CYSTIC FIBROSIS

Karen Vosper

Overview

Cystic fibrosis (CF) is a complex and multisystem disease, characterized by thickened tenacious secretions in the respiratory tract, sweat glands, gastrointestinal tract, pancreas, and other exocrine tissue. It is the most common life-shortening autosomal recessive disorder of the exocrine glands. Typical respiratory manifestations of CF include persistent, productive cough, difficulty clearing secretions, and frequent respiratory infections. Typical gastrointestinal manifestations include large bulky malodorous stools; impaired absorption of fat, protein, and carbohydrates resulting in poor weight gain and growth and malnutrition; as well as excess losses of sodium and chloride in sweat (National Heart, Lung, and Blood Institute, 2016).

Background

In the United States, CF occurs in approximately 1:3,000 Whites, predominately of European descent; 1:9,200 Hispanics; 1:10,900 Native Americans; 1:15,000 African Americans; and 1:100,000 Asian Americans (Lahiri et al., 2016). Gender is not a factor in the disease incidence. Median predicted survival for CF patients in the United States in 2015 was 41.6 years (95% confidence interval: 38.5–44.0 years), and the median age at death was 29.1 years. Overall, 5% of deaths occur in individuals younger than 13 years (CFF Registry Report, 2015). Although CF is a multisystem disease, lung involvement is the major cause of morbidity and more than 90% of mortality.

The gene that causes CF was discovered in 1989 and is located on the long arm of chromosome 7, known as the CF transmembrane regulator (CFTR) gene (Stern, 2006). People with CF either have too few CFTR proteins at the cell surface, CFTR proteins that do not work properly, or both. The defective CFTR proteins result in the poor flow of salt and water in and out of the cells. As a result, abnormally thick and sticky mucus forms and obstructs the epithelial tissues throughout the body, such as the lungs, sinuses, pancreas, intestine, reproductive system, and sweat glands. Thick and sticky airway secretions, a combination of mucus and pus, build up in the lungs causing chronic lung infections and progressive lung damage. In the gastrointestinal tract, there is a reduced ability to absorb nutrients, and digestive enzymes from the pancreas that are critical to the breakdown and absorption of fats, calories, and nutrients do not reach the small intestine, resulting in malabsorption, steatorrhea, and malnutrition (Davis, 2006).

The sweat test is considered the gold standard for diagnosing CF. It can be done on an individual of any age. It has three technical parts: localized sweat stimulation induced by iontophoresis of pilocarpine, collection, and analysis. The preferred site for sweat collection is the flexor surface of the forearm. The chance of urticaria or burn to the skin after iontophoresis is possible. For all 51patients, sweat chloride values greater than or equal to 60 mmol/L is considered positive for CF. A positive result on the sweat chloride test indicates that CF is nearly certain (Collie, Massie, Jones, LeGrys, & Greaves, 2014).

Currently, all 50 states and the District of Columbia screen newborns for CF, but the method for screening may differ from state to state. All screening algorithms in current use in the United States rely on testing for immunoreactive trypsinogen (IRT) as the primary screen for CF. The presence of high levels of IRT, typically elevated in CF-affected infants, indicates the need for a second tier of testing, which determines the positive or negative outcome of the screen. The second tier testing relies on IRT again or DNA testing. All babies with a positive newborn screen (NBS) will be required to have a confirmatory sweat test. As a characteristic of all NBSs, most infants with a positive CF NBS result will not have CF (Farrell et al., 2008).

Clinical Aspects

ASSESSMENT

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