Viral Croup

Acute laryngotracheobronchitis, an inflammatory narrowing of the subglottis producing the characteristic barking cough and inspiratory stridor. It is the commonest cause of acute upper airway obstruction in children.

Why the subglottis

The subglottis is the narrowest part of the paediatric airway and is the only part completely encircled by rigid cartilage, that is the cricoid ring, so mucosal oedema cannot expand outward and instead encroaches directly on the lumen. In a 1 year old the subglottic diameter is around 5 mm.

Poiseuille law dictates that resistance to laminar flow is inversely proportional to the fourth power of the radius, so 1 mm of circumferential oedema reduces the cross sectional area by around 60 percent and increases resistance approximately 16 fold. This is why a small amount of oedema produces dramatic obstruction in a young child and none at all in an adult, and it is the single physiological fact that explains the entire disease.

Epidemiology and aetiology

• Peak age 6 months to 3 years, with a peak at 18 to 24 months.

• Male predominance of around 1.5 to 1.

• Autumn and early winter seasonality.

• Parainfluenza virus types 1, 2 and 3, causing around 75 percent, with type 1 the commonest.

• Influenza A and B, which cause more severe disease.

• Respiratory syncytial virus, adenovirus, rhinovirus, human metapneumovirus, coronavirus and SARS-CoV-2.

• Measles in unvaccinated populations, which causes severe croup.

Variants

• Spasmodic or recurrent croup: sudden nocturnal onset without a viral prodrome or fever, resolving rapidly, recurring, and associated with atopy and reflux. It is thought to have an allergic or hyperreactive rather than a purely infective basis.

• Bacterial tracheitis, that is bacterial laryngotracheobronchitis, in which a viral croup is complicated by bacterial superinfection with Staphylococcus aureus, Streptococcus pyogenes, Moraxella or Haemophilus, producing thick purulent membranous secretions, high fever, a toxic child and a poor response to adrenaline. It is a distinct and dangerous entity requiring intubation in a high proportion.

Westley croup score, used to grade severity and guide management:

• Level of consciousness: normal 0, disoriented 5.

• Cyanosis: none 0, with agitation 4, at rest 5.

• Stridor: none 0, with agitation 1, at rest 2.

• Air entry: normal 0, decreased 1, markedly decreased 2.

• Retractions: none 0, mild 1, moderate 2, severe 3.

Total score: 0 to 2 mild, 3 to 5 moderate, 6 to 11 severe, 12 or above impending respiratory failure.

• Coryzal prodrome of 12 to 72 hours with low grade fever, rhinorrhoea and mild cough.
• Barking, seal like cough, which is the characteristic feature.
• Inspiratory stridor, initially only with agitation, then at rest as obstruction worsens.
• Hoarseness, since the vocal folds are involved.
• Symptoms characteristically worse at night and worse with crying and agitation.
• Low grade fever, usually below 38.5 degrees Celsius.
• Duration of 3 to 7 days with peak severity on days 2 to 3.

Assessment of severity

• Mild: barking cough, no stridor at rest, no or mild retractions, the child is playful and feeding.
• Moderate: stridor at rest, visible retractions, but no agitation and no significant distress.
• Severe: stridor at rest, marked retractions with tracheal tug and sternal recession, agitation, distress, tachycardia, tachypnoea.
• Impending respiratory failure: fatigue, decreasing stridor which paradoxically indicates reducing air movement rather than improvement, decreased air entry, cyanosis, drowsiness, and a falling respiratory rate. A quiet chest in a tiring child with croup is a preterminal sign and is the finding most often misread as improvement.

Red flags indicating a different diagnosis

• Drooling, absence of cough and a toxic appearance, indicating epiglottitis.
• High fever with a toxic child and poor response to adrenaline, indicating bacterial tracheitis.
• Sudden onset without a prodrome in a child who was previously well, particularly with a witnessed choking episode, indicating an inhaled foreign body.
• Neck stiffness, torticollis and refusal to move the neck, indicating retropharyngeal abscess.
• Age under 6 months or over 6 years, in which croup is less likely and a structural cause should be considered.
• Recurrent croup, particularly more than twice, which requires airway endoscopy to exclude subglottic stenosis, subglottic haemangioma, a laryngeal cleft, papillomatosis or a vascular ring.
• Persistent stridor between episodes.
• Failure to respond to standard treatment.
• Facial or lip swelling and urticaria, indicating anaphylaxis or angioedema.

Clinical. Investigations are unnecessary in typical croup and risk distressing the child, which worsens obstruction.

• Do not examine the throat with a tongue depressor in a child with stridor, and do not perform painful procedures such as cannulation or venepuncture unless the airway has been secured or the child is clearly stable, since agitation precipitates deterioration.
• Pulse oximetry, which is normal until late and which therefore provides false reassurance. Hypoxia in croup is a late sign.
• Anteroposterior neck radiography may show the steeple sign of subglottic narrowing, but it is neither sensitive nor specific, adds nothing to management in typical croup, and requires the child to leave a safe area. Reserve it for atypical presentations where a foreign body or another structural cause is suspected, and only in a stable child.
• Lateral neck radiography where epiglottitis or a retropharyngeal collection is considered, with the same caveats.
• Viral polymerase chain reaction on a nasopharyngeal swab where the result affects infection control or cohorting.
• Full blood count, blood cultures and inflammatory markers where bacterial tracheitis or an alternative diagnosis is suspected.
• Direct laryngotracheobronchoscopy under general anaesthesia for recurrent croup, atypical presentation, failure to respond, suspected foreign body, or suspected structural airway abnormality. This is both diagnostic and therapeutic.

