Croup for the USMLE Step 1
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Croup for the USMLE Step 1 Exam
Etiology
- Viral Infection:
- Croup (laryngotracheobronchitis) is most commonly caused by parainfluenza virus type 1, which accounts for the majority of cases. Other viruses associated with croup include:
- Parainfluenza types 2 and 3
- Respiratory syncytial virus (RSV)
- Influenza A and B
- Adenovirus
- Human metapneumovirus
- Age Predilection:
- Croup primarily affects children between the ages of 6 months and 3 years, with peak incidence occurring around 2 years of age. It is less common in children over 6 years old due to the larger airway.
Pathophysiology
- Airway Inflammation:
- Croup is characterized by inflammation and edema of the larynx, trachea, and bronchi. This leads to narrowing of the subglottic space, which is the narrowest part of the pediatric airway, causing the characteristic stridor and barking cough.
- Viral Replication:
- The virus infects the respiratory epithelium, leading to immune activation and the release of pro-inflammatory cytokines, resulting in mucosal edema, increased mucus production, and decreased airway diameter.
Clinical Features
- Prodromal Phase:
- Initial symptoms resemble a common cold, including nasal congestion, rhinorrhea, and low-grade fever. After 1-2 days, more specific signs of upper airway obstruction appear.
- Barking Cough:
- A distinctive harsh, "seal-like" cough is the hallmark of croup and results from turbulent airflow through the narrowed subglottic airway.
- Stridor:
- Inspiratory stridor, a high-pitched sound, occurs due to partial airway obstruction. Stridor is usually worse at night and during agitation or crying.
- Hoarseness:
- Inflammation of the vocal cords leads to hoarseness.
- Fever:
- Mild to moderate fever (typically <39°C) may be present.
- Symptom Severity:
- Symptoms vary in 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 primarily diagnosed clinically, based on the characteristic barking cough, stridor, and hoarseness in a child with an upper respiratory infection.
- Neck X-ray (if performed):
- May show the classic "steeple sign", which represents subglottic narrowing. Imaging is rarely needed unless there is suspicion of an alternative diagnosis.
- Differential Diagnosis:
- Epiglottitis: Rapid onset, high fever, drooling, and toxic appearance.
- Bacterial tracheitis: Severe airway obstruction with fever and purulent secretions.
- Foreign body aspiration: Sudden onset of respiratory distress without preceding viral symptoms.
Management
General Approach
- Supportive Care:
- Mild croup can often be managed at home with cool mist humidifiers or exposure to cool air, which may help reduce swelling and soothe the airway. Ensuring the child remains calm is critical to avoid worsening symptoms due to agitation.
Pharmacologic Treatment
- Glucocorticoids:
- Dexamethasone (single oral or IM dose) is the treatment of choice for all cases of croup, as it reduces airway inflammation. Typical dosing is 0.6 mg/kg.
- Nebulized budesonide is an alternative but less commonly used due to the ease of oral dexamethasone administration.
- Nebulized Epinephrine:
- Used for moderate to severe croup with significant stridor at rest or respiratory distress. It provides rapid but temporary relief by reducing airway edema through vasoconstriction. Patients must be monitored for rebound symptoms after treatment.
Hospitalization
- Criteria for Hospitalization:
- Hospitalization is indicated for children with severe croup, respiratory distress, hypoxia, or those requiring multiple doses of nebulized epinephrine. Oxygen therapy may be needed in severe cases.
Complications
- Bacterial Superinfection:
- Rare but serious complications include secondary bacterial infections like bacterial tracheitis or pneumonia, requiring antibiotics and airway management.
- Airway Obstruction:
- Severe cases can progress to respiratory failure if not treated promptly.
Key Points
- Croup is a viral upper airway infection, primarily caused by parainfluenza virus, that leads to inflammation and narrowing of the subglottic airway, resulting in a barking cough and stridor.
- Diagnosis is clinical, based on the characteristic presentation in a child with a viral upper respiratory infection. Imaging is rarely needed but may show the "steeple sign."
- Treatment includes supportive care, with glucocorticoids (dexamethasone) being the mainstay for all cases. Nebulized epinephrine is used for moderate to severe cases with stridor at rest or respiratory distress.
- Hospitalization is required for severe croup, significant respiratory distress, or recurrent need for epinephrine.