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Croup for the USMLE Step 3

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Croup for the USMLE Step 3 Exam
Etiology
  • Viral Infection:
    • Croup (laryngotracheobronchitis) is most commonly caused by parainfluenza virus type 1. Other viral causes include:
    • Parainfluenza types 2 and 3
    • Respiratory syncytial virus (RSV)
    • Influenza A and B
    • Adenovirus
    • Coronavirus
  • Age Group:
    • Croup primarily affects children between 6 months and 3 years due to their narrower airway, although it can occur in older children and rarely in adults.
Pathophysiology
  • Upper Airway Edema:
    • Viral infection leads to inflammation and edema of the larynx, trachea, and bronchi, particularly the subglottic area. The narrowing of this region produces the characteristic barking cough and stridor.
  • Cytokine Response:
    • Viral replication in the respiratory epithelium induces the release of cytokines, causing mucosal edema, increased mucus production, and further airway narrowing.
Clinical Features
  • Prodromal Symptoms:
    • Initially, children present with non-specific upper respiratory symptoms such as rhinorrhea, nasal congestion, and low-grade fever.
  • Barking Cough:
    • The hallmark symptom is a seal-like barking cough caused by airflow through the narrowed subglottic region.
  • Stridor:
    • Inspiratory stridor is common, especially in moderate to severe cases. Stridor is a high-pitched sound produced during inspiration due to partial airway obstruction.
  • Hoarseness:
    • Laryngeal inflammation leads to hoarseness and voice changes.
  • Symptom Worsening at Night:
    • Symptoms often worsen at night due to the supine position and are aggravated by agitation or crying.
Diagnosis
  • Clinical Diagnosis:
    • Croup is diagnosed clinically based on the classic symptoms of barking cough, stridor, and hoarseness in a child with a viral upper respiratory tract infection.
  • Neck X-ray (if performed):
    • May reveal the "steeple sign"—a narrowing of the subglottic space—though imaging is rarely needed unless there is concern for an alternative diagnosis.
Croup steeple sign
  • Differential Diagnosis:
    • Epiglottitis: Presents with high fever, drooling, and toxic appearance, requiring emergency airway management.
    • Bacterial tracheitis: More severe than viral croup with purulent secretions, high fever, and risk of airway obstruction.
    • Foreign body aspiration: Sudden onset of respiratory distress without prodromal symptoms.
Management
Supportive Care
  • Hydration and Humidified Air:
    • Ensuring proper hydration and using humidified air (e.g., cool mist) may help alleviate symptoms in mild cases. Keeping the child calm is essential to reduce airway narrowing from agitation.
Pharmacologic Treatment
  • Glucocorticoids:
    • Dexamethasone (single oral or IM dose) is the mainstay treatment for croup and is effective in reducing airway inflammation. The typical dose is 0.6 mg/kg, and it improves symptoms within 6 hours.
    • Nebulized budesonide is an alternative but less commonly used than dexamethasone.
  • Nebulized Epinephrine:
    • Indicated for moderate to severe croup with stridor at rest or significant respiratory distress. Epinephrine provides rapid relief by decreasing airway edema via vasoconstriction, but its effects are short-lived, so patients need to be monitored for rebound symptoms.
Criteria for Hospitalization
  • Indications for Hospitalization:
    • Children with severe croup, persistent respiratory distress, hypoxia, or those requiring repeated doses of nebulized epinephrine should be hospitalized. In severe cases, oxygen therapy or close monitoring may be necessary.
Complications
  • Bacterial Superinfection:
    • Secondary bacterial infections such as bacterial tracheitis or pneumonia can develop, especially after severe viral croup, leading to worsening respiratory distress and fever.
  • Respiratory Failure:
    • Severe airway obstruction can progress to respiratory failure if left untreated, requiring intubation and mechanical ventilation in rare cases.
Key Points
  • Croup is most often caused by parainfluenza virus, leading to upper airway obstruction with characteristic barking cough and inspiratory stridor.
  • Diagnosis is clinical, based on symptoms, with imaging rarely required unless alternative diagnoses are suspected.
  • Treatment includes supportive care and dexamethasone for all cases. Nebulized epinephrine is used for moderate to severe croup with respiratory distress or stridor at rest.
  • Hospitalization is necessary for severe croup or if there is a need for repeated doses of epinephrine or persistent respiratory distress.

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