Section 9. Digestive Disorders

Chapter 129. Bowel Movement Disorders

Topics: Introduction | Constipation | Diarrhea | Irritable Bowel Syndrome | Fecal Incontinence | Flatulence

Diarrhea

Diarrhea is an increase in the volume, wateriness, or frequency of bowel movements.

The frequency of bowel movements alone is not the defining feature of diarrhea. Some people normally move their bowels 3 to 5 times a day. People who eat large amounts of vegetable fiber may produce more than a pound of stool a day, but the stool in such cases is well formed and not watery. Diarrhea occurs when not enough water is removed from the stool, making the stool loose and poorly formed. Diarrhea is often associated with gas, cramping, an urgency to move the bowels (defecate), nausea, and vomiting.

Causes

Normally, stool is 60 to 90% water; diarrhea mainly results when the percentage exceeds 90%. Stool may contain too much water if it travels too quickly through the digestive tract, if certain components of the stool prevent the large intestine from absorbing water, or if water is being secreted by the large intestine into the stool. Many different causes, including drugs; infection with viruses, bacteria, or parasites; components of food; stress; chemicals; tumors; chronic disorders such as irritable bowel syndrome and inflammatory bowel disease; and stress can lead to diarrhea. Malabsorption syndromes, in which food cannot be digested normally, can lead to diarrhea as well.

Rapid Transit of Stool: Rapid transit of stool is one of the most common causes of diarrhea. For stool to have normal consistency, it must remain in the large intestine for a certain amount of time. Stool that leaves the large intestine too quickly is watery. Many medical conditions and treatments can decrease the amount of time that stool stays in the large intestine, including an overactive thyroid (hyperthyroidism) or Zollinger-Ellison syndrome; surgical removal of part of the stomach, small intestine, or large intestine; treatment of ulcers in which the vagus nerve is cut; surgical bypass of part of the intestine; and drugs such as antacids containing magnesium, laxatives, prostaglandins, serotonin, and even caffeine. Many foods, especially those that are acidic, can increase transit. Some people are intolerant of specific foods and always develop diarrhea after eating them. Stress and anxiety are also common causes.

Retention of Water in the Stool: Osmotic diarrhea occurs when certain substances that cannot be absorbed into the bloodstream remain in the intestine. These substances cause excessive amounts of water to remain in the stool, leading to diarrhea. Certain foods (such as some fruits and beans) and hexitols, sorbitol, and mannitol (used as sugar substitutes in dietetic foods, candy, and chewing gum) can cause osmotic diarrhea. Also, lactase deficiency can lead to osmotic diarrhea. (Lactase is an enzyme normally found in the small intestine that converts lactose [milk sugar] to glucose and galactose, so that it can be absorbed into the bloodstream.) When people with lactase deficiency drink milk or eat dairy products, lactose is not digested. As lactose accumulates in the intestine, it causes osmotic diarrhea. The severity of osmotic diarrhea depends on how much of the osmotic substance is consumed. Diarrhea stops soon after the person stops eating or drinking the substance.

Blood in the digestive tract also acts as an osmotic agent and results in black, tarry stools (melena). Another cause of osmotic diarrhea is an overgrowth of normal intestinal bacteria or the growth of bacteria normally not found in the intestines. Infection by certain parasites (for example, amebas) can also cause osmotic diarrhea. Antibiotics can cause osmotic diarrhea by destroying the normal intestinal bacteria.

Secretion of Water Into the Stool: Secretory diarrhea occurs when the small and large intestines secrete salts (especially sodium chloride) and water into the stool. Certain toxins--such as the toxin produced during a cholera infection or during some viral infections--can cause these secretions. Infections by certain bacteria (for example, Campylobacter) and parasites (for example, Cryptosporidium) can also stimulate secretions. The diarrhea can be massive--more than a quart of stool an hour in cholera. Other substances that cause salt and water secretion include certain laxatives, such as castor oil, and bile acids (which may build up after surgery to remove part of the small intestine). Certain rare tumors--such as carcinoid, gastrinoma, and vipoma--also can cause secretory diarrhea, as can some polyps.

