Viral Chest Infection

KEY FACTS

 

Terminology

• •Viral infection may involve airways &/or lung parenchyma (alveoli, interstitium)
• •
  Bronchiolitis: Acute inflammation & necrosis of epithelial cells lining small airways with ↑ mucus production

  • ○
    Classically < 2 years of age
• •Other terms: Viral pneumonia, lower respiratory tract infection, peribronchial pneumonia

Imaging

• •
  Primary goal of chest radiography: Differentiate viral airway infection from bacterial pneumonia (which requires antibiotics)

  • ○
    92% negative predictive value for bacterial pneumonia
• •
  Best imaging clues for viral airway infection

  • ○
    ↑ peribronchial markings

    • –
      Radiating linear rope-like or “dirty” perihilar opacities
    • –
      “Doughnuts” of circumferentially thickened bronchial walls (viewed in cross section)
  • ○
    Hyperinflation: Depression of hemidiaphragms with downward sloping on lateral view; ↑ AP chest diameter on lateral view; ± convex bulging of lungs between ribs
  • ○
    Subsegmental atelectasis, possibly multifocal
  • ○
    Lack of focal/lobar consolidation or pleural effusion
• •
  Best imaging clues for viral parenchymal involvement

  • ○
    Interstitial, nodular, or patchy ground-glass opacities

Clinical Issues

• •Pathogens detected in hospitalized pediatric pneumonia patients: Viral 73%, bacterial 15%
• •
  Most common viral etiologies differ by age

  • ○
    RSV < 2 years, rhinovirus > 2 years
• •
  Treatment

  • ○
    Antibiotics for concomitant bacterial infection
  • ○
    Nebulized hypertonic saline in hospitalized infants may shorten length of stay
  • ○
    Antiviral therapy for influenza cases

AP radiograph in a wheezing child shows typical findings of viral airways disease. There are mildly increased perihilar markings with an increased number of “doughnuts” (thickened bronchial walls viewed in cross section) . The lungs show convex bulging between ribs , typical of hyperinflation.

AP radiograph shows viral airways disease with increased perihilar markings of bronchial wall edema . There is no focal lung consolidation or pleural effusion.

AP radiograph in a wheezing child shows hyperinflated lungs with increased rope-like perihilar markings, consistent with viral airways disease. There is no focal lung consolidation.

Lateral radiograph in the same patient shows marked hyperinflation with flattening of the hemidiaphragms (much more evident than on the corresponding frontal view) & widening of the AP diameter of the chest. Also note the increased markings radiating from the hila.

TERMINOLOGY

Definitions

• •Terminology varies, can be confusing
• •Bronchiolitis (as defined by American Academy of Pediatrics): Viral lower respiratory tract infection in infants with “acute inflammation, edema, & necrosis of epithelial cells lining small airways,” & ↑ mucus production
• •Lower respiratory tract infection may describe findings identical to bronchiolitis in patients ≥ 2 years old but may also refer to any infection of lower airways & parenchyma
• •Viral pneumonia may refer to viral infection of lung parenchyma ± airways infection
• •Peribronchial pneumonia sometimes used to differentiate airways infection from parenchymal involvement
• •Lower airways disease includes viral airways infection as well as asthma

IMAGING

General Features

• •
  Best diagnostic clue

  • ○
    ↑ Peribronchial markings with symmetric hyperinflation

Radiographic Findings

• •
  Primary goal: Differentiate viral airways infection from bacterial pneumonia

  • ○
    Debated if radiographic findings correlate with bronchiolitis severity
• •
  Small airways viral infection

  • ○
    Lack of focal dense geographic, round, or “fluffy” lung consolidation (hallmark of bacterial infection)
  • ○
    ↑ peribronchial markings/↑ number of visible bronchi

    • –
      Symmetric, coarse linear markings radiating from hila
    • –
      ↑ thickness of bronchial walls appearing as doughnuts in cross section
    • –
      Central lungs may appear dirty or busy
    • –
      Hila may appear prominent on lateral view
    • –
      Somewhat subjective finding
  • ○
    Hyperinflation

    • –
      Hyperlucency
    • –
      Depression of diaphragm > 10 posterior ribs or > 6 anterior ribs
    • –
      Flattening/downward sloping hemidiaphragms (best seen on lateral view)
    • –
      ↑ AP chest diameter (in infants, chest wider than tall on lateral view)
    • –
      ± convex bulging of lungs between ribs
    • –
      May not be seen > 2 years old: Bronchi larger, less prone to narrowing or obstruction with inflammatory change
  • ○
    Subsegmental atelectasis

    • –
      Wedge-shaped or triangular opacity, often narrow

      • □
        Most common in mid or lower lung
      • □
        Often multifocal
    • –
      Commonly misinterpreted as suspicious for bacterial pneumonia
• •
  Parenchymal involvement

