CYSTIC FIBROSIS - Comprehensive MD Examination Answer (15 Marks)

1. DEFINITION AND EPIDEMIOLOGY

• Incidence: ~1 in 3,000 in Caucasians; 1 in 10,000 in Hispanics; 1 in 15,000 in African Americans
• Murray & Nadel quotes 1 in 1,500-2,500 whites; 1 in 17,000 African Americans
• Carrier frequency: ~1 in 26 in Northern European ancestry
• Most common lethal autosomal recessive disorder in whites
• Median predicted survival has now reached the late 40s (2020 CFF Registry: median ~46 years)
• Majority of patients now live past 18 years of age

2. GENETICS AND MOLECULAR BIOLOGY

• Located on chromosome 7q31.2
• Spans ~230 kb of DNA; contains 27 exons; mRNA is 6.5 kb
• Encodes a 1,480 amino acid integral membrane glycoprotein
• Discovered by positional cloning (chromosome walking/jumping) in 1989
• Expressed in lung, pancreas, sweat glands, intestine, liver, reproductive tract

• F508del (Phe508del): 66-84.7% of CF alleles worldwide - the most common mutation
• More than 2,100 mutations now reported
• G542X: 4.6%; G551D: 4.4%; R117H: 3.0%; N1303K: 2.3%; W1282X: 2.2%

• ABC (ATP-binding cassette) transporter superfamily
• Two membrane-spanning domains (MSD1, MSD2)
• Two nucleotide-binding domains (NBD1, NBD2)
• One unique regulatory (R) domain - phosphorylated by cAMP-dependent PKA to open the channel
• Functions primarily as a cAMP-regulated chloride AND bicarbonate channel
• Also regulates ENaC (epithelial Na+ channel), ORCC (outwardly rectifying chloride channels), and HCO3-/Cl- exchangers
• Predominantly expressed in ionocytes - a rare minority airway epithelial cell type discovered by single-cell sequencing (2018)

3. PATHOPHYSIOLOGY

Airway/Pulmonary Pathophysiology (Murray & Nadel, Ch. 67):

1. ASL depletion: CFTR deficiency reduces Cl- secretion; simultaneously, ENaC is dysregulated (unopposed Na+ and water absorption), depleting the periciliary liquid layer - cilia collapse and cannot beat effectively
2. Abnormal mucus rheology: Low HCO3- leads to acidic, more viscid, hyper-concentrated mucus gel that adheres to the epithelial surface
3. Defective mucosal defense: Altered pH and ion content reduce antimicrobial peptide activity (defensins, lactoferrin); CFTR itself serves as a receptor for P. aeruginosa
4. Pronounced inflammation: Exaggerated neutrophilic response; high IL-8, IL-6, TNF-alpha; massive release of DNA and actin from lysed neutrophils thickens mucus further
5. Early small airway disease: CF lung damage begins in small airways (bronchioles) - abnormal FEF 25-75%, closing volume, and dynamic compliance before FEV1 falls
6. Submucosal gland hyperplasia: Prominent in early CF, leading to inspissated mucus plugs

• Normal sweat gland reabsorbs NaCl in the duct via CFTR
• In CF: CFTR absent in ductal cells → NaCl cannot be reabsorbed → salty sweat (sweat Cl- >60 mEq/L)
• This is the basis of the sweat chloride test (pilocarpine iontophoresis)

• CFTR absent in pancreatic ductal cells → reduced HCO3- and fluid secretion → inspissated protein plugs obstruct ducts → autodigestion → exocrine pancreatic insufficiency (85-90% of patients)
• Pancreatitis occurs in the remaining 10-15% with residual function mutations
• Progressive destruction of islets → CF-Related Diabetes (CFRD)

• CF gut: decreased bicarbonate → increased intraluminal acidity → malabsorption, bacterial overgrowth, impaired motility
• Hyperplasia of mucous glands and goblet cells
• Meconium ileus at birth (10-15%); Distal Intestinal Obstruction Syndrome (DIOS) in older patients
• Gastrointestinal malignancy risk is increased ~23-fold

4. CLINICAL FEATURES

Pulmonary Manifestations (Fishman, Ch. 48):

• Productive cough (chronic, progressive)
• Recurrent respiratory tract infections
• Dyspnea on exertion progressing to rest
• Wheezing, chest tightness
• Hemoptysis (especially in older patients - massive hemoptysis is a life-threatening complication)

