Asthma and Factors Affecting it
Although physicians first recognised asthma more than 1800 years ago, over the past four decades our understanding of underlying pathophysiology and different clinical presentations has developed rapidly.
Asthma remains one of the most important non-communicable diseases. It is a cause of substantial disability and death worldwide. As such, asthma requires global attention and commitment to lessen its burden.
During much of the 20th century, researchers thought that constriction of airway smooth muscle due to excessive sensitivity of the airway to external stimuli (hyper-responsiveness) was the key feature of asthma. In the 1980s, it was recognised that airway inflammation was a cardinal feature, with structural changes in the airway (remodelling) present early in the development of disease.
The definition and classification of asthma has also evolved. The Global Initiative for Asthma (GINA) describes asthma as “a heterogeneous disease, usually characterised by chronic airway inflammation. It is defined by the history of respiratory symptoms such as wheeze, shortness of breath, chest tightness and cough that vary over time and in intensity, together with variable expiratory airflow limitation”. This description captures the essential features of asthma for clinical purposes.
For population-based studies, where doctor diagnosis is not practicable, questionnaires are the tool of choice. Questions about more recent symptoms (in the past 12 months) are more reliable than questions about symptoms further in the past, reducing errors of recall. The most commonly used standard question is “Have you [has this child] had wheezing or whistling in the chest in the past 12 months?”; when the answer is ‘yes’, the term ‘current wheeze’ is commonly used, or ‘current asthma symptoms’. However, asthma can also cause shortness of breath, chest tightness and cough.
In 2018, a Lancet Commission suggested a range of new ways of thinking about asthma, its mechanisms and its treatment, challenging conventional concepts of asthma as a single disease and proposing a more targeted approach. Notwithstanding these novel ideas, it remains clear that most people with asthma symptoms improve with asthma medicines. The use of the term asthma as a clinical diagnosis is still useful in most patients because it opens the door to appropriate management to reduce disease burden; however in low- and middle-income countries (LMICs), where most of the people with asthma live, this basic asthma care may still be non-existent or out of reach.
The two key essential asthma medicines are: (i) relievers (most commonly β2-agonists) that reverse airway narrowing by relaxing airway smooth muscle; and (ii) corticosteroids, which treat the underlying airway inflammation. Inhaled corticosteroids (ICS) are known as preventers (called “controllers” by GINA). The inhaled route is more effective and has fewer side effects than the oral route. In 2017 the World Health Organization added an ICS/long-acting β2-agonist (LABA) combination to its Essential Medicines List. Approaches to management with these medicines, and access to them are discussed in Chapters 5, 13 and 15.
Course of asthma over the lifespan
It is not possible to define a single natural history for asthma and it can develop at any stage in life, including adulthood. However, asthma symptoms most commonly develop for the first time in early childhood. Young children of pre-school age often wheeze with viral infection, but only about half of them go on to have characteristic asthma at school age. Children who have frequent or persistent wheeze are more likely to have evidence of airway inflammation and remodelling, impaired lung function, and persistently troublesome symptoms into adulthood. Some reports raise the possibility that childhood asthma, persisting into adulthood, may predispose people to chronic obstructive pulmonary disease (COPD).
Factors affecting asthma
It has been believed that asthma is an allergic disease in which allergens or certain workplace exposures can trigger attacks of airway narrowing and, through continued exposure, lead to airway inflammation and enhanced airway responsiveness. However when populations are compared around the world, the proportion who are sensitive to common allergens (“atopic” individuals) is not correlated with asthma symptom prevalence and the majority of wheezing even in high-income centres is not atopic. The proportion of wheezing attributable to atopy is greater in high-income centres, but the majority of wheezing even there is not atopic (see Figure). Some occupational causes of asthma do not appear to involve allergy. In many people, asthma probably involves non-allergic inflammation of the airways, although we do not understand well the mechanisms involved.
Research has found that both genetic and non-genetic factors affect asthma. Asthma attacks are often triggered by upper respiratory tract infections, including common colds. Other factors that may provoke asthma attacks include inhaled allergens (dust mites, animal fur, cockroaches, pollens, moulds, allergens in the workplace), inhaled irritants (cigarette smoke, fumes from cooking, heating or vehicle exhausts, cosmetics, aerosol sprays), medicines (including aspirin), exercise, emotional stress, and certain foods or beverages.
However, there is no recognised cause, either biological or environmental, for the underlying asthmatic process. In addition to triggers, asthma may be influenced by genetic susceptibility, environmental tobacco smoke, air pollution, mould and damp, animals, antibiotics and paracetamol (acetaminophen), some occupational exposures, diet and obesity, and (lack of) breastfeeding. Repeated exposure to triggers and/or influencers can contribute to the severity and persistence of asthma.
The understanding of asthma has developed since the 1980s, with new ways of thinking about asthma recently proposed. It is vital that essential asthma medicines are accessible and affordable for all people who have asthma symptoms. At the same time, commitment to and investment in research is essential to increase the understanding of asthma and its causes, potentially leading to improvements in asthma management.
Environmental factors are much more likely than genetic factors to have caused the increases in asthma prevalence in some regions of the world, but we still do not know all the factors and how they interact with each other and with genes. Some of these factors may act in different ways in affluent and non-affluent countries.