Asthma

Asthma is a common long-term inflammatory disease of the airways. It is characterized by variable and recurring symptoms and reduced lung function. Symptoms include episodes of wheezing, coughing, chest tightness, and shortness of breath. A sudden worsening of asthma symptoms sometimes called an 'asthma attack' or an 'asthma exacerbation' can occur when alle

Asthma is a common long-term inflammatory disease of the airways. It is characterized by variable and recurring symptoms and reduced lung function. Symptoms include episodes of wheezing, coughing, chest tightness, and shortness of breath. A sudden worsening of asthma symptoms sometimes called an 'asthma attack' or an 'asthma exacerbation' can occur when allergens, pollen, dust, or other particles, are inhaled into the lungs, causing the bronchioles to constrict and produce mucus, which then restricts oxygen flow to the alveoli. These may occur a few times a day or a few times per week.[2] Depending on the person, asthma symptoms may become worse at night or with exercise.[2]

Asthma is thought to be caused by a combination of genetic and environmental factors.[1] Environmental factors include exposure to air pollution and allergens.[2] Other potential triggers include medications such as aspirin and beta blockers.[2] Diagnosis is usually based on the pattern of symptoms, response to therapy over time, and spirometry lung function testing.[3] Asthma is classified according to the amount of medication required to control symptoms or mechanisms underlying the condition.

There is no known cure for asthma, but it can be controlled.[2] Symptoms can be prevented by avoiding triggers, such as allergens and respiratory irritants, and suppressed with the use of inhaled corticosteroids.[6]: 169–172 [7]: 57, 72–73  Long-acting beta agonists (LABA) or antileukotriene agents may be used in addition to inhaled corticosteroids if asthma symptoms remain uncontrolled.[7]: 77–78, 90 [8] Treatment of rapidly worsening symptoms is usually with an inhaled short-acting beta2 agonist such as salbutamol and corticosteroids taken by mouth.[6]: 214  In very severe cases, intravenous corticosteroids, magnesium sulfate, and hospitalization may be required.[6]: 373–375 

In 2023, asthma affected an estimated 363 million people worldwide and caused 442,000 deaths.[9] Most asthma-related deaths occur in low- and lower-middle-income countries.[9] Asthma often begins in childhood,[2] and the rates have increased significantly since the 1960s.[10] Asthma was recognized as early as Ancient Egypt.[11] The word asthma is from the Greek ἆσθμα (âsthma), which means 'panting'.[12]

Signs and symptoms

Asthma causes recurrent episodes of wheezing, shortness of breath, chest tightness, and coughing.[13] Symptoms are usually worse at night and in the early morning or in response to exercise or cold air.[14]: 8  Some people with asthma rarely experience symptoms, usually in response to triggers, whereas others may react frequently and readily and experience persistent symptoms.[7]: 22–24 

Associated conditions

Asthma is often associated with other conditions both within and outside of the respiratory system. Comorbid medical conditions are more common in severe asthma and can affect the disease severity, symptoms and treatment. Common comorbid conditions include allergic diseases such as allergic conjunctivitis and rhinitis, chronic obstructive pulmonary disease (COPD), exercise-induced bronchoconstriction, bronchiectasis, obstructive sleep apnea, obesity, gastroesophageal reflux disease, diabetes, heart disorders, and mental health conditions such as anxiety and depression.[15][16]

Classification

Due to the diversity in onset, symptoms, outcomes, and response to treatment, asthma is often considered a syndrome — a collection of signs and symptoms — rather than a single condition.[17][18] Historically asthma was classified as being caused by external factors (extrinsic) such as allergens or by internal factors (intrinsic), unrelated to allergies.[18] Currently asthma is most commonly classified according to severity, control of symptoms, phenotypes and endotypes.[18][19]

Asthma and chronic obstructive pulmonary disease (COPD) cause airway restriction and have a wide range of overlapping mechanisms and symptoms. The main difference between the two disorders is that in asthma expiratory airflow fluctuates over time while in COPD airflow obstruction is chronic and can increase in severity over time.[7]: 131–133 