Exudative diarrhea occurs when the lining of the large intestine becomes inflamed, ulcerated, or engorged, and it releases proteins, blood, mucus, and other fluids, which increase the bulk and fluid content of the stool. This type of diarrhea can be caused by many diseases, including ulcerative colitis, Crohn's disease (regional enteritis), tuberculosis, and cancers such as lymphoma and adenocarcinoma. When the lining of the rectum is affected, the person often feels an urgent need to move his bowels and has frequent bowel movements because the inflamed rectum is more sensitive to expansion (distention) by stool.

See the table Foods and Drugs That Can Cause Diarrhea.

Symptoms and Complications

Diarrhea is characterized by frequent loose stools. The consistency of the stool can be anything from soft and pasty to completely watery. The color can range from brown to clear. Black stools may indicate bleeding in the digestive tract, although some drugs used to treat diarrhea (those containing bismuth subsalicylate) turn the stools black as well. When a black color is caused by blood (melena), the stools are usually tarry and foul-smelling.

Cramping may occur before and with a bowel movement, and sometimes large amounts of gas are passed with the stool. Some people experience nausea, especially if the diarrhea is caused by an infectious organism or a toxic substance.

Diarrhea can lead to dehydration and a loss of electrically charged particles (electrolytes), such as sodium, potassium, magnesium, and chloride, from the blood. If large amounts of fluid and electrolytes are lost, blood pressure can drop enough to cause fainting (syncope), heart rhythm abnormalities (arrhythmias), and other serious disorders. At particular risk are the very young, older people, the debilitated, and people with very severe diarrhea. Bicarbonate may be lost in the stool as well, leading to metabolic acidosis (see Section 12, Chapter 159).

Diagnosis

A doctor first tries to establish whether the diarrhea appeared suddenly and has been present for a short time (acute) or whether it is persistent (chronic). If acute diarrhea persists for more than 72 hours (or sooner if blood is present), it should be evaluated by a doctor. A doctor tries to determine whether changes in diet may be the cause; whether the person has other symptoms, such as a fever, pain, and rash; and whether the person has been exposed to people with a similar condition. When diarrhea is not severe and has lasted for less than a week, the symptoms and physical examination alone are usually enough to determine the cause and necessary treatment. If needed, stool samples can be examined. This examination determines if the stool is formed or watery, if it has an unusual odor, and if it contains fat, blood, or undigested materials. The volume of stool over a 24-hour period is also determined.

When diarrhea persists, often a sample of the stool must be examined microscopically for cells, mucus, fat, and other substances. The stool also can be tested for blood and substances that might cause osmotic diarrhea. Samples can be tested for infectious organisms that stimulate secretion, including certain bacteria (for example, Campylobacter and Yersinia) and parasites (for example, amebas, Giardia, and Cryptosporidium). If the person is surreptitiously taking a laxative, it also can be identified in the stool sample. A sigmoidoscopy may be performed, so that a doctor can examine the lining of the anus and rectum. Sometimes a biopsy (removal of a tissue specimen for examination under a microscope) of the rectal lining is performed.

Treatment

Diarrhea is a symptom, and its treatment depends on the cause. Most people with diarrhea only have to remove the cause and suppress the diarrhea until the body heals itself. For example, sometimes chronic diarrhea is cured when a person stops drinking coffee or cola drinks containing caffeine. A viral cause generally resolves by itself in 24 to 48 hours.

Many prescription and over-the-counter drugs are available for the treatment of diarrhea. Over-the-counter drugs include adsorbents (for example, kaolin-pectin), which adhere to chemicals, toxins, and infectious organisms. Some adsorbents can also help firm up the stool. Bismuth helps many people with diarrhea. It has a normal side effect of turning the stool black. Other drugs used are loperamide, codeine, and diphenoxylate.

Prescription drugs used to treat diarrhea include opioids and other drugs that relax the muscles of the intestines. Bulking agents used for chronic constipation, such as psyllium or methylcellulose, can sometimes help relieve chronic diarrhea as well.

When severe diarrhea causes dehydration, hospitalization and treatment with intravenous water and salts may be necessary. As long as the person is not vomiting and does not feel nauseated, drinking liquids containing a balance of water, sugars, and salts can be very effective.

See the drug table Drugs Used to Treat Diarrhea.