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Bronchiolitis in infants and children: Clinical features and diagnosis

Bronchiolitis in infants and children: Clinical features and diagnosis
Author:
Pedro A Piedra, MD
Section Editor:
Morven S Edwards, MD
Deputy Editor:
Diane Blake, MD
Literature review current through: May 2024.
This topic last updated: Oct 16, 2023.

INTRODUCTION — Bronchiolitis is a major cause of illness and hospitalization in infants and young children. The virology, epidemiology, clinical features, and diagnosis of bronchiolitis will be presented here.

Related topics include:

(See "Bronchiolitis in infants and children: Treatment, outcome, and prevention".)

(See "Respiratory syncytial virus infection: Clinical features and diagnosis in infants and children".)

(See "Respiratory syncytial virus infection: Treatment in infants and children".)

(See "Respiratory syncytial virus infection: Prevention in infants and children".)

(See "Acute respiratory distress in children: Emergency evaluation and initial stabilization".)

(See "Evaluation of wheezing in infants and children".)

(See "Treatment of recurrent virus-induced wheezing in young children".)

DEFINITION — For the purposes of this topic review, bronchiolitis is broadly defined as a clinical syndrome of respiratory distress that occurs in children <2 years of age [1]. It is characterized by upper respiratory symptoms (eg, rhinorrhea) followed by lower respiratory tract signs (eg, wheezing and/or crackles). Bronchiolitis is caused by viruses, most commonly respiratory syncytial virus (RSV). In young children, the clinical syndrome of bronchiolitis may overlap with recurrent virus-induced wheezing and acute viral-triggered asthma. (See "Role of viruses in wheezing and asthma: An overview".)

For clinical research, bronchiolitis is typically defined as the first episode of wheezing in a child younger than 12 to 24 months who has physical findings of a viral lower respiratory infection and no other explanation for the wheezing.

PATHOGENESIS — Bronchiolitis occurs when viruses infect the terminal bronchiolar epithelial cells, causing direct damage and inflammation in the small bronchi and bronchioles. Edema, excessive mucus, and sloughed epithelial cells lead to obstruction of small airways and atelectasis. Based upon biopsy or autopsy samples in severe cases and animal studies, pathologic changes begin 18 to 24 hours after infection and include bronchiolar cell necrosis, ciliary disruption, and peribronchiolar lymphocytic infiltration [2-4].

VIRAL ETIOLOGY — Bronchiolitis is caused by viruses. The relative frequencies of different specific viral etiologies vary by season and from year to year. Respiratory syncytial virus (RSV) is generally the most common cause, followed by rhinovirus [5-8]. Less common causes include parainfluenza virus, human metapneumovirus, influenza virus, adenovirus, coronaviruses (including COVID-19), and human bocavirus [5,9-11]. With molecular diagnostics, a viral etiology can be identified in >95 percent of cases; two or more viruses are detected in approximately one-third of young children hospitalized with bronchiolitis [12-15].

RSV – RSV is the most common cause of bronchiolitis and the virus most often detected as the sole pathogen. RSV is ubiquitous throughout the world and causes seasonal outbreaks. In temperate climates, late fall and winter epidemics of bronchiolitis usually are linked to RSV. In tropical and semitropical climates, the seasonal outbreaks usually are associated with the rainy season. (See "Respiratory syncytial virus infection: Clinical features and diagnosis in infants and children".)

Rhinovirus – Human rhinoviruses are the main cause of the common cold. There are more than 170 serotypes. Rhinovirus is associated with lower respiratory tract infection in young children and in individuals with chronic pulmonary disease [16]. Dual viral infections are often detected. Rhinovirus is often associated with bronchiolitis in the spring and fall [8,17]. (See "Epidemiology, clinical manifestations, and pathogenesis of rhinovirus infections".)

