Stated simply, asthma is a disease in which the airways of the lungs become hypersensitive to one or many irritants. When exposed to such irritants, the airways constrict, the lining of the bronchial tubes swells, and mucus production increases, making it hard for air to get into and out of the lungs. Repeated asthma attacks cause permanent scarring and compromise lung function over time.
Our understanding of asthma has continued to evolve, even in just the past 10 years. Historically, doctors thought asthma was a disease of the smooth muscles that wrap around the airways in a lung. A trigger would cause the muscles to go into spasms, narrowing the airways and thus limiting the flow of air. Such triggers included allergens, air pollution, exercise, cigarette smoke, and cold air. Since these episodes waxed and waned over time--and disappeared after treatment--asthma was viewed as an intermittent illness that required minimal treatment between episodes.
Today, that view has been turned on its head. Doctors now know that asthma is a chronic condition and that even without symptoms, the disease is still active. Some other new understandings about the disease include:
- Inflammation of the airways is a persistent feature of asthma--even between attacks--and plays a critical role in changing overall lung function. Translation: Your asthma is changing the way your lungs work every minute of every day, even if you feel just fine and are breathing well. That's why most people with asthma require medication daily, not just during an attack.
- Complex reactions within the airways are a big part of an asthma attack. These reactions are based in the immune system, meaning they involve many of the cells that play a role in allergies: eosinophils, T lymphocytes, macrophages, neutrophils, and basophils as well as mast cells and epithelial cells, which line the airways. It's these reactions that create the mucus and inflammation that further restrict breathing capacity.
Our improved understanding of asthma, however, still leaves us light years away from truly knowing what causes it. One of our greatest questions: Just what is the relationship between inflammation and asthma?
For instance, we know that asthma is always associated with inflammation of the lungs and that the intensity of the inflammation determines the severity of the symptoms. We also know that there can be a spiraling effect in asthma: Inflammation makes the hypersensitivity worse, which triggers more asthma attacks, which brings on more inflammation. No one is quite sure what triggers this inflammatory response. We do know, however, that over time and without proper treatment, the inflammation can eventually change the physical appearance and function of your lungs, leading to the replacement of normal tissue with non-functioning scar tissue that no amount of treatment can reverse.
Currently, asthma is divided into four main types, each of which behaves somewhat differently, is triggered differently, and may respond to different treatments or interventions.
- Allergic asthma (sometimes called extrinsic asthma). This is the most common form; if you have allergies, you probably have allergic asthma. Attacks are triggered by allergens such as seasonal pollens or perennial inhalant allergens such as dust mites and animal dander. Allergic asthma often begins in childhood and stays with you for life.
- Nonallergic asthma (sometimes referred to as intrinsic asthma). This form of asthma results from something within your body, such as a sinus infection or gastroesophageal reflux (heartburn). It generally develops later in life, and very little is known about its causes. One thing that is known: It's often more difficult to treat than allergic asthma.
- Mixed asthma. You can have asthma that's triggered by both allergies and non-llergic factors. For instance, your allergy to grass and ragweed triggers your asthma, but you have symptoms even during the winter, when there is no pollen.
- Acute severe asthma or potentially fatal asthma. If you have experienced this form of asthma, you know it. This life-threatening condition used to be called status asthmaticus. It's a type in which attacks come on suddenly and very intensely but don't respond to the usual treatment. With this form, you often have so much trouble breathing that you become exhausted and collapse. You're also in significant danger of death; people with acute severe asthma can go downhill very fast and die within the first 24 hours of an attack. One regional survey of fatal or near-fatal asthma attacks found that half occurred suddenly and unexpectedly, without any obvious predisposing factors. In the other half of attacks, psychosocial factors (such as stress), running in cold weather, overreliance on inhaled bronchodilators, and delays in seeking care were contributing causes.
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