Hemorrhoid pathophysiology
Hemorrhoid Pathophysiology
Hemorrhoidal venous cushions are a normal part of the human anorectum and arise from subepithelial connective tissue within the anal canal.
Present in utero, these cushions surround and support distal anastomoses between the superior rectal arteries and the superior, middle, and inferior rectal veins. They also contain a subepithelial smooth muscle layer, contributing to the bulk of the cushions. Normal hemorrhoidal tissue accounts for approximately 15-20% of resting anal pressure and provides important sensory information, enabling the differentiation between solid, liquid, and gas.
Most people contain 3 of these cushions. Although classically described as lying in the right posterior (most common), right anterior, and left lateral positions, this combination is found in only 19% of patients. Hemorrhoids can be found at any position within the rectum.
Hemorrhoids are classified by their anatomic origin within the anal canal and by their position relative to the dentate line.
- Internal hemorrhoids develop above the dentate line from embryonic endoderm. They are covered by the simple columnar epithelium of anal mucosa and lack somatic sensory innervation and are therefore painless.
- External hemorrhoids develop from ectoderm and arise distal to the dentate line. They are covered by stratified squamous epithelium and receive somatic sensory innervation from the inferior rectal nerve rendering them painful when irritated.
- Mixed hemorrhoids are confluent internal and external hemorrhoids.
Venous drainage of hemorrhoidal tissue mirrors embryologic origin:
- Internal hemorrhoids drain through the superior rectal vein into the portal system.
- External hemorrhoids drain through the inferior rectal vein into the inferior vena cava.
- Rich anastomoses exist between these 2 and the middle rectal vein, connecting the portal and systemic circulations.
Most symptoms arise from enlarged internal hemorrhoids. Abnormal swelling of the anal cushions causes dilatation and engorgement of the arteriovenous plexuses. This leads to stretching of the suspensory muscles and eventual prolapse of rectal tissue through the anal canal. The engorged anal mucosa is easily traumatized, leading to rectal bleeding that is typically bright red due to high blood oxygen content within the arteriovenous anastomoses. Prolapse leads to soiling and mucus discharge (triggering pruritus) and predisposes to incarceration and strangulation.
Most clinicians use the grading system proposed by Banov et al in 1985, which classifies internal hemorrhoids by their degree of prolapse into the anal canal. This system both correlates with symptoms and guides therapeutic approaches.
- Grade I hemorrhoids project into the anal canal and often bleed but do not prolapse.
- Grade II hemorrhoids may protrude beyond the anal verge with straining or defecating but reduce spontaneously when straining ceases.
- Grade III hemorrhoids protrude spontaneously or with straining and require manual reduction.
- Grade IV hemorrhoids chronically prolapse and cannot be reduced. They usually contain both internal and external components and may present with acute thrombosis or strangulation.
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