General principles

• Keep the child calm and with the parent. Agitation increases turbulent flow and worsens obstruction, and the single most useful intervention early on is to leave the child undisturbed on the parent's lap.
• Avoid unnecessary examination, cannulation and separation from the parent.
• Position the child as they are comfortable, which is usually sitting upright.
• Humidified air and steam, despite being traditional, have no demonstrated benefit in controlled trials and carry a scald risk. They should not be relied upon.
• Oxygen only where there is hypoxia, delivered in the least distressing way, such as blow by oxygen held by the parent.

Corticosteroids, which are given to every child with croup regardless of severity

• Dexamethasone 0.15 mg/kg orally as a single dose, which is as effective as 0.6 mg/kg and is the dose increasingly used. The traditional 0.6 mg/kg dose, to a maximum of 16 mg, remains widely used and is appropriate for severe disease.
• Onset of clinical benefit within 30 minutes to 2 hours, with the effect lasting 24 to 48 hours given the long half life. A single dose is therefore usually sufficient.
• Oral administration is as effective as intramuscular or intravenous, and is far less distressing. Use the oral route unless the child cannot tolerate it.
• Prednisolone 1 to 2 mg/kg is an alternative but has a shorter duration and higher rates of representation, so a second dose the following day is often needed. Dexamethasone is preferred.
• Nebulised budesonide 2 mg is an alternative where oral administration is impossible, such as in persistent vomiting, and is equally effective but considerably more expensive and requires a mask that may distress the child.
• Corticosteroids reduce symptom severity, hospital admission, length of stay, the need for adrenaline and the rate of return visits. Even mild croup benefits, which is why treatment is universal.

Nebulised adrenaline, for moderate to severe croup

• Nebulised adrenaline 1 in 1000 solution at 0.5 mL/kg to a maximum of 5 mL, or 5 mL of 1 in 1000 undiluted in most protocols. Racemic adrenaline 2.25 percent at 0.05 mL/kg to a maximum of 0.5 mL, diluted in saline, is used where available and is equivalent in effect.
• Acts through alpha adrenergic vasoconstriction of the subglottic mucosa, reducing oedema.
• Onset within 10 to 30 minutes, with peak effect at 30 minutes and duration of around 2 hours.
• The effect wanes, and the child may return to the pretreatment state. This is a return to baseline rather than true rebound, but the practical consequence is the same: any child who receives nebulised adrenaline must be observed for at least 2 to 4 hours afterwards before discharge is considered, and must have received a corticosteroid.
• May be repeated as required. A child requiring repeated doses at short intervals requires admission to a high dependency setting.

Heliox

A helium oxygen mixture reduces the density of the inspired gas, converting turbulent to laminar flow through a narrowed segment and reducing the work of breathing. It is a temporising measure in severe croup while corticosteroids take effect, and it is limited by the fact that it cannot deliver a high inspired oxygen fraction.

Antibiotics

Not indicated in viral croup. Reserve for bacterial tracheitis and for documented secondary bacterial infection.

Bacterial tracheitis

• Suspect in a child with croup like features who is toxic, has a high fever, and responds poorly to nebulised adrenaline.
• Admit to intensive care. Intubation is required in a high proportion, both to secure the airway and to permit removal of the thick purulent membranous secretions, which obstruct the tube and require repeated suction and sometimes rigid bronchoscopic clearance.
• Intravenous ceftriaxone 50 mg/kg every 12 to 24 hours plus flucloxacillin 25 mg/kg every 6 hours, or vancomycin where methicillin resistant Staphylococcus aureus is a concern, adjusted to culture.
• Rigid bronchoscopy for diagnosis, culture and clearance of membranes.

Indications for admission

• Moderate or severe croup, or a Westley score of 3 or above.
• Stridor at rest persisting after treatment.
• Requirement for nebulised adrenaline, with observation of 2 to 4 hours minimum before considering discharge.
• Hypoxia.
• Poor oral intake or dehydration.
• Age under 6 months.
• Significant comorbidity: prematurity, chronic lung disease, congenital heart disease, neuromuscular disease, Down syndrome, or a known airway abnormality.
• Toxic appearance or diagnostic uncertainty.
• Social factors, distance from hospital, or parental inability to return, which are legitimate and frequently decisive.
• Represented after an earlier assessment in the same illness.

Intubation

Required in around 1 percent of hospitalised children. Perform in theatre or intensive care by the most experienced operator available, using a tube 0.5 to 1 mm smaller than predicted for age to avoid further subglottic trauma. Extubation is guided by the development of an audible leak around the tube, usually at 2 to 5 days. Prolonged intubation and the use of an oversized tube are the principal causes of acquired subglottic stenosis, which is why tube size selection matters.

Discharge and safety netting

• Discharge when there is no stridor at rest, normal oxygen saturation, adequate oral intake, and a period of observation after any adrenaline.
• Explain the natural course: symptoms are worse at night and typically last 3 to 7 days.
• Advise return immediately for stridor at rest, difficulty breathing, chest recession, drooling, poor colour, unusual drowsiness or reduced fluid intake.
• Advise against steam inhalation over hot water, given scald injuries.
• No antibiotics are needed and this should be stated explicitly, since parents commonly expect them.

Recurrent croup

More than two episodes, or croup outside the usual age range, requires flexible and rigid airway endoscopy to exclude subglottic stenosis, subglottic haemangioma, a laryngeal cleft, recurrent respiratory papillomatosis, a vascular ring or tracheomalacia. Assess for reflux and atopy, which are associated with spasmodic croup.