Introduction
Background
Croup, also termed laryngotracheitis or laryngotracheobronchitis, is a viral respiratory tract infection. It is primarily a pediatric illness and, as its alternative names indicate, generally affects the larynx and trachea but may also extend to the bronchi. It is the most common etiology for stridor in febrile children. It is a common pediatric illness, with the vast majority of children recovering with no consequences; however, it may be life-threatening. Croup manifests as hoarseness, a seal-like barking cough and a variable degree of respiratory distress. However, morbidity is secondary to narrowing of the larynx and trachea below the level of the glottis, causing the hallmark inspiratory stridor.
Stridor is a relatively common complaint; however, it can alarm parents enough to prompt an emergency department (ED) visit. Stridor is an audible harsh, high-pitched musical sound produced by turbulent airflow through a partially obstructed upper airway. During inspiration, areas of the airway that are easily collapsible (eg, supraglottic region) are suctioned closed because of negative intraluminal pressure generated during inspiration. These same areas are forced open during expiration. For this reason, disorders that cause supraglottic obstruction cause stridor that is mostly heard on inspiration. Stridor that occurs due to glottic disorders (eg, croup) and trachea disorders (eg, tracheitis) is often biphasic, or heard during both inspiration and expiration.
Young children who present with stridor require a meticulous evaluation to determine the etiology and, most importantly, to exclude rare life-threatening causes. Although croup is usually a mild, self-limited disease, upper airway obstruction may result in respiratory distress and even death.
Pathophysiology
Acute infectious croup is spread through inhalation of the responsible virus. The initial port of entry is the nose and nasopharynx. The infection spreads and eventually involves the larynx and trachea. Although the lower respiratory tract may also be affected, some authors consider laryngotracheobronchitis a separate entity, with bacterial superinfection as the potential cause.
Inflammation and edema of the subglottic larynx and trachea, especially near the cricoid cartilage, are most clinically significant. Histologically, the involved area is edematous, with cellular infiltration located in the lamina propria, submucosa, and adventitia. The infiltrate contains lymphocytes, histiocytes, plasma cells, and neutrophils. Parainfluenzae virus activates chloride secretion and inhibits sodium absorption across the tracheal epithelium, contributing to airway edema. This is the narrowest part of the pediatric airway. Accordingly, swelling can significantly reduce the diameter, limiting airflow. This narrowing results in the barky cough, turbulent airflow and stridor, and chest retractions. Endothelial damage and loss of ciliary function occurs. A fibrinous exudate partially occludes the lumen of the trachea. Decreased mobility of the vocal cords due to edema leads to the associated hoarseness.
In severe disease, fibrinous exudates and pseudomembranes may develop, causing even greater airway obstruction. Hypoxemia may occur from progressive luminal narrowing and impaired alveolar ventilation and ventilation-perfusion mismatch.
Spasmodic croup (laryngismus stridulus) may be a noninfectious variant of the disorder, with a clinical presentation similar to that of the acute disease but with less coryza. In such cases, subglottic edema occurs without the inflammation typical in viral disease. Although viral illnesses may trigger spasmodic croup, the reaction may be allergic rather than a direct result of infection.
Frequency
United States
Croup is the most common pediatric infection that causes stridor, accounting for approximately 15% of clinic and ED visits for pediatric respiratory infections. In North America, incidence peaks in the second year of life at 5-6 cases per 100 children. Approximately 5% of children experience more than one episode. Croup is most common in late fall and early winter but may be seen at any time of year.
Mortality/Morbidity
Hospitalization rates widely vary among communities, with rates from 1.5-30% and typically averaging 2-5%. Throughout the 1990s, US hospitalizations averaged approximately 41,000 per year but appear to have decreased over the last decade. Fewer than 2% of hospitalized children require intubation. Although exact mortality is unknown, one 10-year study found a mortality rate of less than 0.5% in intubated patients.
Sex
Male-to-female ratio is approximately 1.4:1.
Age
Croup is primarily a disease of infants and toddlers usually younger than 6 years of age, with a peak incidence of 7 and 36 month of age. Although the disease is rare after age 6 years, it may be seen as late as age 12-15 years.
Clinical
History
Croup usually begins with nonspecific respiratory symptoms, including rhinorrhea, sore throat, and cough. Fever is generally low grade (38-39°C) but can exceed 40°C. Within 1-2 days, the characteristic signs of hoarseness, barking cough, and inspiratory stridor develop, often suddenly, along with a variable degree of respiratory distress. Most ED visits occur at night from 10 pm to 4 am, and symptoms are perceived as worsening at night. Symptoms typically resolve within 3-7 days but can last as long as 2 weeks.
