Croup

Croup, also known as laryngotracheobronchitis, is a disease most commonly caused by a viral infection that leads to severe inflammation of the upper airway. It usually presents in children < 5 years of age. Patients develop a hoarse, “seal-like” barking cough and inspiratory stridor. Human parainfluenza viruses account for the majority of cases, followed by respiratory syncytial virus, adenovirus, rhinovirus, and enteroviruses. Croup is usually diagnosed clinically or with the aid of X-ray imaging (steeple sign). Treatment consists of steroids and epinephrine.

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Epidemiology

  • Primarily affects children aged 6–36 months 
    • Potential affected age range: 6 months to 15 years
  • 15% of pediatric emergency hospitalizations are caused by croup
  • More prevalent in the fall and early winter
  • More common in boys, with a male:female ratio of 1.4:1
  • 5–6 cases per 100 toddlers in the second year of life in the United States

Transmission

  • Contagious within the first few days, particularly the first or second day 
  • Transmitted in the form of aerosol droplets released by sneezing and coughing

Risk factors

  • Change of season (fall → winter, winter → spring)
  • Prematurity 
  • Ages 6 months to 6 years
  • Asthma

Etiology and Pathophysiology

Croup is commonly caused by a virus (75% of cases) and less commonly by bacteria.

  • Viral:
    • Most common: parainfluenza virus types 1 and 2
      • 3 virulence proteins: fusion protein, hemagglutinin, neuraminidase
    • Others: respiratory syncytial virus (second most common cause), adenovirus, coronavirus, measles, influenza virus, rhinovirus, enterovirus, herpes simplex virus, metapneumovirus 
  • Bacterial:
    • Usually divided into:
      • Bacterial tracheitis
      • Laryngotracheobronchitis
      • Laryngeal diphtheria caused by Corynebacterium diphtheriae
      • Laryngotracheal bronchopneumonitis
    • Other bacterial agents: 
      • Mycoplasma pneumoniae
      • Staphylococcus aureus
      • Haemophilus influenzae
      • Streptococcus pneumonia
      • Moraxella catarrhalis
  • Pathophysiology:
    • Viruses that cause croup infect the nasal and pharyngeal mucosal epithelia through aerosol droplets.
    • Infection spreads to the larynx and trachea via respiratory epithelium.
    • Infection triggers the infiltration of white blood cells, especially histiocytes, neutrophils, plasma cells, and lymphocytes.
    • Leads to swelling and edema inside the trachea, larynx, and large bronchi
    • Swelling partially obstructs/narrows the airway.
    • Heavy breathing during exercise and other vigorous physical activities results in stridor.
Transmission electron micrograph of a parainfluenza virus

Transmission electron micrograph of parainfluenza virus

Image: “Transmission electron micrograph” by Public Health Image Library. License: CDC/Public Domain

Signs and Symptoms

  • Nasal discharge
  • Congestion
  • Coryza
  • Characteristic barking cough
  • Inspiratory stridor
  • Low-grade fever
  • Hoarseness
  • Acute enlargement of the tonsils
  • Difficulty swallowing and eating (dysphagia)
  • Hoover’s sign (inward movement of the lower rib cage during inspiration)
  • Agitation
  • Pulsus paradoxus secondary to upper airway obstruction
  • 2 clinical phases:
    • Prodromal phase
      • Nasal discharge, congestion, and coryza
    • Inflammatory phase
      • Mild: barking cough, hoarseness, and mild inspiratory stridor
      • Moderate: dyspnea at rest, pronounced thoracic retractions, pallor, tachycardia
      • Severe: severe tachydyspnea at rest with increasing respiratory failure, cyanosis, hypoxemia, bradycardia, and altered mental status

Diagnosis

  • Diagnosis is based on the presence of a “seal-like” barking cough and inspiratory stridor
  • Neither radiographs nor laboratory tests are necessary to make the diagnosis.
  • The severity of croup can be detected by the following laboratory tests:
    • Lateral neck X-ray to rule out any other cause, such as a peritonsillar abscess
    • Anteroposterior X-ray usually shows a “steeple sign,” which represents subglottic narrowing
  • Bronchoscopy may be indicated in patients with recurrent episodes of croup.
  • Severity of the disease can be assessed by Westley score:
FeatureNumber of points assigned for this feature
012345
Chest wall retractionNoneMildModerateSevere
StridorNoneWith agitationAt rest
CyanosisNoneWith agitationAt rest
Level of consciousnessNormalDisoriented
Air entryNormalDecreasedMarkedly decreased
  • Mild: total score ≤ 2 = barking cough and hoarseness but no stridor at rest
  • Moderate: total score of 3–7  
  • Severe: total score of ≥ 8 = obvious stridor and Hoover’s sign
  • Impending respiratory failure: total score ≥ 12

Tip: While a chest X-ray will show a narrowing of the air column in the trachea (steeple sign), a chest X-ray is rarely done in practice and is always the wrong answer if a question stem asks what the most appropriate next step in diagnosis is.

