal Injury

• The temporal relationship between gastroenteritis resolution and bleeding onset suggests persistent mucosal damage from the initial infection 1
• Bacterial pathogens causing acute gastroenteritis can produce deep ulcerations that continue bleeding after diarrheal symptoms resolve 2
• This is particularly relevant if the initial gastroenteritis was severe or associated with bloody diarrhea

Ischemic Colitis

• Dehydration and hypoperfusion during acute gastroenteritis can precipitate ischemic injury to the colon, with bleeding manifesting days later as the mucosa sloughs 1
• The left colon (splenic flexure and descending colon) is most vulnerable due to watershed blood supply
• Consider this especially if the patient had significant volume depletion during the acute illness

Inflammatory Bowel Disease (Crohn's Disease or Ulcerative Colitis)

• Acute gastroenteritis can unmask previously undiagnosed IBD or trigger a flare in subclinical disease 2
• Crohn's disease with acute lower GI bleeding occurs in 85% of cases with colonic involvement, typically from deep ulcers in the left colon 2
• Hemorrhagic Crohn's disease can reveal the diagnosis in 23.5% of cases and occurs during disease flare-up in only 35% of cases, meaning it often presents during quiescent periods 2

Infectious Colitis (Ongoing or Secondary)

• Persistent infection with invasive organisms (Shigella, Campylobacter, Salmonella, E. coli O157:H7, C. difficile) can cause ongoing mucosal ulceration and bleeding 1
• Consider C. difficile if the patient received antibiotics during or after the gastroenteritis episode

Acute Hemorrhagic Rectal Ulcer (AHRU)

• While classically described in elderly bedridden patients with hypoalbuminemia, AHRU should be considered if risk factors are present 3, 4
• Risk factors include: prolonged bed rest during illness, coronary artery disease, hypoalbuminemia, renal failure, and antiplatelet/anticoagulant use 4
• AHRU presents as sudden massive painless rectal bleeding with characteristic endoscopic findings: solitary or multiple rectal ulcers (circumferential, round, or Dieulafoy-like) within 10 cm of the dentate line with normal surrounding mucosa 3, 4

Less Common Considerations

• Dieulafoy's lesion of the rectum: Rare congenital vascular malformation causing massive bleeding, diagnosed by rigid sigmoidoscopy 5
• Medication-induced injury: NSAIDs or aspirin use during the acute illness could cause colonic ulceration
• Hemorrhoids or anal fissures: Straining during diarrheal illness can cause anorectal pathology, though typically presents with bright red blood on tissue rather than mixed with stool

Diagnostic Approach

Initial Hemodynamic Assessment

• Calculate shock index (heart rate/systolic BP): If >1, the patient is hemodynamically unstable and requires urgent CT angiography 6
• Apply Oakland score: If ≤8 points with self-limited bleeding, consider outpatient investigation; if >8 or major bleed, admit for inpatient workup 6, 7

First-Line Investigation Based on Stability

For hemodynamically stable patients:

• Begin with anoscopy/digital rectal examination to exclude anorectal sources (hemorrhoids account for ~14% of lower GI bleeding) 1
• Proceed to colonoscopy with bowel preparation during hospital stay, as there is no evidence that urgent colonoscopy (<24 hours) improves outcomes compared to elective timing (36-60 hours) 6
• Colonoscopy will identify post-infectious ulceration, IBD, ischemic colitis, or AHRU 6

For hemodynamically unstable patients (shock index >1):

• CT angiography is first-line to rapidly localize bleeding without bowel preparation 6, 1
• CTA has 79-95% sensitivity and 95-100% specificity, can identify upper GI or small bowel sources, and guides intervention 6
• If CTA is negative, perform upper endoscopy immediately, as 10-15% of apparent lower GI bleeding originates from upper GI sources 6, 1

Additional Diagnostic Studies

• Stool studies: Culture for bacterial pathogens, C. difficile toxin, ova and parasites if infectious etiology suspected
• Laboratory markers: CBC, coagulation profile, albumin (hypoalbuminemia <3.5 g/dL seen in 92.6% of AHRU patients), inflammatory markers (CRP, ESR) 3
• Consider IBD serologies and fecal calprotectin if inflammatory bowel disease is suspected

Critical Pitfalls to Avoid

• Do not assume all rectal bleeding is from hemorrhoids without direct visualization, especially with a history of recent systemic illness 1
• Do not miss an upper GI source: 10-15% of severe hematochezia originates above the ligament of Treitz, particularly with hemodynamic instability 6, 1
• Do not delay imaging in unstable patients while attempting bowel preparation for colonoscopy 1
• Do not overlook AHRU in patients with risk factors (hypoalbuminemia, bed rest during illness, cardiovascular disease, anticoagulation) as it requires specific endoscopic hemostatic therapy 3, 4
• Consider IBD unmasking: Acute infections can trigger first presentation of Crohn's disease or ulcerative colitis, requiring long-term management beyond treating the acute bleed 2

Management Considerations

• Transfusion thresholds: Use restrictive strategy (Hb trigger 70 g/L, target 70-90 g/L) unless cardiovascular disease present (trigger 80 g/L, target 100 g/L) 6, 7
• Anticoagulation management: If on warfarin, interrupt at presentation and reverse with prothrombin complex concentrate and vitamin K if unstable 6; if on DOACs, withhold temporarily 6, 7
• Endoscopic therapy: Effective for AHRU (hemoclip placement, thermal coagulation), Crohn's ulcers, and post-infectious ulceration 3, 4, 2
• Surgery: Reserved only after exhausting endoscopic and radiological options, except in exceptional circumstances 6