Parainfluenza virus – Parainfluenza virus type 3, which is associated with epidemics in early spring and fall, is another cause of bronchiolitis. Parainfluenza virus types 1 and 2 also can cause bronchiolitis, although croup is the more common presentation [18]. (See "Parainfluenza viruses in children", section on 'Clinical presentation'.)

Human metapneumovirus – Human metapneumovirus sometimes occurs in conjunction with other viral infections and has been identified as an etiology of bronchiolitis and pneumonia in children [19,20]. In two multicenter cohort studies of infants hospitalized with bronchiolitis, human metapneumovirus peaked in March and April [8]. (See "Human metapneumovirus infections".)

Influenza virus – The lower respiratory tract manifestations of influenza are clinically indistinguishable from those due to RSV or parainfluenza viral infections. (See "Seasonal influenza in children: Clinical features and diagnosis", section on 'Clinical features'.)

Adenovirus – Adenovirus may cause lower respiratory tract infections, including bronchiolitis, bronchiolitis obliterans, and pneumonia, though it more typically causes pharyngitis and coryza. Adenovirus can also infect other organs, causing disseminated disease. (See "Pathogenesis, epidemiology, and clinical manifestations of adenovirus infection", section on 'Clinical presentation'.)

Coronavirus

Endemic human coronaviruses typically cause the common cold but also can cause lower respiratory tract infection, including bronchiolitis, throughout the year [8,21,22]. Before the emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), most cases of coronavirus-related bronchiolitis were coinfections with RSV [23].

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) may cause bronchiolitis and other clinical syndromes. (See "COVID-19: Clinical manifestations and diagnosis in children", section on 'Clinical manifestations'.)

Other coronaviruses cause severe acute respiratory syndrome (SARS-CoV-1) and Middle East respiratory syndrome (MERS-CoV). (See "Middle East respiratory syndrome coronavirus: Clinical manifestations and diagnosis", section on 'Clinical features'.)

Human bocavirus – Human bocavirus 1 causes upper and lower respiratory infections during the fall and winter months [9,24-26]. Bronchiolitis and pertussis-like illness can occur. Human bocavirus 2 through 4 are primarily enteric viruses [27].

The clinical syndrome of bronchiolitis can overlap with the clinical findings of atypical bacterial pneumonia (due to Mycoplasma pneumoniae) and pertussis infection. However, these etiologies are far less common causes of respiratory distress in infants and young children. (See "Mycoplasma pneumoniae infection in children", section on 'Clinical manifestations' and "Pertussis infection in infants and children: Clinical features and diagnosis", section on 'Clinical features'.)

EPIDEMIOLOGY — Bronchiolitis typically affects infants and children younger than two years. It occurs principally during the fall and winter (October through March, in the northern hemisphere) [28]. Bronchiolitis is a leading cause of hospitalization in infants and young children [28-30]. Rates of bronchiolitis hospitalization peak between two and six months of age, but it remains a significant cause of respiratory illness during early childhood [29,31].

The epidemiology of bronchiolitis is similar to that of respiratory syncytial virus (RSV) infection because most cases of bronchiolitis are caused by RSV. (See "Respiratory syncytial virus infection: Clinical features and diagnosis in infants and children", section on 'Epidemiology'.)

During the height of the COVID-19 pandemic, the infection control measures that were used (social distancing, masks) substantially decreased the circulation of RSV and other respiratory viruses causing bronchiolitis during the fall and winter months of 2020 through the summer of 2021 [32-35]. However, during the subsequent two RSV seasons (2021 and 2022), there were intense spikes in RSV infections and other respiratory viruses (rhinovirus, parainfluenza virus-3, and human metapneumovirus) that occurred earlier in the season than is usual.

