Cystic fibrosis. 95/2

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Introduction

Cystic fibrosis is a common autosomal recessive disorder in Caucasians. The defect in a single gene results in the production of abnormally viscous mucus secretions causing recurrent chest infections, pancreatic insufficiency, malabsorption of food and intestinal obstruction in the newborn (meconium ileus). Current screening programmes identify six forms (alleles) of the gene, which account for 85% of cases, but over 350 cystic fibrosis alleles have now been reported.

Incidence

4-5% of the British population are carriers of the cystic fibrosis gene. Children have a one in four chance of having cystic fibrosis by inheriting two copies of this defective gene, if both parents are carriers. This happens once in every 2,500 live births, resulting in approximately 250 cystic fibrosis births in England and Wales each year. The incidence of clinical presentations of cystic fibrosis appears to have remained fairly constant in England and Wales though in some areas, eg East Anglia, it has halved in the past decade. This regional fall may be partly explained by the introduction of neonatal screening with prenatal diagnosis in subsequent pregnancies.

Prevalence

In England and Wales, there were an estimated 5,200 people with cystic fibrosis in 1990, representing about 20-30 patients per NHS District. This compares with an estimated 3,700 in 1980. The cystic fibrosis population is currently growing by about 100 cases per year in England and Wales and is projected to rise above 6,000 by 1997. These changes are due to improvements in life expectancy for cystic fibrosis patients. One third of patients are now aged over 16 and the increase will be seen in this age-group. The total number of children with cystic fibrosis will remain fairly constant.

Life expectancy and mortality

Figure 1:Deaths from cystic fibrosis, males and females

The total number of deaths from cystic fibrosis has changed little since 1960 and is currently about 120 deaths per year. However, the distribution of age at death has changed, with a decrease in mortality rates among children and increases among adults.

Mortality from cystic fibrosis is higher in the first year of life than in subsequent years, although first year mortality has also improved. The mortality rate in the first year of life was 40% for the 1960 cohort, 16% for the 1970 cohort, 4% for the 1980 cohort and only 1% for the 1990 birth cohort.

Figure 2: Age-specific death rates from cystic fibrosis males and females

Survival curves for successive cohorts show clearly the dramatic increases in survival rates. Median life expectancy at birth has similarly improved from under 10 years in affected children born in 1960, to 20 years in those born in 1970 and is estimated to be 40 years for the 1990 birth cohort.

Figure 3: Survival curves for cystic fibrosis by birth cohort males and females

The trend towards higher age at death has been attributed to earlier diagnosis, improved management of meconium ileus, diet, physiotherapy, new antipseudomonal antibiotics and the development of specialist centres. The relative contribution of each remains uncertain but the transition from a terminal childhood condition to one producing a substantial adult caseload has significant implications for resource allocation.

Survival differences

Survival is also influenced by patient factors such as sex, social class and region of residence. The median age at death, certified as due to cystic fibrosis, is 2 years higher in males compared to females, and 3 years higher in non-manual social classes compared to manual social classes. After adjusting for sex and social class, differences in age at death between regional areas has also been shown.

Screening

Pilot programmes to detect cystic fibrosis carriers have been mainly concerned with pregnant women and their partners. Testing for the 6 commonest gene mutations in cystic fibrosis will detect 85% of cystic fibrosis genes, that is, 70% (85% x 85%) of couples where both partners are carriers. In such pregnancies the risk of having an affected child is 1 in 4 and pre-natal diagnosis followed by termination can be offered. The uptake rate for such screening is 70-80%.

Heart-lung transplantation

Since the first successful heart-lung transplant for cystic fibrosis in 1985, heart-lung transplantation has become an established treatment for end-stage cystic fibrosis. The best one year survival rates are 70-80%. In 1990-92 less than 40 transplants were carried out each year. However, with the projected increase in the cystic fibrosis population it is estimated that there will be 85-130 such potential transplant recipients each year over the next decade.

Summary

• Cystic fibrosis is a common autosomal recessive disorder in Caucasians, giving rise to approximately 250 affected births each year in England and Wales.

• There were an estimated 5,200 sufferers in 1990; current predictions suggest this will rise to over 6,000 by the year 2000. This expansion will be seen in the adult population, the number of children with cystic fibrosis remaining fairly constant.

• The median age at death is increasing. Cohorts born today have a longer life expectancy at birth compared with cohorts born in the 1960s.

• Heart-lung transplantation is successful. The number of potential recipients is likely to increase over the next decade.

Footnotes

Autosomal recessive. A disease which is not sex-linked, and occurs only when both inherited genes, one from each parent, are defective.
Carrier. A person who has one normal gene and one defective recessive gene. They are completely healthy and do not have the disease but their children may inherit the defective gene. The children of two carriers have a one-in-four chance of getting the disease and a one-in-two chance of being carriers.
Life expectancy. The expected length of life still to be lived at a specific age.
Median. The 'middle' observation when the observations are arranged in ascending or descending order.

Reference

Elborn JS et al. Thorax 1991;46:881-5

We are grateful to Dr John Britton for help and advice in preparing this factsheet.