Introduction
Background
Croup is a generic term that encompasses a heterogeneous group of relatively acute conditions (mostly infectious) that are characterized by a syndrome of distinctive brassy coughs. These conditions may be accompanied by inspiratory stridor, hoarseness, and signs of respiratory distress as a result of laryngeal obstruction. The word croup derives from an old Scottish term roup, which means "to cry out in a shrill voice."
The most common form of croup is acute laryngotracheobronchitis or viral croup, an infection of both the upper and lower respiratory tracts. A reactive inflammatory response causes subglottic edema. Narrowing of the airway can be life threatening in infants and young children because of their small airway.
Pathophysiology
The cells of the respiratory epithelium are infected following viral inhalation. The inflammation is diffuse in the involved airway, but airway narrowing is most marked in the lateral walls of the subglottic larynx because of the surrounding fixed cricoid cartilage. The subglottic larynx and tracheal lumens are normally quite narrow in infants and young children; thus, a small incremental decrease in lumen diameter in infants and small children can be critical, resulting in a large increase in both airway resistance and the work of breathing. Older children have milder symptoms because the airway lumen diameter is beyond the critical size; however, croup can occasionally create severe illness in older children who have congenital or acquired subglottic stenosis.
Of all cases of croup, 75% are caused by parainfluenza virus types 1 and 3.1 Influenza and respiratory syncytial viruses cause most of the remaining cases. Viral croup may be complicated by bacterial tracheitis that is caused by Staphylococcus aureus, Haemophilus influenzae, Streptococcus pneumoniae, or Moraxella catarrhalis. The supraglottic tissues in bacterial tracheitis are normal, but the subglottic mucosa may be ulcerated, may be partly necrotic, and is frequently covered by a thick, purulent exudate (see Image 5).
Frequency
United States
The frequency of croup varies with geography and season. Most cases of viral croup occur in the autumn because of the prevalence of the parainfluenza virus. In the winter, respiratory syncytial virus is more common, and in the spring, influenza virus type B is more common. Viral agents tend not to cause simultaneous epidemics in the same community. The incidence rate of croup varies with climate.
Two studies, summarized in the table below, demonstrate that the frequency of croup peaks during the first 2 years of life.1,2
Table. Frequency of Croup in Children in North Carolina and Seattle
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Table
| Age (y) | North Carolina (per 1000) 1 | Seattle (per 1000 2 |
| <0.5 | 24.3 | 5.2 |
| 0.5-1 | 39.7 | 11 |
| 1 | 47 | 14.9 |
| 2-3 | 31.2 | 7.5 |
| 4-5 | 14.5 | 3.1 |
| Age (y) | North Carolina (per 1000) 1 | Seattle (per 1000 2 |
| <0.5 | 24.3 | 5.2 |
| 0.5-1 | 39.7 | 11 |
| 1 | 47 | 14.9 |
| 2-3 | 31.2 | 7.5 |
| 4-5 | 14.5 | 3.1 |
Mortality/Morbidity
Morbidity: Croup is responsible for 250,000 emergency department visits per year, at an aggregate cost of $50 million. Parainfluenza virus types 1 and 3 are responsible for 70,000 hospitalizations per year, at a total cost of $140 million.
Mortality: The overall mortality rate for croup is unknown. A higher mortality rate is associated with males (male-to-female incidence rate is 1.7:1) and young age (86% in children aged <3 y). Superinfection can complicate croup; superinfection with bronchopneumonia and measles is associated with higher mortality rates in children who have croup.
Race
Croup is more common in white children than in black children, with a relative risk of 1.85 in white children.
Sex
In children younger than age 6 months, males are affected more commonly than females, with a male-to-female incidence rate of 1.43:1. In older children, boys and girls are equally affected.
Age
Viral croup is most common in patients aged 6 months to 5 years, with a peak incidence in the second year of life. Croup is rare in the first 6 months of life; stridor that presents in the first 6 months of life should instigate a search for other causes of the stridor. Congenital anomalies and subglottic hemangiomas should be considered in the differential diagnosis; these conditions narrow the airway and cause stridor in infants. The youngest reported child with croup was aged 3 months.
