Cystic Fibrosis

Cystic fibrosis (CF) is the most common Caucasian lethal genetic disorder (with gene frequency of 1 in 25) in the United States, where 4–5% of population have at least one CF allele.

CF affects approximately 1 in 2500 live births among Caucasians, 1 in 17,000 among African-Americans, and 1 in 90,000 among Asians.

GENETICS/BASIC DEFECTS

1. Inheritance

a. Autosomal recessive

b. CF gene mapped to chromosome 7q31.2 2. Molecular defect

a. Caused by a single gene defect on chromosome 7 that encodes a cAMP-regulated chloride channel known as the cystic fibrosis transmembrane conductance reg- ulator (CFTR)

i. CFTR usually resides in the apical membrane of epithelial cells lining the airway, biliary tree, intestines, vas deferens, sweat ducts, and pancre- atic ducts.

ii. Insufficient fluid secretion secondary to inability of CFTR to transport chloride ion at the above sites causes higher viscidity of the protein por- tions of the secretions and obstructing the ducts, leading to plugging and dysfunction at the organ level

iii. CFTR also regulates the activity of other pro- teins that conduct ions and affects intracellular regulatory processes at the cellular level b. Presence of close to 1000 different disease-causing

mutations of the CFTR gene

i. Deletion mutations including delta F508 which, observed in over 70% of patients with CF, have a deletion of three contiguous base pairs result- ing in the loss of a single amino acid, phenylala- nine at codon 508)

ii. Missense mutation (single base pair exchange within a normal length CFTR protein)

iii. Nonsense mutations (exchange of a single base pair resulting in premature termination of the protein)

iv. Frameshift mutations (the deletions or insertion of a single base pair)

c. Genotype–phenotype correlations

i. Phenotypic presentation of the disease: probably related to the underlying genetic abnormality ii. Patients with homozygous delta F508 typically

have respiratory distress and malabsorption iii. Patients with less severe variations of the disease

or typical CF with borderline normal sweat tests may have other haplotypes

CLINICAL FEATURES

1. Classic clinical triad

a. Exocrine pancreatic insufficiency b. Chronic obstructive pulmonary disease

c. Elevation of sodium and chloride concentration in sweat

2. Chronic sino-pulmonary disease

a. Varying widely in age of onset and rate of progression b. Clinical course

i. Newborn with CF with histologically normal respiratory systems

ii. First few months of age

a) Epithelial chloride channel defect, leading to abnormal respiratory secretions, bron- chopulmonary infections, and airway obstruction

b) Clinical manifestations including cough, wheezing, retractions, and tachypnea c. Persistent endobronchial infection and inflammation

with typical CF pathogens i. Staphylococcus aureus

a) Responsible for the majority of lung disease when CF was first described

b) Commonly isolated in the first year of life from the sputum of patients with CF c) Typically controlled by antibiotic therapy d) Currently only 10% of adult patients with CF

are chronically colonized by the pathogen ii. Mucoid and non-mucoid pseudomonas aerugi-

nosa

a) The most prevalent of the pathogens in CF, causing chronic infection in up to 90% of adults and 80% of children

b) Initial colonization with nonmucoid forms c) Subsequent conversion to mucoid variants iii. Hemophilus influenzae

a) Commonly seen in babies b) Rarely encountered in the adults iv. Other pulmonary pathogens

a) Burkholderia cepacia b) Aspergillus

c) Mycobacteria d) Respiratory viruses d. Upper airway disease

i. Opacified or maldeveloped sinuses ii. Nasal polyps (up to 26% of patients) iii. Sinusitis

a) Facial pain b) Swelling c) Tenderness

d) Air-fluid levels on radiograph

265

e. Pulmonary exacerbations

i. Persistent cough and sputum production ii. Increased dyspnea

iii. Reduction in pulmonary function

iv. Increased hemoptysis: a life-threatening compli- cation but fatal hemoptysis is rare

v. Digital clubbing

vi. Changes in chest radiographic findings vii. Bronchiolitis

viii. Increased rales or rhonchi ix. Decreased air exchange

x. Increased use of accessory muscles of respiration xi. Spontaneous pneumothorax (5–8%), a life-

threatening complication

xii. Obstructive airway disease, leading to respiratory insufficiency associated with bronchiectasis 3. Gastrointestinal abnormalities

a. Intestine

i. Meconium ileus at birth

a) An obstruction of the distal ileum or proxi- mal colon with thickened, viscous meconium in 15–20% of CF patients developed in utero in the second trimester

b) Virtually diagnostic of CF

ii. Meconium peritonitis (rupture of the intestine secondary to complete obstruction)

iii. Meconium plug syndrome in the newborn infants, a more benign condition characterized by blockage of the colon

