The passage of bright red blood is most often seen with cancers of the rectum or sigmoid colon. Melena may result from right-sided colon tumors or obstructing tumors that retard the passage of fecal contents. Unexplained iron deficiency anemia may be the first sign in otherwise asymptomatic patients, especially in tumors located in the proximal colon. Any persistent change in bowel habits should be considered suspicious and deserves further evaluation. Such a change may be newly developed diarrhea, constipation, rectal pressure, or a change in stool caliber. These symptoms may mimic those of other bowel disorders such as diverticular disease, irritable bowel disease, or inflammatory bowel disease. More advanced colorectal cancer may produce unexplained weight loss. When compared with aproximal colon cancers, left sided tumors typically cause obstructive symptoms earlier in the disease course because stool in the distal colon is more solid and, therefore, less likely to pass easily through a narrowed lumen. Conversely, right sided tumors can grow larger and into advanced disease and remain virtually asymptomatic.
Physical examination is usually unrevealing in early colorectal cancer. In advanced disease, a palpable abdominal mass, signs of bowel obstruction or perforation, hepatomegaly, or ascites may be present.
The natural history of colorectal cancer often involves a prolonged period of growth whereby many patients remain asymptomatic until advanced disease is present. Colorectal cancer presents a major health risk to the population, and routine screening of asymptomatic patients with the hope of early detection is recommended. The primary care physician should develop a colorectal screening strategy for adult patients as part of an annual physical examination. The screen examination and a flexible sigmoidoscopy. Stool guaiac for occult blood may be useful as a means of early detection. Stool guaiac testing is not without problems. A negative test does not assure the absence of large bowel cancer and, therefore, should be used for screening purposes in high risk patients.
Routine screening of individuals at average risk should begin at age 50. Although there are published screening guidelines, colonoscpoies are usually performed in clinical practice in place of sigmoidoscopy because sigmoidoscopy may miss up to 50% of potential lesions. If not performed initially, any positive result should be followed by a colonoscopy. The frequency of screening may be determined based on the results of the baseline screening exam and physician recommendations. Individuals with high risk factors should begin screening at an earlier age. Children of a parent with a history of colorectal cancer should begin screening at an age equal to 10 years prior to the age of the parent at diagnosis. Colonoscopy or barium enema may be justified at age 40 or less in first degree relatives. Parents with signs or symptoms of bowel cancer should be referred for a definitive examination and evaluation of their entire large bowel.