  • ○
    Interstitial nodular or patchy opacities
  • ○
    Lobar consolidation rarely occurs
  • ○
    Associated findings of airways infection
• •
  Mild hilar lymphadenopathy

  • ○
    Not alarming in this setting (unlike adults)
• •Gas-distended bowel loops in upper abdomen, typically due to air swallowing

CT Findings

• •Not used primarily to make diagnosis of viral disease but may be obtained in patient with unclear clinical/imaging presentation
• •
  Small airways viral infection

  • ○
    Prominent, ill-defined hila & peribronchial markings radiating into lung
  • ○
    Bronchial & bronchiolar wall thickening
  • ○
    Mosaic attenuation: Patchy heterogeneity of attenuation due to hypoventilation & air-trapping from obstruction/narrowing
  • ○
    Ground-glass opacities
  • ○
    Tree-in-bud nodules
• •
  Parenchymal involvement

  • ○
    Interlobular septal thickening
  • ○
    Patchy, ill-defined consolidation
  • ○
    Nodules, micronodules
  • ○
    ± findings of small airways infection
• •Mild hilar lymphadenopathy

Imaging Recommendations

• •
  Best imaging tool

  • ○
    Chest radiographs (frontal & lateral)

    • –
      Performance in identifying/excluding bacterial pneumonia

      • □
        Positive predictive value: 30%
      • □
        Negative predictive value (NPV): 92%
    • –
      Goal: Antibiotic therapy for all children with bacterial pneumonia while minimizing unnecessary antibiotic administration (for isolated viral infection)

      • □
        High NPV of chest radiograph helpful
    • –
      Difficult to differentiate viral from bacterial pneumonia (i.e., lung parenchymal infection) on imaging

      • □
        One study showed lobar infiltrate in 15% of exclusively viral pneumonia & exclusively interstitial infiltrate in 28% with bacterial pneumonia

DIFFERENTIAL DIAGNOSIS

Bacterial Pneumonia

• •
  Focal/lobar lung consolidation > interstitial infiltrate

  • ○
    Confluent geographic, round, or fluffy opacities
• •Lack of ↑ peribronchial markings, hyperinflation
• •Pleural effusions more common with bacterial infection

Asthma

• •
  ↑ peribronchial markings & hyperinflation

  • ○
    Inflammation of small airways common to asthma & viral disease
• •Primary asthma diagnosis difficult to establish in young children

Bronchial Foreign Body

• •May present with wheezing very similar to viral disease
• •
  Asymmetric hyperinflation characteristic

  • ○
    Affected lung volume static throughout respiratory cycle
• •Foreign body often radiographically occult

Left-to-Right Cardiovascular Shunts

• •
  In infants, left-to-right shunts may have similar appearance

  • ○
    ↑ pulmonary arterial flow may mimic ↑ peribronchial markings
  • ○
    ± hyperinflation
• •Shunts have associated cardiomegaly

Miliary Tuberculosis

• •Diffuse small nodules, thickened interlobular septa
• •Lymphadenopathy, may be low in attenuation
• •History of sick contact with TB
• •Very uncommon in young children

PATHOLOGY

General Features

• •
  Etiology

  • ○
    Acute inflammation & necrosis of epithelial cells lining small airways with ↑ mucus production
  • ○
    ↓ caliber of small, relatively compliant airway lumina significantly ↑ resistance to airflow in infants
  • ○
    Occlusion of airways results in hyperinflation & foci of subsegmental atelectasis
  • ○
    Parenchymal findings vary: Interstitial pneumonitis with lymphocytic infiltration, infection of alveolar epithelium, diffuse alveolar damage, desquamation of pneumocytes, hyaline-membrane formation, alveolar hemorrhage
  • ○
    Typical pathogens

    • –
      Bronchiolitis: RSV > rhinovirus, adenovirus, influenza > coronavirus, human metapneumovirus (hMPV), parainfluenza

      • □
        Multiple viruses in up to 25%
    • –
      Hospitalization-inducing community acquired pneumonia: RSV, rhinovirus > hMPV, adenovirus, Mycoplasma pneumoniae, parainfluenza, influenza, coronavirus, Streptococcus pneumoniae, Staphylococcus aureus, Streptococcus pyogenes

CLINICAL ISSUES

Presentation

• •
  Signs & symptoms

  • ○
    Bronchiolitis: Rhinorrhea, cough, tachypnea, wheezing, rales, ↑ respiratory effort (grunting, nasal flaring, intercostal/subcostal retractions)
  • ○
    Community-acquired pneumonia: Cough, fever, anorexia, dyspnea, wheezing (up to 62%)
• •
  Difficult to differentiate bacterial from viral parenchymal infection based on physical exam or laboratory tests

  • ○
    Procalcitonin level may have diagnostic role: NPV of 95% for bacterial infection if < 2 ng/ml

