Asthma has puzzled and confused physicians from the time of Hippocrates to the present day.


Asthma is a disease of the bronchial tubes (the “airways”) that typically presents with “wheezing”, a high-pitched whistling sound heard during breathing, especially when breathing out. However, wheezing does not always occur, and asthma can also involve shortness of breath or coughing, particularly in children.

Asthma most commonly develops in early childhood, and more than three-quarters of children who develop asthma symptoms before age 7 no longer have symptoms by age 16. However, asthma can develop at any stage in life, including adulthood.

A centuries-old puzzle

Asthma has puzzled and confused physicians from the time of Hippocrates to the present day. The word “asthma” comes from a Greek word meaning “panting”, but reference to asthma can also be found in ancient Egyptian, Hebrew and Indian medical writings. There were clear observations of patients experiencing attacks of asthma in the 2nd century, and evidence of disordered anatomy in the lung as far back as the 17th century.

A subject of controversy

With the advent of anatomic pathology, the differing factors leading to airway obstruction were observed. It was noted that external factors such as allergen exposure could induce attacks. The similarities with anaphylaxis (severe allergic reaction) prompted consideration of asthma as an allergic disease. This evolution in understanding has been reflected in many attempts to define asthma, and such definitions have steadily evolved from clinical descriptions to encompass physiological and pathological features. Nevertheless, the definition and classification of asthma has continued to be a subject of controversy.

1959: The cardinal feature of asthma defined

In modern times, the cardinal clinical feature of asthma, reversible airflow obstruction, has formed the basis of the definition of asthma. For example, 
the definition initially proposed at the CIBA Foundation conference in 1959, and endorsed by the American Thoracic Society in 1962, is that "asthma is a disease characterised by wide variation over short periods of time in resistance to flow in the airways of the lung". In elaborating this definition, the American Thoracic Society introduced the characteristic of hyperreactivity of the airways as a feature that would usually (but not always) be present in asthma.

Subsequently it has been proposed that this phenomenon of bronchial hyperreactivity might be the unifying mechanism underlying the range of disorders encompassed by the term asthma. However, it has been demonstrated that people with clinical asthma may have normal bronchial reactivity, that people without clinical asthma may have enhanced bronchial reactivity, and that there is a poor correlation between current asthma severity and the degree of bronchial hyperreactivity. As a result, while bronchial hyperreactivity may be present in many asthmatics, it is no longer considered to be synonymous with asthma.

1992: Asthma defined as a chronic Inflammatory disorder

  • wheezing
  • breathlessness
  • chest tightness
  • cough
  • sputum production
  • chronic airways inflammation
  • reversible airflow obstruction
  • enhanced bronchial reactivity

Recently the major clinical and physiological characteristics of asthma have been 
incorporated in an operational definition, which also recognises the underlying disease mechanisms. In this way the International Consensus Report on the Diagnosis and Treatment of Asthma defines asthma as "a chronic inflammatory disorder of the airways in which many cells play a role, including mast cells and eosinophils. In susceptible individuals this inflammation causes symptoms which are usually associated with widespread, but variable, airflow obstruction that is often reversible either spontaneously or with treatment, and causes an associated increase in airway responsiveness to a variety of stimuli."

These three components: chronic airways inflammation, reversible airflow obstruction and enhanced bronchial reactivity form the basis of this current definition of asthma. They also represent the major pathophysiological events leading to the symptoms of wheezing, breathlessness, chest tightness, cough and sputum production by which physicians clinically diagnose this disorder.

Asthma is an allergic disease: Yes, no, maybe

In recent decades it has become routine to describe asthma as an allergic disease. A theoretical paradigm has evolved in which allergen exposure produces sensitisation to allergens, and continued exposure leads to clinical asthma through the development of airways inflammation, reversible airflow obstruction, and enhanced bronchial reactivity.

However, it has been acknowledged that not all cases of asthma fit this paradigm, for example, some occupational causes of asthma do not appear to involve allergy. In fact, it is now widely recognised that at most one-half of asthma cases, in both children and adults, involve allergic mechanisms. The majority of the cases probably involve non-allergic mechanisms, including non-allergic inflammation of the airways. These non-allergic mechanisms are currently not well understood.

One disease or multiple conditions resulting in the same effect?

Although asthma’s main characteristic –variable airways obstruction– is well-established and relatively easy to diagnose, the underlying mechanisms (both allergic and non-allergic) are not well understood. It is therefore currently unclear whether asthma is a single disease, with a single underlying causal mechanism, or if it is in fact a grouping of different conditions which all result in the same clinical effect, i.e., variable airways obstruction.

- Neil Pearce, David Strachan

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