iv. Distal intestinal obstruction syndrome later in childhood, adolescence, or adulthood (10%), presenting as crampy abdominal pain, usually with decreased stooling

v. Rectal prolapse occurring in less than 1% of patients but may be the presenting symptom, particularly in infants

vi. Occasional intussusception vii. Gastroesophageal reflux b. Pancreas

i. Pancreatic exocrine insufficiency

a) Failure of the pancreas to produce sufficient digestive enzymes to allow breakdown and absorption of fats and protein

b) Obstruction of the pancreatic duct in utero with resultant progressive loss of exocrine pancreatic acini and their function

c) A hallmark of the disease, occurring in 90%

of patients by 1 year of age

d) Leads to frequent, bulky, foul-smelling, oily stools

e) Steatorrhea (presence of excessive undigested fat in the stool)

f) Failure to thrive is commonly in the patients with CF

ii. Recurrent pancreatitis in few patients

iii. CF-related diabetes mellitus, rare before 10 years of age

c. Liver

i. Prolonged obstructive jaundice in a few affected infants, presumably secondary to obstruction of

extrahepatic bile ducts by thick bile along with intrahepatic bile stasis

ii. Chronic hepatic disease manifested by clinical or histologic evidence of focal biliary cirrhosis or multi-lobular cirrhosis

4. Genitourinary manifestations

a. Congenital bilateral absence of the vas deferens in most males

i. Leading to azoospermia ii. A significant cause of infertility b. Infertility common in women

i. Due to increased amounts of thick mucus in the cervical canal

ii. An increased incidence of amenorrhea

iii. Occasional patients carrying pregnancies to term without significant respiratory deterioration 5. Skeletal manifestations

a. Hypertrophic pulmonary osteoarthropathy i. Rarely seen in children with CF

ii. Increasing frequency with increasing age and severity of disease

b. Triad of skeletal manifestations i. Clubbing of fingers and toes ii. Arthritis

iii. Periosteal new bone formation 6. Nutritional abnormalities

a. Failure to thrive, a common manifestation during infancy and beyond

b. Poor appetite c. Weight loss d. Fatigue

e. Prone to heat prostration

f. Hypoproteinemia with or without edema, anemia, and deficient fat-soluble vitamins A, D, E, and K

g. Peripheral neuropathy secondary to deficient vitamin E h. Delayed puberty largely due to nutritional factors

i. Potentially lethal protein-energy malnutrition in some infants

j. Development of some degree of malabsorption by 4 years of age in roughly 85% of patients

7. Salt losing syndromes a. Acute salt depletion

b. Chronic hypochloremic or hyponatremic alkalosis c. Excessive salt loss in the sweat potentially fatal for

patients exposed to moderate heat or during pro- longed hot weather

8. Prognosis

a. Respiratory failure: the leading cause of death in CF and occurs eventually in nearly all patients

b. Current median survival: approximately 35 years in the United States

c. Current data suggesting a lifespan exceeding 50 years for those diagnosed and treated early

DIAGNOSTIC INVESTIGATIONS

1. Newborn screening

a. Measuring blood immunoreactive trypsinogen in dried blood spots

i. Elevated levels in most CF infants (85–90% sen-

sitive)

ii. Associated with a relatively large number of false positive results

iii. Diagnosis must be confirmed by sweat tests or genotyping (CF multimutational analysis) b. Use DNA-based testing on dried blood spots by

CFTR multimutational analysis 2. Sweat test

a. The traditional method of CF diagnosis

b. Reliably identifies the vast majority of patients with CF who have multiorgan involvement including the lungs and pancreas

c. Using pilocarpine iontophoresis technique to produce sweat for chloride analysis

d. Sweat chloride concentrations i. >60 mEq/L, observed in:

a) CF patients with clinical manifestation of chronic pulmonary disease and/or pancreatic insufficiency

b) CF patients with positive family history c) Untreated Addison disease

d) Ectodermal dysplasia

e) Certain types of glycogen storage diseases f) Untreated hypothyroidism

g) A few normal adults ii. <60 mEq/L, observed in:

a) Very few patients with CF (fewer than 1% of CF patients when they are well)

b) CF patients with malnutrition and edema 3. Indications for sweat testing

a. Meconium ileus or peritonitis (neonate) b. Jaundice (infancy)

c. Hypochloremic alkalosis (infancy)

d. Heat prostration (infancy or adulthood in males) e. Failure to thrive (infancy to childhood)

f. Rectal prolapse (childhood)

g. Nasal polyposis (childhood to adulthood)

h. Panopacification of sinuses or pansinusitis (childhood) i. Pancreatitis (late childhood to early adulthood) j. Unexplained cirrhosis (childhood to adolescence) k. Gallstones (late childhood to early adulthood)

l. Congenital bilateral absence of the vas deferens or azoospermia (any age but more obvious in adults) m. Recurrent or persistent pneumonia (any age)

n. Staphylococcal pneumonia (any age but especially infants)

o. Mucoid Pseudomonas in lung (any age) p. Bronchiectasis (any age)

q. Positive family history in siblings or first cousins (any age)

