Ulcerative colitis (UC) is a long-term condition that results in inflammation and ulcers of the colon and rectum. The primary symptoms of active disease are abdominal pain and diarrhea mixed with blood. Weight loss, fever, and anemia may also occur. Often, symptoms come on slowly and can range from mild to severe. Symptoms typically occur intermittently with periods of no symptoms between flares. Complications may include abnormal dilation of the colon (megacolon), inflammation of the eye, joints, or liver, and colon cancer.

The cause of UC is unknown. Theories involve immune system dysfunction, genetics, changes in the normal gut bacteria, and environmental factors. Rates tend to be higher in the developed world with some proposing this to be the result of less exposure to intestinal infections, or to a Western diet and lifestyle. The removal of the appendix at an early age may be protective. Diagnosis is typically by colonoscopy with tissue biopsies. It is a kind of inflammatory bowel disease (IBD) along with Crohn's disease and microscopic colitis.

Dietary changes, such as maintaining a high-calorie diet or lactose-free diet, may improve symptoms. Several medications are used to treat symptoms and bring about and maintain remission, including aminosalicylates such as mesalazine or sulfasalazine, steroids, immunosuppressants such as azathioprine, and biologic therapy. Removal of the colon by surgery may be necessary if the disease is severe, does not respond to treatment, or if complications such as colon cancer develop. Removal of the colon and rectum generally cures the condition.

Together with Crohn's disease, about 11.2 million people were affected as of 2015. Each year it newly occurs in 1 to 20 per 100,000 people, and 5 to 500 per 100,000 individuals are affected. The disease is more common in North America and Europe than other regions. Often it begins in people aged 15 to 30 years, or among those over 60. Males and females appear to be affected in equal proportions. It has also become more common since the 1950s. Together, ulcerative colitis and Crohn's disease affect about a million people in the United States. With appropriate treatment the risk of death appears the same as that of the general population. The first description of ulcerative colitis occurred around the 1850s.

Gastrointestinal
People with ulcerative colitis usually present with diarrhea mixed with blood, of gradual onset that persists for an extended period of time (weeks). Additional symptoms may include fecal incontinence, increased frequency of bowel movements, mucus discharge, and nocturnal defecations. With proctitis (inflammation of the rectum), people with UC may experience urgency or rectal tenesmus, which is the urgent desire to evacuate the bowels but with the passage of little stool. Tenesmus may be misinterpreted as constipation, due to the urge to defecate despite small volume of stool passage. Bloody diarrhea and abdominal pain may be more prominent features in severe disease. The severity of abdominal pain with UC varies from mild discomfort to very painful bowel movements and abdominal cramping. High frequency of bowel movements, weight loss, nausea, fatigue, and fever are also common during disease flares. Chronic bleeding from the GI tract, chronic inflammation, and iron deficiency often leads to anemia, which can affect quality of life.

The clinical presentation of ulcerative colitis depends on the extent of the disease process. Up to 15% of individuals may have severe disease upon initial onset of symptoms. A substantial proportion (up to 45%) of people with a history of UC without any ongoing symptoms (clinical remission) have objective evidence of ongoing inflammation. Ulcerative colitis is associated with a generalized inflammatory process that can affect many parts of the body. Sometimes, these associated extra-intestinal symptoms are the initial signs of the disease.

Extent of involvement

Classification of colitis, often used in defining the extent of involvement of ulcerative colitis, with proctitis (blue), proctosigmoiditis (yellow), left sided colitis (orange) and pancolitis (red). All classes extend distally to the end of the rectum.
In contrast to Crohn's disease, which can affect areas of the gastrointestinal tract outside of the colon, ulcerative colitis is usually confined to the colon. Inflammation in ulcerative colitis is usually continuous, typically involving the rectum, with involvement extending proximally (to sigmoid colon, ascending colon, etc). In contrast, inflammation with Crohn's disease is often patchy, with so-called "skip lesions."

The disease is classified by the extent of involvement, depending on how far the disease extends: proctitis (rectal inflammation), left sided colitis (inflammation extending to descending colon), and extensive colitis (inflammation proximal to the descending colon). Proctosigmoiditis describes inflammation of the rectum and sigmoid colon. Pancolitis describes involvement of the entire colon, extending from the rectum to the cecum. While usually associated with Crohn's disease, ileitis (inflammation of the ileum) also occurs in UC. About 17% of individuals with UC have ileitis. Ileitis more commonly occurs in the setting of pancolitis (occurring in 20% of cases of pancolitis), and tends to correlate with the activity of colitis. This so-called "backwash ileitis" can occur in 10–20% of people with pancolitis and is believed to be of little clinical significance.

Severity of disease
In addition to the extent of involvement, UC is also characterized by severity of disease. Severity of disease is defined by symptoms, objective markers of inflammation (endoscopic findings, blood tests), disease course, and the impact of the disease on day-to-day life. Mild disease correlates with fewer than four stools daily; in addition, mild urgency and rectal bleeding may occur intermittently. Mild disease lacks systemic signs of toxicity, and exhibits normal levels of serum inflammatory markers (erythrocyte sedimentation rate and C-reactive protein).

Moderate to severe disease correlates with more than six stools daily, frequent bloody stools and urgency.[19] Moderate abdominal pain, low-grade fever, 38 to 39 °C (100 to 102 °F), and anemia may develop (75% of normal). Toxicity is present, as demonstrated by fever, tachycardia, anemia or an elevated ESR or CRP.

Fulminant disease correlates with more than 10 bowel movements daily, continuous bleeding, toxicity, abdominal tenderness and distension, blood transfusion requirement, and colonic dilation (expansion). People with fulminant UC may have inflammation extending beyond just the mucosal layer, causing impaired colonic motility and leading to toxic megacolon. If the serous membrane is involved, a colonic perforation may ensue.

Ulcerative colitis may improve and enter remission. Remission of disease is characterized by formed stools, the absence of bloody diarrhea, resolution of urgency, and normal levels of serum inflammatory markers.

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