• Barrel chest (hyperinflation)
• Digital clubbing (almost universal in advanced disease)
• Crackles/wheezes on auscultation
• Nasal polyps, chronic sinusitis (>90% have radiographic sinus disease)
• Cyanosis in late disease

• Initially: obstructive pattern with small airway abnormalities (FEF25-75% fall first)
• Progressive: FEV1 decline (obstructive; FEV1/FVC reduced)
• Late: mixed obstructive-restrictive; air trapping (RV/TLC elevated)
• V/Q mismatch → hypoxemia → pulmonary hypertension → cor pulmonale
• Late: hypercapnia + respiratory acidosis = respiratory failure
• DLCO reduced even before significant emphysema

• Infancy: S. aureus, H. influenzae
• Childhood onwards: Pseudomonas aeruginosa (mucoid form - signals accelerated decline)
• Chronic colonizers: Mucoid P. aeruginosa (most important), MRSA (associated with FEV1 decline and worse survival), Burkholderia cepacia complex (particularly B. cenocepacia - associated with "cepacia syndrome": rapid fatal deterioration), Stenotrophomonas maltophilia, Achromobacter xylosoxidans
• NTM (nontuberculous mycobacteria) - M. abscessus particularly problematic
• ABPA (allergic bronchopulmonary aspergillosis) in ~10%

• Early: peribronchial thickening, small airway opacities (tree-in-bud)
• Progressive: hyperinflation, bronchiectasis (upper lobes predominant)
• HRCT: bronchiectasis with peribronchial thickening, mucus plugging, air trapping, cysts - predominantly upper lobe distribution

Gastrointestinal & Nutritional Manifestations:

• Meconium ileus in neonates (pathognomonic)
• Exocrine pancreatic insufficiency (85-90%): steatorrhea, malabsorption, fat-soluble vitamin deficiencies (A, D, E, K)
• CFRD (CF-Related Diabetes): 2% children; 19% adolescents; 40-50% adults - associated with worse pulmonary function, lower BMI, decreased survival
• CF liver disease: focal biliary cirrhosis in 5-10%; portal hypertension, esophageal varices
• Rectal prolapse
• DIOS (meconium ileus equivalent)
• Gallstones (30% of adults)

Other Manifestations:

• Male infertility: >98% have bilateral absence of vas deferens (CBAVD) - azoospermia; semen analysis and MESA (microsurgical epididymal sperm aspiration) + IVF possible
• Female infertility: thick cervical mucus impairs conception; delayed menarche due to malnutrition
• Osteoporosis/osteopenia: vitamin D malabsorption, corticosteroid use, poor nutrition
• Arthropathy: episodic CF arthropathy (immune complex-mediated)
• Vasculitis: leukocytoclastic vasculitis
• Metabolic alkalosis: especially in hot weather (salt loss in sweat)
• Hypertrophic osteoarthropathy

5. DIAGNOSIS

Diagnostic Criteria (Fishman; CFF Consensus Guideline 2017):

1. Characteristic sinopulmonary disease
2. GI/nutritional abnormalities
3. Salt-loss syndromes
4. Obstructive azoospermia (CBAVD)
5. Family history with newborn screening

1. Sweat chloride ≥60 mmol/L (pilocarpine iontophoresis) - Gold standard
2. Two disease-causing CFTR mutations identified (one on each allele)
3. Abnormal nasal potential difference (transepithelial nasal PD measurement)

• Normal: Cl- <30 mmol/L
• Intermediate (equivocal): Cl- 30-59 mmol/L (revised downward from 40-59 per CFF updated guidance)
• Diagnostic of CF: Cl- ≥60 mmol/L
• Requires minimum 75 mg sweat; performed by pilocarpine iontophoresis (Gibson-Cooke method)

• Immunoreactive trypsinogen (IRT) on heel prick blood spot; if elevated, reflexed to CFTR mutation panel
• Established in all 50 US states (methods vary by state)
• IRT/IRT or IRT/DNA protocols used

• Newborns with positive screen but sweat Cl- 30-59 + fewer than 2 CF-causing mutations - require monitoring

• Sputum microbiology (BAL in infants)
• HRCT chest
• Pulmonary function tests (from age 6 years)
• Lung Clearance Index (LCI) by multiple breath washout - sensitive early marker, detects ventilation inhomogeneity before spirometry abnormalities
• Annual OGTT for CFRD screening (from age 10 years per CFF guidelines)
• Fecal elastase (pancreatic insufficiency)

