Croup

Imagine this scenario. Your two-year-old son has had a runny nose and an occasional cough for a day or two, but seems no worse to you than everyone else in his preschool class. Two hours after you put him to bed you hear him coughing, only this cough is like none you have ever heard from him before. It sounds like a barking seal at the circus--a brassy, honking noise. In between coughs he is making a strange crowing-like noise. When you snap on the light you see him sitting up in his crib, leaning forward, and coughing that strange cough. You also notice that as he breathes in, the part of his chest below his ribcage is sinking inward with each breath, opposite from the direction that it should ordinarily go. Your little boy has a scared look in his eyes, and you are more than a little scared yourself. He has croup. 

What is croup? 

Croup is a disorder caused by inflammation of the trachea, the main breathing tube in the neck, just below the vocal cords, in an area called the subglottic region. Some say it gets its name from the old Anglo-Saxon word kropan, which means to croak or cry out. If true, such venerable terminology tells us this common childhood ailment has been recognized as a distinct entity by parents for a very long time. Physicians sometimes give it a much fancier name, laryngotracheobronchitis. This learned construction merely describes what croup is: inflammation (hence the "itis") of the breathing tubes extending from the vocal cords (the larynx), through the trachea, and often down to the lower breathing tubes (the bronchi). Even though the inflammation can stretch up and down the airway, it is in the subglottic region that the symptoms happen. The reason for this is a simple law of physics--that is where the airway of a toddler is at its narrowest. The symptoms of croup come from blockage of airflow.

Croup is an extremely common childhood illness. As many as fifteen percent of all children have croup at least once, and five percent have it more than once. The peak time for croup is fall and early winter, but it can occur at any time of year, even summer. The peak risk age for children to get croup is eighteen months, and boys are one-and-one-half times more likely to get it than are girls.

Croup is caused by infection with a respiratory virus. Although there are a few ailments that resemble croup and are caused by something else (more on these below), standard croup symptoms are brought on by viral infection. There are multiple viruses that can do it, but nearly three-quarters of all cases stem from infection from a single family of three, closely-related viruses--the parainfluenza viruses, cousins of true influenza. Less commonly, croup is caused by the true influenza virus, respiratory syncytial virus (RSV), or a few others. All these viruses are spread from child to child in the manner of most respiratory viruses--tiny droplets of infected mucous or saliva.

          Viral inflammation of the subglottic region makes the lining of the trachea swell. Since the trachea is more or less round, this swelling makes the diameter of the airway smaller. Sometimes the swelling of the tissues is so extreme, the size of the child's airway is narrowed to that of a small straw. What happens next is analogous to what happens to cold water pipes if their diameter is narrowed by mineral deposits: loss of half the space inside the pipe from mineral deposits causes only slight reduction in water flow when one turns on the tap, but just a little more blockage severely cuts down flow. This is because flow through a tube is proportional to the fourth power of the radius of the tube. This may sound esoteric, but the principle has important practical implications for small children with croup.

          Imagine an adult whose airway has a diameter of twelve millimeters. Then imagine the lining of this tube develops one millimeter of swelling all around its lining, thereby reducing its diameter to ten millimeters. If one does the calculations, this slight reduction in size reduces airflow by about half, an obstruction easily compensated for by just breathing a little harder. Now consider a toddler with a five-millimeter airway, who develops the same one millimeter of swelling all the way around, reducing it to three millimeters in diameter. The toddler’s airflow is reduced to only thirteen percent of what it was. This reduction is too much to compensate for, although the child tries very hard. This persistent effort causes the symptoms of croup.

The air rushing turbulently through the tiny airway causes the crowing sound characteristic of the breathing of a child with croup. It is called stridor, and an experienced person can often make the diagnosis of croup based upon that sound alone, even over the telephone. Additionally, the front portion of a toddler's ribcage is not yet solid bone--it is still partly cartilage. This means that, since a child's chest is not yet firm in the scaffolding of the ribs, the increased effort of breathing makes the chest cave in with each breath. These are called retractions. They are not specific to croup, but happen in a child with respiratory distress from a variety of causes. The final characteristic of croup, the seal-like barking cough, is from irritation of the vocal cords.

One of the characteristic attributes of croup is the often very sudden onset of the stridor. For some reason croup tends to be worse at night; most visits to emergency departments for croup occur between ten in the evening and four in the morning. A typical story is that parents put their child to bed with just a mild cough only to awaken in the middle of the night to the sound of severe stridor. This is a predictable result of the place where the inflammation is happening. Since airflow is dependent upon the fourth power of the radius of the child's trachea, he may not have much distress during the early stages of the illness. But as the airway gets smaller, subsequent reduction in size becomes critical.

