Cystic Fibrosis

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Chapter 17 Cystic Fibrosis

PATHOPHYSIOLOGY

Cystic fibrosis (CF), inherited as an autosomal-recessive trait, is caused by a mutation of the CF gene on chromosome 7. This results in absence or decreased function of a protein called cystic fibrosis transmembrane regulator (CFTR), which leads to abnormal sodium and chloride transport across the epithelium. The disorder affects the exocrine glands and causes the production of viscous mucus, which leads to obstruction of the small passageways of the bronchi, the small intestine, and the pancreatic and bile ducts. The effects of this biochemical defect on the involved organs are as follows:

a. Bronchial and bronchiolar obstruction from excessive pooling of secretions causes generalized hyperinflation and atelectasis.

b. Pooled mucous secretions increase the susceptibility to bacterial infections. (Pseudomonas aeruginosa and Staphylococcus aureus are the predominant organisms found in sputum and lungs. Burkholderia cepacia is associated with later-life infection in the CF-affected lung, causing devastating infection. B. cepacia is highly transmissible from the infected person with CF to others with CF, and those with known infection should not have contact with an uninfected individual with CF.)

c. Altered oxygen and carbon dioxide exchange can cause varying degrees of hypoxia, hypercapnia, and acidosis.

d. Fibrotic lung changes take place and, in severe cases, pulmonary hypertension and cor pulmonale can occur.

a. Degeneration and fibrosis of acini occur.

b. Secretion of pancreatic enzymes is inhibited, which causes impaired absorption of fats, proteins and, to a limited degree, carbohydrates.

3. Small intestine—absence of pancreatic enzymes (trypsin, amylase, lipase) causes impaired absorption of fats and proteins, which results in steatorrhea and azotorrhea.

4. Liver—biliary obstruction, fibrosis

a. Growth and onset of puberty are retarded.

b. Retardation of skeletal maturation results in delayed bone aging and shortness of stature (38% to 42% of children with CF).

6. Reproductive system

a. Female—late menses; possible infertility because of thickness of cervical mucus

b. Male—vas deferens often absent; sterility but not impotence

INCIDENCE

1. CF affects 1 in every 3700 infants born in the United States, with the birth prevalence varying based on race and ethnicity:

a. 1 in 2500 to 3500 births of non-Hispanic white infants each year

b. 1 in 4000 to 10,000 births of Hispanic infants

c. 1 in 15,000 to 20,000 non-Hispanic blacks

2. CF rarely affects infants of Asian descent (1 in 31,000).

3. Odds are 1 in 4 (25%) that each subsequent pregnancy after the birth of a child with CF will result in a child with CF.

4. CF affects males and females equally.

5. Newborn screening for CF is available and, when positive, could lead to an earlier age at diagnosis compared with when diagnosis is symptom-based. This would lead to earlier nutritional management and result in improved growth and development. At present, newborn screening for CF has improved outcomes for the child with CF but has not been universally adopted.

6. Symptoms vary greatly, resulting in variable life span—95% survival to age 16 years, with a median predicted age of survival of 35.1 years. About 42% of patients with cystic fibrosis are 18 years of age or older. Infants born in the 1990s or later have a predicted median survival age of 40 years.

7. Length and quality of life have greatly increased in recent years, but the disease is ultimately terminal.

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