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Croup for the USMLE Step 2

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Croup for the USMLE Step 2 Exam
Etiology
  • Viral Causes:
    • Croup (laryngotracheobronchitis) is most commonly caused by parainfluenza virus type 1. Other viral pathogens include:
    • Parainfluenza types 2 and 3
    • Respiratory syncytial virus (RSV)
    • Influenza A and B
    • Adenovirus
    • Coronavirus and human metapneumovirus
  • Age Predilection:
    • Croup predominantly affects children between 6 months and 3 years. Rare in children older than 6 years due to a larger airway.
Pathophysiology
  • Airway Inflammation:
    • Croup involves inflammation of the larynx, trachea, and bronchi, primarily affecting the subglottic space. Edema in this area leads to airway narrowing, which produces the characteristic barking cough and stridor.
  • Viral Replication:
    • Viral invasion of the respiratory epithelium triggers an immune response, leading to the release of cytokines that cause mucosal edema and increased mucus production.
Clinical Features
  • Prodromal Phase:
    • Symptoms begin with mild upper respiratory tract infection signs, such as nasal congestion, rhinorrhea, and low-grade fever.
  • Barking Cough:
    • A seal-like barking cough is the hallmark of croup, resulting from turbulent airflow through the narrowed airway.
  • Stridor:
    • Inspiratory stridor is a high-pitched sound produced during inspiration due to partial airway obstruction. It is more prominent during agitation or crying and often worsens at night.
  • Hoarseness:
    • Hoarseness is due to vocal cord involvement caused by laryngeal inflammation.
  • Fever:
    • Mild to moderate fever (typically <39°C) may be present.
  • Symptom Severity:
    • Mild: Occasional barking cough, no stridor at rest.
    • Moderate: Frequent barking cough, stridor at rest, mild respiratory distress.
    • Severe: Continuous stridor, marked respiratory distress, hypoxia.
Diagnosis
  • Clinical Diagnosis:
    • Croup is diagnosed clinically, based on characteristic symptoms such as barking cough, stridor, and hoarseness in children with a viral upper respiratory infection.
  • Neck X-ray:
    • May show the "steeple sign", which indicates subglottic narrowing, though imaging is rarely required unless there is diagnostic uncertainty.
Croup steeple sign
  • Differential Diagnosis:
    • Epiglottitis: Sudden onset, high fever, drooling, and toxic appearance.
    • Bacterial tracheitis: More severe airway obstruction with fever and purulent secretions.
    • Foreign body aspiration: Sudden onset of respiratory distress without viral prodrome.
Management
General Approach
  • Supportive Care:
    • Hydration and keeping the child calm to reduce airway inflammation. Humidified air or exposure to cool mist may help alleviate symptoms, especially in mild cases.
Pharmacologic Treatment
  • Glucocorticoids:
    • Dexamethasone (oral or IM, 0.6 mg/kg) is the treatment of choice for all cases of croup, reducing airway inflammation and improving symptoms within 6 hours. Nebulized budesonide is an alternative but less commonly used.
  • Nebulized Epinephrine:
    • Used for moderate to severe croup with stridor at rest or significant respiratory distress. Epinephrine provides rapid relief by reducing mucosal edema but has a short duration of effect, so patients need to be monitored for rebound symptoms.
Hospitalization
  • Criteria for Hospitalization:
    • Hospitalization is indicated for children with severe croup, persistent respiratory distress, hypoxia, or those requiring repeated doses of nebulized epinephrine. Oxygen therapy may be required in severe cases.
Complications
  • Bacterial Superinfection:
    • Rare but can occur, leading to bacterial tracheitis or pneumonia, which typically present with worsening respiratory distress, fever, and purulent secretions.
  • Respiratory Failure:
    • In severe cases of untreated croup, airway obstruction can progress to respiratory failure, requiring advanced airway management, including intubation.
Key Points
  • Croup is caused by viral infections, primarily parainfluenza virus, leading to subglottic airway narrowing and resulting in a barking cough and inspiratory stridor.
  • Diagnosis is clinical, based on characteristic symptoms. Imaging (e.g., X-ray showing the "steeple sign") is rarely required.
  • Management includes supportive care and dexamethasone for all cases. Nebulized epinephrine is indicated for moderate to severe cases with stridor at rest or respiratory distress.
  • Hospitalization is required for severe cases, recurrent need for epinephrine, or persistent respiratory distress.

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