Before discussing what causes asthma, I would like to first define what asthma is.
Bronchial asthma is a usually reversible disease of the lungs. This chronic and inflammatory disease involves the air passages in the lungs and affects about 25 million people in the United States. Intermittent episodes of wheezing (bronchospasm) can be alleviated by therapy (bronchodilators, corticosteroids). At times, the wheezing stops spontaneously without therapy. Asthma can be caused by extrinsic factors and internal factors (see below). It usually occurs in children and younger adults but can be seen in persons of any age.
Airway inflammation arises due to interaction of a multitude of cells, including:
- mast cells
- epithelial cells
- dendritic cells
and various mediators, including:
- nitric oxide
The methods by which these factors interact with each other to activate inflammation and cause an asthma attack is presently under investigation and is partially, but not completely, understood. The results of this inflammation are constriction of bronchial smooth muscle, swelling or edema of the airways (including mucus in the airways), and hyperresponsiveness of the airways. The results of this situation are patient symptoms of wheezing, cough and shortness of breath.
Extrinsic (external) factors include:
- Allergies (allergens) are common in children and adults and are a common cause of asthma. These allergic reactions are often associated with IgE-related immunologic mechanisms. Some examples of allergens include inhaled materials (e.g. pollens), ingested materials (e.g. foods), and materials that cause problems when they contact the skin. Inhaled (environmental) allergens are some of the most common causes of asthma (e.g. aspirin).
- Occupational asthma occurs in about 10% of asthmatics and results fromexposure to offending dusts, gases, fumes, and vapors that are present in the workplace. These materials are high molecular-weight compounds and low molecular-weight compounds.
Examples of high molecular-weight compounds that cause asthma include animal products (e.g. veterinarians), seafood (e.g. oyster farmers), insects (e.g. entomologists), plants (e.g. bakers), biological enzymes (e.g. detergent industry workers), gums (e.g. printers), and latex (e.g. health care workers). Examples of low molecular-weight compounds that cause asthma include drugs (e.g. antibiotics), wood dust (e.g. saw-mill workers), diisocyanates (e.g. workers with polyurethane), epoxy compounds (e.g. epoxy resin workers), fluxes (e.g. aluminum solderers), and fixatives (e.g. hospital workers).
- Aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) can cause asthma in sensitive patients. This problem usually takes time to develop and is characterized by aspirin sensitivity, nasal polyps, and asthma.
What causes asthma: Intrinsic (internal) factors include:
- Respiratory infections, especially viral infections, are very common causes of exacerbations of asthma.
- Exercise can induce an asthmatic attack and is called exercise-induced asthma (EIA). Exercise usually induces an increase in respiratory rate and depth of breathing and increases the inhalation of atmospheric air which is cooler than the body temperature air in the lungs. This cooler air appears to stimulate receptors in the lung in susceptible patients to induce bronchospasm and an asthmatic attack.
- Genetic factors have been proposed as causes of asthma. The ADAM-33 gene sets on the short arm of chromosome 20 and has been significantly associated with asthma, being called the asthma gene. Four other asthma genes or gene complexes have been identified with the use of positional cloning.
- Emotional factors and in particular stress have been suspected to cause asthma, but the data to support this is not complete.