4. Other clinical laboratory tests

a. Prolonged prothrombin time secondary to deficient vitamin K

b. Shortened red blood cell survival time secondary to deficient vitamin E

c. Hypochloremic alkalosis secondary to high salt loss d. Isolation of organisms from the respiratory tract

i. Pseudomonas aeruginosa ii. Burkholderia cepacia iii. Staphylococcus aureus e. Oxygen saturation

5. Radiographic findings a. Chest X-ray

i. Earliest finding: hyperinflation often with right upper lobe mucus retention

ii. Progression to widespread bronchial dilatation, cysts, linear shadows, and infiltrates

b. Sinuses X-ray: nearly universal presence of pansi- nusitis

c. Abdominal X-ray

i. Uncomplicated meconium ileus: multiple loops of dilated small bowel

ii. Meconium peritonitis

a) Calcifications within the abdominal cavity or even within the scrotum

b) Resulting from an in utero perforation or complicated meconium ileus associated with volvulus

d. Skeletal X-ray i. Arthritis

ii. Periostitis in the diaphysis of the tubular bones 6. CT scan, especially high-resolution CT scan

a. Readily identify air-trapping in the expiratory phase b. Mosaic perfusion, a secondary sign of small airway

abnormality

c. Centilobular nodule characterized by a cluster of ill- defined nodules corresponding to branching terminal airways filled with secretions

d. Classic findings of CF of progressive bronchiectasis involving the large airways

i. Thick parallel bronchial walls seen as linear shadows or tram-line tracts

ii. Ring shadows representing dilated thick-walled bronchi

iii. Multiple nodular densities representing mucus plugging

7. Pulmonary function test 8. Molecular testing

a. More than 1000 different mutations described to date b. ΔF508 mutation (deletion of a phenylalanine residue

at position 508 in the 1480 amino acid protein) i. Present in about 70% of CF alleles in the U.S.

ii. Homozygous ΔF508 mutation in about 50% of American CF patients

c. Higher frequencies of certain mutations in different ethnic groups

i. A455E mutation in Dutch CF patients

ii. W1282X mutation in Ashkenazi Jewish CF patients

d. Presence of two alleles with CF causing mutations confirms the diagnosis

e. Conventional commercial genotyping testing for about 80 specific mutations in CFTR (including ΔF508 muta- tion) identifies about 95% of all CF alleles

9. Carrier testing available for family members of known patients or for couples without a family history

GENETIC COUNSELING

1. Recurrence risk

a. Both parents carrying at least one abnormal copy of

the CF gene

b. Patient’s siblings

i. 25% with CF disease ii. 50% with CF carrier

c. Patient’s offspring: low recurrence risk unless the spouse is affected or a carrier of CF mutation 2. Prenatal diagnosis

a. Ultrasonography

i. Hyperechogenic fetal bowel suggestive of intes- tinal obstruction: perform carrier testing for CF gene mutations in the parents

a) Diagnosis of CF highly likely in the fetus if both parents are carriers of a CF mutation b) Confirmed CF diagnosis by direct mutation

analysis of amniotic fluid cells

ii. Meconium peritonitis secondary to small bowel perforation in utero

a) Associated with CF in only 7% of cases b) Presence of abdominal calcification usually

associated with causes of meconium peri- tonitis other than CF

c) Absence of abdominal calcifications favors the diagnosis of CF, to be confirmed by parental CF carrier testing and mutation analysis of fetal cells in at-risk couples b. Molecular genetic analysis

i. Prenatal diagnosis accomplished in most fami- lies with an affected member by mutation analysis from fetal cells obtained from CVS (10 weeks) or amniocentesis (15–18 weeks)

ii. Preimplantation genetic diagnosis to screen embryos before implantation, an alternative for at-risk couples a) A cleavage stage biopsy of embryo on day

two or three after in vitro fertilization b) Removal of one or two cells for genetic

analysis by nested polymerase chain reac- tion and heteroduplex formation

c) Transfer of normal or carrier embryos to establish pregnancy

d) Followed by CVS or amniocentesis to con- firm the original diagnosis

c. Postnatal sweat testing mandatory in all cases in which the diagnosis of CF has been made or excluded on the basis of prenatal DNA analysis

