Cystic Fibrosis (CF)

Key Features

Essentials of Diagnosis

• Greasy, bulky, malodorous stools; failure to thrive

• Recurrent respiratory infections

• Digital clubbing

• Bronchiectasis on chest imaging

• Sweat chloride > 60 mmol/L

General Considerations

• An autosomal recessive disease that results in a syndrome of chronic sinopulmonary infections, malabsorption, and nutritional abnormalities

• One of the most common lethal genetic diseases in the United States, with an incidence of approximately 1:3000 among whites and 1:9200 in the US Hispanic population

• Caused by a defect in a single gene on chromosome 7 that encodes an epithelial chloride channel called the CF transmembrane conductance regulator (CFTR) protein

  • Most common mutation is F508del, although approximately 1800 other disease-causing mutations been identified

  • Gene mutations lead to absence or defects in CFTR, altering salt and water movement across cell membranes, resulting in abnormally thick secretions in various organ systems and critically altering host defense in the lung

Clinical Findings

• Failure to thrive due to malabsorption from exocrine pancreatic insufficiency

• Respiratory symptoms can include

  • Productive cough

  • Wheezing

  • Recurrent pneumonia

  • Progressive obstructive airways disease

  • Exercise intolerance

  • Dyspnea

  • Hemoptysis

• Meconium ileus

  • Defined as a severe intestinal obstruction resulting from inspissation of tenacious meconium in the terminal ileum

  • Present in approximately 15% of newborns with CF

  • Virtually diagnostic of CF, so the infant should be treated presumptively until a sweat test or genotyping can be obtained

• CF should also be considered in children with

  • Severe dehydration

  • Hypochloremic alkalosis

  • Unexplained bronchiectasis, pancreatitis, or cirrhosis

  • Rectal prolapse

  • Nasal polyps

  • Chronic sinusitis

• Infants with undiagnosed CF may have

  • Hypoproteinemia

  • Anemia

  • Deficiency of the fat-soluble vitamins A, D, E, and K because of ongoing steatorrhea

Diagnosis

Laboratory Findings

• Immunoreactive trypsin (IRT)

  • Elevated in most infants, although false-negative results are possible

  • In newborns with positive newborn screen, the diagnosis must be confirmed by sweat testing, mutation analysis, or both

• Sweat chloride concentration > 60 mmol/L in the presence of one or more typical clinical features confirms diagnosis

• Genotyping that reveals two disease-causing mutations can also confirm diagnosis

• Intermediate sweat chloride values of 30–60 mmol/L may be associated with mild CFTR mutations that result in residual functioning CFTR protein

  • Patients typically have adequate pancreatic exocrine function

  • However, severe lung disease may develop

• Elevated IRT on newborn screen but sweat test < 60 mmol/L and up to two CFTR mutations, at least one of which is not considered disease-causing, appears to have an even milder disease phenotype

Imaging

• Bronchiectasis on chest imaging

Treatment

Medical

• Gastrointestinal treatment

  • Pancreatic enzyme supplementation combined with a high-calorie, high-protein, and high-fat diet

  • Caloric supplements are often added to the diet to optimize growth

  • Daily salt supplementation also is required to prevent hyponatremia, especially during hot weather

• Respiratory treatments

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