CYSTIC FIBROSIS

Cystic fibrosis (CF) is an autosomal recessive disorder occurring in 1 in 2000 live births (1,2).
CF is caused by a single gene that is expressed when a person has two CF alleles and no normal ones (3). It is a gene that creates "transport" proteins.

CF occurs when one of 1480 amino acids has been deleted at position 508. The single deletion changes the function of the protein, inhibiting cell transport of salt and water, causing CF (3). There are more than 20 different mutations that cause CF, four of which account for 85% of CF cases. The most common mutation, at position 508, is responsible for 70% (2,4).

CLINICAL TRIAD

• Chronic obstruction and infection of the respiratory tract.
• Exocrine pancreatic insufficiency.
• Elevated sweat chloride levels.

MANIFESTATIONS OF CYSTIC FIBROSIS

• Meconium ileus is the most common intestinal manifestation, occurring in
  10-15% of cases (1,2). Conversely 80% of neonates with meconium ileus will have cystic fibrosis.
• Ileal or jejunal atresia with or without volvulus (5).
• Meconium plug syndrome. 25% of neonates with meconium plugs will have cystic fibrosis.
• Meconium peritonitis secondary to bowel perforation.

ULTRASOUND

• Hyperechogenic intra-abdominal mass (no posterior shadowing usually).
  Seen in 73% of fetuses (6), but is less specific as a number of normal fetuses may have such an appearance.
• Absent gallbladder during the course of the examination. A collapsed often broad echogenic structure is usually found in the gallbladder fossa, corresponding to the collapsed gallbladder walls (6).
  Duchatel and co-workers (6) found that in 9 of 12 fetuses, no gallbladder could be demonstrated during a routine scan (positive predictive value = 100%, Specificity = 100%, and sensitivity = 75%).
  None of the 353 healthy control fetuses had an absent gallbladder during the scan.
• The presence of both an absent gallbladder and hyperechogenic intra- abdominal mass in the same fetus had a positive predictive value of 100%, a negative predictive value of 95%, a sensitivity of 80% and a specificity of 100%.

Small gallbladder and echogenic meconum impacted bowel

REFERENCES

1. Goldstein R, Filly R, Callen P. Sonographic diagnosis of meconium ileus in utero. J Ultrasound Med 1987;6:663.
2. Nyberg DR, Hastrup W, Watts H et.al. Dilated fetal bowel: A sonographic sign of cystic fibrosis. J Ultrasound Med 1987;6:257.
3. Davies K. Cystic fibrosis: The quest for a cure. New Scientist 1992;132:30.
4. Wilfond BS, Frost N. The cystic fibrosis gene: Medical and social implications for heterozygote detection. JAMA 1990;263:2777.
5. Estroff JA, Parad RB, Benacerraf BR. Prevalence of cystic fibrosis in fetuses with dilated bowel. Radiology 1992;183:677-680.
6. Duchatel F, Muller F, Oury JF et.al. Prenatal diagnosis of cystic fibrosis: Ultrasonography of the gallbladder at 17-19 weeks of gestation. Fetal Diagn Ther 1993;8:28-36.