Cystic Fibrosis¶
Chapter 302 | Part 7: Disorders of the Respiratory System
KEY CLINICAL POINTS¶
- Cystic fibrosis (CF) is an autosomal recessive exocrinopathy caused by mutations in the CFTR gene, leading to defective chloride and bicarbonate transport across epithelial membranes.
- The F508del mutation accounts for ~85% of CFTR defects in the U.S. and is associated with severe pancreatic insufficiency and progressive lung disease.
- CFTR modulators like ivacaftor (for G551D) and triple-combination therapies (elexacaftor/tezacaftor/ivacaftor) have transformed outcomes by correcting CFTR function and reducing sweat chloride levels.
- Diagnosis relies on sweat chloride testing (>60 mEq/L) and CFTR mutation analysis, with newborn screening now widely implemented.
- Management includes pancreatic enzyme replacement, mucolytics (dornase alfa), airway clearance techniques, and targeted antibiotics for Pseudomonas aeruginosa infections.
1. DEFINITION & OVERVIEW¶
Cystic fibrosis (CF) is an autosomal recessive genetic disorder caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. It affects multiple epithelial tissues, leading to defective chloride and bicarbonate transport, resulting in thickened mucus, pancreatic insufficiency, and progressive lung disease. The disease is characterized by chronic respiratory infections, pancreatic exocrine dysfunction, and systemic complications.
CFTR Mutation Classes¶
| Class | Mechanism | Examples |
|---|---|---|
| Class I | Absent synthesis | Nonsense mutations (e.g., G542X) |
| Class II | Protein misfolding | F508del, G551D |
| Class III | Gating defects | G551D |
| Class IV | Conductance defects | R117H |
| Class V | Reduced transcripts | Splice site mutations |
| Class VI | Accelerated turnover | S1251N |
1.1 Genetic Basis¶
CF is an autosomal recessive disorder requiring mutations in both CFTR alleles. Over 2000 CFTR mutations have been identified, categorized into classes I–VI based on molecular mechanisms (e.g., class II defects involve protein misfolding, class III defects involve gating abnormalities).
1.2 Pathophysiology¶
CFTR dysfunction leads to impaired chloride and bicarbonate transport, resulting in hyperviscous airway secretions, pancreatic duct obstruction, and altered exocrine gland function. Defective periciliary fluid layer (PCL) compromises mucociliary clearance, leading to chronic infection and inflammation.
2. EPIDEMIOLOGY¶
CF affects ~70,000 individuals globally, with an incidence of ~1 in 3,500 live births in the U.S. and Europe. Prevalence varies by ethnicity, with higher rates in Caucasians (~1 in 3,300) and lower rates in Asian and African populations. Risk factors include consanguinity and carrier status in populations with high CFTR mutation prevalence.
2.1 Demographics¶
CF is most common in individuals of Northern European descent. In Latin America, median survival is <30 years, compared to >60 years in the U.S. due to disparities in newborn screening and treatment access.
2.2 Complications¶
CF is associated with chronic respiratory infections, pancreatic insufficiency, liver disease, and increased risk of diabetes mellitus. Mortality is historically linked to respiratory failure, but improved therapies have extended survival to >50 years in many patients.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
CF is caused by mutations in the CFTR gene, which encodes an ATP-binding cassette transporter regulating chloride and bicarbonate ion transport. Defective CFTR leads to hyperviscous mucus, impaired mucociliary clearance, and chronic infection. Pathogenesis involves: (1) defective ion transport in airway epithelia, (2) pancreatic duct obstruction, and (3) systemic inflammation from chronic infection.
3.1 Molecular Mechanisms¶
CFTR mutations disrupt chloride and bicarbonate transport, leading to: (1) thickened airway secretions, (2) pancreatic exocrine dysfunction, and (3) altered exocrine gland secretion. Defective periciliary fluid layer (PCL) impairs ciliary function and mucociliary clearance.
3.2 Infection and Inflammation¶
Chronic infection with Pseudomonas aeruginosa, Staphylococcus aureus, and Haemophilus influenzae drives progressive lung damage. Inflammation leads to bronchiectasis, fibrosis, and respiratory failure.
4. CLINICAL FEATURES¶
CF presents with respiratory, pancreatic, and systemic manifestations. Key features include: (1) chronic cough with thick sputum, (2) meconium ileus in newborns, (3) pancreatic insufficiency with steatorrhea, and (4) recurrent respiratory infections. Complications include diabetes mellitus, liver disease, and infertility in males.