RISK FACTORS FOR SEVERE DISEASE — Risk factors for severe or complicated bronchiolitis include [1,36-39]:

Prematurity (gestational age ≤36 weeks)

Low birth weight

Age <12 weeks

Chronic pulmonary disease, particularly bronchopulmonary dysplasia

Immunodeficiency

Anatomic defects of the airways (eg, laryngomalacia, tracheoesophageal fistula)

Hemodynamically significant congenital heart disease

Neurologic disease

Environmental and other risk factors, such as passive smoking, crowded household, daycare attendance, being born approximately two months before or after the start of the epidemic, concurrent birth siblings, older siblings, high altitude (>2500 meters), and outdoor air pollution, can also contribute to more severe disease [37,40-45].

CLINICAL FEATURES

Clinical presentation — Bronchiolitis is a clinical syndrome of respiratory distress that occurs primarily in children younger than two years of age and generally presents with fever (usually ≤38.3°C [101°F]), cough, and respiratory distress (eg, increased respiratory rate, retractions, wheezing, crackles). It often is preceded by a one- to three-day history of upper respiratory tract symptoms (eg, nasal congestion and/or discharge) [46]. Respiratory distress, increased work of breathing, respiratory rate, and oxygenation all can change rapidly with crying, coughing, and agitation. Oxyhemoglobin desaturation can occur under all of these circumstances as well as during sleep when chest wall muscles relax, further narrowing intrathoracic airways.

Clinical course — Typical illness with bronchiolitis begins with upper respiratory tract symptoms (eg, nasal congestion, cough), followed by lower respiratory tract signs (wheezing, crackles) on days 2 to 3, which peak on days 3 to 5 and then gradually resolve. However, the duration of the illness can vary depending on the age of the patient, severity of illness, comorbidities (eg, prematurity, chronic pulmonary disease), and the causative agent. The expected clinical course for infants and children with bronchiolitis is discussed in greater detail separately. (See "Bronchiolitis in infants and children: Treatment, outcome, and prevention", section on 'Typical disease course'.)  

Complications — In most previously healthy infants, bronchiolitis resolves without complications. However, severely affected patients, particularly infants <3 months of age, preterm infants, and those with underlying cardiopulmonary disease or immunodeficiency, are at increased risk for complications, the most serious of which are apnea and respiratory failure.

Dehydration — Infants with bronchiolitis may have difficulty maintaining adequate hydration because of increased fluid needs (related to fever and tachypnea), decreased oral intake (related to tachypnea and respiratory distress), and/or vomiting [47]. They should be monitored for dehydration (eg, increased heart rate, dry mucosa, sunken fontanelle, decreased urine output (table 1)). Parenteral or nasogastric fluid administration may be necessary. (See "Clinical assessment of hypovolemia (dehydration) in children", section on 'Clinical assessment' and "Bronchiolitis in infants and children: Treatment, outcome, and prevention", section on 'Fluid and nutrition management'.)

Aspiration pneumonia — Bronchiolitis may be complicated by aspiration pneumonia. The risk of aspiration increases during active bronchiolitis and resolves weeks later as tachypnea and the work of breathing subside.

Apnea — Bronchiolitis may be complicated by apnea, particularly in infants born prematurely and those younger than two months (ie, those with postmenstrual age <48 weeks) [48-50]. The risk of apnea is not specific to a particular pathogen [48,51]. Presenting with apnea is a risk factor for respiratory failure and the need for mechanical ventilation. (See 'Respiratory failure' below.)

In a three-year multicenter prospective study (2007 to 2010) that included 2156 children <2 years hospitalized with bronchiolitis, apnea was documented in 5 percent [48]. The study focused on sicker patients by aiming to enroll 20 percent of patients from the intensive care unit. Independent risk factors for apnea included age <8 weeks (age was corrected for gestational age if born preterm), caretaker report of previous apnea during the illness, high or low respiratory rate at presentation (ie, respiratory rate <30 or >70 breaths/minute), and room air oxygen saturation <90 percent at presentation. Similar risk factors for apnea were identified in large prospective and retrospective cohorts [50,52]. The risk of apnea was not increased with RSV compared with other viral pathogens [48].

