INTRODUCTION — Bronchiolitis, a lower respiratory tract infection that primarily affects the small airways (bronchioles), is a common cause of illness and hospitalization in infants and young children.
The microbiology, epidemiology, clinical features, and diagnosis of bronchiolitis will be presented here. The treatment, outcome, and prevention of bronchiolitis in children; respiratory syncytial virus; and the emergency evaluation of children with acute respiratory distress are discussed separately:
●(See "Bronchiolitis in infants and children: Treatment, outcome, and prevention".)
●(See "Respiratory syncytial virus infection: Clinical features and diagnosis" and "Respiratory syncytial virus infection: Treatment" and "Respiratory syncytial virus infection: Prevention in infants and children".)
●(See "Acute respiratory distress in children: Emergency evaluation and initial stabilization".)
DEFINITION — Bronchiolitis is broadly defined as a clinical syndrome of respiratory distress that occurs in children <2 years of age and is characterized by upper respiratory symptoms (eg, rhinorrhea) followed by lower respiratory infection with inflammation, which results in wheezing and/or crackles (rales). Bronchiolitis typically occurs with primary infection or reinfection with a viral pathogen [1-3]. In young children, the clinical diagnosis 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 [4,5].
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 [6-8].
MICROBIOLOGY — Bronchiolitis typically is caused by a viral infection. Although the proportion of disease caused by specific viruses varies depending upon the season and the year, respiratory syncytial virus (RSV) is the most common cause, followed by rhinovirus [9-12]. Less common causes include parainfluenza virus, human metapneumovirus, influenza virus, adenovirus, coronaviruses, and human bocavirus [9,13-15]. 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 [16-19]. In addition, lower respiratory tract infection and wheezing episodes in infants infrequently are associated with Mycoplasma pneumoniae and Bordetella pertussis. (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'.)
●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".)
●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 [20]. Dual viral infections are often detected. Rhinovirus is often associated with bronchiolitis in the spring and fall [12,21]. (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 [22]. (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 [23,24]. In two multicenter cohort studies of infants hospitalized with bronchiolitis, human metapneumovirus peaked in March and April [12]. (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 [12,25,26]. Before the emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), most cases of coronavirus-related bronchiolitis were coinfections with RSV [27].
•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 "Severe acute respiratory syndrome (SARS)", section on 'Clinical manifestations' and "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 [13,28-30]. Bronchiolitis and pertussis-like illness can occur. Human bocavirus 2 through 4 are primarily enteric viruses [31].
EPIDEMIOLOGY — Bronchiolitis typically affects infants and children younger than two years, principally during the fall and winter [32]. Bronchiolitis hospitalization has a peak incidence between two and six months of age and remains a significant cause of respiratory disease during the first five years of life [33,34]. It is a leading cause of hospitalization in infants and young children [32,33,35].
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", section on 'Epidemiology'.)
The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic and the nonpharmaceutical intervention implemented to control the pandemic substantially decreased the circulation of RSV and other respiratory viruses causing bronchiolitis during the fall and winter months of 2020-2021 [36-39]. However, in the summer of 2021 an intense spike in RSV, parainfluenza virus-3, and human metapneumovirus activity occurred. It is unclear how the SARS-CoV-2 pandemic and nonpharmaceutical intervention will alter the bronchiolitis epidemiology in the coming years.
RISK FACTORS FOR SEVERE DISEASE — Risk factors for severe or complicated bronchiolitis include [40-47]:
●Prematurity (gestational age ≤36 weeks)
●Low birth weight
●Age less than 12 weeks
●Chronic pulmonary disease, particularly bronchopulmonary dysplasia (also known as chronic lung disease)
●Anatomic defects of the airways
●Hemodynamically significant congenital heart disease
●Immunodeficiency
●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, and high altitude (>2500 meters), can also contribute to more severe disease [45,48-52].
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) [53]. 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 — The duration of the illness due to bronchiolitis depends upon age, severity of illness, associated high-risk conditions (eg, prematurity, chronic pulmonary disease), and the causative agent [19]. Bronchiolitis usually is a self-limited disease. Most children who do not require hospitalization recover by 28 days [54-56].
