Definition

Asthma is a chronic inflammatory pulmonary disorder that is characterized by reversible obstruction of the airways.

Since the 1980s there has been a worldwide increase in the prevalence of asthma in both children and adults. This escalating prevalence has led to significant increases in morbidity and mortality due to the disease. In the United States, asthma is the main reason for the hospitalization of children and for school absenteeism due to a chronic disease; the overall death rate from asthma has increased by 40% from 1982-1992 [Beers and Berkow, 1999]. Additionally, the annual total cost of treating asthma in the United States is more than $6 billion.

The underlying cause of the increasing prevalence of asthma is unknown. However, the airway inflammation that is noted in asthma is due to an immune-mediated process in which inflammatory cells and inflammatory mediators enter airway tissues to cause disease. Many cell-mediated immunologic factors participate in the inflammatory process of asthma. The most important inflammatory cells involved are eosinophils, mast cells, and T lymphocytes.

Important aspects that define asthma include airway hyperreponsiveness and bronchoconstriction. Airway hyperreponsiveness refers to an increased tendency of the asthmatic airway to react to a variety of stimuli that would not cause a response in a normal airway. These asthma triggers can cause an asthma attack in an inflamed airway. Bronchoconstriction refers to a narrowing of the airways that causes obstruction of airflow (sometimes termed airflow limitation). The bronchoconstriction of asthma is unique because it is at least partly reversible, either spontaneously or with treatment.

When inflamed airways respond to an asthma trigger through bronchoconstriction, the characteristic symptoms of asthma appear, namely wheezing, cough, and chest tightness or dyspnea. Wheezing is due to airflow limitation, causing a high-pitched whistling sound, which is usually heard on expiration, but it may also be heard on inspiration. Cough probably results from stimulation of sensory nerves in the airways by inflammatory mediators that are released by various inflammatory cells involved in asthma. Chest tightness or dyspnea is the sensation associated with the increased work needed to breathe when the airways are constricted that patients often feel.

Classification

The most widely accepted classification of asthma in the United States is that recommended by the National Heart, Lung, and Blood Institute's (NHLBI) National Asthma Education and Prevention Program (NAEPP) Expert Panel Report 2 "Guidelines for the Diagnosis and Management of Asthma" [1997]. These guidelines place major emphasis on diagnosis (including classification of asthma) and management (including a stepped approach to asthma treatment).

Other guidelines have also been released:

The Global Initiative for Asthma (GINA) guidelines [1998] were the combined effort of NHLBI and the World Health Organization (WHO). Although these recommendations are essentially similar to the NAEPP guidelines, there are subtle changes in language, including specific details within the stepped approach to asthma therapy.
The Pediatric Asthma: Best Practices guidelines [1999] released by the American Academy of Allergy, Asthma and Immunology (AAAAI), American Academy of Pediatrics (AAP), and NHLBI translated the NAEPP guidelines into recommendations that can be easily used by general pediatricians, who see the bulk of pediatric asthma. These guidelines also acknowledge the differences in assessment and management that exist for younger children with asthma (i.e., age </=5).

The guidelines divide asthma severity into four classes that range from mild intermittent asthma (step 1) to severe persistent asthma (step 4), as shown in the following figure. This classification is based on history of asthma symptoms and measurements of lung function. Specifically, history of asthma symptoms includes the frequency of daytime and night-time symptoms and the effect of the exacerbations on daily activities. Measurements of lung function include predicted values for peak expiratory flow rate (PEFR) and forced expiratory volume in 1 second (FEV₁) and the morning-to-evening variability of PEFR. The presence of one feature of a more severe class is sufficient to place the patient in that class of severity. Importantly, asthma severity is classified before therapy is initiated.

NHLBI, NAEPP, GINA, and AAAAI/AAP Guidelines

Click on image for animation/larger version.

Asthma severity is divided into the same classes in the NAEPP, GINA, and AAAAI guidelines. Classification is based on history of asthma symptoms and lung function before therapy begins and is described as mild intermittent, mild persistent, moderate persistent, and severe persistent (see figures below).

Step 1: Mild Intermittent Asthma Click on image for larger version.	Step 2: Mild Persistent Asthma Click on image for larger version.

Step 3: Moderate Persistent Asthma Click on image for larger version.	Step 4: Severe Persistent Asthma Click on image for larger version.

Below is a simplified way to remember the classification system, in which each individual feature is grouped separately.

Classification of Asthma Severity by Feature

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The presence of just one feature within a given category is sufficient to place a patient in that category. Thus, an individual should be assigned to the most severe grade in which any feature occurs. Note that the classification characteristics noted in the guidelines are general and may overlap over time because asthma is highly variable. Furthermore, an individual's classification may change over time.

Although airway inflammation increases with asthma severity, patients at any level of severity can have mild, moderate, or severe exacerbations. Some patients with intermittent asthma can experience a severe and life-threatening exacerbation. Because symptoms of asthma and the use of asthma guidelines varies, physicians may have difficulty assessing the severity of airways obstruction. This could lead to underclassification of the severity of a patient's asthma with subsequent poor asthma control [Osborne et al., 1999; Shim and Williams, 1980]. It has been estimated that about 2.5 million people with asthma in the United States are underclassified and undertreated [Migliara, 1996]. Underclassification and undertreatment may have contributed to the increased morbidity and mortality rates for asthma over the past two decades [Bousquet et al., 1996].

References

Beers MH, Berkow R, eds. The Merck Manual of Diagnosis and Treatment. 17th ed. Whitehouse Station, NJ: Merck Research Laboratories; 1999.

Bousquet J, Knani J, Henry C, et al. Undertreatment in a nonselected population of adult patients with asthma. J Allergy Clin Immunol. 1996;98:514-521.

Global Initiative for Asthma. Pocket Guide for Asthma Management and Prevention. Bethesda, Md: NIH, Nov 1998. NIH Pub. No. 96-3659B.

National Asthma Education and Prevention Program. Expert Panel Report 2. Guidelines for the Diagnosis and Management of Asthma. Bethesda, Md: NIH, July 1997. NIH Pub. No. 97-4051.

Osborne ML, Vollmer WM, Pedula KL, et al. Lack of correlation of symptoms with specialist-assessed long-term asthma severity. Chest. 1999;115:85-91.

Pediatric Asthma. Promoting Best Practice. Guide for Managing Asthma in Children. American Academy of Allergy, Asthma & Immunology, Inc., 1999.

Shim CS, Williams MH Jr. Evaluation of the severity of asthma: patients versus physicians. Am J Med. 1980;68:11-13.

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