Croup — Laryngotracheobronchitis

• Viral invasion and mucosal inflammation: The respiratory viruses that cause croup directly infect the respiratory epithelium, particularly targeting the columnar epithelial cells of the subglottic region. This area is anatomically predisposed to severe manifestations because the subglottic space is funnel-shaped with a fixed outer layer of cricoid cartilage, unlike the more compliant vocal cords above. Viral replication triggers local production of pro-inflammatory cytokines (IL-6, IL-8, TNF-α, and IL-1β) that activate the innate immune response. This leads to recruitment of neutrophils and lymphocytes to the mucosa, causing infiltration, congestion, and edema of the submucosa and lamina propria. The resulting edema is disproportionately significant in the subglottic region because this narrow space (approximately 4-5 mm in children) has minimal compliance, so even small increases in mucosal thickness dramatically increase airway resistance according to the Hagen-Poiseuille equation (resistance is inversely proportional to the fourth power of the radius).

• Increased mucus production and impaired clearance: Viral infection stimulates goblet cells and mucous glands throughout the laryngotracheobronchial tree to increase mucus secretion. Simultaneously, the inflammatory process damages ciliated epithelial cells and impairs mucociliary clearance. This combination results in accumulation of thick, viscous secretions within the already narrowed subglottic airway. The mucus, combined with fibrin exudate from the inflamed mucosa, creates additional airway obstruction and provides a substrate for secondary bacterial colonization in more severe cases. The accumulated secretions also potentiate cough by stimulating sensory nerve endings (particularly C-fibers) in the laryngeal region, triggering the barky cough characteristic of croup.

• Laryngeal edema and increased airway resistance: The pathophysiologic hallmark of croup is subglottic edema that narrows the air column. This edema increases airway resistance exponentially (as resistance increases with the fourth power of the reciprocal of the radius), creating the characteristic stridor and dyspnea. During inspiration, negative intrathoracic pressure is transmitted across the narrowed segment, causing dynamic compression of the airway and paradoxical inward movement of the narrowed subglottic region (dynamic obstruction). This explains why inspiratory stridor predominates in croup. The work of breathing increases substantially because the patient must generate higher negative intrapulmonary pressures to overcome the increased resistance. In severe cases, this increased work of breathing leads to respiratory muscle fatigue, hypoxemia, and potential respiratory failure.

• Viral-mediated host response and epithelial damage: Beyond direct cytopathic effects, viral infection triggers both adaptive and innate immune responses that contribute to epithelial damage and prolonged inflammation. The balance between pro-inflammatory and anti-inflammatory mediators determines disease severity. Patients with dysregulated immune responses or those who have recently received live viral vaccines may experience more severe croup. Additionally, viral neuraminidase (produced by influenza and parainfluenza viruses) directly damages respiratory epithelium by cleaving sialic acid receptors, exposing underlying layers and perpetuating inflammation.

• Individual host factors affecting severity: Anatomic and physiologic factors in individual patients determine disease severity. Children with intrinsically narrow subglottic airways (such as those with congenital laryngeal stenosis or Down syndrome) develop more severe obstruction with less edema. Atopy and reactive airway disease predispose to greater inflammatory responses and more severe croup. Additionally, age influences severity—the narrowest absolute subglottic diameters occur around age 2-3 years, explaining the peak incidence at this age. Male predominance in croup (2:1 ratio) may relate to anatomically narrower subglottic airways in boys compared to girls.

• Parainfluenza viruses (most common cause): Parainfluenza virus type 1 accounts for 50-75% of all croup cases and is the predominant causative agent of seasonal epidemic croup, with peak incidence in fall months (September-December). Parainfluenza types 2 and 3 cause sporadic cases and are less seasonally restricted. These single-stranded RNA viruses in the Paramyxoviridae family infect the respiratory epithelium and trigger particularly robust subglottic inflammation. The predominance of parainfluenza type 1 as a cause reflects both its virulence and its epidemiology of biennial major epidemics.

