Asthma - extent of asthma in the UK

Written by Dr George Russell, reader in child health, University of Aberdeen

Chronic inflammatory airways disease
The mucous membrane lining the bronchial tree is inflamed in asthma, and the demonstration of such inflammation is perhaps the most robust method of diagnosis.

Unfortunately, this involves obtaining a sample of the mucous membrane, and/or secretions from the lung, an invasive procedure followed by expensive laboratory tests. Such an approach is not practicable for epidemiological purposes.

Simpler tests are becoming available, and may soon reach the stage when they can be applied to populations rather than patients.

A gene-environment interaction
Both academia and the drug industry are currently committing large amounts of money and effort to the search for genes that predispose a person to asthma.

Such genes (of which there are likely to be many) will identify potential asthmatics who:

under appropriate circumstances, will manifest the disease

under a different set of environmental conditions may remain perfectly well.

Summary of asthma diagnosis
It is apparent that it really does depend on what you mean by asthma, and that in the absence of a gold standard diagnostic test, the precise extent of asthma is inevitably uncertain. Nevertheless, we do have a vast amount of information about the prevalence of doctor-diagnosed asthma, and of symptoms suggestive of asthma, and while the data may lack precision, we know a great deal about the epidemiology of asthma.

Asthma in the past
A long time ago, asthma was probably quite uncommon, but in the absence of properly conducted surveys, it is difficult to be sure.

In Victorian times, Hyde Salter, a London physician with a special interest in asthma, felt that it was underdiagnosed, although he made no guess as to its true prevalence.

A century later, medical papers written in the 1950s and 1960s identified it as the commonest chronic disorder of childhood, the commonest cause of absence from school, and a major cause of absence from the workplace.

By the 1970s it was apparent that asthma was badly underdiagnosed in the UK. In the past, this would not have mattered, as there was little treatment to offer milder cases. However, the late 60s and early 70s saw the development of:

selective beta2 bronchodilators, eg salbutamol (Ventolin)

sodium cromoglicate (Intal)

inhaled corticosteroids, eg beclometasone (Becotide).

With these safe and effective therapies available, underdiagnosis became important because now the associated undertreatment really mattered.

The 70s and 80s also saw significant increases in the numbers of patients, particularly children, admitted to hospital with a diagnosis of asthma.

It was unclear at the time if these increased admissions reflected:

a change in hospital diagnostic practice

a change in GP referral habits

a true increase in the prevalence of asthma in the community at large.

Because of the previous underdiagnosis of asthma, which remains a problem [7], my own suspicion at the time was that the hospital statistics reflected nothing more than a change in diagnostic labelling of wheezing episodes. Others (rightly, as it turned out) were persuaded that asthma was becoming more common.

There then followed large numbers of asthma surveys, and in the remainder of this article I will try to summarise the findings from these studies.

Changes in the prevalence of asthma
It is now quite clear that, over the past 30 years or so, there has been an increase in doctor-diagnosed asthma, particularly in the younger age groups [8]. This increase has attracted a great deal of media attention, with talk of:

an asthma epidemic

the asthma generation.

This increase reflects:

changes in the diagnostic labelling of patients with wheeze

Increases in the prevalence of symptoms suggestive of asthma, particularly wheeze [9]

increases in conditions commonly associated with asthma, such as eczema and hay fever.

Numerous studies have now presented data confirming the increase in the prevalence of asthma [10].

No study has shown a decline.

There are some hints that the prevalence might now be stabilising.

More severe asthma may be declining due to:

environmental change

better treatment.

In the forefront of these research endeavours have been workers in the academic Department of Public Health Medicine at St George's Hospital Medical School, where a Lung and Asthma Information Agency (LAIA) website is maintained.

Current prevalence of asthma
The LAIA factsheet on the prevalence of asthma in Great Britain provides data on childhood asthma and shows:

the increase in asthma has been much greater than the increase in wheeze

the proportion of wheezy children diagnosed as asthmatic has risen from well under a half in the 60s to about two-thirds in the 80s.

The true prevalence of asthma probably lies somewhere between the figure for asthma and the figure for wheeze.

Since that factsheet was produced, there have been further rises in the prevalence of wheezing illness in both adults and children. The prevalence of asthma is probably:

at least 15 per cent in children

about half that proportion in adults.

Given the difficulties in obtaining reliable estimates of the prevalence of asthma, these figures are approximate.

Data from the European Respiratory Health Survey suggest that in the UK:

about one quarter of adults between 20 and 44 suffer from wheeze

in 15 per cent there was wheeze with breathlessness

the prevalence of doctor-diagnosed asthma was around 7 per cent.

