Croup (Paramyxoviruses)
Overview
Plain-Language Overview
Croup is a common respiratory illness that mainly affects young children. It involves inflammation of the upper airway, particularly the voice box and windpipe, which can cause a distinctive barking cough and noisy breathing called stridor. The illness is usually caused by viruses from the Paramyxoviridae family, most often the parainfluenza virus. Symptoms often worsen at night and can make breathing difficult due to swelling and narrowing of the airway. While most cases are mild and resolve on their own, severe cases can cause significant breathing problems requiring medical attention.
Clinical Definition
Croup is an acute viral infection characterized by inflammation and edema of the larynx, trachea, and bronchi, leading to upper airway obstruction. It is most commonly caused by parainfluenza viruses (types 1 and 3) from the Paramyxoviridae family. The hallmark clinical features include a barking cough, inspiratory stridor, and hoarseness due to subglottic narrowing. The disease primarily affects children aged 6 months to 3 years, with peak incidence in fall and early winter. The airway inflammation results in increased airway resistance and respiratory distress, which can range from mild to severe. Diagnosis is clinical, and the condition is significant because airway obstruction can progress rapidly, necessitating prompt recognition and management.
Inciting Event
Initial infection with respiratory paramyxoviruses, especially parainfluenza virus types 1 and 2.
Exposure to infected respiratory droplets from close contacts or community outbreaks.
Preceding upper respiratory tract infection symptoms such as rhinorrhea and low-grade fever.
Latency Period
Symptoms typically develop 2 to 6 days after viral exposure.
Initial upper respiratory symptoms precede airway symptoms by 1 to 3 days.
Progression from mild URI to croup symptoms usually occurs within 24 to 48 hours.
Diagnostic Delay
Early symptoms mimic common viral upper respiratory infections, leading to misattribution.
Mild initial symptoms may delay presentation until stridor or respiratory distress develops.
Lack of awareness of characteristic barking cough and inspiratory stridor can delay diagnosis.
Misdiagnosis as allergic reactions or foreign body aspiration may occur.
Clinical Presentation
Signs & Symptoms
Barking cough and inspiratory stridor are classic presenting symptoms.
Hoarseness and laryngotracheal inflammation cause voice changes.
Fever is typically low-grade but may be higher in severe cases.
Respiratory distress with retractions and nasal flaring occurs in moderate to severe disease.
Symptoms worsen at night due to increased airway edema and inflammation.
History of Present Illness
Initial prodrome of rhinorrhea, low-grade fever, and cough lasting 1-3 days.
Onset of barking cough, hoarseness, and inspiratory stridor typically follows.
Symptoms often worsen at night with increased respiratory effort and possible retractions.
Mild to moderate respiratory distress with variable degrees of hypoxia may be present.
Symptoms usually improve over 3 to 7 days with supportive care.
Past Medical History
History of recurrent viral respiratory infections may be present but is not required.
No specific prior conditions are necessary, but prematurity or underlying airway anomalies can worsen severity.
Absence of prior asthma or chronic lung disease helps differentiate from other causes of stridor.
Family History
No known heritable syndromes are associated with croup.
Family history of atopy or asthma may be present but is not directly linked to croup risk.
No genetic predisposition has been established for susceptibility to paramyxovirus infections causing croup.
Physical Exam Findings
Barking cough resembling a seal's bark is a hallmark finding in croup.
Inspiratory stridor due to upper airway obstruction is commonly auscultated.
Hoarseness results from laryngeal inflammation caused by paramyxovirus infection.
Suprasternal and intercostal retractions indicate increased work of breathing.
Tachypnea and mild hypoxia may be present in moderate to severe cases.
Diagnostic Workup
Diagnostic Criteria
Diagnosis of croup is primarily clinical, based on the presence of a barking cough, inspiratory stridor, and hoarseness in a young child with a recent history of upper respiratory symptoms. Physical examination reveals signs of upper airway obstruction without evidence of bacterial infection. Neck or chest X-rays may show the classic steeple sign indicating subglottic narrowing but are not routinely required. Laboratory tests and viral cultures are generally unnecessary unless the diagnosis is unclear or complications are suspected.
Pathophysiology
Key Mechanisms
Infection with paramyxoviruses (most commonly parainfluenza virus) causes inflammation and edema of the subglottic airway, leading to airway narrowing and inspiratory stridor.
Mucosal inflammation results in increased mucus production and laryngotracheal swelling, which contribute to the characteristic barking cough and respiratory distress.
Airway obstruction is dynamic and worsens with agitation or crying due to increased negative intrathoracic pressure.
