Blind loop syndrome occurs when part of the small intestine is bypassed and cut off from the normal flow of food and digestive juices.

The bypassed portion of intestine, known as a blind loop, initiates a cascade of problems. Because food can't pass through the loop, it ferments, encouraging bacterial growth. The bacteria then interfere with absorption of essential nutrients, often leading to diarrhea, weight loss and malnutrition.

Blind loop syndrome — sometimes called stasis syndrome or stagnant loop syndrome — often occurs as a complication of abdominal surgery. But blind loop syndrome can also result from structural defects and some diseases. Although blind loop syndrome occasionally requires surgery, most people respond well to antibiotics.

Because blind loop syndrome affects digestion and absorption, signs and symptoms of the disorder often include:

• Loss of appetite
• Abdominal pain
• Nausea
• Fatty stools (steatorrhea) — frothy, foul-smelling stools indicating poor fat absorption
• Bloating
• An uncomfortable feeling of fullness after eating
• Diarrhea
• Unintentional weight loss

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Digestive system

Although digestion begins in your mouth, the real work of breaking down and absorbing nutrients takes place in your small intestine, the longest section of your digestive tract. Connecting your stomach and large intestine, your small intestine is where food mixes with digestive juices from your pancreas, liver and gallbladder, and where small nutrient molecules — amino acids from proteins, monosaccharides from carbohydrates and most fats — are absorbed into your bloodstream.

Unlike your large intestine, which has millions of bacteria to help break down indigestible fiber, your small intestine normally contains relatively few microorganisms. Because your small intestine is rich in enzymes, it doesn't need the enzymatic action of bacteria; in fact, stomach acid and secretions from your liver and pancreas actually act as antibacterial agents. What's more, the strong muscular contractions (peristalsis) that propel food through your small intestine prevent bacteria from colonizing there.

But in blind loop syndrome, food can't move through the bypassed section of bowel, and the stagnant food becomes an ideal breeding ground for microorganisms — a condition called bacterial overgrowth syndrome. The bacteria may produce toxins as well as interfere with the absorption of nutrients.

What triggers blind loop syndrome
A blind loop can be an inadvertent consequence of gastric surgery, such as Billroth II or Roux-en-Y procedures for ulcers and gastric bypass surgery for obesity. Operations on the small intestine and structural abnormalities sometimes can cause blind loops as well. And a number of medical conditions can lead to bacterial overgrowth, including Crohn's disease and scleroderma and diabetes, which can slow the rate at which food moves through the intestine.

You're at risk of blind loop syndrome if you've had gastric surgery for obesity or ulcers. Other factors that make you more susceptible to the disorder include:

• A structural defect in or an injury to your small intestine
• Crohn's disease, intestinal lymphoma, scleroderma or diabetes
• Diverticulosis of the small intestine, a condition in which small pouches of tissue protrude through the intestinal wall

Bloating, nausea and diarrhea are signs and symptoms of many intestinal problems, ranging from irritable bowel syndrome to stomach flu (gastroenteritis), and for the vast majority of people, they aren't an indication of blind loop syndrome. But see your doctor for a full evaluation if you have:

• Persistent diarrhea
• Rapid, unintentional weight loss

The first step in diagnosing blind loop syndrome is usually an abdominal X-ray or an abdominal computerized tomography (CT) scan, an X-ray technique that produces more detailed images of the body than conventional X-rays do.

You may also have additional tests to check for bacterial overgrowth in your small intestine, for poor fat absorption or for other problems that may be causing or contributing to your symptoms:

• Barium X-ray of the small intestine. This test uses a contrast dye (barium) to coat the lining of your intestine so that it stands out clearly on X-rays. A barium X-ray may reveal a blind loop, diverticulosis, a narrowing (stricture) of the intestine or other anatomical problems as well as slow transit times that can cause bacterial overgrowth.
• Hydrogen breath test. Several breath tests are used to check for bacterial overgrowth. Because fermenting carbohydrates release hydrogen, this test measures the amount of hydrogen in your lungs after you drink a mixture of glucose and water. A rapid rise in hydrogen indicates poor carbohydrate digestion and bacterial overgrowth in your small intestine. Although widely available, the test is less sensitive than other breath tests are.
• D-xylose breath test. In this test, which is more accurate than is a hydrogen breath test, you ingest a type of sugar called xylose. If unusual numbers of bacteria are present in your small intestine, they metabolize the xylose, releasing carbon dioxide in the process. Breath carbon dioxide is measured every half-hour for two hours.
• Bile acid breath test. In your small intestine, bile acids from your liver help emulsify and digest fats (lipids). But proliferating bacteria interfere with this process, leading to poor absorption of both fats and carbohydrates. This test uses a bile salt (glycocholate) bound to a small amount of radioactive material to check for bile salt dysfunction in your small intestine.
• Quantitative fecal fat test. This test may be used to determine how well fats are absorbed. Most often, you eat a high-fat diet for three days. The amount of fat in your stool is then measured. Large amounts of undigested fat indicate malabsorption, one cause of which is bacterial overgrowth.
• Small intestine aspirate and fluid culture. This is the most sensitive test for bacterial overgrowth, but it's invasive and technically difficult to perform. To obtain the fluid sample, doctors pass an endoscope down your throat and through your digestive tract to your small intestine. A sample of intestinal fluid is withdrawn with the endoscope and then placed in a culture medium in a laboratory where it's observed for the growth of bacteria.