In this age group, young enough to make the diagnosis of COPD unlikely, the principal cause of wheeze with breathlessness is asthma. The wide discrepancy between the prevalence of wheeze with breathlessness, and of asthma, illustrates the difficulty in giving precise figures. Absolute numbers are therefore unreliable.

Despite these problems, there is still a great deal that epidemiology can tell us about asthma.

Sex differences influence the prevalence of asthma
In childhood there is a clear sex difference in the prevalence of asthma.

In middle childhood, the male:female ratio may be as high as 2:1, though the male preponderance has been somewhat less in more recent studies.

At puberty, this ratio reverses.

In adults there is a female preponderance.

The role of sex hormones is also indicated by:

the effects of pregnancy on asthma - in some women asthma improves and in other women asthma worsens.

the effects of the monthly hormonal cycle on asthma - there is often pre-menstrual worsening.

Environmental effects on the prevalence of asthma
The effect of environmental factors have been studied extensively, but with few consistent results.

Area of residence

No difference between Newcastle and Cumbria.

No difference between Scottish Highlands and urban areas.

Asthma is more common in rural than in urban settings and in the inner cities than in suburbia.

Increased asthma in association with deprivation

Partly due to environmental tobacco smoke exposure.

Protective effect from being brought up on a farm

A very different matter from simply having an address in the country.

May reflect early exposure to soil bacteria.

Atmospheric pollution

Little or no effect on the prevalence of asthma.

Marked effect on the incidence of attacks of asthma.

Environmental tobacco smoke exposure

Prevalence of asthma in children is about 25 per cent higher in smoking than in non-smoking families.

Intrauterine exposure has an adverse effect that is independent of the effects of exposure after birth.

In adults, the effects of passive smoking on the prevalence of asthma are less dramatic but still important.

Exposure to passive smoking is also an important avoidable factor that can precipitate asthma attacks.

There is a strong association between smoking and deprivation.

Diet
The effects of anti-oxidants:

vitamins A, C and E protect against oxidising chemicals present in the atmosphere.

anti-oxidant consumption has fallen during the period when asthma has been increasingly so dramatically.

the development of asthma in adults is associated with low anti-oxidant levels.

The effects of fatty acids:

studies from Australia have found a protective effect of eating oily fish

supplementing omega-3 polyunsaturated fatty acids given in a special margarine had no beneficial effect.

Infant feeding:

possible protective role of breastfeeding - a controversial topic, with studies showing increased, decreased and unchanged prevalence of asthma.

a recent study demonstrated a beneficial effect [11].

Physical activity and fitness
Risk factors for asthma include both:

obesity

physical inactivity.

During the period when asthma has become more prevalent:

the average weight of children has increased

the average weight of adults has increased

cardiovascular unfitness has become widespread.

Statistically, there is increasing evidence in this chicken and egg situation that asthma may be the egg.

Weight reduction has a beneficial effect on lung function is obese asthmatics [12].

Season of birth

Season of birth affects age of first exposure to seasonal allergens such as grass pollens.

Generally, asthma is increased in children born in the Spring and Summer months.

Birth weight

Children who, at birth, were relatively heavy for their length have a slightly increased prevalence of asthma.

Family position and size
Asthma is more common in firstborn than subsequent children.

May reflect protective effects of upper respiratory tract infection.

Younger children in a family are exposed to additional respiratory viruses brought into the household by older children.

Infection
The allergic reaction is an immune reaction.

Normally stimulated by parasites such as worms.

Th2 reaction involving the production of Immunoglobulin E (IgE).

Lack of parasitic infestation might explain some of the increase in allergies during the past 30 years.

Th2 reaction diverted towards innocuous foreign proteins such as the house dust mite.

'Ordinary' infections stimulate Th1 reaction with production of Immunoglobulin G (IgG).

Lack of IgG stimulation might divert the immune system from Th1 to Th2 reaction (IgG to IgE).

Lack of contact with soil organisms called mycobacteria might be an important factor underlying the increase in asthma.

Infection not entirely benign.

Respiratory virus infection in infancy can precipitate recurrent wheezing.

Upper respiratory tract infection is the most common trigger of asthma attacks.

Immunisation

Pertussis vaccine has been associated with a predisposition to asthma, but other studies have shown no effect.

BCG immunisation (against tuberculosis, using a mycobacterium) may have a protective effect.

These data are insufficiently robust to recommend any change in immunisation practice.

Asthma in athletes

Asthma is more common among elite athletes - females more so than males.

Particularly associated with rigorous training (>20 hours a week).

Prolonged exercise might predispose to asthma.

