Merck Manual

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Rajeev Bhatia

, MD, Phoenix Children's Hospital

Reviewed/Revised Jan 2022 | Modified Sep 2022
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Bronchiolitis is a viral infection that affects the lower respiratory tract of infants and young children under 24 months of age.

  • Bronchiolitis usually is caused by viruses.

  • Symptoms include runny nose, fever, cough, wheezing, and difficulty breathing.

  • The diagnosis is based on symptoms and a physical examination.

  • Most children do well at home and recover in a few days, but some need to be hospitalized.

  • Treatment is primarily supporting the child through the illness with fluids and occasionally with oxygen.

The Bronchi and Bronchioles

The airways resemble an upside-down tree. The trunk is the windpipe (trachea), which branches into large airways called bronchi. The bronchi themselves branch many times into smaller airways, ending in the smallest airways, which are called bronchioles. Bronchioles are as small as one half of a millimeter (or 2/100 of an inch) across. Their walls have a thin, circular layer of smooth muscle that can relax or contract, thus changing airway size.

Bronchiolitis typically affects children younger than 24 months of age and is most common among infants younger than 6 months of age. During the first year of life, bronchiolitis affects about 11 of every 100 children. However, during some epidemics, a much higher proportion of infants are affected. Most cases occur between November and April, with a peak incidence during January and February.

Causes of Bronchiolitis

Bronchiolitis is most often caused by infections with

Infection with any of these viruses can cause inflammation of the airways. The inflammation causes the airways to narrow, obstructing the flow of air into and out of the lungs. In severe cases, children have a low level of oxygen in their bloodstream.

The infection may be more common or more severe among infants whose mothers smoke cigarettes, particularly those who smoked during pregnancy. The infection seems to be less common among breastfed infants. Parents and older siblings can be infected with the same virus, but for them the virus usually causes only a mild cold.

Symptoms of Bronchiolitis

More severely affected infants breathe rapidly and shallowly, use a lot of their respiratory muscles to breathe, and have flaring of their nostrils. They seem fussy and anxious and can become dehydrated because of vomiting and difficulty with drinking. A fever usually is present but not always. Some children also develop an ear infection. Premature infants or infants younger than 2 months sometimes have episodes where they stop breathing briefly (apnea). In very severe and unusual cases, the child may become blue around the mouth because of a lack of oxygen.

Diagnosis of Bronchiolitis

  • A doctor's evaluation

  • Pulse oximetry

  • Sometimes mucus swab or chest x-ray

A doctor bases the diagnosis of bronchiolitis on the symptoms and the physical examination. Doctors measure oxygen levels in the blood by placing a sensor on a finger (pulse oximetry Pulse oximetry Both arterial blood gas testing and pulse oximetry measure the amount of oxygen in the blood, which helps determine how well the lungs are functioning. Arterial blood gas tests are invasive... read more ).

For severe cases, doctors sometimes swab mucus from deep inside the nose to try to identify the virus in the laboratory or do a chest x-ray Chest Imaging Chest imaging studies include X-rays Computed tomography (CT) CT angiography Magnetic resonance imaging (MRI) read more . Other laboratory tests may be done.

Prognosis for Bronchiolitis

Most children recover at home in 3 to 5 days. However, wheezing and coughing may continue for 2 to 4 weeks. With proper care, the chance of developing serious consequences due to bronchiolitis is low, even for children who need to be hospitalized.

Some children have repeated episodes of wheezing after having had bronchiolitis.

Treatment of Bronchiolitis

  • At home, fluids by mouth

  • In the hospital, oxygen therapy and fluids by vein

Home treatment

Most children can be treated at home with fluids and comfort measures.

During the illness, frequent small feedings of clear fluids may be given. Increasing difficulty in breathing, bluish skin discoloration, fatigue, and dehydration indicate that the child should be hospitalized. Children who have congenital heart disease or lung disease or an impaired immune system may be hospitalized sooner and are far more likely to become quite ill from bronchiolitis.

Hospital treatment

Fluids are given by vein if the child cannot drink adequately.

Inhaled drugs that open the airways (bronchodilators) may be tried. Although these drugs relieve wheezing and airway narrowing caused by asthma Asthma in Children Asthma is a recurring inflammatory lung disorder in which certain stimuli (triggers) inflame the airways and cause them to temporarily narrow, resulting in difficulty breathing. Asthma triggers... read more Asthma in Children , their effectiveness in treating bronchiolitis is questionable. Corticosteroids (to suppress inflammation) may be beneficial for some children.

Doctors no longer use the antiviral drug ribavirin (given by nebulizer) except for children whose immune system is extremely weak and whose infection is severe. Antibiotics are not helpful unless the child also has a bacterial infection.


Children who are at high risk of serious complications, such as those who have severe congenital heart disease or who were born very prematurely, may be given palivizumab to help prevent respiratory syncytial virus (RSV) infection Prevention Respiratory syncytial virus infection and human metapneumovirus infection cause upper and sometimes lower respiratory tract infections. Respiratory syncytial virus is a very common cause of... read more . Palivizumab is an antibody to RSV.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Copegus, Moderiba, Rebetol, RibaPak, Ribasphere, Ribasphere RibaPak, RibaTab, Virazole
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