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Cystic Fibrosis

Chapter 302 | Part 7: Disorders of the Respiratory System · Part 7 – Respiratory Disorders · Chapter 302

Key Clinical Points

  1. CF is an autosomal recessive exocrinopathy caused by mutations in the CFTR gene.
  2. Diagnosis requires two disease-causing CFTR variants and/or sweat chloride ≥60 mEq/L.
  3. F508del is the most common mutation, accounting for ~85% of defective alleles in the US (Class II).
  4. CFTR modulators (e.g., ivacaftor, TCT) are highly effective for specific genotypes.
  5. Pulmonary exacerbations require aggressive inpatient antibiotic therapy (aminoglycoside + beta-lactam).
  6. Lung transplantation is indicated for FEV1 <30% predicted or severe decline.
  7. CFTR deficiency leads to hyperviscous mucus, impaired mucociliary clearance, and chronic infection.
  8. Pancreatic insufficiency occurs in ~80% of patients with 'severe' CFTR defects (e.g., F508del, G551D).
  9. CF-related diabetes mellitus affects >30% of adults with the disease.
  10. Median survival in the US has increased to over 60 years due to improved treatments.
  11. Sweat chloride ≥60 mEq/L is the cardinal diagnostic test; hyperviscosity of sweat is not a clinical feature.

DEFINITION & OVERVIEW

• Definition: Cystic fibrosis (CF) is an autosomal recessive exocrinopathy affecting multiple epithelial tissues. • Molecular Basis: ◦ CFTR gene product: Cystic fibrosis transmembrane conductance regulator. ◦ Structure: ~1480-amino-acid molecule. ◦ Function: Integral membrane protein acting as an apical (luminal) plasma membrane anion channel. ◦ Mechanism: Provides a passive conduit for chloride and bicarbonate transport; flow is driven by electrochemical gradient. ◦ Energy: Gating involves conformational cycling and is augmented by ATP hydrolysis (not active transport against concentration gradient). • Physiological Role: ◦ Regulation: Controls volume and composition of exocrine secretion in various tissues. ◦ Respiratory Mucosa: Maintains periciliary fluid layer (PCL) depth; essential for ciliary extension and mucociliary transport. ◦ Pathophysiology: CFTR-deficient cells have depleted PCL → ciliary collapse → failure to clear mucus.

Respiratory Manifestations

• Mucus Dynamics: Copious, hyperviscous, and adherent secretions → obstruction of small/medium airways. • Microbiology: ◦ Common pathogens: Staphylococcus aureus, Haemophilus influenzae, Pseudomonas aeruginosa. ◦ P. aeruginosa: Often shows a stereotypic progression; early colonization → lifelong infection by same strain → evolution to mucoid phenotype (alginate production) → poor prognosis. • Inflammation: ◦ Aggressive, unrelenting, neutrophilic response. ◦ Release of proteases and oxidants → airway remodeling and bronchiectasis. ◦ Driven by pathogens; some inflammatory response occurs even before infection. • Sinus Involvement: Radiographic evidence of sinusitis in most patients; may serve as a reservoir for lower airway seeding.

Pancreatic Findings

• Pathology: Fibric scarring, fatty replacement, cyst formation, and loss of acinar tissue. ◦ Mechanism: Tenacious exocrine secretions (concretions) obstruct ducts → impaired enzyme flow to duodenum. ◦ Sequelae: Chronic malabsorption, poor growth, fat-soluble vitamin deficiency, low fecal elastase-1. ◦ Newborn Screening: High levels of blood immunoreactive trypsinogen used for detection. ◦ Diabetes: CF-related diabetes mellitus in >30% of adults (due to endocrine destruction/dysfunction and potentially insulin resistance).

Additional Organ System Damage

• Liver: ◦ Obstruction of intrahepatic bile ducts → parenchymal fibrosis. ◦ 4–15% develop multilobular cirrhosis; others show steatosis, focal biliary fibrosis, or portal hypertension. • Intestine: ◦ Hyperviscous contents → meconium ileus (neonates) or distal intestinal obstructive syndrome (older individuals). • Reproductive: ◦ Males: ~99% are infertile due to complete involution of the vas deferens (despite functional spermatogenesis). ◦ Females: Higher incidence of infertility due to abnormalities in reproductive tract secretions.

EPIDEMIOLOGY

• Prevalence: Traditionally most common among whites (~1 in 3300 live U.S. births). • Global Diversity: Potential for underdiagnosis in Eastern Europe, Latin America, Asia, and India due to less well-defined/characterized variants. • Impact of Early Diagnosis: Identification in early childhood is critical for improving quality of life and longevity (median survival >60 years in the U.S.). • Barriers to Care: Lack of widespread screening (newborn, sweat, genetics) and limited access to advanced treatments can lead to poorer outcomes.