These findings suggest that low respiratory (ie, <30 breaths/minute) rate in children with bronchiolitis is not necessarily reassuring and that results of virologic studies are not helpful in determining the risk for apnea among hospitalized infants.

Respiratory failure — Approximately 5 to10 percent of patients with bronchiolitis develop severe respiratory compromise requiring advanced respiratory support and 1 to 2 percent require intubation and mechanical ventilation [28,53-55]. Young infants (<2 months old), preterm infants, and those with underlying chronic medical conditions are more likely to require intubation. Intubation rates in children with bronchiolitis have decreased since the early 2000s, likely due to increasing use of noninvasive respiratory support, particularly high-flow nasal cannula. This is discussed separately. (See "Bronchiolitis in infants and children: Treatment, outcome, and prevention", section on 'Respiratory support'.)

Secondary bacterial infection — With the exception of otitis media, secondary bacterial infection is uncommon among infants and young children with bronchiolitis [56]. In a retrospective study of >2000 young children hospitalized for viral respiratory infections, nearly one-third received antibiotic therapy for presumed bacterial infection (chiefly pneumonia), but only 3.5 percent had a bacterial pathogen isolated in culture [56]. The risk of secondary bacterial pneumonia is increased among children who require admission to the pediatric intensive care unit, particularly those who require intubation [57,58]. In a study of 165 infants admitted to the PICU with RSV bronchiolitis, 22 percent had bacterial coinfection [57].

RADIOGRAPHIC FEATURES — In infants and young children with mild disease, radiographs are not necessary as they are unlikely to alter treatment and may lead to inappropriate use of antibiotics [1,59-63]. However, in infants and young children with moderate or severe respiratory distress (eg, respiratory severity score ≥5 (table 2)), radiographs may be warranted, particularly if there are focal findings on examination, the infant has a cardiac murmur, or it is necessary to exclude alternate diagnoses. Radiographs also may be indicated to exclude alternate diagnoses in children who fail to improve as expected [1]. (See 'Differential diagnosis' below.)

Radiographic features of bronchiolitis, which are variable and nonspecific, include hyperinflation and peribronchial thickening (image 1) [62,64]. Patchy atelectasis with volume loss may result from airway narrowing and mucus plugging. Segmental consolidation and alveolar infiltrates are more characteristic of bacterial pneumonia than bronchiolitis, but the radiographic findings can overlap.

The chest radiograph alone is often not sufficient for distinguishing between viral bronchiolitis and bacterial pneumonia; the radiographic findings must be considered in conjunction with other clinical features when making decisions about the need for antibiotic therapy. (See "Pneumonia in children: Inpatient treatment" and "Community-acquired pneumonia in children: Clinical features and diagnosis", section on 'Etiologic clues'.)

EVALUATION — The evaluation of infants and young children with suspected bronchiolitis generally requires only history and physical examination, including pulse oximetry. Laboratory studies and radiographs usually are not necessary for diagnosis but may be warranted to evaluate complications, comorbid infections, or other conditions in the differential diagnosis. The evaluation outlined below is largely consistent with that suggested in clinical practice guidelines from the American Academy of Pediatrics, the National Institute for Care Excellence, and other professional groups [1,65-68]. (See 'Society guideline links' below.)

History — Infants with moderate to severe bronchiolitis typically present for medical attention three to six days after illness onset. Bronchiolitis often is preceded by a one- to three-day history of upper respiratory tract symptoms, such as nasal congestion and/or discharge and mild cough [46]. It typically presents with fever (usually ≤38.3°C [101°F), cough, and respiratory distress (eg, increased respiratory rate, retractions).

Compared with other viruses that cause bronchiolitis, fever tends to be lower with respiratory syncytial virus (RSV) and higher with adenovirus [69]. (See "Respiratory syncytial virus infection: Clinical features and diagnosis in infants and children", section on 'Clinical manifestations' and "Pathogenesis, epidemiology, and clinical manifestations of adenovirus infection", section on 'Clinical presentation'.)