Severity and symptom control

Asthma severity is determined on the basis of how much medication is required to control symptoms and exacerbations while asthma control is the burden of asthma on an individual such as symptoms and inflammation.[7]: 35 [19] Classification of asthma control does not depend on medication usage like severity does, however they often assess similar factors.[19]

Asthma control is assessed on the basis of how well the symptoms are being control and the risk of any future consequences of the disorder.[7]: 36  The frequency and severity of symptoms, impairment caused by symptoms, use of rescue inhalers, questionnaires, healthcare usage, and objective tests such as spirometry, FeNO, sputum eosinophils, and hyperresponsiveness studies are used to evaluate asthma control.[19]

Asthma severity is measured based on how difficult the disorder is to treat. Severity can only be measured once the disorder is under control. Those with mild asthma may only require as needed medication to treat the disease while those with severe asthma require high doses of medication to gain control over the disorder or may be unable to get their symptoms under control even with medications.[7]: 44 

Phenotyping and endotyping

A phenotype is the way in which a condition presents itself, such as when the disease first starts to affect a person and what symptoms an individual displays. An endotype is the mechanisms that underlie the condition.[20] Asthma is most commonly divided into two endotypes, T2-high and T2-low (non-T2). Within the two main endotypes there are subpopulations (phenotypes), some of which overlap or can be categorized under both of the two endotypes.[17]

The two endotypes are distinguished based on the type of inflammation present, with the type-2 high endotype involving the type 2 immune system response and type-2 low involving type 1 immune system response. Type-2 high is characterized by increased eosinophils, increased Fractional exhaled nitric oxide (FeNO), or allergens. Type-2 low asthma is the absence of these inflammatory markers and the mechanisms are not well researched. The phenotypes included under the type-2 high endotype include early-onset allergic asthma, late-onset eosinophilic asthma, and Aspirin-exacerbated respiratory disease. Type-2 low asthma phenotypes include asthma associated with obesity, neutrophilic asthma, asthma associated with cigarette smoke, and paucigranulocytic asthma.[20] Occupational asthma can be further split into separate phenotypes, irritant-induced asthma — caused by exposure to airway irritants such as cleaning products and dust — and sensitizer-induced occupational asthma — developed hypersensitivity. Irritant-induced asthma is a type-2 low phenotype while sensitizer-induced occupational asthma is a type-2 high phenotype.[17][20] Asthma-COPD overlap (ACO) currently lacks a consistent definition making it hard to categorize it into either endotype.[17]

Asthma exacerbation

An asthma exacerbation, commonly referred to as an asthma attack, asthma episode, or acute severe asthma are episodes of increased symptoms (shortness of breath, wheezing, coughing, chest tightness), and decreased lung function. Decreased lung function is measured by peak expiratory flow rate (PEF) or forced expiratory volume in 1 second.[7]: 159–161 

Signs occurring during an asthma exacerbation include the use of accessory muscles of respiration (sternocleidomastoid and scalene muscles), chest retractions with breathing, blue colour of the skin and nails, increased heart rate and changes in respiratory sounds.[21]

An asthma exacerbation is considered mild when there is difficulty carrying out daily activities due to symptoms and a decrease of at least 20% in PEF for over two days. In moderate exacerbations, oxygen desaturation is mild, there is no use of auxiliary respiratory muscles, and there is a 50% or lower decrease in PEF. During severe exacerbations, the person experiencing symptoms becomes restless and may struggle to speak; oxygen saturation drops and there is above a 50% decrease in PEF. Life-threatening exacerbations cause lethargy and clouding of consciousness.[21]

Brittle asthma is a kind of asthma distinguishable by recurrent, severe attacks. However brittle asthma is best regarded as a historical disease descriptor rather than a distinct diagnostic category. While it remains useful for understanding older literature it is no longer routinely used in contemporary clinical practice.[22]