'Western' lifestyle

Asthma is more common in countries with what a Western lifestyle.

Asthma is less common in developing countries.

No evidence that asthma can be reduced by changes in lifestyle.

Factors that have been implicated include:

wall-to-wall carpeting, sealed glazing units and central heating, which together conspire to produce an environment conducive to the reproduction of house mites.

modern furniture materials that release fumes such as formaldehyde.

wall coverings of PVC and other materials containing plasticisers.

reinforced concrete as a building material.

moulds in central heating and air conditioning vents.

use of beds with mattresses, pillows and blankets.

increased cleanliness.

reduced breastfeeding.

use of pesticides and other agrochemicals.

use of bottled gas, kerosene and other heating fuels.

damp housing conditions.

use of gas for cooking.

use of pre-prepared foods.

exposure to cockroach allergens.

increasing pet ownership, particularly of cats.

Environmental influences on the incidence of asthmatic attacks
The factors precipitating attacks of asthma, often known as triggers, are quite different from those that predispose to the development of the asthmatic condition.

Upper respiratory tract infection

By far the most common factor that triggers asthmatic attacks.

An important cause of diagnostic confusion, leading to inappropriate and unnecessary prescription of antibiotics.

Exercise
A common problem that interferes with many everyday activities. The mechanism is described below.

Normal nasal breathing at rest warms and humidifies the inspired air, so that the air reaching the bronchi is more or less fully saturated and at body temperature.

During exercise, the lungs are presented with a much greater volume of air than normal, and mouth breathing ensures that this air is drier and cooler than usual.

Drying and cooling of the airways trigger reflex bronchospasm, and the result is an asthma attack.

Consumption of chilled and acid drinks

Chilled drinks and those with a low pH (especially cola drinks).

Commoner in non-Europeans, and presumably reflects a reflex from oesophageal (gullet) stimulation.

Nocturnal asthma

Asthma often worst in the small hours of the night. Shutting down for the night involves:

increased in bronchial muscle tone

increased irritability as a protection against the possibility of inhaling foreign substances during sleep.

Exposure to dust mites in the bedding may also play a part.

Atmospheric pollution

Urban smog precipitates asthmatic attacks.

Hospital Accident and Emergency departments report marked rises in asthma-related attendances during periods of high pollution.

At its most extreme, this rise can be as much as seven times the level seen during open, breezy weather.

In the north of England, children living near open cast mines consult their doctors about respiratory conditions more often than controls.

Within the home, in addition to tobacco smoke, exposure to a wide variety of chemicals such as paints, furniture polish and cosmetics can precipitate asthma attacks.

Allergens

Best known is pollen asthma.

Asthma can also occur following exposure to animals, such as horses, cats and dogs.

Asthma may also occur on bed-making due to house dust mite .

Asthma can also occur following exposure to foods such as eggs and cow's milk.

Seasonality
On children's units a dip in asthma admissions is seen every summer. This is then followed by an autumn epidemic. Asthma mortality is also increased in the autumn.

In adults there is a similar pattern, though the peaks and troughs are less dramatic.

The reasons for these variations are numerous:

the weather itself may play a part

variations in the concentrations of various aero-allergens. In particular, the house dust mite thrives in the moist, warm conditions of the autumn.

Weather
Many asthmatics identify specific weather conditions that they feel upset their asthma.

Thunderstorms are frequently implicated:

association has been confirmed by epidemiological studies

association weak, and the risk to asthmatics before, during and after thunderstorms is slight.

Psychological and other stressors

Stress can aggravate asthma.

At one time many doctors thought that psychological factors were paramount.

There is a danger in these days when medical therapy is so effective, that this aspect of asthma may be overlooked.

Occupational asthma

A serious problem for both employers and employees.

Numerous substances used in the workplace can trigger allergic reactions, ranging from solder flux to the urine of laboratory animals [6].

Usually clues, though these are often missed:

'ordinary' asthma is often bad first thing in the morning, and lung function tests may show a 'morning dip'

in contrast, industrial asthma may resolve after an overnight absence from the workplace

industrial asthma also tends to improve over the weekend, and to disappear completely during holidays

a common mistake is to blame work-related psychological stress.

Aspirin

Aspirin hypersensitivity relatively uncommon, but important not to miss.

In addition to asthma, these individuals also have nasal polyps.

Conclusions
We are unclear about exactly what we mean by asthma.

We do know that it is becoming more common, especially in children.

Other allergic disorders are also becoming more common - the increase is not confined to asthma.

We know quite a lot about who gets asthma, but not really why, and although we have some ideas of how to avoid getting asthma, we don't know if these ideas work in practice.

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