Steeple Sign

Chest X-ray showing subglottic stenosis, known as steeple sign, commonly seen on anteroposterior X-ray in patients with croup.

Image: “Steeple sign” by Jayshil J. Patel, Emily Kitchin, and Kurt Pfeifer. License: CC BY 4.0

Treatment

  • Supportive care: supplemental oxygen, antipyretics, fluid intake
  • The patient should be made to inhale oxygen via the “blow-by” method (holding an oxygen source near the child’s face).
    • To avoid distress in children, a mask or nasal cannula should not be used.
    • Hot steam or humidified air use does not provide effective relief.
  • Endotracheal intubation is required in < 3% of cases.
  • Corticosteroids (e.g., dexamethasone, budesonide) are given orally or by injection; improvement in airway inflammation/symptoms is usually seen in 6–8 hours.
  • Racemic epinephrine can be given by nebulization to control the severity of the disease.
  • Observe 3–4 hours after initial treatment and pharmacologic intervention
  • Cough medicines with dextromethorphan or guaifenesin should be avoided.
  • Antiviral neuraminidase inhibitors should be given to patients with severe croup.
  • Antibiotics are only prescribed in cases of primary or secondary bacterial infection.
    • Vancomycin and cefotaxime are recommended in these cases.

Prognosis

  • Croup is a self-limiting disease that usually resolves within 3 days (80% of cases).
  • Most cases improve in 3–7 days.
  • Hospitalization is needed in only 8%–15% of cases.
  • Croup is a life-threatening illness but rarely progresses to death; mortality occurs in < 1% of intubated patients; out-of-hospital cardiac arrest and death may occur.
  • Complications (uncommon)
    • Pneumonia
    • Pulmonary edema
    • Secondary bacterial tracheitis (see high fever, toxic appearance, mucopurulent exudates in the trachea)
    • Respiratory failure
    • Pneumothorax
    • Pneumomediastinum
    • Recurrent symptoms (5% of cases)

Clinical Relevance

The following are differential diagnoses of croup:

  • Pneumonia: infection that inflames the air sacs resulting in fluid or pus (purulent material) accumulation. Pneumonia clinically manifests as a cough with phlegm or pus, fever, chills, and difficulty breathing.
  • Pulmonary edema: excess fluid in the lungs. Symptoms can appear acutely or develop over time and include difficulty breathing, cough, chest pain, and fatigue
  • Secondary bacterial tracheitis: caused by a secondary bacterial infection of the trachea which underlies the development of mucopurulent exudates that cause life-threatening complications due to obstruction of upper airways
  • Respiratory failure: a condition in which the respiratory system either fails to adequately oxygenate the blood or fails to adequately eliminate carbon dioxide from the blood
  • Pneumothorax: a collapsed lung which results in the leakage of air in the space between the lungs and chest wall, resulting in significant positive pressure on the parietal pleural membrane and leading to the collapse of the lung
  • Epiglottitis: inflammation of the epiglottis caused by Haemophilus, streptococcal, or staphylococcal infections, causing dyspnea, stridor, and cyanosis, ultimately leading to death due to obstruction of airways
  • Laryngeal diphtheria: manifests as swollen neck and throat, or “bull neck” accompanied by the following clinical symptoms: “barking” cough, stridor, hoarseness, difficulty breathing, and diphtheritic croup
  • Foreign body aspiration: Aspirated food may lodge in the larynx or trachea, which can lead to choking and potentially death.
  • Peritonsillar, parapharyngeal, or retropharyngeal abscesses: Abscesses of the pharynx present with fever, lymphadenopathy, painful neck, neck stiffness, drooling, uvula deviation, “hot potato” voice that is muffled and hoarse, and trouble swallowing.

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