Acute Bronchiolitis

Definition And Etiology

Core Concepts

• Acute inflammatory condition bronchioles secondary viral-induced injury.
• Primarily affects children <2 years age.
• Peak incidence between 3 and 6 months age.
• Occurs predominantly winter and spring seasons.

Causative Organisms

• Respiratory Syncytial Virus (RSV) most common agent, isolated 75% cases (30-70% Indian studies).
• Additional viral agents: Rhinovirus, parainfluenza, adenovirus, human metapneumovirus, bocavirus, influenza, coronavirus (COVID-19).
• Mycoplasma pneumoniae frequently implicated older children.
• Viral co-infections occur; impact clinical severity remains unclear.

Risk Factors

Host And Environmental Predictors Severe Disease

Category	Associated Risk Factors
Host Factors	Prematurity (<32 weeks gestation), low birth weight.
Age	<6-12 weeks age.
Comorbidities	Chronic lung disease (bronchopulmonary dysplasia).
Cardiac	Hemodynamically significant congenital heart disease, moderate-severe pulmonary hypertension, cyanotic heart disease.
Systemic	Immunodeficiency, neuromuscular disorders.
Environmental	Older siblings, household crowding, child care attendance.
Social/Toxic	Passive smoke exposure, lower socioeconomic status.
Nutritional	Lack breastfeeding (three-fold greater risk).

Pathophysiology

Mechanism Airway Obstruction

• Virus infects upper respiratory epithelial cells subsequently sloughed lower respiratory tract.
• Inflammation bronchiolar mucosa leads edema, thickening, mucus plugs formation, cellular debris accumulation.
• Bronchiolar spasm occurs certain cases.
• Peripheral airways contribute up 50% total airway resistance children <5 years.
• Airway resistance inversely related fourth power bronchiolar lumen radius; slight narrowing causes marked airflow reduction.

Ventilatory Consequences

• Obstruction increases resistance during inspiration, expiration.
• Bronchioles partially collapse during expiration causing egress airflow reduction.
• Results air trapping, lung hyperinflation, expiratory wheezing.
• Complete obstruction causes trapped air resorption leading atelectasis.
• Ventilation-perfusion mismatch produces hypoxemia.
• Severe obstructive disease leads hypercapnia, respiratory acidosis.

Clinical Manifestations

Symptomatology

• Prodrome coryza lasting 1-3 days.
• Tachypnea, paroxysmal cough, dyspnea, irritability develop gradually.
• Associated fever usually <39°C.
• Poor feeding, vomiting occur typically after 3-5 days illness.
• Apnea presents early, occasionally preceding lower respiratory signs.
• Apnea represents only presenting feature infants <6 weeks age.
• Premature infants <44 weeks postconceptual age harbor highest apneic event risk.

Physical Examination

• Chest appears hyperexpanded, hyper-resonant percussion.
• Auscultation reveals prolonged expiration, fine crackles, wheezes throughout lungs.
• Increased work breathing: Nasal flaring, suprasternal/intercostal/subcostal retractions.
• Severe obstruction eliminates turbulence; lack audible wheezing (silent chest) combined respiratory distress indicates severe disease.

Disease Severity Grading

Clinical Classification

Severity	Feeding	Respiratory Distress	Oxygen Saturation
Mild	Normal ability feed	Little/no respiratory distress	>92% room air.
Moderate	Reluctant/short breath feeding	Moderate distress, retractions, nasal flaring, +/- apnea	<92% room air, correctable supplemental O2.
Severe	Unable feed	Severe distress, marked retractions, grunting, frequent prolonged apnea	<92% room air, may/may not correct supplemental O2.

