DEFINITION
Asthma is a disorder defined by its clinical, physiological, and pathological characteristics. The predominant feature of the clinical history is episodic shortness of breath, particularly at night, often accompanied by cough. Wheezing appreciated on auscultation of the chest is the most common physical finding. The main physiological feature of asthma is episodic airway obstruction characterized by expiratory airflow limitation. The dominant pathological feature is airway inflammation, sometimes associated with airway structural changes. Asthma has significant genetic and environmental components, but since its pathogenesis is not clear, much of its definition is descriptive. Based on the functional consequences of airway inflammation, an operational description of asthma is:
Asthma is a chronic inflammatory disorder of the airways in which many cells and cellular elements play a role. The chronic inflammation is associated with airway hyperresponsiveness that leads to recurrent episodes of wheezing, breathlessness, chest tightness, and coughing, particularly at night or in the early morning. These episodes are usually associated with widespread, but variable, airflow obstruction within the lung that is often reversible either spontaneously or with treatment. Because there is no clear definition of the asthma phenotype, researchers studying the development of this complex disease turn to characteristics that can be measured objectively, such as atopy (manifested as the presence of positive skin-prick tests or the clinical response to common environmental allergens), airway hyperresponsiveness (the tendency of airways to narrow excessively in response to triggers that have little or no effect in normal individuals), and other measures of allergic sensitization. Although the association between asthma and atopy is well established, the precise links between these two conditions have not been clearly and comprehensively defined. There is now good evidence that the clinical manifestations of asthma symptoms, sleep disturbances, limitations of daily activity, impairment of lung function, and use of rescue medications — can can be controlled with appropriate treatment. When asthma is controlled, there should be no more than occasional recurrence of symptoms and severe exacerbations should be rare.
MECHANISMS OF ASTHMA
Asthma is an inflammatory disorder of the airways, which involves several inflammatory cells and multiple mediators that result in characteristic pathophysiological changes. In ways that are still not well understood, this pattern of inflammation is strongly associated with airway hyper responsiveness and asthma symptoms. Airway Inflammation In Asthma The clinical spectrum of asthma is highly variable, and different cellular patterns have been observed, but the presence of airway inflammation remains a consistent feature. The airway inflammation in asthma is persistent even though symptoms are episodic, and the relationship between the severity of asthma and the intensity of inflammation is not clearly established. The inflammation affects all airways including in most patients the upper respiratory tract and nose but its physiological effects are most pronounced in medium-sized bronchi. The pattern of inflammation in the airways appears to be similar in all clinical forms of asthma, whether allergic, non-allergic, or aspirin-induced, and at all a ges.
I nf lammatory cell s. The characteristic pattern of inflammation found in allergic diseases is seen
in asthma, with activated mast cells, increased numbers of activated eosinophils, and increased numbers of T cell receptor invariant natural killer T cells and T helper 2 lymphocytes (Th2), which release mediators that contribute to symptoms. Structural cells of the airways also produce inflammatory mediators, and contribute to the persistence of inflammation in various wa ys. I nf lammatory mediators. Over 100 different mediators are now recognized to be involved in
asthma and mediate the complex inflammatory response in the airways.
Structural changes in the airways. In addition to the inflammatory response, there are
characteristic structural changes, often described as airway remodelling, in the airways of asthma patients. Some of these changes are related to the severity of the disease and may result in relatively irreversible narrowing of the airways. These changes may represent repair in response to chronic inflammation.
Pathophysiology Airway narrowing is the final common pathway leading to symptoms and physiological changes in asthma. Several factors contribute to the develo pment of airway narrowing in asthma.
Airway hyperresponsiveness, the characteristic functional abnormality of asthma, results in airway narrowing in a patient with asthma in response to a stimulus that would be innocuous in a normal person In turn, this airway narrowing leads to variable airflow limitation and intermittent symptoms. Airway hyperresponsiveness is linked to both inflammation and repair of the airways and is partially reversible with therapy. Its mechanisms are incompletely understood. Airway hyperresponsiveness.
