Viral Pneumonias (General):

Clinical:

Viruses can result in several forms of lower respiratory tract infection including tracheobronchitis, bronchiolitis, and pneumonia [1]. Adenovirus has the greatest effect in the terminal bronchioles [1].

A viral etiology is the most common cause of pneumonia in children under the age of 5 years. Pathogens include: Respiratory syncytial virus (RSV), parainfluenza, adenovirus, influenza, enterovirus, and rhinovirus. Complications include: Subsequent bacterial pneumonia; Bronchiectasis; and Swyer-James Syndrome. The infection primarily affects the AIRWAYS (not the alveoli) therefore will see changes secondary to airblock.

In adults viral pneumonias are usually gradual in onset, have associated non-pulmonary symptoms, and typically only produce mild elevations in the white blood cell count. RSV and parainfluenza virus typically produce only upper airway infections or bronchitis (characterized by evidence of airway inflammation with tree-in-bud opacities, bronchial wall thickening, and peribronchiolar consolidaiton) [2]. Rhinovirus predominantly involves only the upper respiratory tract. Adenovirus infection typically prresents as a multifocal pneumonia pattern with areas of ground-glass opacity or consolidation without airway abnormalities [2].

Influenza A is the most important respiratory viral illness with more than 35,000 deaths and 200,000 hospitalizations annually [1]. Symptoms include rapid onset of high fever, myalgias, headache, lethargy, sore throat, and cough [1].

Avian flu is caused by the H5N1 subtype of influenza A virus and approximately 90% of infections have occurred in patients 40 years old or younger [1]. The fatality rate exceeds 60% [1].

Measles virus is a highly contagious infection with an incubation time of almost 2 weeks [1]. It can result in a fatal pneumonia in immunocompromised and debilitated patients [1].

X-ray:

In children, the CXR characteristically demonstrates HYPERINFLATION with a reticular or airspace pattern. The infection is more commonly bilateral as opposed to lobar. Other findings include perihilar linear densities with loss of hilar and vascular sharpness, patchy atelectasis, and bronchial wall thickening (peribronchial cuffing). Adenopathy (3%) and effusion are uncommon.

In adults, viral infections commonly produce a diffuse, fine reticular pattern. Bronchial wall thickening is also common. Severe infection can result in air space opacities- typically patchy in distribution. Patients that develop a pneumonia secondary to an influenza infection demonstrate a rapidly progressive bilateral pneumonia or patchy bronchopneumonia. Superinfection with Staphylococcus should be suspected if cavitation develops.

On HRCT, bronchial wall thickening, septal prominence, patchy areas of heterogeneous/ground glass attenuation, centrilobular ground glass nodules, airspace consolidation, and/or small branching nodular opacities in the lung periphery (tree-in-bud- indicative of bronchiolar impaction) can all be seen [1].

REFERENCES:
(1) Radiology 2011; Franquet T. Imaging of pulmonary viral pneumonia. 260: 18-39

(2) AJR 2011; Miller WT, et al. CT of viral lower repiratory tract infections in adults: comparison among viral organisms and between viral and bacterial infections. 197: 1088-1095