• Other viral respiratory pathogens: Respiratory syncytial virus (RSV) causes croup in approximately 5-10% of cases and typically occurs in infants under 12 months of age; RSV croup is often more severe. Influenza A and B viruses cause croup during influenza season and may result in particularly severe disease. Adenovirus (particularly types 1, 2, 3, and 5) and rhinoviruses cause milder, sporadic croup. Enteroviruses and metapneumovirus are emerging causes. Measles historically caused severe croup but is now rare in vaccinated populations. Rarely, SARS-CoV-2 causes croup-like illness, typically mild to moderate.

• Age and developmental factors: Children aged 6 months to 3 years are at highest risk, with peak incidence around age 2 years. This age group has narrow subglottic airways with minimal distensibility and is beginning to encounter respiratory viruses without the protection of maternal antibodies but before developing robust adaptive immunity. Croup becomes increasingly uncommon after age 5 years as subglottic growth increases airway diameter and immune tolerance develops.

• Anatomic predisposing factors: Children with congenital subglottic stenosis, laryngomalacia, or hemangiomas have narrower airways and develop more severe obstruction with croup. Down syndrome and other genetic syndromes associated with abnormal laryngeal anatomy are risk factors. Male sex is associated with approximately 2:1 predominance, likely due to anatomically narrower subglottic airways in boys.

• Immunologic and atopic factors: Atopic children with history of atopic dermatitis, food allergies, or reactive airway disease develop more severe croup and are at higher risk for recurrent episodes. Immunocompromised children, including those with severe combined immunodeficiency (SCID) or those on immunosuppressive therapies, may develop more prolonged disease, though severe croup is not dramatically increased in HIV-infected children on antiretroviral therapy.

• Environmental and seasonal factors: Croup shows clear seasonal variation with peak incidence in fall and early winter months (September-December in Northern Hemisphere), coinciding with parainfluenza virus epidemics. Crowded living conditions, daycare attendance, and exposure to older siblings increase risk through increased viral exposure. Recent exposure to a child with croup is a significant risk factor for development of croup in susceptible younger siblings.

• Recent live viral vaccination: Rarely, live attenuated influenza vaccine (LAIV) and live attenuated rotavirus vaccine have been associated with croup-like illness within 1-2 weeks of vaccination, though this is uncommon and usually mild.

• Barky, seal-like cough: This is the pathognomonic clinical finding that results from the combination of subglottic edema narrowing the air column and increased mucus production stimulating sensory fibers. The cough is characteristic and unmistakable—described as a "seal bark," "dog bark," or "hacking" quality. Unlike the cough of bacterial infection or asthma, the croup cough is typically dry and non-productive initially, though patients may produce mucoid sputum. The cough is often worse at night and may be triggered by agitation or crying, as increased inspiratory effort worsens the stridor and triggers the cough reflex. The barky quality persists throughout the illness (typically 3-7 days) and gradually becomes less harsh as subglottic edema resolves.

• Inspiratory stridor: Stridor is the audible manifestation of turbulent airflow through the narrowed subglottic region. In croup, inspiratory stridor predominates because the negative intrathoracic pressure during inspiration causes dynamic compression of the narrowed subglottic segment. Patients with mild croup may have stridor only with agitation or when crying; stridor at rest indicates more severe disease. The presence of biphasic stridor (both inspiratory and expiratory) or expiratory stridor alone suggests additional involvement of intrathoracic airways (subglottic edema extending into the trachea and bronchi) or suggests alternative diagnoses such as foreign body aspiration or epiglottitis. The intensity of stridor correlates imperfectly with severity of airway obstruction but generally indicates more significant disease.

• Dyspnea and increased work of breathing: Due to the exponential relationship between airway radius and resistance, even modest subglottic edema causes substantial increases in airway resistance. Patients develop compensatory tachypnea and increase the depth and force of respiration (increased tidal volume). In mild cases, increased work of breathing is apparent only with exertion or agitation. In moderate to severe croup, patients demonstrate suprasternal and intercostal retractions at rest as accessory respiratory muscles activate to overcome increased airway resistance. Nasal flaring may be present. Severe cases show subcostal retractions and potential for exhaustion and respiratory failure. Dyspnea worsens with agitation, anxiety, and crying—factors that increase minute ventilation requirements and turbulent airflow through the narrowed segment.