6. MANAGEMENT

Airway Clearance Therapy (Fishman, Ch. 48):

• Chest physiotherapy (CPT) with postural drainage - cornerstone
• High-frequency chest wall oscillation (HFCWO/"Vest") - equivalent to CPT
• Active cycle of breathing, autogenic drainage, oscillating PEP devices (Flutter, Acapella)
• Exercise - adjunct to formal airway clearance

1. Dornase alfa (Pulmozyme / rhDNase): Recombinant human DNase I; cleaves extracellular DNA (from neutrophil lysis) that contributes to mucus viscosity; once-daily nebulization; approved 1994; reduces exacerbation risk and maintains FEV1 (especially in patients with FEV1 ≥85%)
2. Hypertonic saline 7%: Restores ASL hydration by osmotic action; twice daily after bronchodilator for ≥48 weeks significantly reduces pulmonary exacerbations and days lost from school/work (large RCT); Cochrane SR (PMID 37319354, 2023) confirms benefit; mannitol is an alternative
3. N-acetylcysteine: Less evidence than above two

Antibiotics (Fishman, Ch. 48):

• EPIC trial: inhaled tobramycin + oral ciprofloxacin can eradicate early P. aeruginosa and delay chronic colonization
• Goal: prevent conversion to mucoid phenotype

• Inhaled tobramycin (TOBI): 300 mg twice daily in alternate months - reduces P. aeruginosa density, improves FEV1, reduces hospitalization
• Inhaled aztreonam lysinate (Cayston): 75 mg three times daily (alternate months) - effective against P. aeruginosa
• Oral azithromycin: 250-500 mg three times/week - anti-inflammatory + anti-biofilm effect; Cochrane SR (PMID 38411248, 2024) confirms improvement in lung function and reduced exacerbations; not recommended if NTM present

• IV antibiotics for severe exacerbation: tobramycin + piperacillin-tazobactam (standard combination)
  • Tobramycin 10 mg/kg/day IV (once-daily preferred - equivalent efficacy, less nephrotoxicity)
  • Serum peaks 25-35 μg/L; troughs <1 μg/L
• Duration: minimum 2 weeks IV (most centers 14-21 days); some use 3-4 weeks to prolong exacerbation-free interval
• MRSA: vancomycin or linezolid
• B. cepacia complex: ceftazidime, meropenem, ciprofloxacin, minocycline, or TMP-SMX combinations
• Augment airway clearance throughout

Anti-inflammatory Therapy (Fishman, Ch. 48):

• Ibuprofen (high-dose): controlled 4-year trial showed improved rate of FEV1 decline in children; limited by GI side effects and narrow therapeutic window (peak serum level 50-100 μg/mL needed)
• Corticosteroids: Not recommended routinely (large multicenter trial showed significant side effects with alternate-day dosing); use restricted to CF+asthma, CF+ABPA, and severe obstructive exacerbation in infants
• Azithromycin: See above

CFTR Modulator Therapies - The Paradigm Shift (Fishman, Ch. 48; Murray Ch. 67):

• Ivacaftor (VX-770 / Kalydeco): First approved CFTR modulator (2012); targets G551D (Class III) and other gating mutations (~4-5% of patients); dramatically improves FEV1 (+10-12 pp), sweat chloride, weight, and exacerbation rate; also first-in-class as a pharmacological chaperone

• Lumacaftor (VX-809): Corrector; used with ivacaftor (Orkambi = lumacaftor/ivacaftor) for F508del homozygotes; modest benefit; significant drug interactions (CYP3A inducer)
• Tezacaftor (VX-661): Next-generation corrector; better tolerated; Symdeko = tezacaftor/ivacaftor; for F508del homozygotes and F508del/residual function heterozygotes

• Elexacaftor/Tezacaftor/Ivacaftor (ETI / Trikafta / Kaftrio): Two correctors (elexacaftor + tezacaftor) + one potentiator (ivacaftor)
  • Approved FDA 2019 (age 12+); now down to age 2 in the US
  • For patients with at least one F508del allele (covers ~90% of CF patients)
  • NEJM trial (Middleton et al., 2019): +14 percentage points in ppFEV1, -86% reduction in sweat chloride, improved BMI, HRQoL, reduced exacerbation rate
  • Systematic review 2024 (PMID 39048464) confirms effectiveness even in patients without F508del
  • Changed the natural history of CF dramatically
  • Side effects: elevated liver transaminases, cataracts (monitoring required), rash
  • CFTR modulator adherence: 2024 systematic review (PMID 39142708) identified barriers to adherence