A blind loop can trigger an escalating series of problems, including:

• Poor absorption of fats. Because bacteria in your small intestine break down (deconjugate) the bile salts needed to emulsify and digest fats, the fat in food as well as the fat-soluble vitamins A, D, E and K aren't well absorbed. This leads to diarrhea and often to steatorrhea — fatty, foul-smelling stools — as well as to weight loss and vitamin deficiency disorders. A lack of vitamin A can cause night blindness, for example, and low levels of vitamin D affect your body's ability to absorb calcium, which can lead to weakened bones.
• Damage to the intestinal lining. Bacterial overgrowth harms the mucous lining (mucosa) of the small intestine both directly and indirectly. Toxic byproducts that are released when bacteria break down stagnant food damage the mucosa, as do bacterial enzymes themselves. This damage means that most nutrients, including carbohydrates and proteins, are poorly absorbed, leading to serious nutritional deficiencies.
• Vitamin B-12 deficiency. Vitamin B-12, which is essential for the normal functioning of your nervous system and the production of blood cells and DNA, is absorbed in your small intestine. But proliferating bacteria actually use up the vitamin, reducing the amount that's available to your body. A severe deficiency can lead to weakness, fatigue, tingling and numbness in your hands and feet, and, in advanced cases, to mental confusion. Damage to your central nervous system resulting from a B-12 deficiency may be irreversible.
• Brittle bones (osteoporosis). Both calcium and vitamin D, which aids in calcium absorption, are metabolized in your small intestine. Damage to your intestine from abnormal bacterial growth causes poor calcium absorption and eventually may lead to bone diseases such as osteoporosis.

Whenever possible, doctors treat blind loop syndrome by dealing with the underlying problem — surgically repairing a postoperative blind loop or stricture, for example. But, the blind loop can't always be reversed. In that case, treatment focuses on correcting nutritional deficiencies and eliminating bacterial overgrowth.

For most people, the best way to treat bacterial overgrowth is with antibiotic therapy, and doctors may start this treatment even when test results are inconclusive. A short course of antibiotics often significantly reduces the number of abnormal bacteria, but because bacteria can return when the antibiotic is discontinued, the therapy may need to be long term.

Doctors also may switch among different drugs to help prevent bacterial resistance. Ironically, because antibiotics wipe out most intestinal bacteria — both normal and abnormal — they can cause some of the very problems they're trying to cure, including diarrhea and an imbalance of bacteria in the digestive tract.

Nutritional support
Addressing nutritional deficiencies is a crucial part of treating blind loop syndrome, particularly in people with severe weight loss. But although malnutrition can be treated, the damage it causes can't always be reversed.

The following measures may improve vitamin deficiencies, reduce intestinal distress and help with weight gain:

• Nutritional supplements. People with blind loop syndrome need intramuscular injections of vitamin B-12 as well as oral vitamin and iron supplements.
• Lactose-free diet. Damage to the intestine may cause some people to lose the ability to digest milk sugar (lactose). In that case, it's important to avoid most lactose-containing products including milk and cheese, or use lactase preparations such as Lactaid that aid in digestion of milk sugar. Some people may tolerate yogurt because the bacteria used in the culturing process naturally breaks down lactose.
• Medium-chain triglycerides. Trigylcerides are a type of fat consisting of a molecule of glycerol to which three hydrocarbon chains are attached. The chains vary in length, and the way your body processes triglycerides depends on the length of the chains. Most dietary fats are long-chain triglycerides. Food sources include many vegetable oils and animal fats, all of which are emulsified and absorbed in the small intestine. On the other hand, medium-chain triglycerides, found in coconut oil, are absorbed without the aid of digestive enzymes. Because they're more readily digested by people with blind loop syndrome, medium-chain triglycerides are sometimes prescribed as a dietary supplement.