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Summary Table

Category	Detail
Risk Factors	Age 6 months to 3 years (peak at 1 to 2 years), male predominance, fall and early winter season
Etiology	Parainfluenza virus (type 1 most common), less commonly RSV, influenza, adenovirus, human metapneumovirus
Patient Presentation	1 to 2 day URI prodrome (rhinorrhea, low-grade fever) followed by sudden onset of barky cough, hoarseness, and inspiratory stridor, classically worse at night
Classic Physical Exam	"Seal-like" barky cough, inspiratory stridor (at rest in moderate to severe cases), suprasternal and intercostal retractions, hoarse voice or cry; the child is typically non-toxic appearing and does not drool
Key Diagnostic Results	Clinical diagnosis; AP neck radiograph shows the steeple sign (subglottic narrowing); lateral neck radiograph shows a distended hypopharynx with a normal epiglottis
Management	Dexamethasone 0.6 mg/kg single dose (PO or IM) for all severities; nebulized racemic epinephrine for moderate to severe distress; observe at least 2 to 4 hours after epinephrine for rebound
Keywords	"Barky cough," "seal-like cough," "worse at night," "steeple sign," "inspiratory stridor in a toddler," "preceded by URI symptoms"

1. Pathophysiology

Croup is fundamentally a disease of subglottic airway edema. Understanding this single concept unlocks every clinical feature tested on exams.

The parainfluenza virus (most commonly type 1) infects the epithelium of the larynx, trachea, and bronchi. The resulting inflammatory response produces mucosal edema and increased mucus secretion. The key anatomic point is that in children, the narrowest portion of the airway is at the cricoid cartilage (the subglottic region), unlike adults whose narrowest point is at the glottis. The pediatric airway is also funnel-shaped rather than cylindrical, which makes it far more vulnerable to obstruction from even mild swelling.

The clinical significance of this anatomy is explained by Poiseuille's Law, which states that airflow resistance is inversely proportional to the fourth power of the radius ($R \propto \frac{1}{r^4}$). This means that 1 mm of circumferential edema in a small pediatric airway causes a dramatic increase in resistance and a corresponding reduction in airflow. This is why a toddler develops stridor and respiratory distress from the same virus that would only cause a hoarse voice in an older child or adult.

Hoarseness results from edema of the vocal cords themselves. The characteristic barky, seal-like cough is produced by turbulent airflow through the narrowed subglottic space and inflamed vocal cords. Inspiratory stridor is present because this is an extrathoracic obstruction: during inspiration, negative intrathoracic pressure causes the already narrowed extrathoracic airway to collapse further inward, worsening the obstruction. Symptoms are classically worse at night because of the natural nocturnal increase in cortisol-mediated anti-inflammatory tone being at its lowest, combined with the supine position and cooler ambient air.

2. Classification and Clinical Manifestation

Westley Croup Severity Score

The Westley score is the most widely used system for grading severity. It guides management decisions and is built around five clinical parameters.

Parameter	0	1	2	3	4	5
Stridor	None	With agitation only	At rest
Retractions	None	Mild	Moderate	Severe
Air Entry	Normal	Mildly decreased	Markedly decreased
Cyanosis	None				With agitation	At rest
Level of Consciousness	Normal					Altered

Severity Classification

Severity	Westley Score	Clinical Features
Mild	0 to 2	Intermittent barky cough, no stridor at rest, no or minimal retractions, child is comfortable and playful
Moderate	3 to 7	Stridor at rest, visible retractions (suprasternal, intercostal), barky cough is frequent, child is restless but consolable
Severe	8 to 11	Prominent stridor at rest, severe retractions, markedly decreased air entry, significant agitation or lethargy
Impending Respiratory Failure	12 or above	Diminishing or absent stridor (ominous sign suggesting minimal air movement), altered consciousness, cyanosis, paradoxical breathing

A critical teaching point: decreasing stridor in a worsening child is not improvement. It signals that air movement has become so poor that there is insufficient flow to generate the turbulent sound. This is a classic exam trap.