Typical illness with bronchiolitis begins with upper respiratory tract symptoms, followed by lower respiratory tract signs and symptoms on days 2 to 3, which peak on days 3 to 5 and then gradually resolve. In a systematic review of four studies including 590 children with bronchiolitis who were seen in outpatient settings and not treated with bronchodilators [5,55-57], the mean time to resolution of cough ranged from 8 to 15 days [58]. Cough resolved in 50 percent of patients within 13 days and in 90 percent within 21 days.
Although discharge criteria vary from center to center, in multicenter studies of children younger than two years hospitalized with bronchiolitis, the median length of stay was two days [19,59]. Length of stay may be shorter in children with bronchiolitis due to rhinovirus and longer in children with bronchiolitis due to respiratory syncytial virus (RSV)-rhinovirus co-infection. The respiratory status typically improves over two to five days [41,60-63]. However, wheezing persists in some infants for a week or longer.
The course may be prolonged in infants younger than six months (particularly those younger than 12 weeks) and those with comorbid conditions (eg, bronchopulmonary dysplasia); these children often are severely affected and may require assisted ventilation [40,64]. (See 'Risk factors for severe disease' above and 'Respiratory failure' below.)
Complications — In most previously healthy infants, bronchiolitis resolves without complications. However, severely affected patients, particularly those born prematurely, <12 weeks of age, or who have underlying cardiopulmonary disease or immunodeficiency, are at increased risk for complications, the most serious of which are apnea and respiratory failure [65]. Infants who require mechanical ventilation for apnea or respiratory failure may develop air leak, such as pneumothorax or pneumomediastinum.
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 [66]. 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 and diagnosis of hypovolemia (dehydration) in children", section on 'Clinical assessment' and "Bronchiolitis in infants and children: Treatment, outcome, and prevention", section on 'Fluid 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) [65,67-73]. The risk of apnea is not specific to a particular pathogen [71,74]. 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 [71]. 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 [70,73]. The risk of apnea was not increased with RSV compared with other viral pathogens [71].
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 — Respiratory failure is another serious complication of bronchiolitis. In a multicenter study, 14 percent of 684 infants younger than 12 months who were hospitalized for management of bronchiolitis required mechanical ventilation for respiratory failure or apnea [65]. In another multicenter study, 16 percent of infants and children younger than two years hospitalized with RSV required intensive care support (with or without mechanical ventilation) [41]. However, the need for intensive care varied depending on the presence and type of risk factors for serious disease:
●No known risk factors – 7 percent
●Congenital heart disease, bronchopulmonary dysplasia, or immunosuppression – 19 to 37 percent
●Age <6 weeks – 29 percent
Hypoxemia, associated with mucus plugging and atelectasis, is common in children with bronchiolitis. It may respond to supplemental oxygen alone, although sometimes it requires additional respiratory support. Hypercapnic respiratory failure, associated with fatigue, usually requires additional respiratory support (eg, intubation and mechanical ventilation).
During 2000 to 2016, 2 to 5 percent of children younger than two years hospitalized with bronchiolitis in the Kids Inpatient Database required mechanical ventilation [32]. Requirement for mechanical ventilation was increased in infants younger than 12 months and high-risk medical conditions.
Secondary bacterial infection — With the exception of otitis media, secondary bacterial infection is uncommon among infants and young children with bronchiolitis or RSV infection. In a nine-year prospective study of 565 children (<3 years) hospitalized with documented RSV infection, subsequent bacterial infection developed in only 1.2 percent and subsequent bacterial pneumonia in 0.9 percent [75]. The risk of secondary bacterial pneumonia is increased among children who require admission to the intensive care unit, particularly those who require intubation [76,77].
RADIOGRAPHIC FEATURES — Chest radiographs are not necessary in the routine evaluation of bronchiolitis [2,3,78]. They should be obtained only if there are clinical findings suggestive of other potential diagnoses [1,79]. (See 'Differential diagnosis' below.)
Radiographic features of bronchiolitis, which are variable and nonspecific, include hyperinflation and peribronchial thickening (image 1) [80,81]. 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 radiographic findings are poor indicators of the etiologic diagnosis and must be used in conjunction with other clinical features in making decisions about diagnosis and treatment. (See 'Differential diagnosis' below and "Community-acquired pneumonia in children: Clinical features and diagnosis", section on 'Etiologic clues'.)