Nutritional Management (ESPEN-ESPGHAN-ECFS Guideline 2024, PMID 38169175):

• High-calorie diet (110-200% of normal energy requirements)
• Pancreatic enzyme replacement therapy (PERT): lipase dosing by meal fat content (1,500-10,000 IU lipase/kg/meal; max 10,000 units/kg/day); enteric-coated microspheres
• Fat-soluble vitamins (A, D, E, K) supplementation mandatory
• Salt supplementation in hot weather, exercise, infants
• Nasogastric/gastrostomy tube feeding if BMI persistently low

CFRD Management:

• Annual OGTT screening from age 10 years
• Insulin is the only recommended treatment (sulfonylureas and metformin not routinely used)
• CFRD management improves both glycemic control and pulmonary outcomes
• Continuous glucose monitoring preferred

Liver Disease:

• Ursodeoxycholic acid (UDCA) 10-20 mg/kg/day: widely used as choleretic (weak evidence for disease modification)
• Annual liver function tests; abdominal US
• Portal hypertension: therapeutic endoscopy (sclerotherapy/banding), TIPS, portosystemic shunts
• Liver transplantation for end-stage liver disease
• GI cancer screening: no formal guidelines yet (elevated 23-fold lifetime risk)

DIOS Management:

• Oral/NG N-acetylcysteine or balanced polyethylene glycol (GoLYTELY)
• If refractory: hyperosmolar contrast enemas (gastrografin) under radiology - patient must be well hydrated
• Surgical consultation if volvulus/intussusception develops

Lung Transplantation:

• Bilateral sequential lung transplantation: definitive treatment for end-stage CF lung disease
• Indications: ppFEV1 <30%, rapidly declining FEV1, frequent life-threatening exacerbations, worsening gas exchange (resting hypoxemia, hypercapnia)
• 5-year survival post-transplant: ~50-55%
• B. cenocepacia colonization is a contraindication at most centers
• CFTR modulators have delayed time to transplantation referral

Reproductive Issues (Fishman, Ch. 48):

• Male: >98% have CBAVD → azoospermia → MESA + IVF possible
• Female: increasingly conceiving as life expectancy improves; prepregnancy counseling essential; CFTR modulators likely safe in pregnancy (isolated reports); premature delivery more common in women with poor FEV1

7. COMPLICATIONS SUMMARY

8. PROGNOSIS AND MONITORING

• Median survival now late 40s (vs. early childhood in 1960s)
• Key prognostic marker: FEV1 - ppFEV1 <30% predicts median survival of 2-3 years without transplant
• FEV1 decline ~2%/year in adults
• Annual monitoring: PFTs, sputum cultures, LFTs, CBC, OGTT (age ≥10), bone density, ophthalmology (for modulator-related cataracts), echocardiogram
• LCI (Lung Clearance Index) by MBW: increasingly used as sensitive early outcome measure, especially in children with normal spirometry
• CFF Patient Registry (USA) provides population-level outcomes tracking

9. RECENT GUIDELINES (ATS/CFF 2023-2026)

• CFF Consensus: Diagnosis of CF (J Pediatr 2017): Updated sweat Cl- intermediate range to 30-59 mmol/L
• ESPEN-ESPGHAN-ECFS Nutrition Guideline 2024 (PMID 38169175): Comprehensive nutritional management
• Cochrane SR: Hypertonic saline (PMID 37319354, 2023): Confirmed benefit in lung function and exacerbation reduction
• Cochrane SR: Azithromycin (PMID 38411248, 2024): Confirmed improved lung function + reduced exacerbations
• Systematic Review: ETI in non-F508del (PMID 39048464, 2024): ETI effective and well-tolerated
• Systematic Review: CFTR modulator adherence (PMID 39142708, 2024): Adherence barriers need addressing
• Liver function biomarkers with CFTR modulators (PMID 39255352, 2024): Modulators improve liver biomarkers; transaminase elevation possible

EXAM SUMMARY TABLE