Spasmodic croup typically presents at night with the sudden onset of "croupy" cough and stridor. The child may have mild upper respiratory complaints but more often appears completely well prior to the onset of symptoms.
Physical
The physical presentation of croup widely varies. Most children have no more than a croupy cough and hoarse cry. Some may have stridor only upon activity or agitation, whereas others have audible stridor at rest and evidence of respiratory distress. Paradoxically, the severely affected child may have "quiet" stridor secondary to the degree of airway obstruction. The child with croup does not appear toxic.
The child's symptoms range from minimal inspiratory stridor to severe respiratory failure secondary to airway obstruction.1 In mild cases, respiratory sounds at rest are normal; however, mild expiratory wheezing may be heard. Children with more severe cases have inspiratory and expiratory stridor at rest with suprasternal, intercostal, and subcostal retractions. Air entry may be poor. Lethargy and agitation may be a result of hypoxemia. Other warning signs of severe respiratory disease include tachypnea, tachycardia out of proportion to fever, and hypotonia. Children may be unable to maintain adequate oral intake, which results in dehydration. Cyanosis is often a late ominous sign.
Many authors have attempted to devise croup scores to assist the examiner in assessing the degree of respiratory compromise. One of the most commonly cited is the Westley score. Although widely used to evaluate treatment protocols, its clinical efficacy has not been extensively studied. According to the Westley scale, a score of less than 3 represents mild disease; a score of 3-6 represents moderate disease; and a score greater than 6 represents severe disease. The score evaluates the severity of croup by assessing the following 5 factors:
- Inspiratory stridor
- None - 0 points
- Upon agitation - 1 point
- At rest - 2 points
- Retractions
- Mild - 1 point
- Moderate - 2 points
- Severe - 3 points
- Air entry
- Normal - 0 points
- Mild decrease - 1 point
- Marked decrease - 2 points
- Cyanosis
- None - 0 points
- Upon agitation - 4 points
- At rest - 5 points
- Level of consciousness
- Normal - 0 points
- Depressed - 5 points
Mild disease consists of occasional barking cough, no stridor at rest, and mild or nonexistent suprasternal or subcostal retractions. Moderate disease includes frequent cough, audible stridor at rest, and visible retractions, but little distress or agitation. Severe disease consists of frequent cough, prominent inspiratory (and, occasionally, expiratory) stridor, conspicuous retractions, decreased air entry on auscultation, and significant distress and agitation. Lethargy, cyanosis, and decreasing retractions are harbingers of impending respiratory failure.
Another clinically useful severity assessment table has been developed by the Alberta Clinical Practice Guideline Working Group.2 Using this classification scheme, 85% of children in 21 general emergency departments in Alberta, Canada, were determined to have mild croup, and less than 1% had severe croup. The assessment is as follows:
- Mild severity - Occasional barking cough, no audible rest stridor, and either mild or no suprasternal or intercostal retractions
- Moderate severity - Frequent barking cough, easily audible rest stridor, and suprasternal and sternal retractions at rest, with little or no agitation
- Severe severity - Frequent barking cough; prominent inspiratory and, occasionally, expiratory stridor; marked sternal retractions; and agitation and distress.
- Impending respiratory failure - Barking cough (often not prominent), audible rest stridor, sternal retractions may not be marked, lethargy or decreased mentation, and often dusky appearance with no supplemental oxygen
Causes
The parainfluenza viruses (I, II, III) are responsible for as many as 80% of croup cases, with parainfluenza I accounting for most episodes and for 50-70% of hospitalizations.
Other infectious causes of croup include adenovirus, respiratory syncytial virus (RSV), measles, coxsackievirus, rhinovirus, echovirus, reovirus, metapneumovirus, varicella, herpes simplex virus, human bocavirus, coronavirus, and influenza A and B. Influenza A is associated with severe disease. Influenza A has been implicated in children with severe respiratory compromise.
Mycoplasma pneumoniae has been implicated in a few cases.
More on Croup |
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Further Reading
Keywords
croup, barking cough, laryngotracheitis, stridor, laryngotracheobronchitis, spasmodic croup, influenza A, inspiratory stridor, parainfluenza virus 1, parainfluenza virus II, parainfluenza virus III, steeple sign, upper respiratory infection