In infants and young children with mild disease, radiographs are unlikely to alter treatment and may lead to inappropriate use of antibiotics [2,80,82]. However, in infants and young children with moderate or severe respiratory distress (eg, nasal flaring, retractions, grunting, respiratory rate >70 breaths/minute, dyspnea, or cyanosis), 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 [2]. Radiographs also may be indicated to exclude alternate diagnoses in children who fail to improve at the expected rate [3]. (See 'Severity assessment' below and 'Differential diagnosis' below and 'Clinical course' above.)
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 [3,83-86]. (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 [53]. 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 [87]. (See "Respiratory syncytial virus infection: Clinical features and diagnosis", 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 'Indications for hospitalization') [3,88]:
●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 [89-91].
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): Outpatient evaluation and 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 2), 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) [1]. (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 [1-3]. (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 [3]. However, they may be necessary to evaluate the possibility of secondary or comorbid bacterial infection, complications, or other conditions in the differential diagnosis, particularly in children who have pre-existing cardiopulmonary disease [1,3,92]. (See 'Complications' above and 'Differential diagnosis' below and 'Laboratory tests or imaging for select patients' above.)
Virology
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, discontinuation of palivizumab prophylaxis, initiation or continuation/discontinuation of antibiotic therapy, anti-influenza therapy, isolation/quarantine for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), or cohorting of hospitalized patients or caregivers) [1,93]. (See "Seasonal influenza in children: Management", section on 'Antiviral therapy'.)
With the exception of SARS-CoV-2, there is debate about whether testing for specific viral agents alters clinical management or outcome, particularly in the outpatient setting [1,92-99]. 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 [95,100-104].
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 [1,84,98,99,105]. Cohorting has the potential to increase the risk of infection with other respiratory viruses leading to prolonged hospitalization [19]. (See "Respiratory syncytial virus infection: Prevention in infants and children", section on 'Infection control in the health care setting'.)
Approach to testing — When an etiologic diagnosis is necessary (eg, for isolating or cohorting hospitalized patients or caregivers, if the results will affect other management decisions such as whether to initiate or continue antibiotic therapy), it can be confirmed with molecular assays (eg, single or multiplex polymerase chain reaction), antigen detection, immunofluorescence, or culture.
●For hospitalized patients, molecular assays are preferred to antigen detection or immunofluorescence given the increased sensitivity and ability to assess a broader panel of respiratory viruses. Results of multiplex panels must be interpreted with caution because they do not differentiate asymptomatic from symptomatic infection [106-108]. In a meta-analysis of case-control studies in children, respiratory syncytial virus (RSV), influenza virus, parainfluenza virus, and human metapneumovirus were associated with lower respiratory tract infection (LRTI) symptoms; rhinovirus was only weakly associated with LRTI symptoms; and adenovirus, bocavirus, and coronaviruses were not associated with LRTI symptoms [106].
●Rapid antigen tests are available for RSV, parainfluenza, adenovirus, influenza viruses, and SARS-CoV-2. Although the sensitivity of rapid antigen tests for RSV and influenza typically ranges from 80 to 90 percent [109,110], it is lower for the other viruses [111,112]. (See "COVID-19: Clinical manifestations and diagnosis in children", section on 'Laboratory tests for SARS-CoV-2'.)
●Direct or indirect immunofluorescence tests also are available for RSV, parainfluenza, adenovirus, influenza virus, and other viruses that cause bronchiolitis.
●Culture is another method that can be used for viral identification, but results may not be available in time for clinical decision-making.
The laboratory diagnosis depends upon the quality and proper handling of the specimen. Virologic testing should be performed on respiratory specimens obtained by nasal wash or nasal aspirate; midturbinate nasal swab is also acceptable [113-115]. Test results, particularly molecular diagnostic test results, should be available within a few hours of collection to ensure their availability for making clinical management decisions.
