Bronchiolitis is an inflammation of the small passages in the lungs (bronchioles), usually caused by a viral infection.

Bronchiolitis usually affects children under the age of 2, with a peak age of 3 - 6 months. It is a common, sometimes severe illness. Respiratory syncytial virus (RSV) is one common cause. Other viruses that can cause bronchiolitis include:

• Adenovirus
• Influenza
• Parainfluenza

The virus is transmitted from person-to-person by direct contact with nasal secretions or by airborne droplets. Although RSV generally causes only mild symptoms in an adult, it can cause a severe illness in an infant.

Bronchiolitis is seasonal and appears more often in the fall and winter months. It is a very common reason for infants to be hospitalized during winter and early spring. It is estimated that by the first year, more than half of all infants have been exposed to RSV.

Risk factors include:

• Being exposed to cigarette smoke
• Being less than 6 months old
• Living in crowded conditions
• Never being breastfed
• Prematurity (born before 37 weeks gestation)

Some children have infections with few or minor symptoms.

Bronchiolitis begins as a mild upper respiratory infection that, over a period of 2 - 3 days, can develop into increasing respiratory distress with wheezing and a "tight," wheezy cough.

The infant's breathing rate may increase a lot (tachypnea), and the infant may become irritable or anxious-looking. If the disease is severe enough, the infant may turn bluish (cyanotic), which is an emergency.

As the effort of breathing increases, parents may see the child's nostrils flaring with each breath and the muscles between the ribs retracting (intercostal retractions) as the child tries to breathe in air. This can be exhausting for the child, and very young infants may become so tired that they have difficulty maintaining breathing.

Symptoms include:

• Bluish skin due to lack of oxygen (cyanosis)
• Cough, wheezing, shortness of breath, or difficulty breathing
• Fever
• Intercostal retractions
• Nasal flaring in infants
• Rapid breathing (tachypnea)

Signs include:

• Decreased blood oxygen
• Wheezing and crackling sounds heard through stethoscope exam of chest

Tests include:

• Blood gases
• Chest x-ray
• Nasal fluid cultures (to determine which virus is present)