ETIOLOGY & PATHOPHYSIOLOGY

• Genetic Diversity: >2000 allelic mutations in CFTR identified worldwide. • Mutation Classification: ◦ Based on molecular mechanism of failure. ◦ Reference: CFTR2 (www.cftr2.org) used to delineate variants. • Specific Mutations: ◦ F508del: Mutation at position 508; leads to protein folding abnormality → arrested in ER → proteasomal degradation (Class II). ◦ G551D: Glycine to aspartic acid at position 551; results in inability to open/gate (Class III); affects ~4% of patients. ◦ W1282X: Common in individuals of Ashkenazi descent and prevalent in Israel (Class I). • Table 302-1 (Categories of CFTR Mutations): ◦ Class I: Absence of synthesis (e.g., G542X, R553X, W1282X). ◦ Class II: Defective maturation/degradation (e.g., F508del; ~85% of US cases). ◦ Class III: Disordered gating/regulation (e.g., G551D; ~4% of patients). ◦ Class IV: Defective conductance through the ion channel pore. ◦ Class V: Reduced number of transcripts (promoter/splicing issues). ◦ Class VI: Accelerated turnover from cell surface.

CLINICAL FEATURES

• General Presentation: Childhood onset with chronic productive cough, malabsorption (steatorrhea), and failure to thrive. • Diagnostic Markers: ◦ Sweat Chloride: ≥60 mEq/L is the cardinal test; high specificity. Note: Hyperviscosity of sweat is NOT a clinical feature. ◦ Genetic Testing: Identification of disease-causing variants on both alleles. • Genotype Impact: ◦ Severe defects (F508del, G551D, nonsense) predict pancreatic insufficiency in ~80% of patients. ◦ Genotype has limited value for predicting specific rates of clinical decline or longevity. • Sub-clinical/Related Conditions: ◦ Forme frustes: Isolated congenital bilateral absence of the vas deferens or isolated pancreatitis. ◦ Carrier Status: Predisposes to non-CF bronchiectasis and chronic rhinosinusitis.

DIFFERENTIAL DIAGNOSIS

• Related Conditions: ◦ Non-CF bronchiectasis and chronic rhinosinusitis (associated with carrier status). ◦ Forme frustes: Isolated congenital bilateral absence of the vas deferens or isolated pancreatitis. ◦ Spectrum of CFTR-related conditions mimicking classic CF.

DIAGNOSTIC APPROACH

  1. Clinical Suspicion: Based on chronic productive cough, malabsorption (steatorrhea), and failure to thrive.
  2. Screening: Newborn screening is the primary method for initial identification.
  3. Cardinal Testing: ◦ Sweat electrolyte measurement (Pilocarpine iontophoresis) → Result ≥60 mEq/L indicates high probability. ◦ CFTR mutation analysis (Genotyping).
  4. Genetic Analysis: ◦ DNA-based panels identifying several hundred variants. ◦ Alternatively: Complete CFTR DNA testing or exonic sequencing with splice junction/regulatory element analysis.
  5. Confirmation Criteria: ◦ Option A: Identification of disease-causing mutations on both alleles. ◦ Option B: Sweat chloride ≥60 mEq/L + characteristic respiratory or other exocrine manifestations.

Table 6.1. Diagnostic Criteria for Cystic Fibrosis: • Sweat Chloride (Pilocarpine Iontophoresis): ≥60 mEq/L (Highly specific; mainstay of diagnosis). • CFTR Genotyping: Disease-causing variants on both alleles (Sufficient for confirmation). • Clinical Features: Characteristic respiratory or other exocrine manifestations (Sufficient for confirmation).

MANAGEMENT & TREATMENT

  1. Nutritional Support: ◦ Exogenous pancreatic enzymes taken with meals. ◦ Nutritional supplementation.
  2. Airway Clearance: ◦ Chest physiotherapy (several times daily). ◦ Recombinant DNAse aerosols (degrade DNA to reduce mucus viscosity). ◦ Nebulized hypertonic saline or mannitol (increase PCL depth, activate mucociliary clearance, mobilize secretions).
  3. Pharmacotherapy: ◦ Anti-inflammatory medications. ◦ Bronchodilators. ◦ Antibiotics: Oral/aerosolized; specifically aminoglycoside + beta-lactam for pulmonary exacerbations.
  4. Advanced Therapies: ◦ CFTR modulators (e.g., ivacaftor, TCT) for specific genotypes.
  5. End-Stage Management: ◦ Lung transplantation: Indicated for FEV1 <30% predicted or severe decline.

COMPLICATIONS & PROGNOSIS

• Survival: Significant improvement with modulators; median survival >60 years in the U.S. • Complications: ◦ Lung: Bronchiectasis, chronic infection (especially P. aeruginosa), and respiratory failure. ◦ Pancreas: Exocrine insufficiency and CF-related diabetes. ◦ Liver: Cirrhosis, portal hypertension, and hepatic insufficiency. ◦ Reproductive: Infertility in both males and females.

SPECIAL CONSIDERATIONS

• Reproductive Health: ◦ Males: ~99% infertile due to vas deferens involution. ◦ Females: Higher infertility rates due to reproductive tract secretion issues. • Aging & Comorbidities: ◦ Management of multi-organ involvement (lung, pancreas, liver) and endocrine abnormalities in adults.

KEY PEARLS & CLINICAL TRAPS

• Cardinal Test: Sweat chloride ≥60 mEq/L is the primary diagnostic threshold. • Mutation Prevalence: F508del (Class II) is the most common mutation (~85% in US). • Pathogen Progression: P. aeruginosa often becomes mucoid over time, leading to a poor prognosis. • Clinical Correlation: Genotype determines eligibility for specific modulators but has limited value in predicting the rate of clinical decline. • Treatment Goals: Management focuses on mucus clearance (physiotherapy, DNAse, hypertonic saline), nutrition (enzymes), and infection control.