Croup is less common in school-aged children, but this condition is occasionally seen in older children who have preexisting subglottic stenosis.
Anatomy
Anatomic differences in the larynx of infants and young children render them more susceptible to respiratory compromise than adults. The larynx of a neonate is located higher in the neck than in older children, and the epiglottis is narrow, omega shaped, and vertically positioned. The submucosa in the subglottic area, the narrowest segment of the larynx, is nonfibrous, resulting in a looser attachment of the mucous membrane than occurs in adults, thus facilitating the accumulation of edema. Additionally, the cartilaginous support of the airways in infants is soft, easily allowing dynamic collapse of the airways during inspiration.
The airway of a neonate measures 5-6 mm in diameter at its narrowest point, which is at the cricoid ring. Therefore, infants are at higher risk for respiratory failure when any compromise to the patency of the airway occurs. In addition, infants are easily fatigued by the work of breathing that is necessary to generate the pressures needed to maintain airflow. The work of breathing increases exponentially with narrowing of the cross-sectional area. Obstruction of the glottis and the subglottic area results in airflow of increased turbulence and velocity. As high-velocity airflow passes across the vocal cords and aryepiglottic folds, these structures vibrate, resulting in stridor.
Presentation
Croup typically begins with symptoms of an upper respiratory infection, rhinorrhea, sore throat, and mild fever for several days. Later, the child develops a characteristic barking cough, hoarseness, and inspiratory stridor. Inspiratory stridor is often the symptom that causes parents to seek medical attention for their affected child.
Physical examination
On physical examination, signs of respiratory distress include increased respiratory and cardiac rates, nasal alar flaring, and retractions. The retractions can be suprasternal, intercostal, and sternal. Increasing chest-wall retraction occurs as the pleural pressure becomes increasingly negative and correlates with the severity of the upper airway obstruction. Rib cage and abdominal asynchrony occur as the respiratory compromise increases. If hypoxemia develops, the anxious or restless child may develop depressed consciousness or cyanosis. On auscultation, the breath sounds are normal, without added sounds except transmission of the stridor. Occasionally, wheezing may be heard, indicating severe airway narrowing, bronchitis, or possibly coexistent asthma.
Most children with croup have a mild illness and require no specific treatment. Symptoms may last 7-14 days. By the time medical attention is sought, the airway obstruction often does not progress but usually lasts for 4 more days; however, in a minority of children, the airway obstruction progresses to become severe.
Hospitalization
Among children who are hospitalized for viral croup, fewer than 1% require intubation. The duration of hospitalization is related inversely to the child's age. Children who are admitted with sternal and chest-wall retractions experience longer hospitalizations; frequently receive medical intervention, such as aqueous mist therapy or racemic epinephrine; and are at increased risk of requiring artificial airway support. Rarely, negative-pressure pulmonary edema occurs in cases of severe airway obstruction. Approximately 50% of patients with croup progress to recurrent croup. Asthma and atopy in children are associated with previous severe or recurrent episodes of croup.
Complications
Croup increases bronchial hyperreactivity and doubles the incidence of developing asthma. Viral croup may be complicated by bacterial tracheitis. The supraglottic tissues in bacterial tracheitis are normal, but the subglottic mucosa may become ulcerated, may become partly necrotic, and is frequently covered by a thick, purulent exudate. A child with bacterial tracheitis may present initially with signs and symptoms that are similar to those of viral croup, but he/she will progress to high fever, toxicity, and progressive respiratory distress.
Epiglottitis
In some children, the viral prodrome of croup is absent, and the clinical features may be confused with epiglottitis. In contrast to croup, epiglottitis is characterized by bacterial cellulitis, primarily of the supraglottic tissues.
