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What Is Cystic Fibrosis?

Cystic fibrosis (CF) is a chronic, progressive condition that primarily affects the body's respiratory and digestive systems. It is due to a gene defect that causes the body to produce abnormally thick mucus.

Approximately 30,000 children and young adults in the U.S. have CF. More than 10 million Americans carry an abnormal copy of the CF gene but aren't sick with the disease. You must have two abnormal copies of the gene, one from your mother and one from your father, to have cystic fibrosis.

In the past, children born with CF died early in childhood. Today, thanks to improved medical treatments, people with CF often live into their 30s and possibly beyond.

People who have CF produce abnormally thick, sticky mucus, which clogs the lungs and leads to recurring lung and sinus infections as well as difficulty breathing. Reduced oxygen in the blood also leads to a characteristic rounding and enlargement of the nail bed in the fingers and toes, called clubbing. Those with the disease may also develop a barrel-shaped chest as a result of their increased work to breathe. These repeated infections often lead to fleshy growths inside the nose, called nasal polyps.

The thick mucus also obstructs the ducts of the pancreas, preventing digestive enzymes from reaching the intestines. So those with CF do not absorb nutrients from their food well, and they eliminate nondigested food through the bowel, resulting in very large stools. Because so little food is absorbed, those with CF have difficulty gaining and maintaining weight, despite a healthy appetite and diet.

CF also affects the reproductive systems of both males and females. Although females with CF have normal fallopian tubes and ovaries, their thick cervical secretions may block sperm entry and prevent them from getting pregnant. Males with CF are almost always sterile because they produce relatively few or no sperm. (Abnormally thick secretions may block the ducts that carry sperm, or the ducts may not develop properly.)

Another hallmark of CF is an unusually high concentration of sodium and chloride (salt) in the sweat. The defective CF gene causes faulty chloride movement in CF cells. As a result, parents often report that their babies "taste very salty" when they kiss them.

What Causes It?

CF occurs in children who have inherited two abnormal copies of a specific gene, one from each parent. A parent who does not have CF but has one normal copy of this gene and one abnormal copy is called a CF carrier. The normal copy dominates, so the parent shows no sign of CF. In some cases, carriers pass on their normal copy of the CF gene to their child. In other instances, they pass on their abnormal copy. A child who receives a normal copy from one parent and an abnormal copy from the other will be a CF carrier and won't have the disease. A child who receives an abnormal gene from both parents will develop CF.

When both parents carry one abnormal copy of the gene, each child they produce has a 25% chance of inheriting two abnormal copies and having the disease, a 25% chance of inheriting two normal copies, and a 50% chance of inheriting one abnormal copy and one normal copy - causing them to be a carrier.

If only one parent has an abnormal gene and the other parent has two normal genes, there is a 50% chance that each child will be a carrier, but it's impossible for any of these children to have the disease.

If one parent has CF and the other parent is a carrier, each child has a 50% chance of inheriting the disease and a 50% chance of being a carrier. If one parent has CF and the other has two normal copies of the gene, the child will be a carrier. Because of the reproductive problems associated with CF, it is highly unlikely a child would be born to two parents with the disease. However, all children born to such a couple would have a 100% chance of inheriting CF.

Males and females are equally likely to inherit CF.

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