Asthma

Environmental conditions and circumstances around pregnancy, birth, and infancy have been associated with the development of asthma in childhood and later in adulthood. Risk appears to be higher if a person's mother became pregnant at a young age or had poor nutrition during pregnancy. Risk may also be higher if someone is born prematurely, had a low weight at birth, or was not breastfed.

Environmental conditions such as exposures to household allergens (such as dust mites, cockroaches, and pet dander) and other environmental allergens have also been associated with the development of asthma in older children and adults.

Diets low in vitamins C and E and in omega–3 fatty acids have been also linked to asthma, as has obesity. There is no evidence that dietary supplements of these substances prevent the development of asthma; however, it has been shown that weight loss can reduce the risk and severity of asthma. Thus, obesity is an important modifiable risk factor for asthma.

Having smaller families with fewer children, cleaner indoor environments, and use of vaccinations and antibiotics in early life may decrease the body's ability to develop resistance to allergens in the environment and may partly explain the increase in asthma in places where these conditions exist (the hygiene hypothesis).

Lower socioeconomic status, active smoking, and a family history of asthma are also risk factors for 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 the following:

• Smooth muscle to contract

• Mucus secretion to increase

• Certain white blood cells to move to the area

Eosinophils, a type of white blood cell found in the airways of people with asthma, release additional substances, contributing to airway narrowing. T helper 2 (Th2) cells, another type of white blood cell, may also be involved in asthma. These cells release chemicals (such as interleukins) that cause overproduction of mucus, narrowing of the airways, and hardening of the respiratory tissues. If inflammation due to eosinophils and Th2 cells is present, doctors may call this asthma the T2-high or the "eosinophilic" type. If inflammation due to these cells is not present, they may call it T2-low asthma.

Eosinophilic asthma is a severe subtype of asthma in which very high levels of eosinophils are present in the blood. The higher the level of eosinophils the more severe the person's symptoms.

In an asthma attack (sometimes called a flare-up or an exacerbation), the smooth muscles of the bronchi contract, causing the bronchi to narrow (called bronchoconstriction). The tissues lining the airways swell due to inflammation and mucus secretion into the airways. The top layer of the airway lining can become damaged and shed cells, further narrowing the airway. A narrower airway requires the person to exert more effort to breathe. In asthma, the narrowing is reversible, meaning that with appropriate treatment or on their own, the muscular contractions of the airways stop, inflammation resolves so that the airways widen again, and airflow into and out of the lungs returns to normal.

In people who have asthma, the airways narrow in response to stimuli (triggers) that usually do not affect the airways in people without asthma. Such triggers include:

• Allergens

• Infections

• Irritants

• Exercise (called exercise-induced bronchoconstriction)

• Stress and anxiety

• Non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin and ibuprofen (called aspirin-exacerbated respiratory disease)

Many inhaled allergens, including pollens, particles from dust mites, body secretions from cockroaches, particles from feathers, and animal dander, can trigger an asthma attack. These allergens combine with immunoglobulin E (IgE, a type of antibody) on the surface of mast cells to trigger the release of asthma-causing chemicals. This type of asthma is called allergic asthma.

Infectious triggers are usually viral respiratory infections, such as colds, bronchitis, and, less commonly, pneumonia.

Irritants that can provoke an asthma attack include smoke from tobacco, marijuana, or cocaine; fumes (such as from perfumes, cleaning products, or air pollution); cold air; and stomach acid in the airways caused by gastroesophageal reflux disease (GERD).

Some people who have asthma can develop airway narrowing when exercising. This type of airway narrowing may be due to breathing drier, colder air through the mouth while exercising.

Stress and anxiety can trigger mast cells to release histamine and leukotrienes and stimulate the vagus nerve (which connects to the airway smooth muscle), which then contracts and narrows the bronchi.

Anger, anxiousness, and crying or hearty laughing may trigger symptoms in some people.

Aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) are triggers for asthma attacks in almost 30% of people with nasal polyps (pearly growths in the interior of the nose that result from similar inflammation as in asthma), but they are triggers in about 9% of people with asthma overall.

Reactive airways dysfunction syndrome (RADS) is a rapid onset and persistent asthma-like disorder that occurs in people with no history of asthma. It is a form of environmental lung disease caused by a single large exposure to nitrogen oxide or volatile organic compounds (such as those in certain bleaches and cleaning products). People have symptoms similar to those of asthma, including cough, wheezing, and shortness of breath. Treatment is similar to usual treatment for asthma.

Asthma attacks occur most often in the early morning hours when the effects of protective medications wear off and the body is least able to prevent airway narrowing.

An asthma attack may begin suddenly with wheezing, coughing, and shortness of breath. At other times, an asthma attack may come on slowly with gradually worsening symptoms. In either 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 sometimes be the only symptom.

