Asthma is defined as episodic and reversible airflow obstruction and is the most common chronic illness of childhood. According to estimates, 9 million children in the United States were affected by asthma in 2002. When taken as a single group, asthma rates for children rose from 2001 until 2004.

However, when evaluated by category, the rates during the period stabilized for white children and black girls, while increasing for black boys. Additionally, the morbidity and mortality of asthma disproportionately affects low-income, minority, and inner-city children.

Acute asthma exacerbations are the third leading cause of pediatric hospitalizations and the estimated cost for pediatric asthma treatment is over $3 billion per year. Because of the magnitude of this burden, much attention has been placed on early diagnosis and prevention of pediatric asthma.

Diagnosis of asthma in the pediatric patient presents a challenge, though, because only half of young children with recurrent wheeze are found to have true asthma. Multiple birth cohorts have supported this finding and have led to the description of different childhood wheezing phenotypes:

• Transient wheezers-those who have recurrent wheezing that remits.
• Non-atopic wheezers-those who are non-allergic and who often have wheezing that is associated with viral infections and that remits by age 5 to 6 years.
• Persistent wheezers-those who are atopic wheezers with persistence.

Additionally, for some patients, the respiratory symptoms may remit only to relapse later in life. The recognition of these different groups has prompted research into identifying and understanding the risk factors for developing persistent asthma.

Atopy

The single greatest risk factor for developing asthma seen in infants and children is atopy or the genetic predisposition for allergic diseases. This includes both atopic dermatitis and allergies. The Tucson Birth Cohort demonstrated that the presence of eczema was a major risk factor in predicting the likelihood of persistent disease.

Based on this cohort, the Asthma Predictive Index (API) was developed . The major risk factors (only one is required) include parental asthma and physician-diagnosed atopic dermatitis. Minor risk factors (two are required) include physician-diagnosed allergic rhinitis, wheezing unrelated to colds, and blood eosinophilia (R4%).

A subsequent study of children at risk for asthma, Prevention of Early Asthma in Kids (PEAK), demonstrated that allergic sensitization could be added to the aforementioned major risk factors in predicting persistent disease.

The importance of allergy in pediatric asthma is further supported by the so-called ‘‘atopic march,’’ as many children initially present with eczema with or without food allergies and then progress to develop asthma and finally allergic rhinitis.

Viral Illnesses

Viral illnesses are tightly linked with asthma both as a risk factor and a trigger. Studies have shown that previous severe infection with respiratory syncytial virus (RSV), including hospitalization and/or oxygen requirement, is a risk factor for the development of asthma.

What remains unclear is whether RSV infection itself results in permanent damage leading to asthma or if a child already predisposed to asthma is more likely to have a severe RSV infection. In either case, a history of a significant RSV infection may support a prediction of persistent disease.

Consequently, repeated viral illness exposure through siblings or daycare may actually lead to less atopy and asthma. Finally, viral illnesses are major triggers for acute asthma symptoms, particularly infection with rhinoviruses, and there is now evidence that patients with asthma lack normal defense mechanisms against rhinovirus infections

Early Allergen Exposures

Investigations into the role of early allergen exposure and its effect on subsequent allergic sensitizations and asthma have had varied results. Several recent studies have presented conflicting results as to whether having a pet in the home is a risk factor for asthma or is protective against developing persistent disease.

In one study of Swedish children, exposure to a dog or cat during the first year of life resulted in lower allergic rhinitis and asthma rates. The etiology of this protective effect from domesticated pets is less clear. It may be the allergen pet dander itself or the endotoxin associated with the animals.

Several studies have also demonstrated that being raised on a farm and being exposed to high levels of endotoxin protects against allergic disorders and wheeze. Early exposure to high levels of dust mites and cockroach allergen is associated with increased rates of asthma in inner-city children. Finally, skin test positivity to Alternaria species is associated with increased risk of persistent asthma