Asthma

Introduction

Asthma is a major health problem worldwide, resulting in significant morbidity and mortality. The prevalence of asthma varies significantly between regions across the world. In Australasia, New Zealand and the United Kingdom (UK), it is thought to affect about 20% of children and 10% of adults. Sufferers tend to present to emergency departments (EDs) when their usual treatment plan fails to control symptoms adequately. The respiratory compromise caused can range from mild to severe and life threatening. For these patients, the main role of the emergency care is therapeutic. Other reasons for patients with asthma to attend an ED include having run out of medication, having symptoms after a period of being symptom- and medication-free and a desire for a ‘second opinion’ about the management of their asthma. For this smaller group, the primary role is one of educating them about the disease, planning an approach to the current level of asthma symptoms and referring them to appropriate health professionals for long-term care.

Epidemiology

Data from the Global Initiative for Asthma suggest that more than 300 million people in the world are currently affected by this disease. Australasia, the UK and North America have a greater prevalence of asthma than the Middle East and some Asian countries. There is also considerable geographical variation in severity, with Australasia reporting the highest proportion of severe disease. The reason for this geographical variation is unclear but may relate in part to ethnicity, rural versus metropolitan environment and air pollution. A number of epidemiological studies suggest that the prevalence and severity of asthma is slowly increasing worldwide.

Aetiology, pathophysiology and pathology

Asthma is characterized by hyper-reactive airways and inflammation leading to episodic, reversible bronchoconstriction in response to a variety of stimuli. It is a complex immunologically mediated disease. There is strong evidence that it is inherited, although no single gene is directly implicated. A polygenic basis is likely to account for asthma’s wide clinical spectrum.

Triggers of the immunological response (e.g. an extrinsic allergen, viral respiratory tract infection, pollutants, occupational exposures, emotion, exercise and drugs such as aspirin and β-blockers) cause an exaggerated inflammatory response with activation of various cell types including mast cells, eosinophils, basophils, Th-2 cells and natural killer cells. This leads to the release of primary mediators, including histamine and eosinophilic and neutrophilic chemotactic factors as well as secondary mediators, including leukotrienes, prostaglandins, platelet-activating factor, interleukins and cytokines. These result in bronchoconstriction via direct and cholinergic reflex actions, increased vascular permeability (resulting in oedema) and increased mucous secretions.

Pathophysiologically, the effects of acute asthma are

• increased physiological dead space

• respiratory muscle fatigue

• intrinsic positive end-expiratory pressure secondary to hyperventilation with air trapping

Thunderstorm asthma

Thunderstorm asthma is rare. Research suggests that it is usually triggered by an uncommon type of thunderstorm that causes grass pollen to be swept up into the clouds as the storm forms. The pollen then absorbs moisture, bursts open and releases large amounts of smaller allergen particles that are blown down to ground level. These particles are very small and, unlike the pollen itself, can be breathed deeply into the lungs, in some people causing irritation and asthma symptoms. Such events usually occur in late spring and early summer when the pollen load is highest.

People at increased risk of thunderstorm asthma may have a history of asthma, unrecognized asthma, hay fever and particularly seasonal hay fever; or they may be allergic to grass pollen. That said, people without these risk factors can also suffer severe symptoms.

Clinical assessment

The aims of clinical assessment are confirmation of the diagnosis, assessment of severity and identification of complications.