Subtypes of Croup

Subtype	Features
Viral croup (acute laryngotracheobronchitis)	Most common form; gradual onset after URI prodrome; self-limited over 3 to 7 days
Spasmodic croup	Sudden onset at night without URI prodrome; recurrent episodes; may have an allergic or atopic component; resolves quickly with cool air or humidity
Bacterial tracheitis (membranous croup)	Appears initially like croup but becomes toxic-appearing; high fever; purulent secretions; does not respond to standard croup therapy; requires IV antibiotics and possibly airway intervention

3. Diagnostic Workup

Test	Role	Expected Finding
Clinical assessment	Primary and best initial approach	Barky cough, hoarseness, inspiratory stridor in a 6-month to 3-year-old with URI prodrome
AP neck radiograph	Supportive (ordered when diagnosis is uncertain)	Steeple sign (symmetric subglottic narrowing)
Lateral neck radiograph	To rule out epiglottitis or retropharyngeal abscess	Normal epiglottis; distended hypopharynx; subglottic narrowing
Pulse oximetry	Assess oxygenation in moderate to severe cases	Hypoxia is a late and concerning finding
Viral studies (PCR, rapid antigen)	Rarely needed clinically	Parainfluenza isolation; more useful for epidemiologic purposes
Direct laryngoscopy	Reserved for atypical or refractory cases	Subglottic edema and erythema; rules out structural anomaly

Croup is a clinical diagnosis. In the vast majority of cases, no laboratory or imaging test is required. A child between 6 months and 3 years presenting with a 1 to 2 day history of URI symptoms followed by the acute onset of a barky cough, hoarseness, and inspiratory stridor, particularly worsening at night, meets the diagnostic criteria based on history and physical examination alone.

Radiographs are not routinely necessary but are ordered when the clinical picture is atypical, when epiglottitis or a foreign body is a concern, or when the child does not respond to initial treatment. The AP neck radiograph is the key imaging study. It demonstrates symmetric subglottic narrowing of the tracheal air column, producing the classic steeple sign (also called the "pencil point" sign or "wine bottle" sign). This replaces the normal squared-off "shouldering" of the subglottic airway. The steeple sign is present in roughly 50% of croup cases, so its absence does not exclude the diagnosis.

The lateral neck radiograph is primarily useful for ruling out epiglottitis. In croup, the lateral view shows a normal, thin epiglottis and distended hypopharynx. In epiglottitis, the lateral view reveals a swollen, rounded epiglottis known as the thumbprint sign.

Pulse oximetry should be monitored in moderate to severe presentations. Significant desaturation ($SpO_2 < 92%$) is a late finding that indicates impending respiratory failure.

Blood work, including a CBC or blood cultures, is not indicated in typical viral croup. If the child appears toxic with a high fever and is not responding to dexamethasone and epinephrine, consider bacterial tracheitis and obtain appropriate cultures and imaging.

4. Management and Treatment

Severity	Treatment	Details
All severities	Dexamethasone	0.6 mg/kg PO or IM, single dose (max 10 mg)
Mild	Dexamethasone alone	Discharge home with return precautions
Moderate	Dexamethasone + nebulized epinephrine	Racemic epinephrine 0.5 mL of 2.25% solution or L-epinephrine 0.5 mL/kg of 1:1000 (max 5 mL); observe 2 to 4 hours
Severe	Dexamethasone + nebulized epinephrine + close monitoring	Repeat epinephrine as needed every 15 to 20 minutes; ICU admission if no improvement
Impending respiratory failure	Intubation	Use an endotracheal tube 0.5 to 1 mm smaller than age-predicted size due to subglottic swelling

Dexamethasone is the cornerstone of croup management and is given to every child with croup regardless of severity, including mild cases. The standard dose is 0.6 mg/kg as a single dose, given orally if the child can tolerate it, or intramuscularly if oral administration is not feasible. The onset of action is within 2 to 4 hours for the oral route and somewhat faster for IM. The long half-life of dexamethasone (36 to 54 hours) means that a single dose provides sustained benefit, and repeat dosing is almost never necessary. Prednisolone (1 mg/kg) can be used as an alternative, though evidence favors dexamethasone for its longer duration and single-dose convenience.

Nebulized epinephrine is the rescue therapy for moderate to severe croup. Racemic epinephrine (0.5 mL of 2.25% solution diluted in 3 mL of normal saline) or L-epinephrine (0.5 mL/kg of the 1:1000 concentration, maximum 5 mL) are equally effective. Epinephrine works through alpha-adrenergic vasoconstriction of the subglottic mucosal vasculature, rapidly reducing edema. The onset is within 10 to 30 minutes, but the effect is temporary, lasting only 1 to 2 hours. After the drug wears off, rebound swelling can occur. This is why any child who receives nebulized epinephrine must be observed for at least 2 to 4 hours before discharge to ensure symptoms do not return. A child who remains well 2 to 4 hours after the last dose of epinephrine and has received dexamethasone can be safely discharged.