Aspects of the history of present illness that help in determining the severity of illness and/or need for hospitalization include (see 'Severity assessment' below and "Bronchiolitis in infants and children: Treatment, outcome, and prevention", section on 'Severity assessment' and "Bronchiolitis in infants and children: Treatment, outcome, and prevention", section on 'Indications for hospitalization') [1,70]:

Assessment of hydration status (eg, fluid intake, urine output)

Symptoms of respiratory distress (tachypnea, nasal flaring, retractions, grunting)

Cyanosis

Episodes of restlessness or lethargy (may indicate hypoxemia and/or impending respiratory failure)

A history of apnea with or without cyanosis or bradycardia

Aspects of the past medical history associated with severe disease include prematurity, chronic pulmonary disease, anatomic abnormalities of the airways, hemodynamically significant congenital heart disease, immunodeficiency, and neurologic disease. (See 'Risk factors for severe disease' above.)

Examination — Characteristic examination findings of bronchiolitis include tachypnea, intercostal and subcostal retractions, expiratory wheezing, and cough. Additional auscultatory findings may include prolonged expiratory phase and coarse or fine crackles (rales). The chest may appear hyperexpanded with increased anteroposterior diameter and may be hyperresonant to percussion. Hypoxemia (oxygen saturation <95 percent) commonly is detected by pulse oximetry. Other findings may include conjunctivitis, pharyngitis, and acute otitis media [71-73].

Severely affected patients have increased work of breathing (subcostal, intercostal, and supraclavicular retractions; nasal flaring; and expiratory grunting). They may appear cyanotic and have poor peripheral perfusion. Wheezing may not be audible if the airways are profoundly narrowed or when increased work of breathing results in exhaustion.

Laboratory tests or imaging for select patients — Laboratory tests are not routinely indicated in the evaluation of infants and young children with bronchiolitis. However, laboratory and/or radiographic evaluation may be necessary to evaluate the possibility of:

Comorbid or secondary bacterial infection in:

Neonates ≤28 days of age with fever – Infants ≤28 days old with fever (temperature ≥38°C [100.4°F]) and symptoms and signs of bronchiolitis have the same risk for invasive bacterial illness (IBI) as young febrile infants without bronchiolitis and should be assessed accordingly. (See "The febrile neonate (28 days of age or younger): Initial management", section on 'Other viral infections'.)

Infants ≥29 to 90 days of age with fever – Extensive laboratory testing is not routinely warranted for infants ≥29 to 90 days of age with fever (temperature ≥38°C [100.4°F]) and symptoms and signs of bronchiolitis, particularly if they have tested positive for a respiratory virus, unless they have another indication for evaluation (ill-appearance, risk factors for IBI (table 3), urologic abnormality). Serious comorbid bacterial infection is uncommon in children with bronchiolitis. (See "The febrile infant (29 to 90 days of age): Outpatient evaluation", section on 'Bronchiolitis' and "The febrile infant (29 to 90 days of age): Outpatient evaluation", section on 'Ancillary studies'.)

Complications or other diagnostic considerations in:

Children of any age with unusual or severe course – CBC and chest radiograph may be warranted to evaluate secondary bacterial infection and other conditions in the differential diagnosis in infants and young children with an unusual or prolonged or severe course (eg, failure to improve after two to five days, wheezing that persists for more than one week) [74]. (See 'Clinical course' above and 'Differential diagnosis' below.)

Children of any age with severe disease – In infants and young children with severe disease, arterial or capillary blood gas measurements may be necessary to evaluate respiratory failure. (See 'Respiratory failure' above.)

DIAGNOSIS

Clinical diagnosis — Bronchiolitis is diagnosed clinically. Characteristic features include a viral upper respiratory prodrome followed by increased respiratory effort (eg, tachypnea, nasal flaring, chest retractions) and wheezing and/or crackles in children younger than two years of age [1,59,74]. (See 'History' above and 'Examination' above.)

