Cystic fibrosis Flashcards
What is cystic fibrosis?
Cystic fibrosis (CF) is an autosomal recessive disorder causing increased viscosity of secretions (e.g. lungs and pancreas).
What causes CF?
It is due to a defect in the cystic fibrosis transmembrane conductance regulator gene (CFTR), which codes a cAMP-regulated chloride channel
Which chromosome has a defect in CF patients?
Chromosome 7
What is the epidemiology of cystic fibrosis?
Cystic fibrosis affects 1 per 2500 births, and the carrier rate is c. 1 in 25
What is the most comon mutation in CF?
delta-F508
What are the 3 biggest complications of cystic fibrosis?
- Thick pancreatic and biliary secretions that cause blockage of the ducts, resulting in a lack of digestive enzymes such as pancreatic lipase in the digestive tract
- Low volume thick airway secretions that reduce airway clearance, resulting in bacterial colonisation and susceptibility to airway infections
- Congenital bilateral absence of the vas deferens in males. Patients generally have healthy sperm, but the sperm have no way of getting from the testes to the ejaculate, resulting in male infertility
What 4 organisms commonly colonise in CF patients?
- Staphylococcus aureus
- Pseudomonas aeruginosa
- Burkholderia cepacia
- Aspergillus
What are the symptoms of CF?
- Chronic cough
- Thick sputum production
- Recurrent respiratory tract infections
- Loose, greasy stools (steatorrhoea) due to a lack of fat digesting lipase enzymes
- Abdominal pain and bloating
- Parents may report the child tastes particularly salty when they kiss them, due to the concentrated salt in the sweat
- Poor weight and height gain (failure to thrive)
What are the 4 main presenting features?
- neonatal period (around 20%): meconium ileus, less commonly prolonged jaundice
- recurrent chest infections (40%)
- malabsorption (30%): steatorrhea, failure to thrive
- other features (10%): liver disease
What are the signs of cystic fibrosis?
- Short stature
- diabetes mellitus
- delayed puberty
- Rectal prolapse (due to bulky stools)
- nasal polyps
- Male infertility, female subfertility
What is the gold standard investigation for CF?
the sweat test
What is involved in the sweat test?
- A patch of skin is chosen for the test, typically on the arm or leg.
- Pilocarpine is applied to the skin on this patch.
- Electrodes are placed either side of the patch and a small current is passed between the electrodes. This causes the skin to sweat.
- The sweat is absorbed with lab issued gauze or filter paper and sent to the lab for testing for the chloride concentration.
- The diagnostic chloride concentration for cystic fibrosis is more than 60mmol/l.
Name some causes of a false positive sweat test?
- malnutrition
- adrenal insufficiency
- glycogen storage diseases
- nephrogenic diabetes insipidus
- hypothyroidism, hypoparathyroidism
- G6PD
- ectodermal dysplasia
What is the most common cause of a false positive result in a sweat test?
skin oedema, often due to hypoalbuminaemia/ hypoproteinaemia secondary to pancreatic exocrine insufficiency.
What two other diagnostic test for CF are there?
- Newborn blood spot testing
- Genetic testing for CFTR gene
