What Causes Asthma?
According
to recent estimates, asthma affects 300 million people in the world and more
than 22 million in the United States. Although people of all ages suffer from
the disease, it most often starts in childhood, currently affecting 6 million
children in the US. Asthma kills about 255,000 people worldwide every year.
Children at Risk
Asthma is
the most common chronic disease among children - especially children who have
low birth weight, are exposed to tobacco smoke, are black, and are raised in a
low-income environment. Most children first present symptoms around 5 years of
age, generally beginning as frequent episodes of wheezing with respiratory
infections. Additional risk factors for children include having allergies, the
allergic skin condition eczema, or parents with asthma.
Young
boys are more likely to develop asthma than young girls, but this trend
reverses during adulthood. Researchers hypothesize that this is due to the
smaller size of a young male's airway compared to a young female's airway,
leading to a higher risk of wheezing after a viral infection.
Allergies
Almost all asthma sufferers have allergies. In
fact, over 25% of people who have hay fever (allergic rhinitis) also develop
asthma. Allergic reactions triggered by antibodies in the blood often lead to
the airway inflammation that is associated with asthma.
Common sources of indoor allergens include animal
proteins (mostly cat and dog allergens), dust mites, cockroaches, and fungi. It
is possible that the push towards energy-efficient homes has increased exposure
to these causes of asthma.
Tobacco Smoke
Tobacco smoke has been linked to a higher risk of
asthma as well as a higher risk of death due to asthma, wheezing, and
respiratory infections. In addition, children of mothers who smoke - and other
people exposed to second-hand smoke - have a higher risk of asthma prevalence.
Adolescent smoking has also been associated with increases in asthma risk.
Environmental Factors
Allergic reactions and asthma symptoms are often
the result of indoor air pollution from mold or noxious fumes from household
cleaners and paints. Other indoor environmental factors associated with asthma
include nitrogen oxide from gas stoves. In fact, people who cook with gas are
more likely to have symptoms such as wheezing, breathlessness, asthma attacks,
and hay fever.
Pollution, sulfur dioxide, nitrogen oxide, ozone,
cold temperatures, and high humidity have all been shown to trigger asthma in
some individuals.
During periods of heavy air pollution, there tend
to be increases in asthma symptoms and hospital admissions. Smoggy conditions
release the destructive ingredient known as ozone, causing coughing, shortness
of breath, and even chest pain. These same conditions emit sulfur dioxide,
which also results in asthma attacks by constricting airways.
Weather changes have also been known to stimulate
asthma attacks. Cold air can lead to airway congestion, bronchoconstriction
(airways constriction), secretions, and decreased mucociliary clearance
(another type of airway inefficiency). In some populations, humidity causes
breathing difficulties as well.
Obesity
Overweight adults - those with a body mass index
(BMI) between 25 and 30 - are 38% more likely to have asthma compared to adults
who are not overweight. Obese adults - those with a BMI of 30 or greater - have
twice the risk of asthma. According to some researchers, the risk may be
greater for non-allergic asthma than allergic asthma.
Pregnancy
The way you enter the world seems to impact your
susceptibility to asthma. Babies born by Caesarean sections have a 20% increase
in asthma prevalence compared to babies born by vaginal birth. It is possible
that immune system-modifying infections from bacterial exposure during Cesarean
sections are responsible for this difference.
When mothers smoke during pregnancy, their children
have lower pulmonary function. This may pose additional asthma risks. Research
has also shown that premature birth is a risk factor for developing asthma.
Stress
People who undergo stress have higher asthma rates.
Part of this may be explained by increases in asthma-related behaviors such as
smoking that are encouraged by stress. However, recent research has suggested
that the immune system is modified by stress as well.
Genes
It is
possible that some 100 genes are linked to asthma - 25 of which have been
associated with separate populations as of 2005.
Genes
linked to asthma also play roles in managing the immune system and
inflammation. There have not, however, been consistent results from genetic
studies across populations - so further investigations are required to figure
out the complex interactions that cause asthma.
Mom and
Dad may be partially to blame for asthma, since three-fifths of all asthma
cases are hereditary. The Centers for Disease Control (USA) say that having a
parent with asthma increases a person's risk by three to six times.
Genetics
may also be interacting with environmental factors. For example, exposure to
the bacterial product endotoxin and having the genetic trait CD14 (single
nucleotide polymorphism (SNP) C-159T) have remained a well-replicated example
of a gene-environment interaction that is associated with asthma.
Airway Hyper-reactivity
Researchers
are not sure why airway hyper-reactivity is another risk factor for asthma, but
allergens or cold air may trigger hyper-reactive airways to become inflamed.
Some people do not develop asthma from airway hyper-reactivity, but hyper-reactivity
still appears to increase the risk of asthma.
