A reversible lung disease, asthma is characterized by obstruction or narrowing of the airways, which are typically inflamed and hyperresponsive to various stimuli. It may resolve spontaneously or with treatment.
Although this common condition can strike at any age, half of all cases first occur in children younger than age 10; in this age-group, asthma affects twice as many boys as girls. The sex ratio equalizes by age 30.
Extrinsic and intrinsic asthma
Asthma that results from sensitivity to specific external allergens is referred to as extrinsic (atopic). In those cases where the allergen isn’t obvious, asthma is referred to as intrinsic (nonatopic). Allergens that cause extrinsic asthma include pollen, animal dander, house dust or mold, kapok or feather pillows, food additives containing sulfites,
and any other sensitizing substance.
and any other sensitizing substance.
Extrinsic asthma usually begins in childhood and is accompanied by other manifestations of atopy (type I, immunoglobulin [Ig] E–mediated allergy), such as eczema and allergic rhinitis.
With intrinsic asthma, no extrinsic allergen can be identified. Most cases are preceded by a severe respiratory tract infection. Irritants, emotional stress, fatigue, exposure to noxious fumes, and endocrine, temperature, and humidity changes may aggravate intrinsic asthma attacks.
For many asthmatics, intrinsic and extrinsic asthma coexist.
Other asthma triggers
Several drugs and chemicals may provoke an asthma attack without using the IgE pathway. Apparently, they trigger release of mast cell mediators via prostaglandin inhibition. Examples of these substances include aspirin, various nonsteroidal anti-inflammatory drugs (such as indomethacin and mefenamic acid), and tartrazine, a yellow food dye.
Exercise may also provoke an asthma attack. With exercise-induced asthma, bronchospasm may follow heat and moisture loss in the upper airways.
Two-phase allergic response
When the patient inhales an allergenic substance, sensitized IgE antibodies trigger mast cell degranulation in the lung interstitium, releasing histamine, cytokines, prostaglandins, thromboxanes, leukotrienes, and eosinophil chemotaxic factors. Histamine then attaches to receptor sites in the larger bronchi, causing irritation, inflammation, and edema. In the late phase, inflammatory cells flow in. The influx of eosinophils provides additional inflammatory mediators and contributes to local injury.
Signs and symptoms
An acute asthma attack begins dramatically, with simultaneous onset of severe multiple symptoms, or insidiously, with gradually increasing respiratory distress. Asthma that occurs with cyanosis, confusion, and lethargy indicates the onset of life-threatening status asthmaticus and respiratory failure. Symptoms of bronchial airway obstruction may persist between acute episodes.
Signs and symptoms of asthma include:
sudden dyspnea, wheezing, and tightness in the chest
coughing that produces thick, clear, or yellow sputum
tachypnea, along with use of accessory respiratory muscles
hyperresonant lung fields
diminished breath sounds.
In 1997, the National Heart, Lung, and Blood Institute of the National Institutes of Health identified four levels of asthma severity based on the frequency of symptoms and exacerbations, effects on activity level, and lung function study results: mild intermittent, mild persistent, moderate persistent, and severe persistent.
Findings for mild intermittent asthma include the following:
Symptoms occur less than twice per week.
The patient is asymptomatic with normal peak expiratory flow (PEF) between exacerbations.
Brief exacerbations (from a few hours to a few days) vary in intensity.
Nighttime symptoms occur less than twice per month.
Findings for mild persistent asthma include the following:
Symptoms occur more than twice per week, but less than once per day; exacerbations may affect activity.
Nighttime symptoms occur more than twice per month.
Lung function studies show FEV1 or PEF greater than 80% of normal values; PEF may vary by 20% to 30%.
Findings for moderate persistent asthma include the following:
Symptoms occur daily.
Exacerbations occur more than twice per week and may last for days; exacerbations affect activity.
A bronchodilator is used daily.
Nighttime symptoms occur more than once per week.
Lung function studies show FEV1 or PEF 60% to 80% of normal values; PEF may vary by greater than 30%.
Findings for severe persistent asthma include the following:
Symptoms occur on a continuous basis.
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