Table of Contents
Page created on May 15, 2021. Last updated on April 19, 2022 at 12:38
Definition and epidemiology
Asthma is a chronic inflammatory disorder of the airways characterised by airway hyperresponsiveness which causes recurrent asthmatic attacks. These are episodes of bronchoconstriction, leading to wheezing, dyspnoea, and coughing, sometimes after being exposed to a certain trigger.
It’s one of the most common chronic diseases worldwide. It affects around 300 million people, affect Western countries more than non-Western ones. Prevalence is increasing. In the UK every 5th person has asthma.
Uncontrolled asthma can be fatal, but the mortality rate of asthma is highest in non-Western countries. Asthma diagnosis is not easy, especially in children, and it is frequently overdiagnosed.
Etiology
Risk factors for asthma:
- Hereditary factors
- Family history of asthma
- Atopy
- Airway hyperresponsiveness
- Male sex (in children)
- Female sex (in adults)
- Acquired factors
- Early exposure to inhaled allergens (indoor and outdoor)
- Irritants (smoke, pollution)
- Respiratory infections
- Obesity
- High socio-economic status
Factors which can trigger an asthmatic attack:
- Allergens
- Irritants
- Strong odours
- Respiratory infections
- Exercise
- Cold air
- Drugs (aspirin, non-selective beta blockers)
- Laughing, crying
Pathomechanism
In asthma, the smooth muscles of the airway become hypertrophic and oedematous, thickening the wall of the bronchi and narrowing the lumen. Hypersecretion of the mucous glands produces thick mucous, which the respiratory cilia can’t remove.
During an asthmatic attack, air is trapped in the lung, causing lung hyperinflation.
Classification
We can subdivide asthma into allergen-induced asthma, non-allergen-induced asthma, and intrinsic asthma. There can be overlap between these types.
Allergen-induced asthma (allergic asthma) is more related to allergens, and usually has onset in childhood. It’s usually a part of the atopic march, which includes the development of atopic dermatitis first, then hay fever, and lastly allergic asthma. These patients usually have TH2-dependent and eosinophil inflammation.
Non-allergen-induced asthma is more related to airway irritants, drugs, and infections, and usually has onset in adulthood. These patients usually do not have TH2-dependent inflammation or eosinophil inflammation, but rather neutrophil inflammation.
In intrinsic asthma, there is airway hyperreactivity without inflammation or allergy.
Aspirin-exacerbated respiratory disease (aspirin asthma) is a special type of asthma which occurs in people taking aspirin or NSAIDs.
There are other types of asthma as well, like:
- Exercise-induced asthma (EIA)
- Obesity-associated asthma
- Smoking-associated asthma
- Occupational asthma
- Cough-variant asthma
Clinical features
Asthma is characterised by asthmatic attacks, episodes of bronchoconstriction, leading to symptoms like wheezing, dyspnoea, chest tightness, and coughing. These episodes typically occur at night or in the morning.
Severe asthmatic attacks are characterised by:
- Dyspnoea in rest
- Agitation
- High respiratory rate
- Use of accessory muscles for breathing
- Loud wheezing
- Tachycardia
- Pulsus paradoxus
Diagnosis and evaluation
During the evaluation of asthma, it’s important to take a precise history to determine the type of asthma the patient has. It’s important to ask about the specific symptoms the patient experience, when they occur, if there are any triggers, and if the patient has any of the risk factors.
Physical examination may be normal or it may reveal wheezing and prolonged expiration on auscultation, but these findings are not specific for asthma.
The diagnosis is based on characteristic findings on spirometry together with typical symptoms. The characteristic findings on spirometry are either of the following:
- If spirometry shows obstruction, administration of a bronchodilator improves the obstruction by > 12%
- This shows that the airway obstruction is reversible, unlike in COPD
- Obstruction is defined as decreased FEV1/FVC ratio
- If spirometry does not show obstruction, a challenge test can be used, where we try to trigger airway obstruction and see whether spirometry shows obstruction afterward
- The trigger can be either inhalation of 10% KCl for 140 breaths or methacholine (a bronchoconstrictor)
- If the FEV1 drops by more than 20%, the test is positive
- If the FEV1 or the peak expiratory flow (PEF) varies by more than 20% over time
- Repeat measurements at the office or at home over time are necessary
- A peak flow meter can be used for this
A peak flow meter is a small device which the patient can use at home to measure the peak expiratory flow (PEF). It is used to monitor changes in PEF over time. It can be used to diagnose asthma, see whether treatment improves the PEF, or whether a person has worse PEF in the weekdays than the weekends, which would suggest occupational asthma.
Treatment
Inhaled bronchodilators can reverse the episode of bronchoconstriction, but they don’t treat the underlying cause, the chronic inflammation of the airway. The aim of the treatment is to reverse asthmatic episodes with inhalers, as well as to decrease the chronic inflammation to prevent asthmatic episodes.
The treatment of asthma should be personalized to the type of asthma, but we’re not going to go into such detail here.
Drugs used to reverse asthmatic episodes are called relievers. These are inhaled rescue medications used to alleviate symptoms and prevent bronchoconstriction. These can be:
- Short-acting beta-2 agonists (SABA) like salbutamol
- Long-acting beta-2 agonists (LABA) which have rapid onset (RABA) like formoterol
- Salmeterol is a LABA with slow onset and is not suitable
Drugs used to reduce the airway inflammation are called controllers. These are inhalational corticosteroids (ICS), especially budesonide and beclomethasone.
In 2019, the guidelines on asthma treatment changed. Until then, mild asthma was treated with only relievers and not controllers. However, it was found that this increases mortality. The 2019 guidelines recommend both relievers and controllers for all.
Other drugs which may be used in more severe asthma include leukotriene receptor antagonists (LTRA), tiotropium, oral glucocorticoids, or biological therapy against IgE, IL-5, IL-5R, or IL-4R.
Complications
Long-term untreated asthma can cause suboptimal lung development in children and loss of lung function over time.
Cardiac asthma
Patients with left-sided heart failure may experience asthma-like symptoms, like dyspnoea, wheezing, and coughing, which may be called cardiac asthma. Differentiating cardiac and bronchial asthma is usually not a problem.
Patients with cardiac asthma usually have other symptoms of heart failure, the presence of cardiac risk factors, frothy sputum when coughing, are older, have abnormal ECG, chest x-ray, etc. They usually don’t have allergy, and spirometry does not show the characteristic features of bronchial asthma.