Pectinate Line & Anorectal Disorders
One line decides pain, drainage, nodes, and tumor type
The pectinate line: an embryologic seam
Where two origins meet. The pectinate (dentate) line marks the junction between endoderm of the hindgut and ectoderm of the proctodeum, the anal pit. It runs along the anal valves at the bases of the anal columns; behind each valve is a small crypt into which anal glands open. Because the two halves of the canal come from different sources, nearly every feature switches at this line.
| Feature | Above | Below |
|---|---|---|
| Origin | Hindgut endoderm | Proctodeum ectoderm |
| Epithelium | Simple columnar | Stratified squamous (keratinized at the anal verge) |
| Nerves | Visceral afferents: stretch only, painless | Inferior rectal nerve (pudendal, S2–S4): painful |
| Artery | Superior rectal (from IMA) | Inferior rectal (from internal pudendal, internal iliac) |
| Vein | Superior rectal → IMV → splenic → portal | Inferior rectal → internal pudendal → internal iliac → IVC |
| Lymph | Internal iliac nodes | Superficial inguinal nodes |
| Cancer | Adenocarcinoma | Squamous cell carcinoma |
A portosystemic junction. The superior rectal vein belongs to the portal system, while the middle and inferior rectal veins drain to the IVC. In portal hypertension, blood diverts through their anastomoses and dilates them into anorectal varices, which can bleed.
Anorectal varices are not hemorrhoids. Hemorrhoids are normal vascular cushions that enlarge with straining; portal hypertension does not cause them.
Hemorrhoids
Internal hemorrhoids. These are enlarged vascular cushions of the superior (internal) hemorrhoidal plexus above the line, covered by viscerally innervated mucosa. They cause painless bright red blood coating the stool or on the paper, and prolapse. Grading: I, bleeding without prolapse; II, prolapse that reduces spontaneously; III, prolapse that needs manual reduction; IV, irreducible. Straining, constipation, a low-fiber diet, and pregnancy promote them. Fiber and fluids come first; office rubber band ligation treats most grade I–III disease, and hemorrhoidectomy is reserved for large or refractory grade III–IV disease.
External hemorrhoids. These arise from the inferior hemorrhoidal plexus below the line, under somatically innervated anoderm. Acute thrombosis produces a tense, exquisitely painful bluish perianal lump; old thromboses leave skin tags. When seen within about 72 hours of onset, a thrombosed external hemorrhoid may be excised; later, pain is already easing and care is conservative.
Anal fissure
Typical fissure. A linear tear in the anoderm below the dentate line, about 90% in the posterior midline, where blood supply is poorest; anterior midline fissures come next, mainly after childbirth. Passage of hard stool starts the tear, and a hypertonic internal anal sphincter reduces perfusion and prevents healing. Patients report tearing pain during and after defecation and bright red blood on the paper.
Chronic fissure and treatment. A fissure lasting more than about 6–8 weeks shows a sentinel skin tag, a hypertrophied anal papilla, and exposed internal sphincter fibers. Treatment aims to relax the sphincter: fiber and sitz baths, topical nitroglycerin (a nitric oxide donor) or calcium channel blockers (diltiazem, nifedipine), botulinum toxin, and, if these fail, lateral internal sphincterotomy.
A lateral, multiple, painless, or nonhealing fissure is not idiopathic until proven otherwise: evaluate for Crohn disease, HIV, tuberculosis, syphilis, and anal cancer, with biopsy.
Perianal abscess and fistula-in-ano
Cryptoglandular origin. Most perianal abscesses begin when an anal gland draining into a crypt at the dentate line becomes obstructed and infected. Infection starts in the intersphincteric plane and can spread to the perianal (most common), ischioanal, intersphincteric, or supralevator spaces, causing throbbing pain, swelling, and fever. Treatment is incision and drainage; antibiotics are added for cellulitis, systemic illness, diabetes, or immunosuppression.
Fistula-in-ano. When the drained cavity leaves an epithelialized tract between the crypt and perianal skin, the result is a fistula with chronic drainage. Simple fistulas are laid open (fistulotomy); fistulas crossing much of the sphincter are managed with a seton to protect continence. Crohn disease causes complex, multiple, and recurrent fistulas, and perianal disease may precede intestinal symptoms.
Goodsall rule: an external opening posterior to a transverse line through the anus follows a curved tract to the posterior midline; an anterior opening usually runs straight to the nearest crypt.
Anal squamous cell carcinoma
HPV-driven. Most anal cancers are squamous cell carcinomas caused by high-risk HPV, chiefly HPV-16, whose E6 protein degrades p53 and E7 inactivates Rb. They progress through anal intraepithelial neoplasia (high-grade squamous intraepithelial lesion). Risk factors are HIV infection, receptive anal intercourse, other immunosuppression such as organ transplantation, prior cervical or vulvar HPV neoplasia, and smoking. Low-risk HPV-6 and -11 cause benign anogenital warts instead.
Presentation and spread. Bleeding, a mass, pain, or itching; tumors below the line spread to superficial inguinal nodes. First-line treatment is chemoradiation with 5-FU and mitomycin (Nigro regimen), which preserves the sphincter; abdominoperineal resection is reserved for persistent or recurrent disease. HPV vaccination prevents it, and high-risk groups such as people with HIV may be screened with anal cytology and high-resolution anoscopy.