Definition And Etiology

Core Concepts

Acute bronchiolitis is an acute inflammatory condition of the bronchioles occurring secondary to a viral-induced injury.
The condition primarily affects children under 2 years of age.
The peak incidence is observed between 3 and 6 months of age.
It occurs predominantly during the winter and spring seasons.

Causative Organisms

Respiratory Syncytial Virus (RSV) is the most common causative agent.
RSV is isolated in 75% of cases globally, and 30-70% in Indian studies.
Additional viral agents include rhinovirus, parainfluenza, adenovirus, human metapneumovirus, bocavirus, influenza, and coronavirus (COVID-19).
Mycoplasma pneumoniae is frequently implicated in older children.
Viral co-infections can occur, though their impact on clinical severity remains unclear.

Pathophysiology

Mechanism Of Airway Obstruction

The virus infects the upper respiratory epithelial cells.
These infected cells are subsequently sloughed into the lower respiratory tract.
Inflammation of the bronchiolar mucosa leads to edema and thickening.
Mucus plugs form and cellular debris accumulates within the airways.
Bronchiolar spasm occurs in certain cases.
Peripheral airways contribute up to 50% of total airway resistance in children under 5 years.
Airway resistance is inversely related to the fourth power of the bronchiolar lumen radius.
Even slight narrowing causes a marked reduction in airflow.

Ventilatory Consequences

Airway obstruction increases resistance during both inspiration and expiration.
Bronchioles partially collapse during expiration, causing a reduction in egress airflow.
This dynamic collapse results in air trapping, lung hyperinflation, and expiratory wheezing.
Complete airway obstruction causes resorption of trapped air, leading to atelectasis.
The resulting ventilation-perfusion mismatch produces hypoxemia.
Severe obstructive disease leads to hypercapnia and respiratory acidosis.

Risk Factors For Severe Disease

Host And Environmental Predictors

Category	Associated Risk Factors
Host Factors	Prematurity (<32 weeks gestation), low birth weight.
Age	<6-12 weeks of 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 of breastfeeding carries a three-fold greater risk.

Clinical Manifestations

Symptomatology

The illness begins with a prodrome of coryza lasting 1 to 3 days.
Tachypnea, paroxysmal cough, dyspnea, and irritability develop gradually.
Associated fever is usually low-grade, typically <39°C.
Poor feeding and vomiting typically occur after 3 to 5 days of illness.
Apnea presents early and may occasionally precede lower respiratory signs.
Apnea represents the only presenting feature in some infants <6 weeks of age.
Premature infants <44 weeks post-conceptual age harbor the highest risk for apneic events.

Physical Examination Findings

The chest appears hyper expanded and is hyper-resonant on percussion.
Auscultation reveals prolonged expiration, fine crackles, and wheezes scattered throughout the lungs.
Increased work of breathing is evidenced by nasal flaring and suprasternal, intercostal, or subcostal retractions.
Severe obstruction can eliminate airflow turbulence.
A lack of audible wheezing (silent chest) combined with severe respiratory distress indicates extremely severe disease.

Disease Severity Grading (modified Tal staging)

Score*	Respiratory Rate (<6 months)	Respiratory Rate ($\ge$ 6 months)	Wheezing/crackles	Pulse oximetry in room air (%)†	Use of accessory muscles
0	$\le$ 40	$\le$ 30	None	$\ge$ 95	None
1	41 - 55	31 - 45	Expiration only	92 - 94	Mild intercostal recessions
2	56 - 70	46 - 60	Expiration and inspiration with stethoscope	90 - 91	Moderate intercostal recessions
3	>70	>60	Expiration and inspiration without stethoscope	$\le$ 89	Marked intercostal recessions, tracheal tug or head bobbing

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

Differential Diagnosis

Distinguishing Clinical Features

Condition	Differentiating Characteristics
Bronchial Asthma	Unusual <1 year of age, positive family history, recurrent attacks, consistent bronchodilator response, lacks preceding URI.
Congestive Heart Failure	Cardiomegaly on CXR, tachycardia, large tender liver, raised JVP, basilar lung rales.
Foreign Body Aspiration	Sudden onset, localized wheeze, localized obstructive emphysema or collapse, absence of 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 history of atopy.

Diagnostic Evaluation

Specific Modalities

Acute bronchiolitis is primarily a clinical diagnosis based on age, seasonal occurrence, and typical presentation.
Routine blood investigations and radiology are not indicated for typical cases.
Pulse oximetry is essential for identifying hypoxia and establishing admission requirements.
Chest X-Ray shows hyperinflation, minimal infiltrates, depressed diaphragm, and abnormally translucent lung fields.
Areas of atelectasis on X-Ray can be difficult to distinguish from bacterial pneumonia.
Viral identification (Antigen detection, immunofluorescence, multiplex PCR) does not alter management for the majority of patients.
Viral identification is primarily useful in hospital settings for preventing nosocomial transmission and avoiding antibiotic abuse.
Arterial Blood Gas (ABG) is indicated in severe cases for assessing respiratory failure and hypercapnia.

Management Strategies

Supportive Care Priorities

Management is focused on symptomatic relief, maintaining hydration, and ensuring adequate oxygenation.
Paracetamol is used to control fever.
Nasal block should be cleared using normal saline drops and gentle suctioning.
Nurse the infant propped up, with the head elevated 30 to 40 degrees.
Supplemental humidified oxygen is indicated if SpO2 <90% in infants >6 weeks of age.
Supplemental oxygen is indicated if SpO2 <92% in infants <6 weeks of age or those with underlying health issues.
Intravenous fluids are indicated for impending respiratory failure that prevents oral tolerance.
Orogastric tube feeding is preferred for admitted patients who can tolerate enteral intake.

Advanced Respiratory Support

High-Flow Nasal Cannula (HFNC) is used as a rescue therapy to reduce intensive care requirements.
Continuous Positive Airway Pressure (CPAP) is used for impending respiratory failure management.
Intubation and Mechanical Ventilation are reserved for severe refractory respiratory failure and severe patient exhaustion.

Pharmacotherapy Guidelines

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

Complications And Prognosis

Disease Course

Acute bronchiolitis is a self-limiting illness where symptoms typically subside in 3 to 7 days.
The median ambulatory symptom duration is 14 days.
Approximately 10% of patients remain symptomatic for up to 3 weeks.
The case fatality rate is <1% in developed nations.
Fatalities primarily affect infants possessing complex systemic comorbidities.

Known Complications

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

Prevention And Immunoprophylaxis

General Measures

Ensure strict hand hygiene.
Avoid passive smoking.
Encourage exclusive breastfeeding.

Passive Immunization

Palivizumab is a monoclonal antibody administered intramuscularly (15 mg/kg) monthly during seasonal epidemics.
Palivizumab is administered for a maximum of five doses.
Indications for Palivizumab include infants <12 months possessing prematurity <29 weeks, chronic lung disease of prematurity, or hemodynamically significant congenital heart disease.
Nirsevimab acts as an alternative single-dose prophylaxis providing 5 months of protection.