Cystic Fibrosis and Bronchiectasis
Core concepts
Cystic fibrosis is autosomal recessive, caused by CFTR dysfunction, and affects multiple organs. Sweat chloride of 60 mmol/L or greater is diagnostic on two occasions; 30–59 mmol/L is intermediate; below 30 mmol/L is normal in most patients. Newborn screening typically uses immunoreactive trypsinogen followed by DNA testing. Pulmonary disease involves dehydrated, thick mucus, impaired mucociliary clearance, chronic infection with Staphylococcus aureus and Pseudomonas aeruginosa, and bronchiectasis. Airway clearance, hypertonic saline, dornase alfa, azithromycin, and CFTR modulators are key therapies. Pancreatic insufficiency, meconium ileus, salt wasting, and male infertility also occur.
Bronchiectasis is chronic airway dilation with a chronic wet cough. Causes include post-infectious damage, cystic fibrosis, primary ciliary dyskinesia, immunodeficiency, and aspiration. Primary ciliary dyskinesia causes chronic sinusitis and neonatal respiratory distress; about half of patients have situs inversus, and the Kartagener triad combines situs inversus, chronic sinusitis, and bronchiectasis.
Traps
A single sweat test in an infant with edema may be misleading. Normal pancreatic function does not exclude cystic fibrosis. Primary ciliary dyskinesia can occur without situs inversus. Do not treat a chronic wet cough as asthma. CFTR modulators are genotype-specific, not universal.
Worked example and rule of thumb
An infant with meconium ileus and failure to thrive should have sweat chloride testing; a value of 72 mmol/L confirms cystic fibrosis. Rule of thumb: chronic wet cough with bronchiectasis warrants a sweat test and genetic evaluation for cystic fibrosis.