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 muscles in the neck and chest to help in breathing, but may still struggle for air. Sweating is a common reaction to the effort and anxiety. The heart rate 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 actually diminish, however, because hardly any air is moving in and out of the lungs. Confusion, lethargy, and a blue skin color (cyanosis, which may appear as grayish discoloration in dark skin) 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 identification (such as a medical alert bracelet or necklace).

Severity is a measure of how bad a disease is. Asthma severity is usually assessed before treatment is started, because people who have responded well to treatment have few symptoms. Asthma severity is categorized as:

• Intermittent: The person's symptoms occur 2 days per week or less and do not interfere with activities of daily life

• Mild persistent: The person's symptoms occur more than twice per week but only slightly limit activities of daily life

• Moderate persistent: The person's symptoms occur daily and limit some activities of daily life

• Severe persistent: The person's symptoms occur throughout the day and interfere excessively with activities of daily life

It is important to remember that the severity category does not predict how serious an attack a person may have. Even a person who has mild asthma with long periods of no or mild symptoms and normal lung function may have a severe, life-threatening asthma attack.

The most severe form of asthma is called status asthmaticus. It is severe, intense, prolonged airway narrowing that is resistant to treatment. In status asthmaticus, the lungs are no longer able to provide the body with adequate oxygen or to remove carbon dioxide adequately.

Without oxygen, many organs begin to malfunction. The buildup of carbon dioxide leads to acidosis, an acidic state of the blood that affects the function of almost every organ. Blood pressure may fall to dangerously low levels. The airways are so narrowed that it is difficult to move air in and out of the lungs.

Status asthmaticus may require that an artificial airway be passed through the person’s mouth and throat into the main airway leading to the lungs (the trachea) and that a mechanical ventilator be used to assist breathing. Sometimes breathing can be assisted by a machine without inserting a breathing tube (called noninvasive ventilation). Higher-than-normal doses of several medications are also needed.

Control is the degree to which symptoms, effects on daily life, and risks of severe asthma attacks are minimized by treatment. Asthma control is similar to severity, but it is assessed after treatment has started. The goal is for all people to have well-controlled asthma regardless of disease severity. Control is classified as:

• Well controlled: Symptoms occur twice per week or less often

• Not well controlled: Symptoms occur more than twice per week but not every day

• Very poorly controlled: Symptoms occur daily

Impairment refers to the limitations symptoms place on daily life. Impairment due to asthma is determined by asking:

• How often symptoms are experienced

• How often the person awakens at night

• How often the person uses a short-acting beta-2 agonist for symptom relief

• How often asthma interferes with normal activity

Other factors, such as measures of lung function, responses to standardized questionnaires, and what medications are used to treat asthma, are also used to determine the severity, control, and impairment of asthma.

Risk refers to the likelihood of future asthma attacks, decreases in lung function, and side effects related to the medications taken to control asthma. Doctors monitor risk with spirometry measurements (which measure lung function) over time as well as factors such as how often the person needs to receive certain oral steroids (sometimes referred to as glucocorticoids or corticosteroids) or to be hospitalized to control asthma symptoms.

Determining what triggers a person’s asthma is often difficult.

Allergy testing is appropriate when there is a suspicion that some avoidable substance (for example, exposure to cat dander) is provoking attacks. Skin testing can help identify allergens that may trigger asthma symptoms. However, an allergic response to a skin test does not necessarily mean that the allergen being tested is causing the asthma. The person still has to note whether attacks occur after exposure to this allergen. If doctors suspect a particular allergen, a blood test that measures the level of antibody produced in response to the allergen (the radioallergosorbent test [RAST]) can be done to determine the degree of the person's sensitivity to the allergen.

Because people who are having a severe asthma attack commonly have low blood oxygen levels, doctors may check the level of oxygen by using a sensing monitor on a finger or ear (oximetry). In severe attacks, doctors also need to measure levels of carbon dioxide in the blood, and this test typically requires obtaining a sample of blood from an artery or, occasionally, a vein. However, carbon dioxide levels can sometimes be monitored in the person's breath using a sensor placed in front of the nose or mouth.

Doctors may also check lung function, usually with a spirometer (a mouthpiece and tubing connected to a recording device that is used to measure air flow in the lungs) or with a peak flow meter. Usually, a chest x-ray is needed only when asthma attacks are severe, in order to rule out other serious conditions (such as a lung collapse).

Older adults are more likely to have other lung diseases that also cause shortness of breath (such as chronic obstructive pulmonary disease), so doctors have to determine how much of the person's breathing difficulty is related to asthma and reversible with the appropriate anti-asthma therapy. Often, in these people diagnosis involves a brief trial of medications that are used to treat asthma to see whether the person's condition improves.