Acute diseases and diffuse alveolar damage
In acute conditions such as acute respiratory distress syndrome (ARDS / ARDS) or acute eosinophilic pneumonia, ground-glass opacity is often combined with areas of consolidation.
A special place is occupied by acute interstitial pneumonia (AIP / AIP), which is considered an idiopathic form of ARDS. Histologically, it is represented by diffuse alveolar damage (DAD). On HRCT, it is characterized by:
- Focal or diffuse ground-glass opacities, which occur in all three phases of the disease (exudative, organizing, and fibrotic).
- Consolidation or collapse, localized predominantly in the gravitationally dependent (lower/posterior) parts of the lungs.
- Traction bronchiectasis, reflecting the early onset of fibrosis.
Viral pneumonias
Various viral agents cause specific CT patterns involving ground-glass opacities:
- Cytomegalovirus pneumonia (CMV): Develops in transplant recipients. Characterized by diffuse and focal areas of ground-glass opacity, as well as small nodules surrounded by a zone of decreased attenuation (halo sign / halo).
- Human metapneumovirus (hMPV): Manifests as bilateral but asymmetric focal areas of ground-glass opacity combined with small nodules and multifocal consolidation.
- SARS coronavirus (SARS-CoV): Causes unilateral or bilateral opacities (often in the lower lobes). With pronounced thickening of the intralobular interstitium or interlobular septa against the background of ground-glass opacity, a characteristic «crazy paving» pattern (crazy paving) is formed.
Idiopathic Interstitial Pneumonias (IIPs)
In chronic idiopathic processes, the presence and severity of ground-glass opacity help stratify the diagnosis:
- Non-specific interstitial pneumonia (NSIP / NSIP): The main sign is areas of ground-glass opacity, often associated with traction bronchiectasis, while honeycombing is minimal.
- Cryptogenic organizing pneumonia (COP / COP): Ground-glass opacity is combined with peripheral/peribronchial consolidation and a perilobular pattern.
- Lymphocytic interstitial pneumonia (LIP / LIP): Includes areas of ground-glass opacity, centrilobular nodules, thin-walled cysts, and septal thickening.
Relation to the UIP pattern (UIP): Ground-glass opacity is not characteristic of typical and probable usual interstitial pneumonia (it may be present only to a mild degree). If this sign predominates on HRCT, it indicates an alternative diagnosis pattern and requires searching for other pathology.
Smoking-related interstitial lung diseases (SR-ILD)
This group includes pathologies that have significant clinical and radiological overlap in smoking patients:
- Respiratory bronchiolitis-associated interstitial lung disease (RB-ILD): Manifests as subtle, poorly defined centrilobular ground-glass nodules.
- Desquamative interstitial pneumonia (DIP / DIP): Characterized by extensive non-specific ground-glass opacities (usually in the lower zones), which may be accompanied by mild reticulation.
As these conditions progress, areas of ground-glass opacity can transform into a diffuse lesion with established pulmonary fibrosis.
If there is a lot of ground-glass opacity on CT, this is a reason to reject the diagnosis of typical UIP and look for an alternative cause (infection, NSIP, or smoking-related reaction).
Class and exam questions
Does the ground-glass opacity sign exclude the diagnosis of usual interstitial pneumonia (UIP)?
Mild ground-glass opacity is acceptable in UIP. However, its predominance on HRCT is regarded as a sign of an alternative diagnosis.
What is the crazy paving pattern?
This is a CT pattern in which pronounced thickening of the intralobular interstitium or interlobular septa is visualized against the background of ground-glass opacity areas (found, for example, in SARS).
What is the difference between RB-ILD and DIP on CT?
In RB-ILD (respiratory bronchiolitis-associated interstitial lung disease), ground-glass opacity is represented by indistinct centrilobular nodules, and in DIP (desquamative pneumonia), by extensive non-specific opacities, predominantly in the lower parts.
What else to study on this topic
- Diagnostic criteria for the UIP pattern on HRCT
- Differential diagnosis of the honeycombing pattern
- Stages and HRCT presentation of diffuse alveolar damage (DAD)
- Smoking-related interstitial lung diseases (SR-ILD)
- The impact of a multidisciplinary approach on diagnosing ILD
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