Vaccination has reduced the number of cases of epiglottitis caused by H influenzae type b; streptococci are the most common cause of bacterial epiglottitis in the immunized population. Children with epiglottitis are usually aged 2-4 years and were previously healthy; the symptoms of sore throat with painful swallowing, fever, and toxicity develop within hours. Drooling is frequent, and the neck is hyperextended in an attempt to maintain airway patency. Airway edema can progress rapidly to occlusion. Signs and symptoms of epiglottitis usually provide clinical distinction from croup; however, the distinction is not always possible.
Note: Epiglottitis is a life-threatening medical emergency. In children who have symptoms that are suggestive of epiglottitis, direct visualization of the epiglottis must be performed in a controlled setting by a physician who is experienced in airway management.
Preferred Examination
- Most children with clinical croup require no testing beyond a thorough history and physical examination. Observation and frequent physical examination remain the best ways to monitor affected children. Pulse oximetry is useful if the patient also has bronchiolitis or pneumonia. The oral cavity and oropharynx are examined in the emergency department to exclude other causes of stridor or respiratory distress such as peritonsillar or retropharyngeal abscess or uvulitis.
- Laryngoscopy and airway support in a well-controlled environment is required if complete airway obstruction is imminent. Flexible nasopharyngoscopy can be used safely during the acute episode to evaluate the glottic and supraglottic areas. The subglottic area can frequently be visualized by looking through the vocal cords — take care not to pass the scope below the glottis.
- Endoscopy has a role in atypical, severe, or recurrent cases of laryngotracheobronchitis. In addition, endoscopy may be used to evaluate children in whom extubation has failed and in whom evidence is seen of severe subglottic trauma, in which case reintubation may not be advisable.
- Neck radiographs may be helpful to evaluate the various causes of stridor.
Limitations of Techniques
- Pulse oximetry: Most children with croup have normal pulse oximetry findings unless they have severe bronchospasm. Hypoxia that results in low oxygen saturation is detected in severe croup. Frequent decreases in oxygen saturation are caused by movement artifacts.
- Direct endoscopy: The indications for endoscopy in patients who have laryngotracheitis are not well defined. Instrumentation of the already edematous subglottic area may precipitate the need for intubation and should be reserved for children with historic evidence that suggests a diagnosis other than viral croup. Noisy breathing or an abnormal cry between episodes of croup, progressively more severe or frequent episodes of croup, intubation in the neonatal period, and choking or gagging before the onset of symptoms are possible indications for endoscopy.
- Radiographic studies: The diagnosis of croup is primarily clinical and requires no further testing. Anteroposterior (AP) and lateral soft-tissue technique radiographs of the neck can help the clinician to differentiate croup from other causes of stridor and respiratory distress, such as foreign body, epiglottitis, and retropharyngeal abscess. Lateral neck radiographs detect croup with up to 93% sensitivity and 92% specificity. The steeple sign on AP radiographs is not specific for croup and may be seen in some children with epiglottitis. The steeple sign can also be absent in some children with croup. A pseudo-steeple sign, which is a normal variant, may be seen at times during the respiratory cycle in some children without croup.
Patient Education:
For excellent patient education resources, visit eMedicine's Lung and Airway Center. Also, see eMedicine's patient education articles Croup and Coughs.
Differential Diagnoses
Epiglottitis, Acute
Subglottic Stenosis
Other Problems to Be Considered
Conditions that cause obstruction in the region of the larynx include the following:
Laryngeal foreign body aspiration
Acute angioedema (presents with other evidence of swelling of face and neck)
Retropharyngeal abscess
Parapharyngeal abscess
Bacterial tracheitis
Infectious mononucleosis
Laryngeal diphtheria
Paraquat poisoning
Burns or thermal injuries
Smoke inhalation
Neoplasm or hemangioma
Acute laryngeal fracture
Chiari I Malformation
Chiari II Malformation
Dandy-Walker Malformation
Laryngomalacia
Laryngeal papillomatosis
Extrinsic obstruction by a vascular ring
More on Croup |
 Overview: Croup |
| Imaging: Croup |
| Follow-up: Croup |
| Multimedia: Croup |
| References |
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Further Reading
Keywords
acute laryngotracheitis, acute laryngotracheobronchitis, inspiratory stridor
