Prevalence of Asthma in the United States
DR. SOPHOCLES: What is the prevalence and incidence of asthma in the United States?
DR. COX: According to multiple data sources, the estimated prevalence of asthma is roughly 7.5%. The
population of the U.S. in 2005 was about 296 million, so this makes the prevalence of asthma about
22 million people. The age distribution shows
that 33% of asthmatics are between 0 and 17
years, whereas 67% are 18 and above. In terms of
stratification for asthma severity, approximately
28% have mild or intermittent asthma, 25% have
mild persistent, 31% have moderate persistent,
and 16% have severe persistent asthma. In terms
of race, approximately 7.6% of the Caucasian
population has asthma, 5% of the Hispanic
population and 9.3% of the African-American
population. In terms of gender, 57% of the asthmatics are female and 42% are male. Now the gender
difference is not the same across all ages. Before puberty, the incidence of asthma is greater in boys than
girls, but at puberty, that changes and it’s slightly greater in women than men. And in terms of asthma
mortalities, there have been about 4,000 deaths a year. Actually in the past 10 years this number has
stayed fairly consistent.
I
t’s estimated that there are 1.8 million asthma emergency room visits and 500 thousand hospitalizations
a year for asthma. In a cross-sectional study of households in 29 countries, they found that asthma
control fell far short of goals world-wide and that
the use of anti-inflammatory therapies was low,
even in patients with persistent asthma. In Japan
it was 9% of patients and Western Europe, 26%
of patients used anti-inflammatory therapies.
There’s a correlation between perception of
control and control appears to be poor in many
asthmatics, and many asthmatics still smoke.
The onset of asthma for most patients begins
early in life. The incidence after infancy remains
100 in 100,000 patients throughout the lifespan. In some patients, the development of chronic
inflammation may be associated with permanent alterations in airway structure leading to irreversible
airflow obstruction, referred to sometimes as airway remodeling, which I discussed earlier. And this may
not be prevented by, or fully responsive to the current treatments that we have available. This loss of
lung function may occur early in the course of asthma. An asthma predictive index has been developed
from a prospective study that I mentioned earlier.
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DR. SOPHOCLES: Using the asthma predictive
index, can you prevent high risk individuals
from developing asthma with early treatment?
DR. COX: Children were followed from birth and
a number of parameters have been measured over
different time periods including a serum IgE, skin
testing, and pulmonary function tests.
What they have identified is in children younger than age three, who have more than three wheezing
episodes in the previous years, who had one of the major criteria—either a parent history with
asthma, evidence of aeroallergen sensitization, or atopic dermatitis—or two of minor criteria which is
eosinophilia equal to or greater than 4%, food
sensitization, or wheezing outside of respiratory
tract infections. children who met either one
major or two minor criteria and had those three
wheezing episodes had a 76% chance of being
diagnosed with asthma after six years of age, if
they had this positive asthma predictive index
before age three. If they did not have this positive
predictive ndex, 97% of children would not
develop asthma with the negative asthma
predictive index.
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DR. SOPHOCLES: Can we use the asthma predictive index for early intervention and
modify the disease?
DR. COX: Well, one clinical trial did attempt to identify children by this asthma predictive index who
were at high risk for developing asthma and treated them with inhaled corticosteroids for two years.
Unfortunately they found that the treatment did not alter the development of asthma symptoms or
modify lung functions during the third, treatment-free year. So this study did not support a subsequent
disease modifying-effect of inhaled corticosteroids because they tended to go back to the same disease
state after treatment was discontinued.
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DR. SOPHOCLES: What happens to lung function in asthmatics over time?
DR. COX: There have been a few studies that have looked at large populations of asthmatics and
non-asthmatics and followed them over time measuring lung function. One was published in the New England Journal of Medicine and followed
patients over a 15 year period of time. What they
found was asthmatics lost lung function at roughly
38 milliliters a year, whereas non-asthmatics lost
lung function at 22 milliliters a year. They also
noted that if one smoked and had asthma, the
loss of lung function was much greater and gave
the example of a 60-year-old man who was 175
centimeters, who was non-smoking, would have
an FEV1 of 3.05L, whereas the same man who
smoked and had asthma would have an FEV1 of
1.99L. So smoking and asthma in combination
appear to really significantly increase the loss of
lung function over time.
Another very long term study on children age
7-10 and up to age 35, and measuring lung
function at several time points, did also find that
the loss of lung function could occur early in the
diagnosis of asthma. In this study, they actually
characterized the asthma in four categories.
They were referred to as mild forms which were
wheezy bronchitis and mild wheezy bronchitis.
I’m presuming that this is wheezing in association with respiratory tract infections. And then there was the asthma and severe asthma group.
What they found was, as one might expect, the more severe classification of asthmatics had the lower lung function, but the loss of lung function appeared to occur early in the diagnosis of asthma and then plateaued throughout the course of the subsequent twenty-some years, because they were followed up to age 35. This is referred to as the Melbourne Asthma Study.
Another study was a population study in New
Zealand. It looked at children and adults age 9
to 26, following them with questionnaires,
pulmonary function tests, bronchial challenge
and allergy testing, and evaluating them every
two years with pulmonary function tests, with
skin testing at age 13 and 21, and collecting a
respiratory diary. They assessed 1139 children
and had complete data on 613 and what they
found was 75% of the respondents reported
wheezing on at least one occasion and 51% had wheezed on more than one occasion.
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