When is recurrent croup not croup?

By Cecille G. Sulman, MD

Croup season is just around the corner. This illness affects approximately 3 percent of children per year, usually between the ages of 6 months and 3 years. The word croup derives from the Scottish term "roup," which means "to cry out in a shrill voice." The most common form of croup is acute laryngotracheobronchitis, and the frequency varies with geography and season. (See Table 1.)

Figure 1. Steeple sign associated with croup.

Although humidification and oral decongestants often are used in children with mild to moderate croup, there is no evidence to support their use in clinical practice. Antibiotics do not improve symptoms in croup of any severity, as croup is usually viral in origin. In moderate to severe croup, oral dexamethasone, nebulized adrenaline and nebulized bedesonide reduce symptoms. Supplemental oxygen is indicated with respiratory distress. A child who progresses with high fever, toxicity and progressive respiratory distress may have viral croup complicated by bacterial tracheitis, which may be confirmed by radiographs demonstrating marked irregularity and edema of the walls of the trachea. If severe obstruction is present, endoscopic removal of the obstructing membrane may improve the clinical condition of the patient.

When do you start considering diagnoses other than croup?

When the patient history is inconsistent with croup, other diagnoses for stridor should be considered. (See Table 2.) Red flags include stridor that occurs for short durations (hours rather than days), stridor that persists beyond the infectious period or stridor not associated with infection. Other concerning history includes episodes severe enough to require multiple hospitalizations or intubations, or a history of coughing or choking before the onset of stridor. Croup is also less common in school-age children but occasionally is seen in older children with preexisting subglottic stenosis. (See Table 3.)

Infants and young children are more susceptible to respiratory compromise than adults. The larynx is located higher in the neck, 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, facilitating the accumulation of edema. Additionally, the cartilaginous support of the airways in infants is soft, easily allowing dynamic collapse of the airway during inspiration. A mere 1 mm of edema reduces the cross-sectional area of the airway by 35 percent. The work of breathing increases exponentially with narrowing of the cross-sectional area resulting in increased turbulence and velocity of air flow. As high-velocity airflow passes across the vocal cords and aryepiglottic folds, these structures vibrate, resulting in stridor. 

On physical examination, signs of respiratory distress include increased respiratory and cardiac rates, nasal ala flaring and retractions. Retractions can be suprasternal, intercostal and subcostal. Increasing chest wall retraction occurs as pleural pressure becomes increasingly negative and correlates with the severity of the upper airway obstruction. Rib cage and abdominal asynchrony occur as respiratory compromise increases. If hypoxemia develops, the anxious or restless child may develop depressed consciousness or cyanosis. On auscultation, transmission of the stridor or wheezing may be heard. 

Flexible laryngoscopy allows visualization of the supraglottic structures and occasionally the subglottis. Care must be taken in deciding when to perform a laryngoscopy, since this procedure in a child with significant airway distress may worsen an already tenuous airway. High kilovolt anteroposterior and lateral neck radiographs can help differentiate croup from other causes of stridor and respiratory distress, such as a foreign body, epiglottitis, retropharyngeal abscess or subglottic hemangiomas. Airway fluoroscopy also may provide information on a dynamic process such as bronchomalacia or tracheomalacia. Virtual bronchoscopy is a new computed tomography technique that is useful in determining tracheobronchial anatomy beyond a region of small stenosis, length of a stenotic segment and greatest region of narrowing.

Figure 2. Left to right, normal larynx, normal subglottis, normal trachea/bronchus.

The most common etiology of stridor is laryngomalacia, which is readily diagnosed with flexible laryngoscopy in the office. Laryngomalacia commonly resolves by 12 to 14 months. Severe laryngomalacia associated with failure to thrive or respiratory distress should be evaluated further with endoscopy to rule out the presence of other disorders, such as subglottic narrowing or vascular compression. Severe laryngomalacia may be surgically managed with a supraglottoplasty. 

Many patients are diagnosed with recurrent croup prior to a final diagnosis of subglottic stenosis. Acquired subglottic stenosis is the most common acquired anomaly of the larynx in children and the most common abnormality requiring tracheotomy in children younger than 1 year. (See Figure 3.) Aquired subglottic stenosis usually is related to prolonged intubation, however, the frequency has decreased due to improved techniques in managing prolonged ventilation. Congenital subglottic stenosis is the third most common congenital anomaly of the larynx and usually is associated with a small or malformed cricoid cartilage with or without thickening of the underlying submucous layer. Diagnosis is made by bronchoscopy and using the Myer-Cotton Classification based on the child's age and the size of the endotracheal tube that can be placed in the airway with an air leak pressure of less than 20 cm of water. (See Table 4.) Mild subglottic stenosis (Grades I and II) typically improves without intervention. Moderate to severe subglottic stenosis (Grades III and IV) typically requires surgical intervention by bypassing the stenosis with a tracheotomy or by expanding the lumen of the cricoid. Expansion may take place endoluminally with dilation or laser techniques, or with open techniques that may involve cartilage grafting. 

Figure 3. Subglottic narrowing as seen through the true vocal folds (left). Narrowing of the subglottis (right).

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