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 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. Depending on the person, asthma symptoms may become worse at night or with exercise.
Asthma is thought to be caused by a combination of genetic and environmental factors. Environmental factors include exposure to air pollution and allergens. Other potential triggers include medications such as aspirin and beta blockers. Diagnosis is usually based on the pattern of symptoms, response to therapy over time, and spirometry lung function testing. 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. Symptoms can be prevented by avoiding triggers, such as allergens and respiratory irritants, and suppressed with the use of inhaled corticosteroids. Long-acting beta agonists or antileukotriene agents may be used in addition to inhaled corticosteroids if asthma symptoms remain uncontrolled. Treatment of rapidly worsening symptoms is usually with an inhaled short-acting beta agonist such as salbutamol and corticosteroids taken by mouth. In very severe cases, intravenous corticosteroids, magnesium sulfate, and hospitalization may be required.
In 2019, asthma affected approximately 262million people and caused approximately 461,000 deaths. Most of the deaths occurred in the developing world. Asthma often begins in childhood, and the rates have increased significantly since the 1960s. Asthma was recognized as early as Ancient Egypt. The word asthma is from the Greek ἆσθμα, which means 'panting'.
Signs and symptoms
Asthma is characterized by recurrent episodes symptoms of wheezing, shortness of breath, chest tightness, and coughing. Symptoms are usually worse at night and in the early morning or in response to exercise or cold air. Some people with asthma rarely experience symptoms, usually in response to triggers, whereas others may react frequently and readily and experience persistent symptoms.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, bronchiectasis, obstructive sleep apnea, obesity, gastroesophageal reflux disease, diabetes, heart disorders, and mental health conditions such as anxiety and depression.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 one single condition. Historically asthma was classified as caused by external factors such as allergens or by internal factors, unrelated to allergies. Currently asthma is most commonly classified according to severity, control of symptoms, phenotypes and endotypes.Although asthma is a chronic obstructive condition, it is not considered as a part of chronic obstructive pulmonary disease, as this term refers specifically to combinations of disease that are irreversible such as bronchiectasis and emphysema. Unlike these diseases, the airway obstruction in asthma is usually reversible; however, if left untreated, the chronic inflammation from asthma can lead the lungs to become irreversibly obstructed due to airway remodelling. In contrast to emphysema, asthma affects the bronchi, not the alveoli. The combination of asthma with a component of irreversible airway obstruction has been termed the asthma-chronic obstructive disease overlap syndrome. Compared to other people with "pure" asthma or COPD, people with ACOS exhibit increased morbidity, mortality, and possibly more comorbidities.
Asthma exacerbation
An asthma exacerbation, commonly referred to as an asthma attack, are episodes of increased symptoms, and decreased lung function.Signs occurring during an asthma attack include the use of accessory muscles of respiration, there may be a paradoxical pulse, and over-inflation of the chest. A blue colour of the skin and nails may occur from lack of oxygen.
The level of peak expiratory flow rate is determined as follows:
- A mild exacerbation is ≥200 L/min, or ≥50% of the predicted best.
- Moderate is defined as between 80 and 200 L/min, or 25% and 50% of the predicted best.
- Severe is defined as ≤ 80 L/min, or ≤25% of the predicted best.
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.
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. Asthma is most commonly divided into two endotypes, T2-high and T2-low. Within the two main endotypes there are subpopulations, some of which overlap or can be categorized under both of the two endotypes.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, 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. 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. Asthma-COPD overlap currently lacks a consistent definition making it hard to categorize it into either endotype.
Exercise-induced
Exercise can trigger bronchoconstriction both in people with and without asthma. It occurs in most people with asthma and up to 20% of people without asthma. Exercise-induced bronchoconstriction is common in professional athletes. The highest rates are among cyclists, swimmers, and cross-country skiers. Asthma symptoms may occur under a wide range of weather conditions but are more frequently triggered by changes in weather. Inhaled beta2 agonists do not appear to improve athletic performance among those without asthma; however, oral doses may improve endurance and strength.Alcohol-induced asthma
Alcohol may worsen asthmatic symptoms in up to a third of people. This may be even more common in some ethnic groups, such as the Japanese and those with aspirin-exacerbated respiratory disease. Other studies have found improvement in asthmatic symptoms from alcohol.Infectious asthma
Infectious asthma is an easily identified clinical presentation. When queried, asthma patients may report that their first asthma symptoms began after an acute lower respiratory tract illness. This type of history has been labelled the "infectious asthma" syndrome, or as "asthma associated with infection" to distinguish infection-associated asthma initiation from the well known association of respiratory infections with asthma exacerbations. Reported clinical prevalences of IA for adults range from around 40% in a primary care practice to 70% in a specialty practice treating mainly severe asthma patients.Causes
Asthma is caused by a mixture of genetic and external factors. The disease manifests when those with a genetic susceptibility to asthma are exposed to specific environmental factors. Environmental factors can also trigger asthma symptoms.Risk factors
Factors during pregnancy that have been linked to the development of asthma include weight gain or obesity in the mother, stressful pregnancy, smoking while pregnant, the use of certain medications while pregnant and caesarean section. Early childhood exposure to secondhand smoke, high levels of stress in parents, respiratory infections, and indoor mold or fungi have also been associated with asthma development.Respiratory tract infections, especially during early childhood or if they are severe and recurring can lead to decreased lung function and subsequent asthma. Conversely, there has been research suggesting that certain infections during childhood may lessen the risk of developing asthma. This theory is known as the “hygiene hypothesis”.
Prenatal or childhood exposure to cigarette smoke increases the likelihood of a child developing asthma. Children whose maternal grandmother smoked during pregnancy are also more likely to develop asthma, regardless of if their mothers developed asthma or smoked. Nicotine is believed to be the cause of these effects and nicotine is linked to changes in DNA.
Chronic exposure to air pollution increases the risk of developing asthma. Outdoor air pollution includes nitrogen dioxide and traffic pollution while indoor air pollution includes biomass, pesticides, building materials such as asbestos and formaldehyde, mold, dust mites, cockroaches, and endotoxins.
Asthma is more commonly seen in urban environments than in rural environments. This is believed to be due to the higher presence of certain risk factors for asthma in urban settings such as traffic pollution, secondhand smoke, social inequality, lack of green spaces, and industrialization as well as protective factors associated with rural environments such as less air pollution, early protective exposure to allergens and bacteria, and higher levels of physical activity.
In those who are affected by allergies, exposure to allergens can trigger asthma symptoms. However, some research has suggested that early exposure to allergens in childhood may help desensitize individuals from allergies. Other studies have shown that early exposure may increase risk of allergies and the development of allergies is multifactorial. Allergens also play a role in the development of adult-onset asthma.
Adult-onset asthma is caused by relations between genetics, lifestyle factors such as obesity and smoking, and environmental factors such as an urban or rural environment, occupational exposures, and air pollution. Unlike childhood asthma, which is more prevalent in males, in adults asthma is more prevalent in females. Over 400 occupational exposure have been linked to asthma. Exposure to asthmagens, allergens and substances that are known to cause asthma; the amount and length of time that an individual was exposed to the substance; genetics; allergies; and smoking can affect the development of occupational asthma.