Almost 50 percent of infants will have an episode of wheezing in the first year of life. A wheezing baby can lead to many questions and concerns for parents and for pediatricians.

Katharine Kevill, MD, who specializes in pediatric pulmonology, explains the causes, symptoms, and risk factors for infant wheezing and educates parents about when to take their wheezing child to the pediatrician.

-- Dennis Clements MD, PhD, MPH

What is a wheeze?

Katharine Kevill, MDThe term wheezing may mean different things to different people.

The Merriam-Webster Dictionary defines the verb wheeze as "to breathe with difficulty usually with a whistling sound." Physicians often have a more specific definition such as "a musical and continuous sound that originates from oscillations in narrowed airways." 

Air moving through a narrowed airway can make a whistling sound, in the same way that the wind whistles as it moves through a tunnel.

Sometimes, wheezes are only heard with the stethoscope, but other times they are heard with the naked ear.

The wheezing sound can originate at any points from the upper to the lower airways. The upper airways start at the pharynx and extend down to the level of the larynx (sometimes called the voicebox).

The lower airways start with the largest airway, the trachea. The airways become smaller as they branch out into bronchi and then bronchioles. Finally, air reaches the alveoli, or air sacs.

The tone of the wheeze depends upon where the airway is narrowed. Babies with a lot of upper airway congestion may have coarse, noisy breathing.

Wheezes that come from multiple places in the lower airway may have a musical tone, with several different pitches (polyphonic).

Usually wheezing occurs when the baby exhales, but it can also occur when he or she inhales (inspiration). The noise made due to airway obstruction during inspiration is called stridor.

Why are infants more susceptible to wheezing than older children?

Obstruction to airflow depends upon the width of the airway. Babies have very narrow airways, so even a small decrease in the width can cause a large obstruction to airflow.  

Furthermore, babies have very compliant chest walls relative to older children. When babies breathe out forcefully, this can cause the chest wall to move inward and place pressure on the airways, which can cause the airways to narrow.

What are the causes for wheezing in infants?

Many wheezing episodes in babies are caused by bronchiolitis, or inflammation of the small airways, called bronchioles. Usually, bronchiolitis is caused by viral respiratory infections.

Other problems may cause babies to have noisy breathing, even when they don’t have a cold. Gastroesophageal reflux (food going from the stomach back up the esophagus) or swallowing problems can cause babies to have a lot of nasal congestion and noisy breathing. 

Babies may also be born with variations in the structure (or anatomy) of their respiratory tract that can cause wheezing. One common anatomic variation is tracheomalacia, where the cartilage that supports the trachea is not yet firm enough. In this case, the trachea narrows as the baby breathes out, causing an airflow obstruction. 

Accidental aspiration of an object into the airway can also lead to airway obstruction and wheezing. Other possible causes for wheezing in infancy include diseases such as cystic fibrosis.

If a baby wheezes, will he or she develop asthma?

Although almost 50 percent of infants have an episode of wheezing in the first year of life, most of them do not go on to develop asthma.   

For children who have had at least three episodes of wheezing within 12 months, some risk factors have been associated with an increased likelihood of developing asthma. Three major risk factors for developing asthma include:

• A parent with asthma
• Eczema in the child
• Allergy in the child to any inhaled allergen (such as dust, mold, etc.)

When should a parent take a wheezing baby to the pediatrician?

Certainly any concerns that parents have about their infant’s breathing should be discussed with the pediatrician.

If the infant is having a lot of trouble breathing, then it may be necessary to bring the baby to the pediatrician or the emergency room right away. 

Signs of respiratory distress or trouble breathing may include:

• Severe coughing
• Blue color of the face or lips
• Crying but not making much noise
• Flaring out at the nostrils
• Sucking in at the base or the top or the ribs
• Breathing really fast

In most cases, infants who wheeze do very well. Usually infants with bronchiolitis stop wheezing after the viral infection runs its course. Other problems such as reflux or swallowing problems typically improve with age. 

Most infants with wheezing are treated by their general pediatricians and require few, if any, studies. In some cases, pediatricians may refer wheezing infants to a physician that specializes in respiratory issues such as a pediatric pulmonologist or an otolaryngologist for further evaluation. 

-- Katharine Kevill, MD, is a pediatric pulmonologist with Duke Department of Pediatric's Division of Pulmonary and Sleep Medicine.

-- Dennis Clements, MD, PhD, is the chief of primary care pediatrics at Duke Children's Hospital.