Humidified air or cool mist has been traditionally recommended but has no proven benefit in randomized controlled trials. It should not be relied upon as a therapeutic intervention, though it is commonly encountered in older references.

Heliox (a helium-oxygen mixture) may be used in severe cases to reduce turbulent airflow through the narrowed airway. It is a temporizing measure and does not treat the underlying inflammation.

Intubation is reserved for children with impending respiratory failure who do not respond to medical therapy. The tube selected should be 0.5 to 1.0 mm smaller than the standard age-predicted size to account for subglottic swelling.

Antibiotics are not indicated in viral croup. If bacterial tracheitis is suspected (toxic appearance, high fever, purulent secretions, failure to respond to croup therapy), IV antibiotics covering Staphylococcus aureus (including MRSA) and Streptococcus species should be initiated.

5. Differential Diagnosis and Distractors

Differential	Why It Looks Similar	Key Discriminator
Epiglottitis	Both cause stridor and respiratory distress in young children	Epiglottitis presents with acute high fever, drooling, dysphagia, tripod positioning, and a toxic-appearing child; the onset is rapid (hours, not days); the cough is muffled or absent (not barky); lateral neck X-ray shows the thumbprint sign; the child is typically older (2 to 7 years in the unvaccinated, older in the vaccinated) and refuses to lie down
Foreign body aspiration	Can present with sudden-onset stridor and respiratory distress	There is no URI prodrome, no fever, and onset is abrupt (seconds to minutes); history often includes choking or gagging while eating or playing; findings may be unilateral (asymmetric breath sounds, unilateral wheezing or hyperinflation); inspiratory and expiratory films or decubitus films show air trapping
Bacterial tracheitis	Initial presentation may mimic severe croup	The child is toxic-appearing with high fever and purulent tracheal secretions; croup therapy (dexamethasone and epinephrine) fails to provide improvement; often occurs as a secondary bacterial infection after an initial viral croup episode
Retropharyngeal abscess	Fever, stridor, and neck symptoms in a young child	The child has neck stiffness, refusal to extend the neck, drooling, and a muffled (hot potato) voice; lateral neck X-ray shows widened prevertebral soft tissue (greater than half the width of the adjacent vertebral body at C3 to C4)
Peritonsillar abscess	Sore throat, fever, and muffled voice	Typically affects older children and adolescents; physical exam shows unilateral tonsillar swelling with uvular deviation; trismus is present; stridor is uncommon
Angioedema	Acute onset of upper airway swelling and stridor	There is no viral prodrome or fever; look for lip, tongue, or facial swelling; history of ACE inhibitor use or known allergic triggers; urticaria may be present
Laryngomalacia	Chronic stridor in an infant	Stridor is present since birth or shortly after, is typically worse with feeding, crying, and supine positioning, and improves with prone positioning; there is no acute viral illness

6. Traps and High-Yield Pearls

The single most common way students lose points on croup questions is by confusing croup with epiglottitis. Both feature stridor in a young child, but the clinical profiles are very different. The croup child has a barky cough and is non-toxic; the epiglottitis child is drooling, toxic, and unable to swallow. If a vignette describes drooling, tripod positioning, or a "muffled" voice, the answer is epiglottitis, not croup.

The second trap involves the management sequence. Dexamethasone is given to every child with croup, including mild presentations. Students frequently reserve corticosteroids for moderate or severe cases only. This is incorrect. Another common error is discharging a child immediately after epinephrine administration. The mandatory observation period of 2 to 4 hours must be respected due to the risk of rebound symptoms.

A third trap is the disappearing stridor scenario. A vignette will describe a child with worsening croup whose stridor suddenly becomes quiet. Students may interpret this as clinical improvement. In reality, the loss of audible stridor in a child who is becoming lethargic or agitated represents critically reduced airflow and is a sign of impending respiratory failure.

Finally, remember that croup is a clinical diagnosis. The steeple sign on X-ray is a classic teaching point but is not required to make the diagnosis. If a vignette gives you a textbook history of barky cough, hoarseness, and inspiratory stridor in a toddler after a URI, the next step is treatment with dexamethasone, not an X-ray.

The core competency being tested in croup questions is the ability to recognize a classic presentation and act on it decisively, distinguish it from surgical or life-threatening airway emergencies (epiglottitis, foreign body, bacterial tracheitis), and apply the correct treatment hierarchy: corticosteroids for all, epinephrine for distress, and observation before discharge.

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