General dynamics of imaging features
The radiological picture of viral pneumonias undergoes consistent changes as the disease progresses:
- Initial stages (radiography): pulmonary tissue densification (consolidation) and diffuse opacities are detected. Initially, they are localized peripherally in the middle and lower zones, but over time spread centrally.
- First week (CT): ground-glass opacity patterns (ground-glass opacities), areas of consolidation, interlobular septal thickening, and pleural effusion accumulation dominate.
- Subsequent weeks (CT): the picture transforms. Centrilobular nodules form, and the crazy-paving pattern (crazy-paving) appears. Signs of bronchiolitis obliterans, peribronchial air trapping (air trapping), and an organizing pneumonia pattern frequently join.
Herpesviruses and Cytomegalovirus (CMV)
These pathogens pose the greatest threat to patients with severe immunosuppression.
Cytomegalovirus pneumonia (CMV) Is a leading cause of mortality among hematopoietic stem cell and solid organ transplant recipients, as well as in AIDS patients (with a CD4 count drop below 100 cells/mm³).
- Clinical timeline: develops in the post-transplant engraftment period (median 50–60 days).
- CT signs: diffuse and focal ground-glass opacity zones, lobar consolidation, irregular reticular shadows. The presence of miliary nodules with the halo sign (halo) is characteristic.
Herpes simplex virus type 1 (HSV-1) Causes life-threatening pneumonias in mechanically ventilated individuals. Frequently presents as a polymicrobial infection. Radiographs show massive bilateral airspace consolidation, while CT reveals subsegmental consolidation and ground-glass opacity areas.
Varicella-zoster virus (Varicella-zoster virus) Pneumonia is the most serious complication of primary chickenpox in adults. CT visualizes multicentric hemorrhages around bronchi, multiple 5–10 mm nodules with a ground-glass halo. As the disease resolves, these foci may calcify, turning into small (2–3 mm) densely calcified granules.
Hantaviruses and Metapneumovirus
- Hantavirus pulmonary syndrome (HPS):
This is a severe zoonosis transmitted via rodents. It causes diffuse alveolar damage in the exudative and proliferative phases. In early stages, the lungs may appear normal, but ARDS then rapidly develops: CT records extensive bilateral ground-glass opacity zones, bronchial and interlobular septal thickening, and basal diffuse consolidations.
- Human metapneumovirus (hMPV):
A recently discovered RNA virus causing bronchiolitis and pneumonia. Particularly dangerous for premature infants with congenital heart defects and adults with COPD exacerbations. On CT, it manifests as asymmetric ground-glass opacity areas, small nodules, and multifocal consolidation. In severe cases, it leads to pulmonary parenchyma fibrosis.
Influenza viruses and Coronaviruses
Swine (H1N1) and avian (H5N1) influenza
- With the H1N1 virus, changes have a peribronchovascular and subpleural distribution. A specific imaging sign is considered to be the reverse halo sign (reverse halo sign) — when a ground-glass opacity area is bordered along its contour by a dense ring of consolidation.
- Avian influenza (H5N1) runs an extremely severe course (mortality exceeding 60%). CT is dominated by multifocal consolidations, with pseudocavitations, pneumatoceles frequently forming, and lymph nodes enlarging.
Coronaviruses (SARS-CoV and MERS) During the development of severe acute respiratory syndrome (SARS), against the background of pronounced interlobular septal thickening within ground-glass opacity zones, the crazy-paving pattern is visualized. Middle East respiratory syndrome (MERS) yields a 30–40% mortality rate and is characterized by ill-defined bilateral consolidations predominantly in the upper lobes.
HIV-associated pulmonary lesions
With the introduction of highly active antiretroviral therapy (HAART), the spectrum of infections has changed significantly. AIDS-defining malignancies (such as Kaposi sarcoma) are much less common. In developed countries, bacterial pneumonias (Streptococcus pneumoniae predominates) have become the leading cause of chest involvement. However, in patients initiating HAART in the setting of latent mycobacterial infections, a paradoxical immune reconstitution inflammatory syndrome (IRIS) may develop.
Briefly: answers to topical questions
How does the immune reconstitution inflammatory syndrome (IRIS) manifest in HIV-infected patients?
Immune reconstitution inflammatory syndrome (IRIS) in HIV-infected patients manifests as a paradoxical worsening of clinical status and radiological picture against the background of viral replication control and CD4 level growth. This is due to an intense inflammatory response to dead or latent microorganisms (e.g., Mycobacterium tuberculosis, Cryptococcus, Toxoplasma gondii).
Pulmonary imaging manifestations:
- Lymph node enlargement (low attenuation) — mediastinal node involvement with central X-ray attenuation reduction.
- Pulmonary nodules (nodules) — diffuse bilateral changes.
- Organizing pneumonia — an inflammatory process that may be accompanied by small pleural effusion.
In central nervous system involvement (e.g., PML-IRIS), MRI determines confluent hyperintense white matter changes involving subcortical U-fibers and hypointense foci with ill-defined peripheral contrast enhancement.
The reverse halo sign (a ring of dense tissue surrounding clear ground-glass opacity) is a classic CT finding in swine flu (H1N1) and organizing pneumonia.
Class and exam questions
What does varicella-zoster virus pneumonia look like on CT?
In the acute phase, multiple 5–10 mm diameter nodules are determined, surrounded by a ground-glass halo (halo). Later, they calcify into dense, randomly scattered 2–3 mm granules.
Within what timeframe does CMV pneumonia develop post organ transplantation?
The disease is typical for the post-transplant engraftment period — usually 30 to 100 days post-surgery (on average at 50–60 days).
What is the crazy-paving pattern in viral pneumonia?
This is a CT pattern (crazy paving) occurring when ground-glass opacity zones combine with pronounced thickening of the intralobular interstitium and interlobular septa. Frequently encountered in SARS and at late stages of other viral infections.
What else to study on this topic
- Imaging patterns of pneumonias in immunocompromised patients (HIV, transplantation).
- Differential diagnosis of the reverse halo sign and simple halo.
- Hantavirus pulmonary syndrome (HPS): pathogenesis of diffuse alveolar damage.
- Immune reconstitution inflammatory syndrome (IRIS) in HIV-infected individuals.
- Tree-in-bud pattern in herpes simplex virus type 2 infection.
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