What are the signs that blood in stool is caused by an anal fissure?

How to Identify Blood from an Anal Fissure

Blood from an anal fissure typically appears as bright red blood that is scanty in amount, often visible on toilet paper or as streaks on the stool surface, and is accompanied by significant pain during and after defecation. 1

Characteristic Features of Bleeding from Anal Fissures

• Anal fissures cause bright red bleeding that is usually scanty in amount, not mixed within the stool 1
• The bleeding is typically accompanied by severe pain during and after bowel movements, with pain sometimes lasting for hours after defecation 1
• The pain is often described as sharp, tearing, or burning in nature and is the predominant symptom due to the rich somatic innervation of the anal canal 2
• Blood is most commonly noticed on toilet paper after wiping or as streaks on the stool surface 3

Distinguishing Features from Other Causes of Rectal Bleeding

• Unlike hemorrhoidal bleeding, which can occur without pain, anal fissure bleeding is almost always accompanied by significant pain 1, 4
• Blood from anal fissures appears as bright red streaks on the stool surface or toilet paper, whereas hemorrhoidal bleeding may be more copious and can squirt into the toilet bowl 1
• Up to 20% of patients with hemorrhoids have concomitant anal fissures, making careful examination essential 5
• Rectal bleeding that is dark, mixed within the stool, or associated with guaiac-positive stools or anemia should prompt evaluation for other causes beyond anal fissures 1

Physical Examination Findings

• Anal fissures are best visualized by gently effacing the anal canal with opposing traction on the buttocks 1
• Most anal fissures (90%) occur in the posterior midline of the anal canal 1
• Chronic fissures may have associated findings including:
  • A sentinel skin tag just distal to the fissure 1
  • A hypertrophied anal papilla at the proximal margin 1
  • Visible fibrosis or exposed internal sphincter muscle at the base 1
• Fissures located outside the midline (anterior or lateral) should raise suspicion for underlying conditions such as Crohn's disease, HIV/AIDS, ulcerative colitis, or malignancy 1, 3

Diagnostic Approach

• The diagnosis of anal fissure should be suspected based on the characteristic history of pain with defecation and bright red bleeding 1
• Physical examination with gentle traction on the buttocks is usually sufficient to confirm the diagnosis 1
• Anoscopy may be helpful but should be avoided if there is marked pain or tenderness 1
• If significant anal pain prevents adequate examination, examination under anesthesia may be warranted 1
• All patients reporting rectal bleeding should undergo sigmoidoscopy to rule out other causes 1
• Further evaluation of the proximal colon (colonoscopy or barium enema) is indicated if:
  • The bleeding is not typical of hemorrhoids or fissures 1
  • There are guaiac-positive stools 1
  • The patient has anemia 1
  • The patient has risk factors for colorectal cancer 1

Common Pitfalls

• Attributing all rectal bleeding to hemorrhoids without adequate examination for anal fissures 5
• Failing to recognize that multiple anorectal conditions can present simultaneously 5
• Not considering other causes of anal pain and bleeding such as anorectal abscess, inflammatory bowel disease, or malignancy 1
• Attempting instrumentation of the anal canal in the setting of marked pain, which can be traumatic and rarely yields diagnostic information 1
• Neglecting to evaluate off-midline fissures for underlying systemic conditions 1, 3