The increased pressure forces the anterior rectal lining against the contracted puborectalis and frequently the lining prolapses into the anal canal during straining and then returns to its normal position afterwards. The repeated trapping of the lining can cause the tissue to become swollen and congested. Ulceration is thought to be caused by resulting poor blood supply (ischemia), combined with repeated frictional trauma from the prolapsing lining, and exposure to increased pressure are thought to cause ulceration. Trauma from hard stools may also contribute. The site of the ulcer is typically on the anterior wall of the rectal ampulla, about 7–10 cm from the anus. However, the area may of ulceration may be closer to the anus, deeper inside, or on the lateral or posterior rectal walls. The name "solitary" can be misleading since there may be more than one ulcer present. Furthermore, there is a "preulcerative phase" where there is no ulcer at all.Pathological specimens of sections of rectal wall taken from SRUS patients show thickening and replacement of muscle with fibrous tissue and excess collagen. Rarely, SRUS can present as polyps in the rectum.SRUS is therefore associated and with internal, and more rarely, external rectal prolapse. Some believe that SRUS represents a spectrum of different diseases with different causes.Another condition associated with internal intussusception is colitis cystica profunda (also known as CCP, or proctitis cystica profunda), which is cystica profunda in the rectum.
Mucosal prolapse (partial rectal mucosal prolapse) refers to prolapse of the loosening of the submucosal attachments to the muscularis propria of the distal rectummucosal layer of the rectal wall. Most sources define mucosal prolapse as an external, segmental prolapse which is easily confused with prolapsed (3rd or 4th degree) hemorrhoids (piles). However, both internal mucosal prolapse (see below) and circumferential mucosal prolapse are described by some. Others do not consider mucosal prolapse a true form of rectal prolapse.Internal mucosal prolapse (rectal internal mucosal prolapse, RIMP) refers to prolapse of the mucosal layer of the rectal wall which does not protrude externally. There is some controversy surrounding this condition as to its relationship with hemorrhoidal disease, or whether it is a separate entity. The term "mucosal hemorrhoidal prolapse" is also used.Solitary rectal ulcer syndrome (SRUS, solitary rectal ulcer, SRU) occurs with internal rectal intussusception and is part of the spectrum of rectal prolapse conditions. It describes ulceration of the rectal lining caused by repeated frictional damage as the internal intussusception is forced into the anal canal during straining. SRUS can be considered a consequence of internal intussusception, which can be demonstrated in 94% of cases. Mucosal prolapse syndrome (MPS) is recognized by some. It includes solitary rectal ulcer syndrome, rectal prolapse, proctitis cystica profunda, and inflammatory polyps. It is classified as a chronic benign inflammatory disorder.