Pathophysiology
Summary
The rectum and anus are contiguous structures demarcated by the dentate (pectinate) line. Superior to this line, the mucosa is columnar and derived from endoderm, with innervation primarily via autonomic fibers from the inferior hypogastric plexus, which detect stretch only. Inferior to the dentate line, the mucosa is squamous & arises from ectoderm, with somatic innervation through the inferior rectal nerve, making pathologies in this region painful.
Venous drainage also differs above and below the dentate line. Superiorly, venous blood flows into the portal system via the superior rectal vein, which drains into the inferior mesenteric artery. Inferiorly, venous blood drains into the systemic circulation via the inferior rectal vein, ultimately joining the internal pudendal vein and internal iliac vein. The upper anus and lower rectum serve as a portocaval junction, making them susceptible to conditions like portal hypertension, which can contribute to conditions like hemorrhoids.
Internal hemorrhoids are painless and occur above the dentate line, while external hemorrhoids are painful occur below the dentate line. Risk factors for hemorrhoids include older age, chronic constipation, & pregnancy.
Rectal prolapse is associated with a weak pelvic floor and commonly occurs in older women. It is associated with conditions that increase intrabdominal pressure, such as whooping cough, chronic constipation, and cystic fibrosis. Anorectal fissures often occur posteriorly near the midline, and are initiated by trauma to the anal sphincter. Anorectal fistulas are epithelialized tunnels connecting the anus or rectum to the surface of the skin, and are associated with IBDs like Crohn’s disease and pelvic radiation.
Neoplasms such as rectal adenocarcinoma, which arises above the dentate line, and squamous cells carcinoma can occur in this region. Squamous cell carcinoma (SCC) is characterized by keratinization, and is the most prevalent type of anal cancer. Basaloid carcinoma is a nonkeratinizing variant of SCC that often occurs near the dentate line. Risk factors for SCC include high-risk HPV infection (16, 18), HIV infection, immunocompromised states, smoking, and receptive anal intercourse.
Lesson Outline
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FAQs
The dentate line serves as a critical anatomical landmark where the rectum and anus converge. Superior to the dentate line, the mucosa is columnar and derived from the endoderm. Autonomic fibers from the inferior hypogastric plexus innervate the rectum above the dentate line, and detect stretch only. Venous blood in this region drains into the portal circulation. Below the dentate line, the mucosa is squamous and originates from ectoderm, with innervation from the inferior rectal branch of the pudendal nerve. Pathologies in this area can cause pain due to somatic nerve involvement, and venous blood drains into the systemic circulation.
Hemorrhoids are engorged vascular structures in the anal canal's submucosa, often resulting from increased intra-abdominal pressure that leads to venous pooling. They are commonly associated with aging, chronic constipation, and pregnancy. Internal hemorrhoids occur above the dentate line and are generally painless but may cause bleeding. In contrast, external hemorrhoids are found below the dentate line, and can result in pain if thrombosis occurs.
Rectal prolapse occurs when the rectum protrudes through a weakened pelvic floor and exits through the anus. This condition is commonly associated with chronic constipation, increased intra-abdominal pressure due to conditions like whooping cough, and cystic fibrosis. Elderly women with a history of multiple pregnancies and a weakened pelvic floor are also at higher risk.
An anal fissure is a tear in the lower rectal lining that often causes pain during bowel movements. It is commonly initiated by trauma to the anal sphincter due to hard stool, severe diarrhea, or vaginal delivery. Anorectal fistulas are epithelialized tunnels connecting the anus or rectum to the skin surface and are frequently associated with inflammatory conditions like Crohn's disease, as well as pelvic radiation.
The most common form of anal cancer is squamous cell carcinoma, which often displays keratinization. Risk factors include high-risk HPV infection (16, 18), HIV infection, immunocompromised states, smoking, and receptive anal intercourse. Other types include basaloid (non-keratinized) carcinoma, which arises near the dentate line, and rectal adenocarcinoma, which occurs above the dentate line and behaves similarly to colon adenocarcinoma. Anorectal cancers frequently present with bleeding.
