Asthma is a condition in which the airways narrow – usually reversibly – in response to certain stimuli. Asthma affects more than 100 million people worldwide, and is becoming more common.
Although it is one of the most common chronic diseases of childhood, adults can also develop asthma, even at an old age. Asthma affects more than 6 million children and occurs more frequently in boys before puberty and in girls after puberty. It also occurs more frequently in blacks and Puerto Ricans.
The reason for the increase in asthma in children is not known, but it may relate to more widespread use of vaccines and antibiotics, to the fact that children are spending more time indoors, or to both. Increased use of vaccines and antibiotics may have shifted the activity of a special subgroup of white blood cells (called lymphocytes) in the body from fighting infection to releasing chemical substances that promote the development of allergies. Alternatively, because children are spending more time indoors and living in better-insulated homes than they were in the past, the exposure to potentially allergic substances is increased. There are few data to support either theory.
The most important characteristic of asthma is narrowing of the airways that can be reversed. The airways of the lungs (the bronchi) are basically tubes with muscular walls. Cells lining the bronchi have microscopic structures, called receptors. There are two main types of receptors: Beta-adrenergic and cholinergic. These receptors sense the presence of specific substances and stimulate the underlying muscles to contract and relax, thus altering the flow of air. Beta-adrenergic receptors respond to chemicals such as epinephrine and make the muscles relax, thereby widening (dilating) the airways and increasing airflow. Cholinergic receptors respond to a chemical called acetylcholine, making the muscles contract, thereby decreasing airflow.
Cause Of Asthma
Narrowing of the airways is often caused by abnormal sensitivity of cholinergic receptors, which cause the muscles of the airways to contract when they should not. Certain cells in the airways, particularly mast cells, are thought to be responsible for initiating the response. Mast cells throughout the bronchi release substances such as histamine and leukotrienes, which cause smooth muscle to contract, mucus secretion to increase, and certain white blood cells to migrate to the area. Eosinophils, a type of white blood cell found in the airways of people with asthma, release additional substances, contributing to airways narrowing.
In an asthma attack, the smooth muscles of the bronchi contract, causing the bronchi to narrow (called bronchoconstriction), and the tissues lining the airways swell from inflammation and mucus secretion into the airways. The top layer of the airways lining can become damaged and shed cells, further narrowing the diameter of the airway. A narrower airway requires the person to exert more effort to move air in and out of the lungs. In asthma, the narrowing is reversible, meaning that with appropriate treatment or on their own, the muscular contractions of the airways stop, the airways widen again, and the airflow into and out of the lungs returns to normal.
In people who have asthma, the airways narrow in response to stimuli that usually do not affect the airways in normal lungs. The narrowing can be triggered by many inhaled allergens, such as pollens, particles from dust mites, body secretions from cockroaches, particles from feathers, and animal dander. These allergens combine with immunoglobulin E (a type of antibody) on the surface of most cells to trigger the release of asthma-causing chemicals from these cells. (This type of asthma is called allergic asthma). Although food allergies include asthma only rarely, certain foods (such as shellfish and peanuts) can induce severe attacks on people who are sensitive to these foods.
Cigarette smoke, cold air, and viral infections can also provoke asthma attacks. Additionally, people who have asthma can develop bronchoconstriction exercising. Stress and anxiety can trigger mast cells to release histamine and leukotrienes and stimulate the vague nerve (which connects to the airway smooth muscle), which then contracts and narrows the bronchi. Gastroesophageal reflux disease (GERD) is a common trigger of asthma.
Symptoms Of Asthma
Asthma attacks vary in frequency and severity. Some people who have asthma are symptom-free most of the time, with only an occasional, brief, mild episode of shortness of breath. Other people cough and wheeze most of the time and have severe attack after viral infections, exercise, or exposure to allergens or irritants, including cigarette smoke. Coughing may be the only symptom in some people (cough-variant asthma). Crying or hearty laughing may bring on symptoms in some people. Some people with asthma produce clear, sometimes sticky (mucoid) phlegm, (sputum). Asthma attacks occur most often in the early morning hours when the effects of protective drugs wear off and the body is least able to prevent bronchoconstriction.
An asthma attack may begin suddenly with wheezing, coughing, and shortness of breath. Wheezing is particularly noticeable when the person breaths out. At other times, an asthma attack may come on slowly with gradually worsening symptoms. In case, people with asthma, usually first notice shortness of breath, coughing or chest tightness. The attack may be over in minutes, or it may last for hours or days. Itching on the chest or neck may be an early symptom, especially in children. A dry cough at night or while exercising, may be the only symptom. Symptoms of asthma can also be caused by other disorders. Symptoms are reversible with timely treatment and typically occur after exposure to one or more triggers.
During an asthma attack, shortness of breath may become severe, creating a feeling of severe anxiety. The person instinctively sits upright and leans forward, using the neck and chest muscles to help in breathing, but still struggles for air-sweating is a common reaction to the effort and anxiety. The pulse usually quickens, and the person may feel a pounding in the chest.
In a very severe asthma attack, a person is able to say only a few words without stopping to take a breath. Wheezing may diminish, however, because hardly any air is moving in and out of the lungs. Confusion, lethargy, and a blue skin color (cyanosis) are signs that the person’s oxygen supply is severely limited, and emergency treatment is needed. Usually, a person recovers completely with appropriate treatment, even from a severe asthma attack. Rarely, some people develop attacks so quickly that they may lose consciousness before they can give themselves effective therapy. Such people should wear a medical alert bracelet and carry a cellular phone to call for emergency medical assistance.
Prevention and Treatment Of Asthma
An array of drugs can be used to prevent and treat asthma. Most of the drugs used to prevent asthma are also used to treat an asthma attacks but in higher doses or in different forms. Some people need to use more than one drug to prevent and treat their symptoms.
Therapy is based on two classes of drugs: anti-inflammatory drugs and bronchodilators. Anti-inflammatory drugs suppress the inflammation that narrows the airways. Bronchodilators help to relax and widen (dilate) the airways. Anti-inflammatory drugs include corticosteroids (which can be inhaled, taken by mouth, or given intravenously) leukotriene modifiers, and mast cell stabilizers. Bronchodilators include beta-adrenergic agonists (both those for quick relief of symptoms and those for long-term control, drugs with anticholinergic effects and methylxanthines.
Education about how to prevent and treat asthma attacks is beneficial for all people who have asthma and often for their family members. Proper use of inhalers is essential for effective treatment. People should know what can stimulate an attack, what helps to prevent an attack, how to use drugs properly and when to seek medical care. Many people use a handheld peak flow meter to evaluate their breathing and determine when they need intervention, before their symptoms get extreme. People who experience frequent, severe asthma attacks should know how to reach help quickly.
Many people have a written treatment plan that was devised in collaboration with their doctor. Such a plan allows them to take control of their own treatment and has been shown to decrease the number of times people need to seek care for asthma in the emergency department.