ASTHMA: Chronic Inflammatory Airway Disease With Reversible Airflow Obstruction
Asthma is a chronic inflammatory disorder of the airways characterised by episodic, reversible airflow obstruction and bronchial hyperresponsiveness. The disease is defined not only by airway narrowing, but by an underlying inflammatory process that makes the airways excessively sensitive to otherwise harmless stimuli. This hyperreactivity leads to recurrent episodes of wheeze, breathlessness, chest tightness and cough, particularly at night or in the early morning.
Although asthma can present at any age, it commonly begins in childhood and follows a variable course. Some individuals experience infrequent, mild symptoms, while others develop severe, persistent disease with significant morbidity. Understanding the pathophysiology of asthma explains why symptoms fluctuate, why triggers vary between patients and why treatment focuses on inflammation as much as bronchoconstriction.
What You Need to Know
Asthma is a chronic inflammatory disorder of the conducting airways in which inflammation leads to narrowing of the airway lumen and increased resistance to airflow, most evident during expiration. Even when symptoms are not present, the airways remain hyperresponsive, meaning they react excessively to triggers such as allergens, viral infections, cold air, or exercise. During an asthma exacerbation, several processes occur simultaneously, including contraction of airway smooth muscle, swelling of the airway wall due to inflammation and oedema, and increased mucus production, all of which sharply reduce airflow and make breathing out particularly difficult.
At the core of asthma is the interaction between inflammation and variable airflow obstruction.
This process is driven by:
Chronic airway inflammation that increases baseline airway sensitivity
Acute bronchoconstriction from smooth muscle contraction during exposure to triggers
Excess mucus that further narrows already inflamed airways
A key feature that distinguishes asthma from many other obstructive lung diseases is reversibility. Between exacerbations, airway calibre often improves, either spontaneously or with bronchodilator therapy, allowing airflow to return close to normal. Over time, however, repeated inflammation and poor symptom control can cause structural changes in the airway wall, including smooth muscle hypertrophy and thickening of the basement membrane. These changes reduce the degree of reversibility and contribute to more persistent airflow limitation, increasing disease severity and the risk of long-term respiratory impairment.
Beyond the Basics
Airway inflammation and immune activation
The inflammatory process in asthma arises from immune dysregulation within the airway mucosa. In many individuals, asthma is associated with a type 2 immune response involving eosinophils, mast cells, and T helper 2 lymphocytes. When the airway is exposed to allergens or irritants, these immune cells become activated and release inflammatory mediators such as histamine, leukotrienes, and cytokines. These substances act locally within the airway wall and rapidly amplify the inflammatory response.
Inflammatory mediators increase vascular permeability, allowing fluid to leak into the airway wall and cause mucosal oedema. They also stimulate mucus secretion and promote contraction of airway smooth muscle. Importantly, inflammation does not fully resolve between acute attacks. Low-grade, persistent inflammation maintains airway hyperresponsiveness, meaning the airways remain primed to constrict in response to stimuli that would not affect healthy lungs.
Bronchial hyperresponsiveness
A hallmark feature of asthma is an exaggerated airway response to a wide range of triggers, including allergens, cold air, exercise, respiratory infections, and environmental irritants such as smoke or pollution. This heightened sensitivity reflects both ongoing inflammation and intrinsic changes in airway smooth muscle behaviour.
In asthma, airway smooth muscle is more prone to contraction and less responsive to inhibitory signals that would normally limit bronchoconstriction. As a result, relatively mild stimuli can provoke significant narrowing of the airways. This altered responsiveness explains the sudden onset of symptoms and the unpredictable nature of exacerbations, even when baseline lung function appears near normal.
Bronchoconstriction and expiratory flow limitation
During an asthma exacerbation, contraction of airway smooth muscle markedly narrows the bronchial lumen. Because airway resistance increases exponentially as airway diameter decreases, even small reductions in calibre can cause substantial airflow limitation. This mechanical effect is central to the rapid development of breathlessness during an attack.
Expiration is affected more than inspiration. As intrathoracic pressure rises during exhalation, narrowed airways are more likely to collapse prematurely, trapping air within the alveoli. This dynamic hyperinflation increases the work of breathing and contributes to the sensation of chest tightness. Many individuals instinctively prolong expiration or recruit accessory muscles in an attempt to improve airflow.
Mucus hypersecretion and airway obstruction
Chronic airway inflammation promotes goblet cell hyperplasia and enlargement of submucosal glands, leading to increased mucus production. During acute exacerbations, mucus becomes thicker and more viscous, and may accumulate within the airway lumen to form obstructive plugs.
Mucus plugging further limits ventilation and contributes to ventilation–perfusion mismatch, as affected alveoli receive reduced airflow while pulmonary blood flow is relatively preserved. This imbalance worsens hypoxaemia and increases the severity of acute symptoms, particularly when combined with bronchoconstriction and airway oedema.
Airway remodelling in chronic asthma
With long-standing or poorly controlled asthma, repeated inflammatory injury leads to structural changes in the airway wall, known as airway remodelling. These changes include thickening of the basement membrane, hypertrophy of airway smooth muscle, increased connective tissue deposition, and persistent enlargement of mucus-secreting glands.
Airway remodelling reduces airway elasticity and narrows the lumen even in the absence of acute inflammation. Over time, this limits the degree of reversibility that characterises early asthma and contributes to more persistent airflow limitation in severe disease, altering the long-term course of respiratory function.
Clinical Connections
Asthma commonly presents with episodic wheeze, breathlessness, cough, and chest tightness that vary over time and in intensity. Symptoms are often worse at night or in the early morning, reflecting circadian variation in airway tone and inflammation, and may be triggered by allergens, respiratory infections, exercise, cold air, or irritants such as smoke. During an acute exacerbation, physical examination may reveal widespread wheeze, prolonged expiration, and use of accessory muscles, all indicating increased effort to overcome airway narrowing.
Diagnosis is supported by spirometry demonstrating variable and reversible airflow obstruction, typically shown by an improvement in forced expiratory volume in one second after bronchodilator administration or by significant peak expiratory flow variability over time. These findings help distinguish asthma from other causes of obstructive lung disease where airflow limitation is more fixed. Assessment commonly integrates symptom patterns, trigger history, and objective lung function to confirm the diagnosis and assess severity.
In practice, clinical assessment and ongoing monitoring focus on:
Symptom frequency, nocturnal waking, and activity limitation as indicators of control
Lung function trends, including spirometry or peak flow measurements
Early features of deterioration, such as increasing reliever use or reduced exercise tolerance
Management centres on reducing airway inflammation with inhaled corticosteroids and relieving bronchoconstriction with bronchodilators, alongside trigger identification and avoidance. Clear recognition of symptom patterns, objective changes in airflow, and early deterioration supports timely treatment adjustment and reduces the risk of severe exacerbations.
Concept Check
Why is asthma considered a primarily inflammatory airway disease rather than just bronchoconstriction?
How does bronchial hyperresponsiveness contribute to asthma symptoms?
Why is expiration more affected than inspiration during an asthma attack?
How does mucus hypersecretion worsen airflow obstruction and gas exchange?
Why can poorly controlled asthma lead to irreversible airway changes?