Chest radiographs and laboratory studies are not necessary to make the diagnosis of bronchiolitis and should not be routinely performed [1]. However, they may be necessary to evaluate the possibility of bacterial co-infection, complications, or other conditions in the differential diagnosis, particularly in children who have pre-existing cardiopulmonary disease [1,74,75]. (See 'Complications' above and 'Differential diagnosis' below and 'Laboratory tests or imaging for select patients' above.)

Viral testing

Indications — We test for specific viral agents in children with bronchiolitis when the results of such testing will alter management of the patient or patient's contacts (eg, decisions regarding antibiotic therapy, treatment for influenza, isolation precautions or cohorting of hospitalized patients or caregivers) [74,76]. (See "Seasonal influenza in children: Management", section on 'Antiviral therapy'.)

There is debate about whether testing for specific viral agents alters clinical management or outcome, particularly in the outpatient setting [74-82]. However, the identification of a viral etiologic agent during emergency department evaluation or in hospitalized patients has been associated with a decreased utilization of antibiotic treatment in some studies [78,83-86].

Identification of the responsible virus in hospitalized patients may help to avoid health care-associated transmission by permitting cohorting of patients and/or caregivers. However, direct evidence that this strategy prevents transmission of respiratory viruses in children is lacking, and it may be more logical to isolate all infants with bronchiolitis [66,74,81,82,87]. Cohorting has the potential to increase the risk of infection with other respiratory viruses leading to prolonged hospitalization [15]. (See "Respiratory syncytial virus infection: Prevention in infants and children", section on 'Infection control in the health care setting'.)

Approach to testing — When the decision is made to perform testing, the two main testing methods are molecular assays (eg, single or multiplex polymerase chain reaction [PCR]) and rapid antigen tests. Other methods include immunofluorescence and viral culture, but these are less commonly performed.

Multiplex PCR tests (respiratory viral panel) – For hospitalized patients, PCR testing is preferred over antigen detection given the increased sensitivity and ability to assess a broader panel of respiratory viruses. Results of multiplex panels must be interpreted with caution because given their high sensitivity, they can be positive even in the absence of symptomatic infection [88-90].

Rapid antigen tests – Rapid antigen tests are available for respiratory syncytial virus (RSV), parainfluenza, adenovirus, influenza viruses, and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The sensitivity of rapid antigen tests for RSV, influenza, and SARS-CoV-2 typically ranges from 70 to 90 percent [91,92]; however, it is lower for the other viruses [93,94]. (See "Respiratory syncytial virus infection: Clinical features and diagnosis in infants and children", section on 'Laboratory confirmation' and "Seasonal influenza in children: Clinical features and diagnosis", section on 'Approach to testing' and "COVID-19: Clinical manifestations and diagnosis in children", section on 'Laboratory tests for SARS-CoV-2'.)

Other tests

Direct or indirect immunofluorescence tests also are available for RSV, parainfluenza, adenovirus, influenza virus, and other viruses that cause bronchiolitis. With increased availability of PCR and antigen tests, these tests are less commonly performed.

Culture is another method that can be used for viral identification, but results are generally not available in time for clinical decision-making.

The quality of the specimen and how it is handled can impact the sensitivity of testing. Testing should be performed on respiratory specimens obtained by nasal aspirate, nasal swab (preferably a midturbinate swab), or nasal wash [95,96]. Rapid antigen and PCR test results are usually available within a few hours of collection to ensure their availability for making clinical management decisions.

Severity assessment — For infants diagnosed with bronchiolitis, characterizing the severity of illness informs management decisions. The severity assessment is based chiefly on the degree or respiratory distress (table 2). This is discussed in greater detail separately. (See "Bronchiolitis in infants and children: Treatment, outcome, and prevention", section on 'Severity assessment'.)

