Acute bronchiolitis

Etiology and pathophysiology

The disease is primarily of viral origin. In addition to the main causative agents (RSV and rhinovirus), the etiology may involve metapneumovirus, influenza and parainfluenza viruses, adenovirus, coronavirus, bocavirus, and the atypical pathogen Mycoplasma pneumoniae. In severe cases, a mixed infection with two or more viruses is detected in approximately one-third of patients.

The development of pronounced respiratory failure is due to the anatomical and physiological characteristics of infants. Their airways are very narrow, so even minor wall thickening leads to diffuse airflow obstruction.

Mechanism of obstruction is based on three factors:

• Bronchiolar wall edema.
• Epithelial layer necrosis.
• Impaired mucociliary clearance (accumulation of secretions with inflammatory cells and desquamated epithelium in the lumen).

Impaired airflow occurs via a valvular mechanism. Airway resistance increases during both phases of respiration, but during expiration, the radius of the bronchioles physiologically decreases, leading to air trapping in the lungs (hyperinflation). The fundamental physiological shift in moderate and severe bronchiolitis is hypoxia, accompanied by hypercapnia in extremely severe cases.

Clinical presentation

The disease debuts as a typical mild-to-moderate acute viral respiratory infection with body temperature rising to 38.5–39 °C. However, within a few hours, the clinical picture changes rapidly and the fully developed stage sets in.

Main clinical manifestations:

• Progressive respiratory failure and dyspnea (expiratory or mixed in character).
• Minimally productive cough.
• Perioral cyanosis and a drop in oxygen saturation level.
• Episodes of apnea, refusal to feed, and signs of dehydration.

Auscultation reveals abundant fine moist and crepitant rales. Sometimes they can be heard even at a distance (transmitted rales).

Diagnostics and differential search

The diagnosis is established clinically. It is based on a combination of signs of a current respiratory infection and severe obstructive syndrome in a child under 2 years of age.

Complete blood count usually shows no specific abnormalities. Chest X-ray visualizes enhanced vascular markings and signs of generalized hyperinflation of the lung fields.

The main disease for differential diagnosis is bronchial asthma. Unlike acute bronchiolitis, asthma is characterized by recurrent obstruction, eosinophilia, a family history of allergic diseases, and a rapid response to inhaled sympathomimetics. The condition must also be differentiated from foreign body aspiration, pneumonia, pertussis, and heart failure.

Treatment approaches

Absolute indications for admission to the pediatric intensive care unit include all children under 1 year of age, as well as patients older than one year with a severe course of the disease.

Main areas of therapy:

1. Oxygen therapy: a baseline requirement for most patients. Target saturation should be maintained at no less than 92%. In severe respiratory failure, mechanical ventilation is indicated.
2. Rehydration: fluid and electrolyte correction often requires parenteral administration of solutions.
3. Bronchodilator therapy: nebulized inhalations (salbutamol, fenoterol combined with ipratropium bromide) are prescribed.
4. Antibiotic therapy: is not routinely indicated and is used exclusively when bacterial superinfection is proven.

With timely initiation of treatment, the prognosis is favorable, with a mortality rate of less than 1%.

Bronchiolitis obliterans

A distinct chronic form of small airway pathology is bronchiolitis obliterans. This disease involves concentric narrowing up to complete obliteration of the bronchioles and arterioles with intact alveoli.

This leads to local perfusion disturbances and the development of emphysema. Clinically, the condition presents with persistent irreversible obstruction, dyspnea, minimally productive cough, and auscultatorily with crepitation and fine rales. Instrumental confirmation includes radiography (mosaic pattern of lung markings, "air trapping") and lung scintigraphy (areas of impaired blood flow).

Class and exam questions

What else to study on this topic

• Pathophysiology of the valvular mechanism and "air trapping" formation
• Features of respiratory support and criteria for mechanical ventilation
• Specifics of respiratory viral co-infection and atypical microflora
• Radiological and scintigraphic findings in bronchiolitis obliterans

Other topics in "Chapter 4. Respiratory Diseases"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.