Asthma is a Disease of Different Phenotypes
Asthma is a chronic condition characterized by inflammation in the airways and airway hyper-reactivity. For years allergy specialists and other clinicians thought that all asthma was the same and treated all patients with asthma with similar treatment regimens. However, researchers and clinicians have recently learned that this is not the case. There are differences in the types of underlying inflammation observed in the lung and differences in the clinical features of the disease (e.g. what triggers it) between different patients with asthma. These differences in asthma are called “phenotypes”. There are some phenotypes that seem to be more common in males than females or in children than adults. Identifying the specific asthma phenotype of an individual patient helps allergy / asthma / immunology specialists better manage the disease.
The current treatment of asthma is now moving towards trying to treat the more specific phenotype of asthma for individual patients rather than using the same treatment regimen for all patients. This is part of the medical model known as personalized, or precision, medicine. Several different types or what we call “phenotypes” of asthma have been described. These include: allergic, non-allergic, aspirin-sensitive, severe, exercise-induced, neutrophilic, fixed-obstruction, and occupational. See below for a brief description of some of the more commonly identified asthma phenotypes in children and adults.
Allergic asthma is a type of asthma where environmental allergies (e.g. pollen allergy, dust mite allergy) trigger the asthma. This is the most common asthma phenotype and has been described to occur in about 40-50% of patients with asthma. This phenotype is also one that is most easily identified. In the evaluation of a patient with asthma, testing may be done to determine if that patient has an “allergic phenotype” or not. These tests may include allergy testing for environmental allergens, a complete blood count to look for an increase in eosinophils (an allergic type cell), blood IgE antibody level, and exhaled nitric oxide testing. If you have allergic asthma your allergy specialist may modify your treatment plan, add additional medications, and/or recommend that you make changes in your home to decrease allergen exposure. Controlling allergies with allergy medications (antihistamines, nasal steroids) and avoiding allergy triggers may be recommended as an important part of your asthma management. Allergy immunotherapy (allergy shots or sublingual tablets) may also be recommended. Recent advances and new medications to treat asthma have been focused on this phenotype. Biological therapies that target the allergic response pathway (including the medications omalizumab, mepolizumab, dupilumab and reslizumab) may also be considered.
Patients with the non-allergic asthma phenotype do not demonstrate evidence of allergic sensitivity. These patients have an asthma diagnosis but allergy tests do not indicate that they are sensitive to indoor or outdoor allergies; they do not have elevated eosinophil counts and/or IgE levels, and may not have an elevated exhaled nitric oxide test. If you are diagnosed with this type of asthma your doctor may choose a different asthma regimen for you.
Aspirin sensitive asthma phenotype is a less common asthma phenotype that is observed in both children and adults. Patients with aspirin sensitive asthma will have a flare up of their asthma when they take aspirin or a non-steroidal anti-inflammatory drug (NSAID). Patients with this asthma phenotype may also have chronic rhinitis (nasal inflammation) symptoms and/or or nasal polyps. A complete blood count evaluation may also reveal an elevated eosinophil count. Patients with this type of asthma may benefit from medications that target the immunologic pathway stimulated by aspirin or NSAIDs such as leukotriene modifiers and certain biological therapies.
Neutrophilic asthma is an asthma phenotype which has been described most commonly among patients that have severe asthma. This type of asthma may be diagnosed by your allergy / asthma / immunology specialist by obtaining a sputum sample (saliva and mucous coughed up from the respiratory tract) and observing a large number of the neutrophil type of cell in your sputum. If your allergy / asthma / immunology specialist determines that you have this phenotype of asthma your medication regimen may be modified and additional medications such as macrolide antibiotics may be recommended.
Exercise-induced asthma is a type of asthma where a patient has a flare up when exercising. Most patients with asthma will have an exercise induced flare up if they exercise to a certain capacity. To make a diagnosis of this type of asthma your allergy / asthma / immunology specialists may ask you questions about when your symptoms occur most frequently or consistently (e.g. do you have an asthma flare up every time you exercise?) or they may perform an exercise test. An exercise test consists of having you exercise (e.g. running on a treadmill or the clinic hallway) and checking your lung function after exercising to determine if exercise induces a drop in your lung function. Patients with this type of asthma may be treated with specific types of medications and/or may be instructed to use their short-acting beta-agonist (albuterol) inhaler a few minutes before exercise to prevent an attack.
Although researchers and allergy / asthma / immunology specialists now recognize that there are different phenotypes of asthma among patients, it may still be difficult to accurately determine the phenotype of certain patients. A patient may also demonstrate more than one phenotype of asthma (e.g. allergic asthma and exercise induced asthma). And although certain tests may aid in the diagnosis, most asthma phenotypes do not have a reliable confirmatory test. Even if your asthma specialist is able to determine which type of asthma you have it may not lead to a significant change in medications since there is a lack of targeted treatments for many asthma phenotypes. More research is needed to understand asthma phenotypes, how they differ, how to identify them, and to continue to develop new medications that target each phenotype successfully.
Nevertheless, it is important to think about what asthma phenotype you may have and to discuss this with your asthma / allergy / immunology specialist.