DIFFERENTIAL DIAGNOSIS — Bronchiolitis must be distinguished from a variety of acute and chronic conditions that affect the respiratory tract, including recurrent viral-triggered wheezing or asthma, bacterial pneumonia, pertussis, chronic pulmonary disease, foreign body aspiration, aspiration pneumonia, congenital heart disease, heart failure, and vascular ring [73,97]. Severe bronchiolitis also can unmask underlying airway obstruction that existed before the infection (eg, vascular ring). Clinical features (eg, lack of preceding upper respiratory tract symptoms, witnessed episode of choking, differential aeration, poor growth) may help to distinguish some of these conditions from bronchiolitis; for others, radiographic or laboratory studies may be necessary.

Recurrent viral-triggered wheezing – Recurrent viral-triggered wheezing/recurrent wheezing is a major consideration in the differential diagnosis of bronchiolitis in older infants and toddlers. A history of recurrent wheezing episodes and a family or personal history of asthma, eczema, and atopy help to support a diagnosis of asthma. However, during the first episode of wheezing, it is difficult to distinguish bronchiolitis from asthma. (See "Role of viruses in wheezing and asthma: An overview" and "Wheezing phenotypes and prediction of asthma in young children" and "Evaluation of wheezing in infants and children".)

Bacterial pneumonia – It can be difficult to distinguish bacterial pneumonia from bronchiolitis in young children because the symptoms and signs of both conditions are nonspecific. Children with bacterial pneumonia may be more ill appearing (eg, higher fever), but clinical features cannot reliably differentiate bacterial from viral lower respiratory tract infection (table 4). (See "Community-acquired pneumonia in children: Clinical features and diagnosis", section on 'Diagnosis'.)

B. pertussis infection – The clinical presentation of B. pertussis infection in infants may be similar to that of bronchiolitis. Infants with pertussis may lack the characteristic "whoop" and may have a nonparoxysmal cough. Microbiologic testing is necessary for diagnosis. (See "Pertussis infection in infants and children: Clinical features and diagnosis", section on 'Diagnosis'.)

Airway abnormalities and pulmonary disease – An underlying airway or pulmonary condition should be suspected in children with prolonged or recurrent symptoms, stridor, recurrent respiratory infections, poor weight gain, or recurrent aspiration. (See "Approach to the child with recurrent infections" and "Poor weight gain in children younger than two years in resource-abundant settings: Etiology and evaluation" and "Congenital anomalies of the larynx".)

Children with underlying airway abnormalities (eg, laryngomalacia, tracheomalacia) or pulmonary disease may have a superimposed acute episode of bronchiolitis, and, in some cases, the underlying disorder is unrecognized before the acute episode. The clinical course of bronchiolitis in these children tends to be severe and may require prolonged hospitalization. (See "Congenital anomalies of the larynx", section on 'Laryngomalacia' and "Congenital anomalies of the intrathoracic airways and tracheoesophageal fistula", section on 'Tracheomalacia'.)

Foreign body aspiration – Clinical features of foreign body aspiration may include a history of choking (not always present), focal monophonic wheezing, decreased air entry, or regional variation in aeration. A high index of suspicion should be maintained for foreign body aspiration so that definitive treatment can be provided. (See "Airway foreign bodies in children".)

Aspiration pneumonia – Aspiration pneumonia may occur secondary to gastroesophageal reflux disease and/or swallowing dysfunction. It also may occur as a complication of bronchiolitis; the risk of aspiration increases during active bronchiolitis and resolves weeks later as tachypnea and the work of breathing subside. Clinical features associated with aspiration may include coughing with feeds, weak suck reflex, cyanosis during feeding, and recurrent or chronic stridor. (See "Gastroesophageal reflux in infants" and "Aspiration due to swallowing dysfunction in children".)

