Transcript for Asthma
Asthma is an episodic, chronic respiratory disorder characterized by airway obstruction caused by inflammation and hyperresponsiveness of the bronchial smooth muscle.
The two golden words in asthma are “reversible”, which means the obstruction can virtually disappear with medications like bronchodilators, and “inducible”, which means the obstruction can occur in response to a variety of stimuli; including allergens like house dust mites, pet animal dander, like cat dander, and pollen, or irritants like tobacco smoke, respiratory tract infections, like a common cold or pneumonia, cold or dry air, and even emotional stress.
Now, atopy is the genetic predisposition of an individual towards developing IgE antibodies to otherwise harmless environmental antigens, making that individual strongly predisposed to developing three allergic disorders, eczema or atopic dermatitis, allergic rhinitis, and asthma, collectively coined the atopic triad.
Because of this genetic component, asthma is usually diagnosed in early childhood, and may or may not carry on into adulthood.
So say a genetically predisposed child is exposed to a potential allergen for the first time. First off, dendritic cells, take up the allergen and present it to a type 2 helper T-cell, or Th2 cell.
IgE antibodies then prime mast cells, which cautiously anticipate the next event.
When the child is re-exposed to the allergen, the mast cells spill out vasoactive mediators like histamine and leukotrienes, which cause bronchoconstriction and inflammation.
Once the child is no longer exposed to the allergen, the immune system relaxes, and everything goes back to normal, until the next event.
Now, not all asthma episodes are triggered this way by an allergen.
Aspirin opposes prostaglandin production and tips the balance between prostaglandins and leukotrienes in favor of increased leukotriene production, which promotes smooth muscle contraction of the airways.
Alright, patients with asthma have intermittent episodes of dry cough, chest tightness, and shortness of breath.
Some patients have just a single symptom - like cough, making the diagnosis more difficult.
These symptoms usually occur in the daytime, but can also happen at night.
On auscultation there will likely be wheezing during expiration, because that’s when the airways collapse and become narrow.
Percussion may reveal hyperresonance, which indicates that the bronchoconstriction has led to air trapping and hyperinflation.
Severe episodes are ones where a patient struggles to breathe - they may only be able to speak in short phrases, and may need to use accessory muscles like the scalene muscles of the neck, or the intercostal muscles of the ribs to help with breathing.
But “all that wheezes isn’t asthma”.
For example, chronic obstructive pulmonary disease, or COPD, can also cause wheezing, but it’s more common in the older patient who smokes and in COPD the symptoms are often irreversible because there are permanent changes to the architecture of the lungs.
Another one is aspiration of a foreign body which might obstruct an airway, and cause wheezing in one area of the lungs. This usually occurs in young children, and there’s often a sudden onset of wheezing and cough along with a history of the child being unobserved or potentially swallowing something.
Bronchiolitis, or inflammation of the small bronchioles, is often seen in children less than 2 years of age, and presents with acute cough, wheeze and shortness of breath following an upper respiratory tract infection like a common cold.
Gastroesophageal reflux disease, or GERD can also mimic asthma with symptoms like chronic cough and wheezing, however the patient is often an obese adult, and symptoms often improve with proton-pump inhibitors.
Allergic bronchopulmonary aspergillosis, or ABPA, is an allergic response to aspergillus antigens and usually occurs in the setting of asthma or cystic fibrosis.
ABPA presents with wheezing and high serum IgE levels and eosinophil counts, however chest radiographs often show recurrent pulmonary infiltrates.
Okay - now when asthma is suspected, the first step in diagnosis is conducting pulmonary function tests, or PFTs.
The most important measurements are the forced vital capacity, or FVC, which is total volume of gas exhaled after a forced maximum expiration, and the forced expiratory volume at 1 second, known as FEV1, both of which are decreased in asthma.
Now, because asthma is an obstructive lung disease, the FEV1 is decreased to a greater degree than the FVC1, so the ratio of FEV1 to FVC is usually less than 80%.
Also, because asthma is episodic in nature, the PFTs are only abnormal when the patient is having symptoms.
If a person is symptomatic, the next step is to give a bronchodilator like albuterol, which reverses the PFT measurements back towards normal, confirming the diagnosis of asthma.
A significant reversal is defined an increase in the FEV1 by more than 12% from baseline.
If the reversal is less than 12% after giving a bronchodilator, then the patient is less likely to have asthma.
Now, if you do PFTs on asymptomatic patients, they would be normal, since asthma is an episodic disease. So, in this case we try to induce asthma symptoms by performing the “methacholine challenge test”.
Methacholine is like acetylcholine, so it binds to muscarinic receptors on bronchial smooth muscle, causing mild bronchoconstriction.
In a healthy individual who doesn't have asthma, this bronchoconstriction is negligible, and doesn’t cause a significant change in PFT measurements. But because an asthmatic patient’s bronchial smooth muscle is “hyperresponsive”, methacholine causes a 20% decrease in their FEV1 from the baseline.
Remember the golden asthma words; reversible and inducible!
Now some additional tests are not specific to asthma, but may help with the diagnosis.
A complete blood count, or a CBC may show an increased eosinophil count.
Serum IgE levels may also be increased.
And skin testing may help identify specific allergens that the patient can subsequently avoid. That involves injecting small quantities of known allergens into the patient’s skin, and then observe to see if the immune system overreacts to it.
Okay, so after diagnosing asthma, the next step is to determine how bad it is.
Asthma severity is classified as intermittent, mild persistent, moderate persistent, or severe persistent.
As the severity of the asthma escalates, so does the complexity of treatment plan - specifically the type of chronic treatment the patient requires.
In terms of medications, the first class are bronchodilators, which include short-acting beta agonists, or SABAs, like albuterol, and long-acting beta agonists, or LABAs, like salmeterol.
The second class are anti-inflammatory medications, which include inhaled corticosteroids, like budesonide or fluticasone, leukotriene antagonists, like montelukast and zileuton, and oral corticosteroids, like prednisone.
The third and final class of medications are mast cell stabilizers, like cromolyn.
Alright, classification of asthma severity is determined by four parameters; the frequency at which a patient experiences daytime symptoms, the frequency at which the patient experiences night time symptoms, the FEV1 in between exacerbations, and the degree to which the asthma interferes with the patient’s normal daily activities, like going to work.
Children less than 5 years of age can’t usually follow PFT instructions, so the FEV1 parameter is eliminated.
Now, intermittent asthmatics have less than 3 daytime symptomatic episodes per week, nocturnal symptoms less than 3 times per month, an FEV1 more than 80%, and no interference with normal activity.