What are the X-ray features of influenza?

X-Ray Features of Influenza

In acute uncomplicated influenza, the chest X-ray is usually normal, but when pneumonia develops, radiographic findings vary dramatically depending on whether the patient has primary viral pneumonia (bilateral interstitial infiltrates predominantly in mid-zones) or secondary bacterial pneumonia (lobar consolidation). 1

Uncomplicated Influenza

• The chest X-ray is typically normal in patients with uncomplicated influenza infection without pneumonia. 1
• Most outpatients with mild, self-limited influenza have normal chest radiographs (67% in pediatric studies). 2

Primary Viral Pneumonia

When primary viral pneumonia complicates influenza (typically within 48 hours of fever onset), characteristic radiographic patterns emerge:

• Bilateral interstitial infiltrates are the most common finding, predominantly affecting the mid-zones of the lungs. 1
• Ground-glass opacities appear frequently, often in a patchy, multifocal distribution. 3, 4, 5
• Focal consolidation is also well-recognized despite the predominantly interstitial pattern. 1
• Peribronchovascular and subpleural consolidation may be present, particularly in the middle and upper lung zones. 3
• Bilateral lung shadowing on chest X-ray consistent with primary viral pneumonia should be considered a feature of severe pneumonia regardless of CURB-65 score. 1

CT Findings in Primary Viral Pneumonia

• Ground-glass opacities are the predominant finding on CT (present in 84.5% of patients with H1N1 pneumonia). 3
• Interlobular septal thickening and centrilobular nodules are common secondary findings. 4
• Consolidation areas demonstrate peribronchovascular and subpleural distribution. 3

Secondary Bacterial Pneumonia

Secondary bacterial pneumonia typically develops 4-5 days after initial symptom onset during the early convalescent period:

• Lobar pattern of consolidation is the characteristic finding. 1
• Unilateral or focal consolidation is more typical than the bilateral pattern seen in viral pneumonia. 1
• Cavitations or pleural changes strongly suggest bacterial superinfection rather than pure viral pneumonia. 1
• Pleural effusions may be present and indicate bacterial involvement. 1

Combined Viral-Bacterial Pneumonia

• Clinical and radiographic features appear later than primary viral pneumonia. 1
• Cavitation or pleural effusions are frequently observed. 1
• The radiographic pattern may show features of both interstitial and consolidative disease. 1

Distribution and Extent Patterns

• In children with severe influenza requiring hospitalization or ICU admission, bilateral, symmetric, multifocal areas of consolidation associated with ground-glass opacities are the predominant findings. 2
• Lower lung zone involvement is common (observed in interstitial patterns). 5
• Prominent peribronchial markings with hyperinflation may be the only finding in mild cases, particularly in children. 2
• The total volume of affected lung is typically less than one lobe, even when involvement appears extensive. 4

Prognostic Radiographic Features

• Greater number of lobes involved and higher radiographic severity scores correlate with need for mechanical ventilation and worse outcomes. 3
• Multifocal bilateral opacities indicate more severe illness. 3
• Extensive disease (>1 lobe involvement) was observed in 37.5% of hospitalized patients with H1N1 pneumonia. 5

Clinical Indications for Chest X-Ray

• A chest X-ray should be obtained during assessment of suspected influenza cases seen in hospital settings (emergency department or acute admissions ward). 1
• In children, chest X-ray should be performed if they are hypoxic, have severe illness, or are deteriorating despite treatment. 1
• Chest X-ray is indicated when pneumonia is clinically suspected based on new or worsening dyspnea, focal chest signs, or respiratory distress. 1, 6
• Repeat chest X-ray at approximately 6 weeks is recommended if respiratory symptoms or signs persist or where there is concern. 1

Important Caveats

• Chest radiography is too insensitive to differentiate bacterial from non-bacterial pneumonia reliably, though consolidation is reasonably specific (74%) for bacterial etiology. 1
• There is significant inter-observer and intra-observer variability in interpreting pediatric chest radiographs for pneumonia. 1
• Nodular opacities, reticular opacities, and lymphadenopathy are uncommon findings in influenza pneumonia. 2
• The absence of radiographic abnormalities does not exclude influenza infection, as most uncomplicated cases have normal chest X-rays. 1