How is croup diagnosed?

         Croup is entirely a clinical diagnosis; there is no specific test for it. This means the doctor decides it is croup based upon a typical story (cough, congestion, stridor, and mild fever). Sometimes, though, a doctor will get an x-ray of the child's neck, which often shows some narrowing of the airway. The figure below shows such a case. Air on an x-ray appears black, bones are white, and tissue is grey. The central black column of this child's trachea is narrowed abnormally at the point of the arrowhead in something called the "steeple sign," since it resembles a steeple. (The bones stacked like coins in the neck are part of the spinal column.) Doctors do not always get such an x-ray, especially if everything points to croup. If the story is atypical, a common reason for getting the x-ray is to make sure the child's symptoms are not from something else. These other possibilities are divided into those that are infectious and those that are non-infectious.

Serious bacterial infection can block a child’s airway. The principal one of these is epiglottitis, a severe and rapid swelling of the epiglottis, a structure that sits just above the opening of the trachea at the back of the throat. The epiglottis is what keeps food from going into the trachea during swallowing. When it becomes severely swollen, it can completely block the airway and cause a life-threatening emergency. Another infection that can mimic croup is bacterial tracheitis, a severe infection of the entire trachea that causes so much infected pus that a child's airway can become obstructed. It, too, can be life threatening.

Fortunately, both epiglottitis and bacterial tracheitis are extremely rare. Epiglottitis was once not uncommon, but near universal vaccination of children against the bacterium Hemophilus influenzae, the main causative organism, has dramatically reduced the incidence of the disorder. Both these serious conditions usually behave quite differently from croup. The main difference is that both cause high fever (croup's fever is nearly always low-grade) and the children appear quite ill. The key distinction between croup and epiglottitis is that the latter not only makes breathing difficult but also makes swallowing painful or even impossible for the child. Thus a child with epiglottitis will not only have stridor, but will sit hunched forward and drool, unable to swallow.

An x-ray of the neck can help distinguish croup from these more serious infections. However, if the doctor thinks epiglottitis is possible, the standard way to proceed is for the child to be given a sedative and to have his airway directly inspected using a procedure called laryngoscopy. If this is necessary, it is usually done by an airway specialist, such as an otolaryngologist, commonly called an ENT doctor.

There also are non-infectious processes that can cause upper airway obstruction and stridor, since anything blocking the airway may produce the same symptoms. Overall, what distinguishes these non-infectious causes of upper airway obstruction from the infectious ones is the lack of any other evidence of infection, such as nasal congestion, fever, or malaise.

If the onset of a child's breathing problems is quite sudden, the doctor might consider the possibility of a foreign body stuck in the airway. Toddlers put anything into their mouths--toys and bits of food are frequent offenders. On the other hand, if the progression of a child's symptoms occurs over days or weeks, the doctor might think about several kinds of tissue growths that can occur within the airway. If either of these possibilities is likely, the child usually needs laryngoscopy or bronchoscopy, inspection of the trachea and lower airway, for diagnosis.

A few children have recurrent, sudden episodes of croup symptoms without any other evidence of viral infection. These attacks from what is called spasmodic croup also generally happen at night. The cause is unknown, but it may be related to allergies. It is generally treated the same way as viral croup (see below).

The walls of the trachea are stiffened with bands of cartilage; this is what holds them open and keeps them that way. Some children have an airway that is intrinsically less stiffened with cartilage than most, causing it to collapse a bit when the child breathes, resulting in stridor that can sound like croup. In this condition, called tracheomalacia, the symptoms are chronic, and are often worse when the child is lying on his back because the weight of the tissue in the neck compresses the airway more. It requires bronchoscopy to diagnose definitively.

Croup ranges in severity from quite mild to the rare case of near-total obstruction of the airway. To help categorize this severity doctors have devised various scoring systems to rate the child's symptoms. One commonly used "croup score" is the Westley scale. The scale assigns points for various symptoms and groups children into "mild," (less than three points), "moderate," (three to six points), and "severe" (more than six points). It uses five criteria to do this: severity of retractions, degree of stridor, how well the air is getting into the child's lungs as assessed with the examiner's stethoscope, if the child is dusky-colored (slightly gray) from insufficient air, and if the child is becoming poorly responsive from lack of oxygen. Generally mild croup can be treated at home; moderate and severe croup require medical attention, and usually the more ill children will be admitted to the hospital.

How is croup treated?