Congenital heart disease (CHD) and heart failure – Clinical findings that are suggestive of congenital heart disease (CHD) and/or heart failure include (see "Suspected heart disease in infants and children: Criteria for referral"):

History of feeding difficulties (especially tiring with feeds) and poor weight gain

Abnormalities on cardiovascular examination (eg, pathologic murmur, abnormal second heart sound, gallop rhythm, decreased lower extremity pulses)

Hepatomegaly

Cardiomegaly and/or pulmonary edema on chest radiograph

Children with underlying CHD may have a superimposed acute episode of bronchiolitis, and, in some cases, the underlying cardiac disease is unrecognized before the acute episode. The clinical course of bronchiolitis in children with underlying CHD tends to be severe and may require prolonged hospitalization.

Vascular rings – Although stridor is more common, children with vascular rings may also have wheezing (typically with pulmonary artery slings). Anterior bowing of the trachea in the lateral chest radiograph may be a clue, but other imaging modalities are usually necessary for definitive diagnosis. (See "Vascular rings and slings", section on 'Clinical manifestations'.)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Bronchiolitis in infants and children".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or email these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword[s] of interest.)

Basics topic (see "Patient education: Bronchiolitis and RSV in children (The Basics)")

Beyond the Basics topic (see "Patient education: Bronchiolitis and RSV in infants and children (Beyond the Basics)")

SUMMARY AND RECOMMENDATIONS

Definition and clinical features – Bronchiolitis is broadly defined as a clinical syndrome of respiratory distress that occurs in children <2 years of age. It is characterized by upper respiratory symptoms (eg, rhinorrhea) followed by lower respiratory tract signs (eg, wheezing and/or crackles). (See 'Definition' above and 'Clinical features' above.)

Etiology – Bronchiolitis is caused by a viruses. Respiratory syncytial virus (RSV) is the most common cause, followed by rhinovirus; less common causes include parainfluenza virus, human metapneumovirus, influenza virus, adenovirus, coronaviruses (including severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2]), and human bocavirus. (See 'Viral etiology' above.)

Epidemiology and risk factors – Bronchiolitis typically affects infants and children younger than two years. In the northern hemisphere, it occurs principally during the fall and winter. (See 'Epidemiology' above.)

Risk factors for severe disease and/or complications include gestational age ≤36 weeks, age <12 weeks, chronic pulmonary disease, congenital and anatomic defects of the airways, hemodynamically significant congenital heart disease, immunodeficiency, and neurologic disease. (See 'Risk factors for severe disease' above.)

Evaluation – The evaluation of infants and young children with suspected bronchiolitis generally requires only history and physical examination. Chest radiographs and laboratory tests are not necessary for diagnosis but may be warranted to evaluate complications, comorbid infections, or other conditions in the differential diagnosis. (See 'Evaluation' above.)

Diagnosis – Bronchiolitis is diagnosed clinically. Characteristic features include a viral upper respiratory prodrome followed by increased respiratory effort (eg, tachypnea, nasal flaring, chest retractions) and wheezing and/or crackles in children younger than two years. (See 'Clinical diagnosis' above and 'History' above and 'Examination' above.)

Viral testing is performed when the results of such testing will alter management of the patient or patient's contacts (eg, decisions regarding antibiotic therapy, isolation precautions or cohorting of hospitalized patients or caregivers). When testing is indicated, the two main testing methods are molecular assays (eg, single or multiplex polymerase chain reaction [PCR]) and rapid antigen tests. Multiplex PCR testing is generally preferred for hospitalized patients. (See 'Viral testing' above.)

Differential diagnosis – The differential diagnosis of bronchiolitis includes recurrent viral-triggered wheezing, bacterial pneumonia, foreign body aspiration, underlying airway or pulmonary disease, aspiration pneumonia, congenital heart disease, heart failure, and vascular rings. Clinical features (eg, lack of preceding upper respiratory tract symptoms, witnessed episode of choking, differential aeration, poor growth) may help to distinguish some of these conditions from bronchiolitis; for others, radiographic or laboratory studies may be necessary. (See 'Differential diagnosis' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Ann R Stark, MD, who contributed to earlier versions of this topic review.

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Topic 6018 Version 57.0

References

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