• Hoarseness and voice changes: Inflammation of the vocal cords produces hoarseness that is often described as a "weak" or "muffled" voice quality, distinct from the "hot potato" voice of pharyngitis or epiglottitis. Severe laryngeal involvement can progress to voice loss. Hoarseness typically develops early in the illness and persists as long as vocal cord inflammation remains.

• Low-grade fever and constitutional symptoms: Croup is typically a systemic viral illness with fever that is usually modest (99-102°F or 37.2-38.9°C), though parainfluenza virus and RSV can occasionally cause higher temperatures. The fever pattern is variable but often shows daily variation with higher temperatures in afternoon/evening hours. Associated constitutional symptoms include malaise, myalgias, and decreased appetite. However, young children may appear relatively non-toxic despite significant airway obstruction, which is a distinguishing feature compared to bacterial epiglottitis.

• Prodrome of upper respiratory infection symptoms: Most cases of croup are preceded by 1-3 days of nonspecific upper respiratory infection (URI) symptoms including rhinorrhea, nasal congestion, pharyngitis, and mild cough. This prodrome reflects the fact that croup is part of a generalized viral infection of the respiratory tract, not localized to the larynx. Recognition of this prodrome before the onset of stridor is important for diagnosis.

• Cough quality during sleep versus wakefulness: An important clinical observation is that croup symptoms often improve when children are calm and recur or worsen with agitation, anxiety, or crying. Many parents report that their child sleeps relatively well and appears less symptomatic during sleep when the child is calm and breathing is not turbulent. This pattern helps distinguish croup (which worsens with agitation) from epiglottitis or foreign body aspiration (which may not show this pattern).

• Classic presentation in different severity categories:
• Mild croup: Barky cough, occasional inspiratory stridor with agitation, minimal dyspnea at rest, normal activity level, child plays and drinks normally
• Moderate croup: Barky cough, stridor at rest, mild to moderate retractions, increased respiratory rate, some difficulty with fluids, reduced activity
• Severe croup: Continuous stridor at rest, significant retractions (suprasternal, intercostal, subcostal), substantial dyspnea, difficulty managing secretions, lethargy or agitation, risk of respiratory failure

• Subglottic involvement extending into trachea and bronchi: When inflammation extends beyond the subglottic region into the trachea and lower airways, patients may develop additional findings including biphasic stridor, productive cough with sputum production, and wheezing or decreased breath sounds if significant tracheobronchitis develops. This represents a spectrum toward laryngotracheobronchitis proper (the full form of the disease) and may correlate with more severe systemic viral infection.

• Clinical diagnosis based on presentation and history: Croup is primarily a clinical diagnosis based on the characteristic history of barky cough, inspiratory stridor, recent URI prodrome, and typical age group (6 months-3 years). No laboratory or imaging tests are required to confirm straightforward cases. The combination of a young child with recent URI symptoms, barky cough, and inspiratory stridor in the fall/winter months is virtually pathognomonic for croup. This clinical approach is supported by major guidelines (AAP, IDSA) and is appropriate for the vast majority of cases seen in primary care and emergency department settings.

• Croup severity assessment and scoring systems: Multiple scoring systems exist to stratify disease severity and guide treatment decisions. The most commonly used is the Westley Croup Score, which combines five parameters: stridor (0-5 points based on presence at rest or with agitation), retractions (0-3 points for suprasternal, intercostal, or subcostal retractions), level of consciousness (0-5 points from normal to unconscious), cyanosis (0-5 points), and cry quality (0-2 points for normal to weak/hoarse). Scores range from 0-17; mild croup is typically scored as 0-2, moderate as 3-5, severe as 6-11, and impending respiratory failure as ≥12. While not universally used in practice, understanding this scoring system helps standardize severity assessment in research and educational contexts. For Step 2 CK preparation, recognize that severe croup is defined by rest stridor with retractions or altered mental status, not simply the presence of stridor.

