external hemorrhoids
Summary
External hemorrhoids are dilated vascular cushions arising from the inferior rectal venous plexus, located below the dentate line and covered by squamous epithelium. They are innervated by somatic nerves, making them painful, especially when thrombosed. Common risk factors include straining during defecation, constipation, pregnancy, and prolonged sitting.
Detail
Hemorrhoids are normal vascular structures that become symptomatic when engorged, prolapsed, or thrombosed. External hemorrhoids originate distal to the dentate line (anatomic/pectinate line) from the inferior hemorrhoidal venous plexus, which drains into the internal pudendal vein and ultimately the internal iliac vein (systemic circulation). Because this region is covered by anoderm (modified squamous epithelium) and innervated by the inferior rectal nerve (branch of pudendal nerve, S2-S4), external hemorrhoids are somatically innervated and thus painful—unlike internal hemorrhoids, which arise above the dentate line from the superior hemorrhoidal plexus (portal circulation via superior rectal vein), are covered by columnar epithelium, and are visceral (painless) due to autonomic innervation.
Pathophysiology: Increased intra-abdominal pressure (straining, constipation, pregnancy, obesity, chronic cough, low-fiber diet) leads to venous engorgement and downward displacement of the anal cushions, causing dilation and potential thrombosis of the external hemorrhoidal plexus.
Clinical presentation: Perianal lump, pain (especially with thrombosis, presenting as a tender bluish nodule at the anal verge), pruritus, bleeding (usually less than internal hemorrhoids), and skin tags after resolution of thrombosed hemorrhoids.
Complications: Acute thrombosis leads to sudden severe pain and a palpable, tender, purplish perianal mass. This is managed with excision (if within 72 hours and severe pain) or conservative management (sitz baths, analgesics, stool softeners) if presenting later.
Diagnosis: Primarily clinical, via inspection and digital rectal exam. Anoscopy can help evaluate internal hemorrhoids.
Management: Conservative (increased fiber and fluid intake, stool softeners, sitz baths, topical analgesics) for uncomplicated cases. Surgical excision for thrombosed external hemorrhoids presenting acutely or for large symptomatic hemorrhoids.
Key USMLE associations: Portal hypertension does NOT cause hemorrhoids (common misconception)—anorectal varices are different from hemorrhoids and are associated with portal hypertension, but the two are pathophysiologically distinct entities that are often conflated in questions. Differentiate hemorrhoids (venous cushion dilation, common, related to straining) from anorectal varices (portosystemic shunt due to portal HTN, seen in cirrhosis).
Sources
- First Aid for the USMLE Step 1
- UpToDate: Overview of hemorrhoids
- Sabiston Textbook of Surgery
- Robbins and Cotran Pathologic Basis of Disease
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