The severity of the disease is directly related to post-conceptual age. Infants under six month of age are the most severely affected due to smaller, more easily obstructed airways and decreased ability to clear secretions. Hypoxia is the best predictor of severe illness and correlates best with the degree of tachypnea. The degree of wheezing or retractions correlates poorly with hypoxia. First infections are usually most severe.
Subsequent attacks are generally milder, particularly in older children.
It occurs early in the course of the disease and may be the presenting symptom. The apnea is non-obstructive central apnea occurring during quiet sleep.