• Lateral neck radiograph findings (if diagnosis uncertain): While not required for typical cases, lateral neck radiography can be helpful in atypical presentations or when diagnosis is uncertain. The classic radiographic finding is the "subglottic narrowing" or "funnel sign" on the anteroposterior (AP) neck film (not lateral), showing loss of the normal subglottic shoulder where the trachea widens below the cricoid cartilage. The narrowing is typically symmetric and concentric. However, subglottic narrowing on radiography correlates poorly with clinical severity and does not change management. Radiographs should not delay treatment in suspected moderate or severe croup. Radiographs are most useful for excluding alternative diagnoses such as epiglottitis (thumbprint sign on lateral view), foreign body aspiration (obvious radiopaque object or air trapping on expiratory film), or bacterial tracheitis (narrowed subglottic region with irregular walls and intraluminal air-fluid levels, though this is rare).

• Upper airway imaging and visualization: Direct visualization of the larynx using laryngoscopy is generally not indicated for straightforward croup because it is unnecessary for diagnosis, is distressing to the child, and risks precipitating acute airway obstruction if the child becomes agitated during the procedure. Laryngoscopy should be reserved for atypical presentations where alternative diagnoses are suspected or when considering intubation in severe cases. On direct visualization, the subglottic region appears edematous and narrowed, with possible mucoid exudate. The vocal cords typically appear normal or mildly inflamed.

• Laboratory testing: No laboratory tests are required for diagnosis of typical croup. Complete blood count, blood cultures, throat cultures, and viral cultures are not indicated for uncomplicated croup. In research settings or for epidemiologic purposes, respiratory virus detection (by RT-PCR, rapid antigen testing, or multiplex PCR panels) can confirm the causative virus, but this does not change management. If croup is atypical or severe, or if the diagnosis is unclear, testing may be pursued to exclude alternative diagnoses. For example, if bacterial superinfection is suspected (high fever, purulent secretions, rapid progression

Immediate priorities

• Keep the child calm: agitation increases inspiratory effort, worsens dynamic subglottic collapse, and can convert compensated obstruction into failure. Examine the child in a parent's lap; defer IVs, throat swabs, and radiographs in moderate–severe disease.
• Oxygen: give blow-by humidified oxygen only for hypoxemia. Cool mist and humidified air are not effective and should not be relied upon — this conclusion rests on a Cochrane review of humidified air in croup and is reflected in the Canadian Paediatric Society position statement, not on a formal AAP croup guideline.

First-line therapy — corticosteroids for every severity

• Glucocorticoids (dexamethasone): a single dose of dexamethasone 0.6 mg/kg PO (IM or IV if vomiting) is standard; oral and parenteral routes are equally effective. Its anti-inflammatory action reduces submucosal edema and capillary permeability in the subglottis, with clinical benefit beginning within a few hours and lasting beyond the drug's biologic half-life, covering the typical illness course. Dexamethasone is recommended even for mild croup (barky cough without rest stridor) — a favorite exam point.
• Nebulized budesonide: an alternative when the child is vomiting and parenteral access is undesirable; no proven advantage otherwise.

Escalation for moderate–severe croup (stridor at rest, retractions)

• Nebulized epinephrine: racemic epinephrine or L-epinephrine produces alpha-adrenergic vasoconstriction of subglottic mucosal vessels, shrinking edema within 10–30 minutes. Effect wanes in roughly 2 hours, so observe for 2–4 hours (commonly cited; the exact interval varies by source) for return of stridor before discharge; a child who remains stable after observation and has received dexamethasone may go home.
• Heliox: low-density helium–oxygen reduces turbulent flow; evidence is limited and it is a temporizing measure only.
• Intubation: reserved for impending or actual respiratory failure (exhaustion, altered mental status, hypoxemia/hypercapnia). Use an endotracheal tube 0.5–1 mm smaller than predicted for age to avoid trauma to the inflamed subglottis.

Avoid

• Antibiotics: no role in viral croup; reserve for suspected bacterial tracheitis or pneumonia.
• Sedatives, opioids, and cough suppressants: mask respiratory deterioration and depress ventilation.
• Routine imaging or laryngoscopy in a distressed child with stridor at rest.

