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Croup for the AMIB

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Croup for the American Board of Internal Medicine Exam
Etiology
  • Viral Infection:
    • Croup (laryngotracheobronchitis) is most commonly caused by parainfluenza virus type 1, which accounts for the majority of cases. Other viral etiologies include:
    • Parainfluenza types 2 and 3
    • Respiratory syncytial virus (RSV)
    • Influenza A and B
    • Adenovirus
    • Coronavirus
    • Less commonly, measles virus can cause severe croup in unvaccinated populations.
  • Age Predilection:
    • Croup predominantly affects children between 6 months and 3 years of age. It is rare beyond age 6, as the airway becomes larger and less prone to significant obstruction.
Pathophysiology
  • Upper Airway Obstruction:
    • Croup is characterized by inflammation of the larynx, trachea, and bronchi, resulting in edema of the subglottic region. This narrowing of the airway leads to the hallmark symptoms of croup: inspiratory stridor, barking cough, and hoarseness.
    • The small diameter of the pediatric airway makes children more susceptible to significant airway narrowing when edema occurs.
  • Viral Invasion and Inflammatory Response:
    • Viral replication leads to local immune activation, causing release of pro-inflammatory cytokines (e.g., IL-6, TNF-α) and recruitment of immune cells. This leads to mucosal edema, increased mucus production, and epithelial damage.
Clinical Features
  • Prodromal Symptoms:
    • Croup typically begins with mild upper respiratory symptoms such as nasal congestion, rhinorrhea, and low-grade fever, resembling a common cold.
  • Characteristic Symptoms:
    • Barking cough: A distinctive harsh, "seal-like" cough that occurs due to turbulent airflow through the inflamed and narrowed subglottic region.
    • Inspiratory stridor: A high-pitched sound heard during inspiration, caused by airflow obstruction in the upper airway.
    • Hoarseness: Inflammation of the larynx leads to voice changes and hoarseness.
    • Fever: Mild to moderate fever (typically <39°C) is common but not always present.
  • Symptom Severity:
    • Symptoms often worsen at night and improve during the day. The cough and stridor tend to peak on days 2-3 of illness and usually resolve within 3-7 days.
  • Mild, Moderate, and Severe Croup:
    • Mild: Occasional barking cough and stridor, no signs of respiratory distress (normal oxygen saturation, no retractions).
    • Moderate: Frequent barking cough, stridor at rest, mild retractions, normal oxygen saturation.
    • Severe: Stridor at rest, significant respiratory distress (marked retractions, nasal flaring, tachypnea), and hypoxia.
Diagnosis
  • Clinical Diagnosis:
    • Croup is primarily diagnosed based on clinical presentation. The characteristic barking cough, stridor, and hoarseness, along with the patient's age and seasonal prevalence (late fall and early winter), strongly suggest the diagnosis.
    • Imaging is not routinely required but may be considered in atypical cases.
  • Neck X-ray:
    • If performed, a frontal neck X-ray may show the classic "steeple sign", which represents subglottic narrowing. However, imaging is not necessary for most cases.
Croup steeple sign
  • Differential Diagnosis:
    • Epiglottitis: Presents with high fever, drooling, and rapid progression of airway obstruction. Patients often appear toxic and assume a tripod position.
    • Bacterial tracheitis: More severe than viral croup, with high fever, toxic appearance, and purulent secretions. It may follow viral croup as a secondary bacterial infection.
    • Foreign body aspiration: Sudden onset of respiratory symptoms with no preceding upper respiratory infection. Stridor may be unilateral.
    • Allergic reaction: Acute airway obstruction with stridor, often accompanied by angioedema or urticaria.
Management
General Approach
  • Supportive Care:
    • Mild croup is typically managed at home with supportive care, including hydration and a calm environment to prevent worsening of symptoms due to agitation. The use of a cool mist humidifier or exposure to cool air may alleviate symptoms.
Pharmacologic Treatment
  • Glucocorticoids:
    • Dexamethasone is the cornerstone of treatment for all cases of croup, regardless of severity. A single dose (oral, IM, or IV) has been shown to reduce airway inflammation and improve symptoms. Typical dosing is 0.6 mg/kg (maximum of 10 mg).
    • Nebulized budesonide may be used as an alternative to dexamethasone, though systemic glucocorticoids are preferred due to ease of administration and longer-lasting effects.
  • Nebulized Epinephrine:
    • Indicated for moderate to severe croup with significant stridor at rest or respiratory distress. Epinephrine reduces airway edema through vasoconstriction. Its effects are rapid but short-lived, so it is used alongside dexamethasone for sustained improvement.
    • Rebound symptoms may occur after epinephrine administration, so patients should be monitored for at least 2-4 hours after treatment for signs of recurrence.
Hospitalization Criteria
  • Indications for Hospitalization:
    • Patients with severe croup, requiring repeated doses of nebulized epinephrine, or those who exhibit persistent respiratory distress, hypoxia, or poor oral intake should be hospitalized for close monitoring and supportive care.
    • In severe cases, oxygen therapy may be required, and rare instances of impending respiratory failure may necessitate intubation.
Complications
  • Bacterial Superinfection:
    • Although uncommon, bacterial tracheitis or pneumonia can develop as secondary complications. These typically present with worsening fever, respiratory distress, and purulent secretions. Antibiotics and airway management may be required.
  • Airway Obstruction:
    • Severe airway obstruction can lead to respiratory failure, particularly in very young children. Early recognition and management with epinephrine and corticosteroids are critical to preventing this outcome.
Prevention
  • Vaccination:
    • The influenza vaccine is recommended for children and at-risk individuals to prevent influenza-associated croup, especially during the flu season.
  • Hygiene Practices:
    • As croup is spread via respiratory droplets, practicing good hand hygiene and avoiding close contact with infected individuals can reduce transmission.
Key Points
  • Croup is a viral respiratory illness, most commonly caused by parainfluenza virus, that results in upper airway obstruction due to inflammation of the subglottic region.
  • Clinical features include a barking cough, inspiratory stridor, hoarseness, and fever. Symptoms often worsen at night and typically resolve within 3-7 days.
  • Diagnosis is clinical, based on characteristic symptoms. Imaging is not routinely required but may show the "steeple sign" on X-ray.
  • Treatment includes supportive care and pharmacologic therapy with dexamethasone for all cases. Nebulized epinephrine is used for moderate to severe cases with stridor at rest or respiratory distress.
  • Hospitalization is indicated for severe croup, recurrent need for epinephrine, or respiratory distress with hypoxia.

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