Symptoms of thrombosed external haemorrhoids
External haemorrhoids originate below the dentate line and are covered by squamous epithelium, which is pain sensitive.
An external haemorrhoid may become thrombosed, sometimes as a result of straining during defecation. A thrombosed external haemorrhoid (perianal haematoma) appears as an extremely painful tense blue swelling at the anal margin. Although sometimes called a perianal haematoma, the clot is contained within an endothelium-lined blood vessel, so is not a true haematoma.