The immune response to viral infection involves infiltration of mononuclear cells and release of inflammatory mediators that exacerbate airway edema.
| Involvement | Details |
|---|---|
| Organs | Larynx is the primary organ affected in croup, with inflammation causing the classic stridor and hoarseness |
Trachea involvement leads to airway narrowing and respiratory distress | |
Lungs may be secondarily affected if lower respiratory tract infection or obstruction occurs | |
| Tissues | Subglottic mucosa is the key site of inflammation and edema leading to the hallmark airway narrowing in croup |
Laryngeal tissue involvement causes the characteristic barking cough and stridor | |
Tracheal mucosa inflammation contributes to airway obstruction and respiratory symptoms | |
| Cells | Respiratory epithelial cells are the primary site of infection by paramyxoviruses causing croup and contribute to airway inflammation |
Neutrophils infiltrate the subglottic mucosa causing edema and airway narrowing characteristic of croup | |
Lymphocytes mediate the adaptive immune response to viral infection and help clear the virus | |
| Chemical Mediators | Histamine released from mast cells contributes to mucosal edema and bronchoconstriction in the airway |
Cytokines such as interleukin-6 and tumor necrosis factor-alpha promote inflammation and recruit immune cells to the airway | |
Leukotrienes enhance vascular permeability and bronchoconstriction worsening airway obstruction |
Treatments
Pharmacological Treatments
Dexamethasone
- Mechanism:
Reduces airway inflammation and edema by suppressing immune response and cytokine production
- Side effects:
Hyperglycemia
Immunosuppression
Gastrointestinal upset
- Clinical role:
First-line
Nebulized Epinephrine
- Mechanism:
Stimulates alpha-adrenergic receptors causing vasoconstriction and decreased airway mucosal edema
- Side effects:
Tachycardia
Hypertension
Transient pallor
- Clinical role:
Rescue
Non-pharmacological Treatments
Provide humidified air or cool mist to soothe inflamed airway mucosa and reduce airway resistance
Ensure adequate hydration and oxygen supplementation if hypoxia is present
Monitor respiratory status closely for signs of airway obstruction or respiratory distress
Prevention
Pharmacological Prevention
No specific vaccine is currently available for parainfluenza viruses causing croup.
Palivizumab prophylaxis is not indicated as it targets respiratory syncytial virus, not paramyxoviruses.
Non-pharmacological Prevention
Hand hygiene and respiratory droplet precautions reduce transmission of paramyxoviruses.
Avoiding exposure to infected individuals during peak viral seasons decreases risk.
Maintaining good overall health and nutrition supports immune defense against viral infections.
Outcome & Complications
Complications
Severe airway obstruction leading to hypoxia and respiratory failure is the most serious complication.
Bacterial tracheitis can superimpose, causing worsening airway obstruction and fever.
Secondary bacterial pneumonia may develop in some cases.
Dehydration from poor oral intake due to respiratory distress is common.
| Short-term Sequelae | Long-term Sequelae |
|---|---|
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Differential Diagnoses
Croup (Paramyxoviruses) versus Epiglottitis
Croup (Paramyxoviruses) | Epiglottitis |
|---|---|
Commonly affects children aged 6 months to 3 years | Typically affects children aged 2 to 7 years |
Gradual onset of barking cough, hoarseness, and low-grade fever | Rapid onset of high fever, severe sore throat, and drooling with toxic appearance |
Neck X-ray shows steeple sign (subglottic narrowing) | Lateral neck X-ray shows thumbprint sign (swollen epiglottis) |
Responds well to nebulized epinephrine and corticosteroids | Requires immediate airway management and intravenous antibiotics |
Croup (Paramyxoviruses) versus Bacterial Tracheitis
Croup (Paramyxoviruses) | Bacterial Tracheitis |
|---|---|
Caused by viral paramyxoviruses like parainfluenza virus | Caused by bacterial pathogens such as Staphylococcus aureus |
Usually presents with gradual onset and low-grade fever | Often follows viral upper respiratory infection with worsening symptoms and high fever |
Normal or mildly elevated white blood cell count with lymphocytic predominance | Elevated white blood cell count with neutrophilic predominance |
Improves with supportive care including corticosteroids and nebulized epinephrine | Requires intravenous antibiotics and possible airway support |
Croup (Paramyxoviruses) versus Foreign Body Aspiration
Croup (Paramyxoviruses) | Foreign Body Aspiration |
|---|---|
No history of choking; gradual symptom onset | History of sudden choking episode or witnessed aspiration |
Progressive onset of barking cough and inspiratory stridor | Abrupt onset of coughing, stridor, and respiratory distress |
Neck X-ray shows steeple sign without localized air trapping | Chest X-ray may show localized air trapping or radiopaque foreign body |
Responds to medical management with corticosteroids and nebulized epinephrine | Requires urgent bronchoscopy for removal |
Croup (Paramyxoviruses) versus Allergic Laryngitis
Croup (Paramyxoviruses) | Allergic Laryngitis |
|---|---|
No allergen exposure or atopic history | Associated with allergen exposure and history of atopy |
Continuous symptoms with low-grade fever and viral prodrome | Intermittent symptoms triggered by allergens, often without fever |
Normal IgE and no eosinophilia | Elevated serum IgE and eosinophilia |
Improves with corticosteroids and nebulized epinephrine | Improves with antihistamines and avoidance of allergens |