         Once a doctor decides a child has croup, treatment is fairly well-accepted. Therapy is directed at two things: making the child feel better and reducing the airway inflammation to improve airflow. Mist has been a mainstay of treatment for mild croup for many years. Most physicians believe it often gives a child significant relief from the pain and raspy, dry feeling in the throat, although whether it actually helps reduce the inflammation of the airway itself and improves air flow is doubtful. Mist may also help loosen airway mucous and allow the child to cough it up easier. Throat pain and fever are helped by treatment with acetaminophen or ibuprofen.

 

          The traditional home remedy for mild croup is to close the bathroom door and run a tap until the room is completely steamy, then turn it off and sit with the child in the mist. A parent needs to be careful with this, of course; children have been burned from scalding water. Exposure to cool night air (since croup happens mostly at night) is also a traditional remedy. Although widely practiced and certainly benign, it, too, has never been validated.

 

          Doctors typically use one or both of two ways to reduce the inflammation and swelling in the child's airway. Direct application of the drug epinephrine (adrenaline) to the swollen tissues shrinks them by constricting the tiny blood vessels under their surface; it is the virus-induced engorgement of these vessels and leakage of fluid out of them that causes the swelling in the first place. The drug is given by nebulization, blowing high-flow air or a mixture of air and oxygen through the liquid epinephrine and thereby dispersing it into a fine mist, which the child then breathes through a mask to carry the drug to the subglottic area. Epinephrine works within minutes and usually gives a child prompt relief from the stridor and retractions. Unfortunately, the effects of epinephrine only last a few hours at most. It can then be repeated, although dose after dose of epinephrine can rarely lead to worse swelling when the drug wears off.

 

         The subglottic swelling of croup is from inflammation in the area, so standard treatment of moderate or severe croup also consists of using a drug to reduce the inflammation--a steroid. Steroids are also being used increasingly for mild croup, both to make the child feel better and to interrupt its early stages and arrest progression of the swelling. Steroids can be given orally, by intramuscular injection, or even by nebulization, like the epinephrine. A commonly used steroid for croup is dexamethasone (Decadron), a single dose of which is usually sufficient to reduce the inflammation. Unfortunately, steroids do not act immediately, as does inhaled epinephrine--they take at least four to six hours to work. 

 

          A typical treatment scenario for a child coming to the emergency department with croup would be to have him breathe some cool mist, followed by a nebulized epinephrine treatment. Usually the best way to do this is to have the child sit in a parent's lap, since he is most comfortable there and agitation makes the stridor and retractions worse. Then, the child receives a dose of steroids. Often by then the child's symptoms are much better, but it is important to keep the child in the emergency department for at least an hour or two more to make sure the symptoms do not recur after the epinephrine wears off. A child who continues to have symptoms after epinephrine or who needs repeated doses of epinephrine should be admitted to the hospital. What doctors particularly look for is continued stridor when the child is completely calm; called "stridor at rest," it is a standard indication for hospital admission.

          A child with severe croup needs more complicated management, although this is very uncommon. If the child is clearly not getting enough air to stay alert and keep his blood oxygen levels up he needs immediate placement of a breathing tube, called an endotracheal tube. It is placed by a procedure known as intubation. A child with less severe croup, but who remains in significant distress and begins to tire from the effort of breathing also may need intubation.

What is the typical course of a child with croup?

          Croup usually runs its course in five to seven days, typically with a day of worse symptoms and several more of cough and hoarseness. Since the symptoms characteristically get better during the day, it is common during the middle of the illness for a child to have minimal symptoms during the day but several nights of worse cough. 

What is the risk of a child getting croup again and are there any long-lasting effects? 

          There is no clear-cut evidence that children who have one episode of croup are more likely to get it again. There is some evidence that children who have croup are more at risk to later develop reactive airways disease--asthma--than children who have never had croup. However, if true, this may not be a cause-and-effect association; the propensity for a child to get croup when infected by a respiratory virus may reflect the same innate tendency to develop asthma. They may be different manifestations of the same thing. There are no long-term after-effects of typical viral croup.

Useful references and links 

Malhotra A, Krilov, LR. Viral croup. Pediatrics in Review 2001. 22:5-12.

Scolnik D, Coates AL, Stephens D, et al. Controlled delivery of high vs low humidity vs mist therapy for croup in emergency departments. Journal of the American Medical Association 2006. 295:1274-80.

Bjornson CL, Klassen TP, Williamson J, et al. New England Journal of Medicine 2004. A randomized trial of a single dose of oral dexamethasone for mild croup. 351:1306-13.

Marchesault V. Historical review of croup. Canadian Journal of Infectious Diseases 2001. 12:337-9. 

Croup (Kids Health) 

Croup (eMedicine) 

Croup (Informed Parent)