3. Management

a. Antibiotic treatment of acute respiratory infections and exacerbations of chronic endobronchial infections

i. Purpose

a) To improve pulmonary function b) To improve exercise tolerance c) To improve quality of life

ii. Short term oral ciprofloxacin for mild exacerba- tions by pseudomonas agent with caution in the younger pediatric age patients

iii. Intravenous antibiotics, a mainstay of treatment of acute exacerbations of pulmonary disease a) Aminoglycoside (usually tobramycin) and

β-lactam antibiotics

b) Implanted central lines or peripheral vein catheters to maintain IV access

iv. Aerosol (inhaled) antibiotics such as tobramycin v. Other aerosol antibiotics such as colistin

b. Airway clearance and exercise

i. Percussion and postural drainage in an attempt to keep the airways free of infected and obstructive secretions

ii. Positive expiratory pressure (PEP) devices to promote mucous clearance by preventing airway closure and increase collateral ven- tilation

iii. Therapy vest, a chest wall compression and oscillation system comprising a fitted vest cou- pled to a pneumatic compressor

iv. Flutter valve vibrating devices v. Directed exhalation techniques

c. Sodium cromolyn (Intal

) or nedocromil (Tilade

) beneficial in some patients

d. Use of inhaled (aerosolized) bronchodilators such as β-adrenergics

e. Use of inhaled alpha dornase (DNase) (Pulmozyme

) to reduce sputum viscosity by cleav- ing extracellular neutrophil-derived DNA and brings about sustained improvement in pulmonary function in many patients with CF

f. Avoid cough depressants g. Oxygen therapy if indicated h. Anti-inflammatory therapies

i. Glucocorticoids: useful in selected patients ii. Ibuprofen

i. Nutritional support to prevent malnutrition

i. Provide pancreatic enzyme supplements and vitamins to correct malabsorption

ii. Provide normal or high-fat diets with essential fatty acids necessary for growth and development iii. Provide fat-soluble vitamin supplements j. Management of gastrointestinal obstructive compli-

cations

i. Gastrograffin enema for treatment of fecal impaction with distal intestinal obstruction syndrome

ii. Surgery for intussusception if indicated

iii. Surgery necessary to correct obstruction due to meconium ileus in most newborns

k. Management of biliary tract disease

i. Treatment of severe focal or multi-lobular biliary cirrhosis, portal hypertension, and hyper- splenism

a) Sclerosing of esophageal varices b) Hepatic shunt procedures c) Liver transplantation ii. Cholecystectomy for gallstones

iii. Urodeoxycholic acid for selected cases of liver dysfunction

l. Management of CF-associated diabetes mellitus i. Dietary management

ii. Oral hypoglycemic agents for some patients iii. Insulin often required

m. Management of pneumothorax i. Chest tube drainage

ii. Followed by simple surgical oversewing of blebs if the initial air leak does not resolve

iii. Followed by definitive pleural ablation if

necessary

n. Management of hemoptysis i. Reassurance in most cases ii. Additional vitamin K

iii. Discontinue drugs that may interfere clotting iv. Bronchial artery embolization by interventional

radiologic techniques in unusual cases o. Management of respiratory failure

i. Oxygen administration

ii. Tracheal intubation and mechanical ventilation in terminally ill patients on the lung transplant waiting list

iii. Lung transplantation, the ultimate approach to irreversible respiratory failure

iv. Heart-lung and lung transplantation successfully applied to patients with end-stage CF patients p. Gene therapy in CF patients by in vivo gene transfers

(targeted lung delivery of aerosolized gene vectors)

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Fig. 1. This 5-month-old infant has history of failure to thrive, foul smelling stools and excessive spitting. The patient is positive for two copies of the ΔF508 mutation.

Fig. 2. A 16-year-old boy with cystic fibrosis showing an emaciate

chest, club fingers, and a honeycomb-like chest with air trapping

illustrated by chest radiographs.

Fig. 3. Photomicrograph of the pancreas of a 16-year-old male who died of bronchiectasis and bronchopneumonia. In the top photomicro- graph, the exocrine glands (acini) are mostly absent. Irregularly dilated exocrine ductules are lined by flattened atrophic epithelium. Many unaffected islets of Langerhans are present at left lower corner. In the lower photomicrograph with higher magnified, there are many islets of Langerhans but normal exocrine glands and ductules are absent.

Fig. 4. Photomicrograph of the duodenum of the same patient. The Brunner’s glands in the submucosa are distended by inspissated secre- tion. Three mucosal crypts are also slightly dilated.

Fig. 5. Fragment size separation of CFTR mutations using capillary gel electrophoresis (ABI 310 genetic analyzer, Applied Biosystems).

A mutation in ΔF508 is noted in red.