Bronchiolitis overview

# Overview
Bronchiolitis is the most common lower respiratory tract infection in pediatric patients between 1 month and 2 years of age.  It is usually caused by the respiratory syncytial virus (RSV) and is characterized by inflammation, edema, and necrosis of the bronchiole's epithelium. It is classified according to histological features as bronchiolitis obliterans, proliferative bronchiolitis, diffuse panbronchiolitis, or respiratory bronchiolitis. The bronchiolitis severity score is used to classify bronchiolitis into 4 classes. Typical clinical manifestations include rhinitis, cough, wheezing, respiratory rales (crackles), use of respiratory accessory muscles, and/or nasal flaring. In adults, common risk factors in the development of bronchiolitis include exposure to cigarette smoke, living in crowded areas, and being immunocompromised. In infants, the risk factors include age < 6 months, lack of breastfeeding, prematurity, and having congenital heart diseases. The mainstay of treatment of bronchiolitis is supportive therapy.

# Historical Perspective
Bronchiolitis was first reported in 1899 when it was discovered by researchers at the University of Minnesota. The disease was fully described in 1901 by Dr. Lange.

# Classification
Bronchiolitis should be classified in order to understand how it may occur and the clinical manifestations that may be observed. Bronchiolitis is classified based on the patient's age and the different histological forms of the disease. According to age, bronchiolitis is classified as either adult or infant. Based on the different histological features, it can be classified as acute infectious bronchiolitis, bronchiolitis obliterans, proliferative bronchiolitis, diffuse panbronchiolitis, or respiratory bronchiolitis. Based on the Bronchiolitis Severity Score (BSS), bronchiolitis is classified into 4 classes.

# Pathophysiology
Bronchiolitis is transmitted by air droplets. It is caused by RSV, which infects the nasopharyngeal mucosa. After the infection, the virus spreads to the lower airway tracts until it reaches the bronchioles, where viral replication takes place. The viral infection induces inflammation, which leads to edema and necrosis of the bronchiolar epithelium. Cough reflex occurs due to exposure of the subepithelial tissue and nerve fibers. Vascular permeability increases, leading to edema and swelling. Histopathologically, bronchiolitis obliterans shows intraluminal polyps, inflammatory infiltration, and macrophages. Constrictive bronchiolitis shows thickening of the airways and interluminal narrowing.

# Causes
Bronchiolitis usually affects children under the age of 2, with a peak age of 3 - 6 months. Bronchiolitis is a common disease in children and sometimes causes severe illness. Respiratory syncytial virus (RSV) is the most common cause of bronchiolitis. Other viruses that can cause bronchiolitis include adenovirus, influenza, and parainfluenza. It may be caused by bacterial organisms like Legionella pneumophilia and Mycoplasma pneumonia. Other noninfectious causes include smoking, collagen vascular disease, and being post bone marrow transplant.

# Differentiating Bronchiolitis from Other Diseases
Bronchiolitis must be differentiated from other respiratory and cardiac diseases that present with similar clinical manifestations. Based on cough and dyspnea, bronchiolitis should be differentiated from asthma, COPD, pneumonia, congestive heart failure, diffuse idiopathic neuroendocrine cell hyperplasia, tuberculosis, pertussis, foreign body aspiration, pulmonary embolism, and Harman-Rich syndrome.

# Epidemiology and Demographics
Bronchiolitis is one of the most common acute respiratory diseases that affects infants and children. Bronchiolitis affects around 3,000 per 100,000 children in the United States. It occurs mostly during fall, winter, and early spring. Bronchiolitis can affect any age group, but mostly affects infants, especially those under 2 years. Bronchiolitis occurs more often in boys than girls and is more common among Native Americans, Alaskans, and Hispanics. Bronchiolitis has a low mortality rate despite the high number of hospitalizations associated with the illness.

# Risk Factors
Bronchiolitis has a different range of risk factors that can be differentiated based on the age. In adults, common risk factors in the development of bronchiolitis include exposure to cigarette smoke, living in crowded areas, and being immunocompromised. In infants, the risk factors include age < 6 months, lack of breastfeeding, prematurity, and having congenital heart diseases.

# Natural History, Complications, and Prognosis
If left untreated, in the first 2-3 days, a patient with bronchiolitis presents with mild upper respiratory symptoms, shortness of breath, wheezing, persistent prominent cough, and tachypnea. Chest wall retraction and nasal flaring usually develop between the third and seventh day. Symptoms gradually disappear within the next 2 weeks. Complications are usually observed among patients younger than 2 months of age, premature infants, and patients with other medical conditions (congenital heart disease, chronic pulmonary disease, and immunodeficiencies). Severity scores can be used to estimate the prognosis.

# Diagnosis

## History and Symptoms
Common symptoms of bronchiolitis include fever, cough, dyspnea, and Nasal discharge. Other symptoms include post tussive vomiting and dehydration.

## Physical Examination
Patients infected with bronchiolitis have a toxic appearance and may be cyanotic. Fever is one of the signs of the disease, but a lack of it does not exclude the diagnosis. Lung examination shows abnormalities in inspection and auscultation. On inspection, intercostal and substernal retractions can be observed. On auscultation, wheezing and crackles can be clearly heard with a decrease in respiratory sounds. Extrapulmonary manifestations can occur as well, including pharyngitis, conjunctivitis, arrhythmias, tachycardia, and seizures.

## Laboratory Findings
Bronchiolitis diagnosis depends mainly on the symptoms and physical examination, as the laboratory diagnosis is not specific for the disease. Commonly used lab tests include viral pathogen tests like ELISA, immunofluorescent assays, and optical immunoassays. Complete blood count is also not specific for bronchiolitis. Pulmonary function tests may be helpful in supporting the diagnosis and excluding other obstructive lung diseases.

## X-ray
Chest X-ray in cases of bronchiolitis is usually nonspecific and may be inefficient for differentiating bronchiolitis from other lower respiratory tract infections. A chest X-ray may show atelectasis and consolidations. It is also used in excluding other medical conditions like pneumonia.

## CT
CT scan shows nonspecific findings that can be found in other diseases. These findings are centrilobular nodules, bronchiolar wall thickening, ground glass appearance, and parenchymal cysts.

## MRI
There are no MRI findings associated with bronchiolitis.

## Other Imaging Findings
There are no additional imaging findings for bronchiolitis.

## Other Diagnostic Studies
There are no additional diagnostic findings for bronchiolitis.

# Treatment

## Medical Therapy
The predominant therapy for bronchiolitis is providing supportive measures. Supportive therapy includes frequent, small feeding and oxygen therapy. In severe cases, infants may require intravenous fluids and food via a nasogastric tube. In extreme cases, mechanical ventilation or the use of continuous positive airway pressure (CPAP) might be necessary. Prophylaxis is indicated in infants with hemodynamically significant heart disease and preterm infants who require >21% oxygen for at least the first 28 days of life. The drug of choice for prophylaxis is palivizumab.

## Surgery
Surgical intervention is not recommended for the management of bronchiolitis.

## Primary Prevention
Effective measures for the primary prevention of bronchiolitis include washing hands, avoiding contact with patients with symptomatic respiratory infections, and prevention of tobacco smoke exposure. These preventive measures are to prevent viral dissemination during the RSV season. In patients with a high risk of developing severe infection, passive immunization with palivizumab is recommended.

## Secondary Prevention
There are no secondary preventive measures available for bronchiolitis.