4.1 Respiratory Manifestations¶
Chronic bronchial infection, airway remodeling, and progressive lung disease. Common findings: copious mucus, bronchiectasis, and Pseudomonas aeruginosa colonization. Radiographic evidence of sinusitis is common.
4.2 Pancreatic Findings¶
Pancreatic duct obstruction leads to exocrine insufficiency (steatorrhea, malabsorption), pancreatic fibrosis, and loss of islet cells. CF-related diabetes occurs in >30% of adults.
5. DIFFERENTIAL DIAGNOSIS¶
Differential diagnoses include: (1) non-CF bronchiectasis, (2) congenital bilateral absence of the vas deferens (CBAVD), (3) pancreatic insufficiency due to other causes, and (4) chronic sinusitis. CF must be distinguished from other genetic disorders with similar phenotypes (e.g., congenital neutropenia).
5.1 Non-CF Bronchiectasis¶
Non-CF bronchiectasis lacks CFTR mutations and is often associated with immunodeficiency or prior infections. CF is distinguished by pancreatic insufficiency and characteristic sweat test abnormalities.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnosis combines clinical findings, sweat chloride testing, and CFTR mutation analysis. Newborn screening has improved early detection. Diagnostic criteria include: (1) sweat chloride ≥ 60 mEq/L, (2) CFTR mutations, and (3) characteristic clinical features.
CF Diagnostic Criteria¶
| Criteria | Details |
|---|---|
| Sweat Chloride | ‡60 mEq/L |
| CFTR Mutations | Two disease-causing variants |
| Clinical Features | Respiratory/pancreatic manifestations |
6.1 Sweat Test¶
Pilocarpine iontophoresis measures sweat chloride levels. CF patients show >60 mEq/L, while non-CF individuals have <40 mEq/L. Hyperviscosity of eccrine sweat is not a diagnostic feature.
6.2 Genetic Testing¶
CFTR mutation analysis identifies ~90% of CF cases. Panels detect over 2000 mutations, with F508del being the most common. CFTR2.org provides variant interpretation.
7. MANAGEMENT & TREATMENT¶
Management includes CFTR modulators, pancreatic enzyme replacement, airway clearance techniques, and antimicrobial therapy. Triple-combination therapies (elexacaftor/tezacaftor/ivacaftor) are FDA-approved for ≥ 1 F508del mutation. Supportive care includes nutritional support, vitamin supplementation, and pulmonary rehabilitation.
CFTR Modulator Approval¶
| Drug | Indications | Mechanism |
|---|---|---|
| Ivacaftor | G551D, G178R, G1244E | Potentiates CFTR gating |
| Tezacaftor | F508del | Corrects CFTR processing |
| Elexacaftor | F508del | Corrects CFTR processing |
| Triple-Combination | ‡1 F508del | Corrects and potentiates CFTR |
7.1 CFTR Modulators¶
Ivacaftor (for G551D), tezacaftor (for F508del), and triple-combination therapies (elexacaftor/tezacaftor/ivacaftor) improve ion transport. These are HEMT (highly effective modulator therapy) for specific mutations.
7.2 Antimicrobial Therapy¶
Inhaled antibiotics (colistin, tobramycin) and oral antibiotics (azithromycin) target chronic Pseudomonas infections. Dornase alfa (DNAse) reduces mucus viscosity.
8. PROGNOSIS & COMPLICATIONS¶
Median survival has improved to >50 years with modern therapies. Complications include respiratory failure, pancreatic cancer, liver cirrhosis, and CF-related diabetes. Prognosis is influenced by modulator therapy, infection control, and nutritional status.
8.1 Lung Transplantation¶
Lung transplantation is indicated for end-stage lung disease, with median survival >9 years. Transplantation is considered for FEV1 <30% predicted and refractory respiratory failure.
9. SPECIAL CONSIDERATIONS¶
Pregnancy in CF women requires close monitoring due to increased maternal and fetal risks. Males with CF are typically infertile due to congenital absence of the vas deferens. Pediatric management emphasizes early nutritional support and modulator therapy. Elderly patients face increased risks of cancer and comorbidities.
9.1 Reproductive Health¶
Males with CF are infertile (99% of cases). Females may have infertility due to abnormal secretions. Assisted reproductive technologies are critical for CF carriers.
10. KEY POINTS & CLINICAL PEARLS¶
CF is a monogenic disorder caused by CFTR mutations. Early diagnosis via newborn screening and modulator therapy improves outcomes. Management requires multidisciplinary care, including pulmonary, nutritional, and genetic support. CFTR modulators are the cornerstone of modern treatment, with triple-combination therapies offering the most benefit for F508del carriers.