Bronchiolitis is an inflammation of the bronchioles of the lung, most of only due to viral infection.
It is the common lower respiratory tract infection (LRTI) in the first year of life with 90% of cases occurring from 1-9 months of age. Peak incidence is between 3-6 months old. It is, however, rare in children over 2 years old. It is more common in the winter months.
Children who develop bronchiolitis are also more likely to have a viral wheeze during their childhood.
Pathophysiology
The bronchioles are the airways in the lungs that are <2mm wide, and are absent of cartilage and absent of submucosal glands. They begin after the segmental bronchi. First we have terminal bronchioles (which mark the end of the conducting division of the airways) and then we have the respiratory bronchioles (which mark the beginning of the respiratory division of the airways).
The majority of cases of bronchiolitis are caused by respiratory syncytial virus (RSV). Less commonly it may be due to adenoviruses and mycoplasma or it may be secondary to bacterial infection. Adenovirus is more commonly associated with bronchiolitis obliterans.
It begins as an upper respiratory tract infection (URTI) before spreading to the lower respiratory tract after 1-3 days. The virus infects the epithelial cells and goblet cells of the bronchioles, leading to inflammation, smooth muscle constriction, necrosis, oedema and mucus secretion. Neutrophils are the inflammatory cells that invade in severe cases of RSV and lead the inflammatory processes.
It is the combination of cellular destruction and inflammation that leads to obstruction of the bronchioles. This obstruction causes subsequent hyperinflation, atelectasis, wheezing and ventilation-perfusion mismatch.
It takes about 2 weeks before the epithelial cells of the bronchioles begin to regrow. This recovery process takes 4-8 weeks to complete.
Respiratory viral infections in adults also lead o swelling and mucus production, similar to what we see in children. However, the key difference is that the relative effect on the lumen of the airway is much smaller considering the lumen is much wider in adults compared to children. The airways of infants are small to begin with and a small amount of inflammation and mucus creates a relatively larger effect. This greatly impeded the ability of the infant to move air in and out of the alveoli.
⚠️ Risk factors
- Formula fed baby or breastfed for less than 2 months - this is because the IgG found in breast milk is protective against RSV.
- Smoke exposure
- Siblings in nursery or school - increases the exposure risk.
- Chronic lung disease (bronchopulmonary dysplasia)
- Reduced lung function at birth
- Immunodeficiency
- Congenital heart disease that is haemodynamically significant
- <3 months old.
😷 Presentation
Initially it may present similar to any mild URTI - with coryzal symptoms.
Coryzal symptoms include:
- Low grade fever
- Rhinorrhoea
- Blocked nose
- Watery eyes
After 1-2 days is when we start noticing more severe symptoms typical of bronchiolitis:
💡 These symptoms are worst on days 3-4.
- Feeding difficulties
- Dry cough
- Dyspnoea
- Wheezing
- Grunting - this represents breathing against a partially closed glottis. It increases the positive end expiratory pressure (PEEP). This is the pressure which the alveoli have at the end of expiration to remain open. A raised PEEP results in less air being expired out of the alveoli. Grunting is the sound that is produced when the glottis close to prevent the functional residual capacity from dropping too low and causing alveolar atelectasis.
- Fine crackles on auscultation.
- Tachypnoea
- Accessory muscle usage - accessory muscles include the sternocleidomastoid, abdominal muscles and intercostal muscles.
- Suprasternal, intercostal and subcostal recession
- See-sawing - this is when there is paradoxical breathing (chest pulling inwards and abdomen being pushed out when inspiring).
- Nasal flaring
- Tracheal tugging
- Central cyanosis
- Abnormal airway noises
- Tripoding
Symptoms usually last 7-10 days and most children recover within 2-3 weeks.
🔍 Investigations
⭐️ It is a clinical diagnosis.
If there is diagnostic doubt or if the child is very unwell we can do some tests:
- Nasopharyngeal aspirate - to test for RSV and to perform a viral culture.
- Throat swabs - to test for RSV and to perform a viral culture.
- Blood and urine cultures - if we are suspecting sepsis.
- FBC
- ABG - if there signs of respiratory distress and respiratory failure.
- Chest X-ray (CXR) - it is not routinely done but it may show hyperinflation, focal atelectasis, a flattened diaphragm and peribronchial cuffing (bronchial wall thickening).
- PEWS score
🧰 Management
We first have to see whether the child needs be managed at home or needs admission into hospital.
An urgent referral should be made if we see anything of the following:
- Apnoea
- Severe respiratory distress (consistent with the features mentioned above)
- Central cyanosis
- Oxygen saturations <92%
- Child looks seriously unwell
If any of the following are seen, we can consider a hospital referral:
- Respiratory rate >60
- Inadequate fluid intake (50-75% of the usual volume) or clinical dehydration
- If the oxygen saturations are ≤92%:
- High-flow humidified oxygen via a nasal cannula. This helps deliver air and oxygen with additional pressure to prevent alveolar atelectasis.
- Continuous positive airway pressure (CPAP) - if there is impending respiratory failure.
- Intubation and mechanical ventilation - if the child begins to develop respiratory acidosis or a rising pCO2
- Ensure adequate feeding - this can be orally or via a nasogastric/orogastric tube or even with IV fluids depending on the severity. It is important to avoid over feeding the child as a full stomach can restrict breathing.
- Upper airway suctioning - if there are upper airway secretions or apnoea.
- Ribavirin - this is an antiviral that may be considered in severe cases.
- Clinically stable for 24 hours
- Are taking adequate oral fluids
- Maintained oxygen saturations at the following levels for 4 hours, including a period of sleep:
- ≥90% for children ≥6 weeks old.
- ≥92% for babies ≤6 weeks or any age with an underlying health condition.
- Palivizumab - a monoclonal antibody targeting RSV. It works by targeting the fusion protein that RSV normally needs to bind to allow it to bind to the host cell. Palivizumab binding to the fusion protein disables the ability of RSV binding to it and therefore leads to an inability to bind to the host cell.
- Ex-premature babies
- Congenital heart disease
It is given as a prophylaxis to high risk babies. High risk babies are babies that are/have:
It provides passive immunity by circulating the body until the virus is encountered. It does not stimulate production of the antibody by our own immune system and its levels decrease over time. For this reason we need to give it as a monthly injection.
🚨 Complications
- Hypoxia
- Dehydration
- Fatigue
- Respiratory faiure
- Persistent cough or wheeze - parents should be notified that this is very common and it may continue for several weeks.
- Bronchiolitis obliterans
Also known as popcorn lung or obliterative bronchiolitis.
It occurs when there is injury to the bronchioles due to infection or inhalation of harmful substances which causes overactive cellular repair and scar tissue build-up. The scar tissue obstructs the bronchioles and prevents oxygen transfer. Scarring and narrowing of bronchioles continues to worsen over time which can potentially lead to respiratory failure. This narrowing is permanent
It is particularly common in lung transplant patients as about 50% of patients who receive lung transplant develop bronchiolitis obliterans within 5 years of transplant due to organ rejection.
Outcomes are poor and most individuals dying in months-years.
⭐️ It is more commonly associated with adenovirus.