Demographics

• •
  Age

  • ○
    Typical, striking radiographic findings of viral disease more often in young children (< 5 years of age)
• •
  Epidemiology

  • ○
    Bronchiolitis: Most common cause of hospitalization in 1st year of life
  • ○
    Viruses cause majority of chest infections in preschool children (4 months-5 years of age)

    • –
      < 2 years old: RSV most common

      • □
        Typical in late autumn, winter
    • –
      > 2 years old: Rhinovirus most common

      • □
        Typical in autumn, spring
  • ○
    Pneumonia-related hospitalization from multicenter prospective study

    • –
      Greatest in children < 5 years old
    • –
      Viral in 73%, bacterial in 15%
  • ○
    Predisposing conditions for more severe viral chest infections: Age < 6 months, prematurity, congenital heart disease, immunosuppression/immunodeficiency, asthma

Natural History & Prognosis

• •Resolution of symptoms over time, typically days to weeks

Treatment

• •Antibiotics for concomitant bacterial infection only (not isolated viral infection)
• •
  Bronchiolitis

  • ○
    Nebulized hypertonic saline in infants may shorten hospitalization by increasing mucociliary clearance
  • ○
    Oxygen supplementation if saturations < 90%
  • ○
    Palivizumab (RSV prophylaxis, not vaccine): Given in 1st year of life to infants born before 29 weeks gestation or infants with hemodynamically significant heart disease or chronic lung disease of prematurity
  • ○
    Routine use of albuterol & nebulized epinephrine not recommended
  • ○
    Corticosteroid use not supported
• •
  Clinical pneumonia with viral pathogen

  • ○
    Hospitalization if hypoxemia or respiratory distress
  • ○
    Antiviral therapy for influenza cases

DIAGNOSTIC CHECKLIST

Consider

• •Consider aspirated foreign body if lung volumes asymmetric

AP radiograph in a child with viral illness shows symmetric hyperinflation, increased peribronchial markings, & a linear band of atelectasis in the right upper lobe.

Axial CECT from a 25-year-old man with known influenza A infection shows tree-in-bud opacities in the right lower lobe , consistent with impaction of inflamed airways (bronchioles) by mucus or cells.

Frontal chest radiograph of a 5-month-old patient with cough & difficulty breathing shows bilateral “dirty” perihilar peribronchial opacities & lung hyperinflation with diaphragm depression , consistent with bronchiolitis.

Lateral radiograph from the same patient shows linear opacities extending from the hila , thickened bronchial walls , & downward sloping of the flattened hemidiaphragms . Note the widened AP dimension of the chest.

AP chest radiograph of a 7-year-old girl with influenza A infection shows patchy bilateral pulmonary opacities superimposed on bronchial wall thickening, consistent with viral parenchymal & airways disease.

Axial CECT from the same patient shows bilateral patchy, consolidative & ground-glass opacities, which can be seen with influenza A chest infection.

Lateral radiograph in an infant with bronchiolitis shows hyperinflation with ↑ anterior to posterior thoracic diameter & flattened hemidiaphragms . The hila are prominent with ↑ perihilar markings.

AP radiograph in an infant with bronchiolitis shows ↑ perihilar markings & generalized hyperinflation with atelectasis focally in the right upper lobe .

AP radiograph in an ill child shows ↑ peribronchial markings as prominent & indistinct linear densities radiating from hila. The lungs are hyperinflated, typical of bronchiolitis.

Frontal radiograph shows ↑ perihilar markings with hyperinflation, though it is difficult to fully appreciate the degree of hyperinflation on the frontal view.

Lateral radiograph of the same patient shows marked hyperinflation with flattening of the hemidiaphragms . The degree of hyperinflation is much easier to appreciate here than on the frontal view.

Frontal radiograph shows marked hyperinflation & ↑ peribronchial markings in this infant with bronchiolitis.

Lateral radiograph in the same patient shows marked hyperinflation with flattening of the hemidiaphragms , seen much better in this projection than on the frontal view.

Frontal radiograph shows increased perihilar markings & subsegmental atelectasis in the right middle lobe.

Lateral radiograph in the same patient shows marked hyperinflation with an ↑ AP diameter of the chest. ↑ perihilar markings are also noted, typical of bronchiolitis.

Axial CECT (shown in lung windows) obtained for other reasons shows CT findings of viral disease: ↑ prominence & indistinctness of interstitial & vascular markings, most pronounced about the hila, with geographic regions of parenchymal lucency due to air trapping. Some of the hilar fullness is due to mild adenopathy.

Frontal radiograph in a child with bronchiolitis shows ↑ peribronchial markings as rope-like densities radiating from the hila bilaterally. The lungs are generally hyperinflated with bibasilar subsegmental atelectasis. Gas-distended bowel in the upper abdomen is likely due to air swallowing.