• Respiratory failure (emergency): progressive subglottic obstruction increases work of breathing until respiratory muscles fatigue. The ominous signs are paradoxically diminishing stridor with decreasing air entry, lethargy, and cyanosis — a quiet child with a rising PaCO₂ is deteriorating, not improving. Requires immediate nebulized epinephrine and preparation for intubation.
• Bacterial tracheitis (emergency): bacterial superinfection of virus-damaged tracheal epithelium, most often Staphylococcus aureus, also group A streptococcus, Moraxella, and Haemophilus species. Signalled by high fever, toxic appearance, purulent secretions, and failure to respond to nebulized epinephrine and dexamethasone. Management is airway control with bronchoscopic removal of pseudomembranes plus antistaphylococcal antibiotics; when vancomycin is used for suspected MRSA, the 2020 IDSA/ASHP consensus dosing target is a 24-hour AUC/MIC of 400–600, not a trough goal.
• Post-obstructive (negative-pressure) pulmonary edema: sustained high negative intrathoracic pressure against a narrowed airway increases venous return and transcapillary fluid flux; pink frothy secretions and hypoxemia appear soon after obstruction is relieved or the airway is secured. An emergency requiring positive-pressure support.
• Hypoxemia and pneumonia: extension of viral infection to the lower airways, or secondary bacterial pneumonia; suspect with focal findings, persistent fever, or worsening oxygen requirement.
• Dehydration: tachypnea plus poor oral intake from dyspnea; look for reduced urine output and delayed capillary refill.
• Rebound stridor after nebulized epinephrine: not true rebound worsening but return of baseline edema as the vasoconstrictor effect wanes at roughly 2 hours — the rationale for observation before discharge.
• Post-intubation subglottic stenosis: mucosal injury from a tube in an already inflamed, cricoid-bounded subglottis; presents later as recurrent stridor or exertional noisy breathing.
• Corticosteroid effects: a single dexamethasone dose is well tolerated; transient hyperglycemia and behavioral change may occur, and caution applies in the immunocompromised child or recent varicella exposure.
• Recurrent (spasmodic) croup: repeated episodes should prompt evaluation for fixed subglottic narrowing, hemangioma, or gastroesophageal reflux.

• The stem's buzzwords: a 1–3 year old in the fall with 2 days of rhinorrhea, then a barky, seal-like cough, hoarseness, and inspiratory stridor worse at night and with crying. Low-grade fever, non-toxic appearance, drooling absent.
• The imaging buzzword: steeple sign — symmetric subglottic narrowing on the AP neck film. Contrast with the thumbprint sign on the lateral film in epiglottitis. Imaging is optional and never delays treatment.
• Single best next step in almost every vignette: a single dose of oral dexamethasone, including for mild croup with no stridor at rest. Add nebulized epinephrine when there is stridor at rest or retractions.
• The association examiners test: parainfluenza virus type 1 is the leading cause of croup.
• Two mimics worth separating cleanly:
• Epiglottitis: abrupt stridor with high fever, drooling, tripod posture, muffled "hot potato" voice, and a toxic-appearing child — Haemophilus influenzae type b in the unimmunized, and Staphylococcus aureus, Streptococcus pneumoniae, or group A streptococcus in the immunized.
• Bacterial tracheitis: a croup-like prodrome that turns toxic with high fever, purulent tracheal secretions, and no response to nebulized epinephrine or dexamethasone — most often S. aureus, and unrelated to Hib immunization status.
• Disposition rule: after nebulized epinephrine, observe 2–4 hours (commonly cited; the exact interval varies by source); if stridor at rest has not returned and the child has had dexamethasone, discharge is appropriate. Epinephrine alone is never a discharge plan.
• Deterioration trap: decreasing stridor with decreasing air entry, lethargy, and cyanosis means impending respiratory failure, not recovery.
• Common distractors to avoid: humidified/cool mist air (not effective), antibiotics (no role in viral croup), racemic epinephrine as monotherapy, routine laryngoscopy or blood draws in a distressed child, and albuterol (croup is an upper-airway, not bronchospastic, obstruction).
• Recurrent croup is a red flag: frequently repeated episodes, or croup presenting at an atypical age (<6 months or >6 years), should prompt evaluation for subglottic stenosis, hemangioma, or foreign body rather than another course of steroids.