Nasal wash specimens are obtained by holding the infant or child upright at a 45° angle. A bulb syringe or a soft plastic catheter attached to suction is used to aspirate nasal secretions after a small amount of normal saline (1 to 3 mL) is instilled in each nostril.
Severity assessment — Severe bronchiolitis is indicated by persistently increased respiratory effort (tachypnea; nasal flaring; intercostal, subcostal, or suprasternal retractions; accessory muscle use; grunting), hypoxemia, apnea, or acute respiratory failure [85]. Repeated observations are necessary to adequately assess disease severity because examination findings may vary substantially over time [3]. Infants and young children with severe disease usually require hospitalization for frequent observation as well as respiratory and/or fluid support. (See "Bronchiolitis in infants and children: Treatment, outcome, and prevention", section on 'Indications for hospitalization'.)
Other factors that have been associated with increased illness severity include toxic or ill appearance, oxygen saturation <90 percent by pulse oximetry while breathing room air, respiratory rate ≥70 breaths/minute, and atelectasis on chest radiograph [40,41,116]. However, there is limited and/or conflicting evidence relating these clinical findings to clinical outcomes [3,40,41,60,81,117,118].
Several scoring instruments have been developed to assess the clinical severity of bronchiolitis in research settings [119-123], but few have demonstrated validity in predicting outcomes [78,124]. In clinical practice, clinicians generally rely on history and clinical findings.
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 [91,125]. 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 [126]. (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 3). (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'.)
●Chronic pulmonary disease – Chronic underlying pulmonary conditions should be suspected in children with prolonged or recurrent symptoms, such as recurrent wheezing, poor weight gain, recurrent aspiration, stridor, or recurrent respiratory infection. (See "Assessment of stridor in children" and "Approach to the child with recurrent infections" and "Poor weight gain in children younger than two years in resource-abundant countries: Etiology and evaluation", section on 'Causes'.)
Children with underlying 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 children with underlying pulmonary disorders tends to be severe and may require prolonged hospitalization.
●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 – Associated clinical findings of congenital heart disease may include poor weight gain, poor peripheral perfusion, and abnormalities on cardiac examination (eg, pathologic heart murmur, abnormal second heart sound, gallop, rub, active precordium). (See "Suspected heart disease in infants and children: Criteria for referral".)
Children with underlying cardiac conditions 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 children with underlying cardiac disorders tends to be severe and may require prolonged hospitalization.
●Heart failure – Associated clinical findings of heart failure in infants may include easy fatigue and/or diaphoresis with feeding, poor weight gain, heart murmur or gallop rhythm, and hepatomegaly. (See "Heart failure in children: Etiology, clinical manifestations, and diagnosis", section on 'Clinical manifestations'.)
●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 modalities (barium contrast esophagogram, bronchoscopy, magnetic resonance angiography) usually are 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 and is characterized by upper respiratory symptoms (eg, rhinorrhea) followed by lower respiratory infection with inflammation, which results in wheezing and or crackles (rales). (See 'Definition' above and 'Clinical features' above.)
●Microbiology – Bronchiolitis typically is caused by a viral infection. Respiratory syncytial virus 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 'Microbiology' above.)
●Epidemiology and risk factors – Bronchiolitis typically affects infants and children younger than two years, principally during the fall and winter. It is unclear how the SARS-CoV-2 pandemic and nonpharmaceutical intervention will alter the bronchiolitis epidemiology in the coming years.
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 'Epidemiology' above and '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 and severity assessment – 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.)
Severe bronchiolitis is indicated by persistently increased respiratory effort (tachypnea; nasal flaring; intercostal, subcostal, or suprasternal retractions; accessory muscle use; grunting), hypoxemia, apnea, or acute respiratory failure. Children with severe disease usually require hospitalization for respiratory and fluid support. (See 'Severity assessment' above and "Bronchiolitis in infants and children: Treatment, outcome, and prevention", section on 'Severe bronchiolitis'.)
●Differential diagnosis – The differential diagnosis of bronchiolitis includes recurrent viral-triggered wheezing or recurrent wheezing, pneumonia, foreign body aspiration, chronic pulmonary disease, aspiration pneumonia, congenital heart disease, heart failure, and 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. (See 'Differential diagnosis' above.)
