Viral-induced wheeze describes is an acute wheezy illness caused by a viral infection. Small children (typically under 3 years) have small airways. When these small airways encounter a virus (commonly RSV or rhinovirus) they develop a small amount of inflammation and oedema, swelling the walls of the airways and restricting the space for air to flow. This inflammation also triggers the smooth muscles of the airways to constrict, further narrowing the space in the airway.
This swelling and constriction of the airway caused by a virus has little noticeable effect on the larger airways of an older child or adult, however due to the small diameter of a child’s airway, the slight narrowing leads to a proportionally larger restriction in airflow. This is described by Poiseuille’s law, which states that flow rate is proportional to the radius of the tube to the power of four. Therefore, halving the diameter of the tube decreases flow rate by 16 fold.
Air flowing through these narrow airways causes a wheeze, and the restricted ventilation leads to respiratory distress. For some reason, certain children are much more prone to this airway swelling than others. There seems to be a hereditary element, so when assessing a wheezy child ask about a family history of viral-induced wheeze. These children are at higher risk of developing asthma in later life.
Viral Induced Wheeze or Asthma?
The distinction between a viral-induced wheeze and asthma is not definitive. Generally, typical features of viral-induced wheeze (as opposed to asthma) are:
- Presenting before 3 years of age
- No atopic history
- Only occurs during viral infections
Asthma can also be triggered by viral or bacterial infections, however it also has other triggers, such as exercise, cold weather, dust and strong emotions. Asthma is historically a clinical diagnosis, and the diagnosis is based on the presence of typical signs and symptoms along with variable and reversible airflow obstruction.
Evidence of a viral illness (fever, cough and coryzal symptoms) for 1-2 days preceding the onset of:
- Shortness of breath
- Signs of respiratory distress
- Expiratory wheeze throughout the chest
TOM TIP: Neither viral-induced wheeze or asthma cause a focal wheeze. If you hear a focal wheeze be very cautious and investigate further for a focal airway obstruction such as an inhaled foreign body or tumour. These patients will require an urgent senior review.
Management of viral-induced wheeze is the same as acute asthma in children.
Last updated August 2019