Croup 

Croup is a common, primarily pediatric viral respiratory tract illness. As its alternative names, laryngotracheitis and laryngotracheobronchitis, indicate, croup generally affects the larynx and trachea, although it may also extend to the bronchi. It is the most common etiology for hoarseness, cough, and onset of acute stridor in febrile children. The vast majority of children with croup recover without consequences or sequelae; however, it may be life-threatening (see the image below). (See Etiology, Epidemiology, Prognosis, Clinical, and Treatment.)

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 (subglottic region), causing the hallmark marked inspiratory stridor. (See Prognosis, Clinical, and Workup.)

Stridor

Stridor^[1] is a common symptom in patients with croup. The acute onset of this abnormal sound alarms parents enough to prompt an urgent care or emergency department visit. Stridor is an audible harsh, high-pitched, musical sound produced by turbulent airflow through a partially obstructed upper airway. This partial airway obstruction can be present at the level of the supraglottis, glottis, subglottis, and/or trachea. 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.

Depending on timing within the respiratory cycle, stridor can be heard on inspiration, expiration, or in both (biphasic; inspiratory and expiratory). Inspiratory stridor suggests a laryngeal obstruction, whereas expiratory stridor suggests tracheobronchial obstruction. Biphasic stridor indicates either a subglottic or glottic anomaly. An acute onset of marked inspiratory stridor is one of the hallmarks of croup; however, there also may be less audible expiratory stridor. (See Clinical.)

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. (See Prognosis, Clinical, and Workup.)

Patient education

For patient education information, see the Lung Disease and Respiratory Health Center, as well as Croup.

Viruses causing acute infectious croup are spread through either direct inhalation from a cough and/or sneeze or by contamination of hands with then touching the mucosa of the eyes, nose, and/or mouth. The most common viral etiologies are parainfluenza viruses. The primary ports of entry are 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 seal-like 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. This type of croup always occurs at night and has the hallmark of reoccurring in children; hence it has also been called “frequently recurrent croup.” In spasmodic croup, subglottic edema occurs without the inflammation typical in viral disease. Although viral illnesses may trigger this variant, the reaction may be allergic rather than a direct result of infection.

Causes

The parainfluenza viruses (types 1, 2, 3) are responsible for as many as 80% of croup cases, with parainfluenza type 1 accounting for about 66% of cases, as well as for the majority of hospitalizations.

Other infectious causes of croup include:

• Adenovirus
• Respiratory syncytial virus (RSV)
• Enterovirus
• Coronavirus
• Rhinovirus
• Echovirus
• Reovirus
• Metapneumovirus^[2]
• Influenza A and B
• Rarer causes - Measles virus, herpes simplex virus, varicella

Influenza A is associated with severe disease; it has been implicated in children with severe respiratory compromise. Mycoplasma pneumoniae has been implicated in a few cases of croup.

Croup is the most common pediatric illness that causes acute stridor, accounting for approximately 15% of clinic and emergency department visits for pediatric respiratory tract infections. It is primarily a disease of infants and toddlers, with a peak incidence from age 6 months to 36 months (3 years). In North America, incidence peaks in the second year of life, at 5-6 cases per 100 children. Although the disease is rare after age 6 years, it may be seen as late as ages 12-15 years.

The male-to-female ratio for croup is approximately 1.4:1. The disease is most common in late fall and early winter but may be seen at any time of year. Approximately 5% of children experience more than 1 episode.

The prognosis for croup is excellent, and recovery is usually complete. The majority of patients are managed successfully as outpatients, without the need for inpatient hospital care. Hospitalization rates vary widely among communities, ranging from 1.5-30% and typically averaging 2-5%. Throughout the 1990s, US hospitalizations averaged approximately 41,000 per year but appear to have subsequently decreased. 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.^[3]

Some evidence suggests that hospitalization for croup may be associated with a future development of asthma. In at least 1 study, children hospitalized for croup later demonstrated higher levels of bronchial hyperresponsiveness and an allergic response to skin testing.

Complications

Complications in croup are rare. In most series, less than 5% of children who were diagnosed with croup required hospitalization and less than 2% of those who were hospitalized were intubated. Death occurred in approximately 0.5% of intubated patients.

Bacterial superinfection may result in pneumonia or bacterial tracheitis, a life-threatening infection that can arise after an acute viral respiratory infection.^{[4, 5, 6, 7]} The child usually has a mild to moderate illness for 2-7 days, but then develops severe symptoms. These patients usually have a toxic appearance and do not respond well to nebulized racemic epinephrine.

Treatment in these cases requires close observation, broad-spectrum antibiotics, and, occasionally, endotracheal intubation. The most frequent organism is Staphylococcus aureus, followed by group A streptococcus (Streptococcus pyogenes), Moraxella catarrhalis, Streptococcus pneumoniae, Haemophilus influenzae, and anaerobes.

Pulmonary edema, pneumothorax, lymphadenitis, and otitis media have also been reported in croup. An inability to maintain adequate oral intake and increased insensible fluid losses can lead to dehydration; as such, patients may require intravenous fluid hydration to stabilize their fluid volume.