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The internal hemorrhoidal plexus of veins is located in the submucosal space above the valves of Morgagni. The external hemorrhoidal plexus is separated from the internal hemorrhoidal plexus by the anal canal; but the two are connected under the anal canal by underlying tissue to form the interhemorrhoidal depression. Whenever the internal hemorrhoidal plexus is enlarged, there is associated increase in supporting tissue mass, and the resultant venous swelling is called an internal hemorrhoid. When veins in the external hemorrhoidal plexus become enlarged or thrombosed, the resultant bluish mass is called an external hemorrhoid.(1)
In the external versus internal classification, the differentiation of external versus internal is based upon the location of the dentate (pectinate) line.(2) This landmark is where stratified squamous epithelial tissues of the anus become columnar epithelial tissues of the rectal mucosa. Hemorrhoids occurring above the dentate line are internal; those below are external.(3) In addition to the classifications of internal versus external, there are also classifications of prolapsing and nonprolapsing.
Both types of hemorrhoids are very common and may be associated with straining at stool, pregnancy, or with cirrhosis. All of these circumstances cause increased hydrostatic pressure in the portal venous system. When internal hemorrhoids enlarge, they are generally not associated with pain. The most common complaint is bright red blood on the toilet tissue or coating the stool, and the individual may have a vague feeling of anal discomfort. It is not until an internal hemorrhoid is thrombosed, infected, or when there has been erosion of the overlying mucosal surface that pain is a feature.
Internal hemorrhoids may be classified additionally by degree of severity. First-degree internal hemorrhoids enlarge but do not prolapse.(4) Second-degree hemorrhoids bleed and prolapse with defecation, but return spontaneously.(5) Third-degree hemorrhoids prolapse with each bowel movement and sometimes with exertion related to work or simply when standing. They can be replaced with manual manipulation.(6) Fourth-degree hemorrhoids prolapse irreducibly with thrombosing, profuse bleeding, and great pain.(3) When prolapse is not treated, hemorrhoids will usually becomes chronic as the muscularis stays stretched.
External hemorrhoids, because they lie under the skin, are quite often painful, particularly if there is a sudden increase in their mass. These episodes result in a tender, blue swelling at the anal verge due to thrombosis of a vein in the external plexus and need not be associated with enlargement of internal veins.(1)
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