Differential Diagnosis

Distinguishing Clinical Features

Condition	Differentiating Characteristics
Bronchial Asthma	Unusual <1 year age, positive family history, recurrent attacks, consistent bronchodilator response, lacks preceding URI.
Congestive Heart Failure	Cardiomegaly CXR, tachycardia, large tender liver, raised JVP, basilar lung rales.
Foreign Body Aspiration	Sudden onset, localized wheeze, localized obstructive emphysema/collapse, absence infectious prodrome.
Bacterial Pneumonia	High fever (>39°C), pronounced adventitious sounds, focal crackles, severe toxemia.
Episodic Viral Wheeze	Persistent wheeze lacking crackles, recurrent episodes, family atopy history.

Evaluation And Investigations

Diagnostic Approach

• Primarily clinical diagnosis based age, seasonal occurrence, typical presentation.
• Routine blood investigations, radiology not indicated.
• Pulse oximetry essential identifying hypoxia, establishing admission requirement.

Specific Modalities

• Chest X-Ray: Hyperinflation, minimal infiltrates, depressed diaphragm, abnormally translucent lung fields. Areas atelectasis difficult distinguish bacterial pneumonia.
• Viral Identification: Antigen detection, immunofluorescence, multiplex PCR respiratory secretions. Does not alter management majority patients. Useful hospital settings preventing nosocomial transmission, avoiding antibiotic abuse.
• Arterial Blood Gas: Indicated severe cases assessing respiratory failure, hypercapnia.

Management

Supportive Care Priorities

• Focused symptomatic relief, maintaining hydration, oxygenation.
• Paracetamol controls fever.
• Nasal block cleared normal saline drops, gentle suctioning.
• Nurse infant propped up/head elevated 30-40 degrees.
• Supplemental humidified oxygen indicated SpO2 <90% (>6 weeks age) or <92% (<6 weeks age/underlying health issues).
• Intravenous fluids indicated impending respiratory failure preventing oral tolerance.
• Orogastric tube feeding preferred admitted patients tolerating enteral intake.

Advanced Respiratory Support

• High-Flow Nasal Cannula (HFNC): Rescue therapy reducing intensive care requirement.
• Continuous Positive Airway Pressure (CPAP): Impending respiratory failure management.
• Intubation/Mechanical Ventilation: Severe refractory respiratory failure, exhaustion.

Pharmacotherapy Guidelines

Intervention	Recommendation Status	Clinical Context
Nebulized Hypertonic Saline	Limited role	Considered children hospitalized >3 days.
Nebulized Adrenaline	Rescue medication	0.1-0.3 mL/kg/dose (1:1,000); inconsistent short-lived improvement.
Beta-Agonists	Optional single trial	Discontinue lacking objective clinical response.
Antibiotics	NOT recommended	Strictly reserved documented secondary bacterial infection.
Systemic/Inhaled Steroids	NOT recommended	Lack outcome impact.
Chest Physiotherapy	NOT recommended	Exacerbates distress.
Ribavirin	NOT recommended	Minimal impact, high toxicity, challenging administration.

Complications And Prognosis

Disease Course

• Self-limiting illness, symptoms typically subside 3-7 days.
• Median ambulatory symptom duration 14 days; 10% symptomatic 3 weeks.
• Case fatality <1% developed nations, primarily affecting infants possessing complex comorbidities.

Known Complications

• Acute Respiratory Distress Syndrome (ARDS).
• Congestive heart failure, myocarditis, arrhythmias.
• Secondary bacterial infection, acute otitis media.
• Bronchiolitis obliterans.
• Predisposition childhood asthma (relationship observed 25% cases).

Prevention

General And Immunoprophylaxis

• Strict hand hygiene.
• Avoidance passive smoking.
• Encourage breastfeeding.
• Palivizumab: Monoclonal antibody administered intramuscularly (15 mg/kg) monthly during seasonal epidemics. Maximum five doses.
• Palivizumab Indications: Infants <12 months possessing prematurity <29 weeks, chronic lung disease prematurity, hemodynamically significant congenital heart disease.
• Nirsevimab: Single dose providing 5 months protection.