Pathogenesis and imaging findings
The pathogenesis of IRIS is based on an excessive inflammatory reaction of the recovering immune system to antigens present in the body.
Radiological and CT manifestations of the syndrome:
- Mediastinal lymphadenopathy. A characteristic feature is central low attenuation (low attenuation).
- Appearance of diffuse bilateral pulmonary nodules (nodules).
- Pattern of organizing pneumonia.
- Presence of a small pleural effusion.
The role of mycobacteria as antigens
Mycobacterial infections frequently serve as a background for the development of IRIS. More than 95% of these are caused by Mycobacterium tuberculosis, with the remaining cases attributed to nontuberculous mycobacteria (NTM) such as M. kansasii and Mycobacterium avium-intracellulare complex (MAC). Risk factors include HIV infection, immunodeficiencies, diabetes mellitus, alcoholism, silicosis, malignancies, as well as social factors (poverty, homelessness, stay in closed institutions).
Primary tuberculosis
Traditionally detected in children, but in developed countries up to 23–34% of cases occur in adults. Bacilli spread to regional lymph nodes and throughout the body within 2–6 weeks. Pathomorphologically, the process is characterized by the accumulation of macrophages, monocytes, and exudate in the pneumonia zone.
Radiological semiotics:
- In 15% of cases, the radiograph remains normal.
- Parenchyma: homogeneous lobar consolidation, visually indistinguishable from bacterial pneumonia.
- Lymphadenopathy: the most frequent sign in children.
- Effusion: often massive, unilateral.
During healing, one-third of patients develop a Ghon focus (a clearly demarcated shadow in the lung), which, combined with calcification of the ipsilateral lymph node, forms the Ranke complex.
Secondary tuberculosis (reactivation)
Occurs predominantly due to the reactivation of dormant foci. The immune response leads to caseous necrosis. Unlike primary tuberculosis, there is no lymphadenopathy, the upper lobes are primarily affected, and cavities often form (in 40–80% of active cases; fluid levels within them are atypical—less than 20%).
Specific manifestations and complications:
- Rasmussen aneurysm: a life-threatening weakening of the pulmonary artery wall. On CT, it appears as a contrast-filled aneurysm within consolidation near a cavity.
- Endobronchial spread produces the "tree-in-bud" sign (tree-in-bud).
- Miliary tuberculosis: multiple small (1–2 mm) nodules scattered throughout both lungs (in elderly patients, more often as post-primary).
- Tuberculoma: a stable lesion, surrounded by satellite nodules in 80% of cases.
- Empyema necessitatis: involvement of the soft tissues and ribs of the chest wall.
Nontuberculous mycobacteria (NTM)
Account for up to 13% of pulmonary mycobacterial infections (mostly MAC). These are free-living saprophytes. Notably, the infection is not transmitted from person to person, but is acquired from the environment through inhalation or ingestion by susceptible patients.
Briefly: answers to topical questions
What is the morphological substrate of the "tree-in-bud" sign on CT?
The morphological substrate of the "tree-in-bud" sign (tree-in-bud) on CT consists of small centrilobular nodular and linear branching opacities. This sign indicates inflammatory and infectious bronchiolitis; in secondary tuberculosis, it is associated with endobronchial spread.
Which lung segments are most frequently affected in secondary tuberculosis?
In secondary (post-primary) tuberculosis, the upper lobes of the lungs are predominantly affected, although specific segments are not specified.
The radiological picture of secondary tuberculosis has the following distinctive features:
- Absence of lymphadenopathy — the main difference from the primary form of the disease.
- Cavity formation — a frequent sign of an active process, occurring in 40–80% of cases.
- Endobronchial spread — radiologically produces the "tree-in-bud" sign (tree-in-bud).
Also, during healing, pronounced loss of lung volume and pleural thickening are observed.
To differentiate secondary tuberculosis from primary: Secondary = Upper lobes + Pronounced cavities + Absence of lymphadenopathy.
Class and exam questions
What is the essence of the paradoxical response in IRIS?
The patient's condition and radiological picture worsen against the background of objective laboratory improvement (increased CD4, virus suppression) due to a vigorous reaction to present antigens.
What is a Rasmussen aneurysm?
This is a rare granulomatous weakening of the pulmonary artery wall near a tuberculous cavity, which can lead to life-threatening hemorrhage.
What does the Ranke complex include?
The Ranke complex represents a combination of a Ghon focus (a calcified shadow in the lung parenchyma) and calcification of the ipsilateral root or mediastinal lymph node.
What else to study on this topic
- Pathogenesis and differential diagnosis of organizing pneumonia in IRIS
- "Tree-in-bud" sign on CT: endobronchial spread of infection
- Features of chest wall involvement (empyema necessitatis)
- Miliary tuberculosis: mechanisms of hematogenous dissemination
- Predisposing factors for infections caused by nontuberculous mycobacteria
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