Atopy
Atopy - such as eczema (atopic dermatitis),
allergic rhinitis (hay fever), allergic conjunctivitis (an eye condition) - is
a general class of allergic hypersensitivity that affects different parts of
the body that do not come in contact with allergens. Atopy is a risk factor for
developing asthma.
Some 40% to 50% of children with atopic dermatitis
also develop asthma, and it is probable that children with atopic dermatitis
have more severe and persistent asthma as adults.
Diagnosing Asthma
Asthma
diagnoses are based on three core components: a medical history, a physical
exam, and results from breathing tests. A primary care physician will
administer tests and, if you have asthma, determine your level of asthma
severity as intermittent, mild, moderate, or severe.
Medical History
A
detailed family history of asthma and allergies can help your doctor make an
accurate asthma diagnosis. Your own personal history of allergies is also
important as many are closely linked to asthma.
Information
about asthma symptoms is also useful. Be prepared to divulge when and how often
they occur and what factors seem to exacerbate or worsen symptoms. Common
symptoms and signs include:
§ Wheezing
§ Coughing
§ Breathing difficulty
§ Tightness in the chest
§ Worsening symptoms at night
§ Worsening symptoms due to cold air
§ Symptoms while exercising
§ Symptoms after exposure to allergens
It is
also wise to make note of health conditions that can interfere with asthma
management such as runny nose, sinus infections, acid reflux disease,
psychological stress, and sleep apnea.
It is
often somewhat harder to diagnose young children who may develop their first
asthma symptoms before age 5. Symptoms are likely to be confused with those of
other childhood conditions, but young children with wheezing episodes during
colds or respiratory infections are likely to develop asthma after 6 years of
age.
Physical Exam
A
physical examination will generally focus on the upper respiratory tract,
chest, and skin. A doctor will use a stethoscope to listen for signs of asthma
in your lungs as you breathe. The high-pitched whistling sound while you exhale
- or wheezing - is a key sign of both an obstructed airway and asthma.
Physicians
will also check for a runny nose, swollen nasal passages, and nasal polyps. Skin
will be examined for conditions such as eczema and hives, which have been
linked to asthma.
Physical
symptoms are not always present in asthma sufferers, and it is possible to have
asthma without presenting any physical maladies during an examination.
Asthma Tests
Lung
function tests, or pulmonary function tests, are the third component of an
asthma diagnosis. To measure how much air you breathe in and out and how fast
you can blow air out, physicians administer a spirometry test.
Spirometry
is a noninvasive test that requires you to take deep breaths and forcefully
exhale into a hose connected to a machine called a spirometer. The spirometer
then displays two key measurements:
Forced
vital capacity (FVC) - the maximum amount of air one can inhale and exhale
Forced expiratory volume (FEV-1) - the maximum amount of air exhaled in one
second
The
measurements are compared against standards developed for a person's age, and
measurements below normal may indicate obstructed airways.
Children
younger than 5 years of age are difficult to test using spirometry, so asthma
diagnoses will rely mostly on symptoms, medical histories, and other parts of
the physical examination. It is common for doctors to prescribe asthma
medicines for 4 to 6 weeks to see how a young child responds.
Other Tests
A
"Challenge Test" (or bronchoprovocation test) is when a physician
administers an airway-constricting substance (or something as simple as cold
air) to deliberately trigger airway obstruction and asthma symptoms. Similarly,
a challenge test for exercise-induced asthma would consist of vigorous exercise
to trigger symptoms. A spirometry test is then administered, and if
measurements are still normal, an asthma diagnosis is unlikely.
Physicians
use allergy tests to identify substances that may be causing or worsening
asthma. These tests cannot be used to diagnose asthma, but they can be used to
understand the nature of asthma symptoms.
Doctors
may also test for another disease with similar symptoms as asthma, such as
reflux disease, heartburn, hay fever, sinusitis, sleep apnea, chronic
obstructive pulmonary disease (COPD), airway tumors, airway obstruction,
bronchitis, lung infection (pneumonia), blood clot in the lung (pulmonary
embolism), congestive heart failure, vocal cord dysfunction, and viral lower
respiratory tract infection.
Tests may
be administered for these ailments such as chest x-rays, EKGs
(electrocardiograms), complete blood counts, CT (computerized tomography) scans
of the lungs, gastroesophageal reflux assessment, and sputum induction and
examination.
A new
test using exhaled nitric oxide is being evaluated since physicians are looking
for a test that is more accurate than spirometry. Higher levels of nitric oxide
are linked to higher degrees of asthma severity. The current drawback lies in
the high cost of the test and the specialized equipment required to measure
this chemical marker.
An asthma
specialist can usually be avoided, as most primary care physicians are capable
of diagnosing asthma. An asthma specialist may be necessary, however, if you
need special asthma tests or have had a life-threatening asthma attack in the
past. In addition, specialists can be of use if you need more than one kind of
medicine or higher doses of medicine in order to control your asthma, if you
have overall difficulty controlling asthma, or if you will be receiving allergy
treatments.
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