Main causes of the sign
The pathological increase in density of the ground-glass type is non-specific and occurs in a number of conditions. Depending on the clinical picture, this pattern may indicate various diseases:
- Fibrotic non-specific interstitial pneumonia. Often found in patients with an established systemic connective tissue disease. CT scans reveal traction bronchiectasis (traction bronchiectasis) — subsegmental bronchi that do not taper toward the periphery. Their presence directly indicates the development of microscopic fibrosis, the size of which is below the resolution limit of the scanner.
- Acute intra-alveolar hemorrhage. This pathology is characterized by the appearance of extensive and visually "homogeneous" opacification of the lung tissue.
- Pulmonary edema. Accompanied by a mild ground-glass opacity sign combined with smooth thickening of the intralobular septa (intralobular septa).
Consolidation and the "dark bronchus" sign
It is important to distinguish ground-glass opacity from consolidation (consolidation). A classic example of consolidation is acute respiratory distress syndrome (ARDS). Non-contrast CT in ARDS reveals bilateral dense consolidation located predominantly in the gravitationally dependent lung regions.
The main radiological difference lies in the visualization of structures. In consolidation, the airways become very clearly visible against the surrounding airless tissue. However, the walls of the bronchi themselves, as well as blood vessels passing within the consolidation zone, are completely invisible.
Additionally, CT can reveal the "dark bronchus" sign (Black Bronchus Sign). It occurs because the air inside the segmental and subsegmental airways has a significantly lower X-ray attenuation compared to the air partially displaced by fluid or cells in the surrounding affected lung parenchyma.
Assessment of interlobular septa and Kerley lines
Interstitial changes are often accompanied by thickening of the interlobular septa. On conventional radiographs, they correspond to Kerley lines:
- Kerley B lines: correspond to thickened interlobular septa. They are 30–60 mm long and 1–2 mm wide. They are best visualized in subpleural regions, where they lie strictly perpendicular to the pleural surface.
- Kerley A lines: longer (up to 80–100 mm) and sometimes curved structures. They cross the inner two-thirds of the lung field in various directions, but are directed predominantly medially toward the lung root (hilum). They are much harder to visualize in practice.
In left ventricular failure, these septal lines become clearly visible because the septa are stretched by accumulating extravascular fluid. Important nuance: if adjacent alveoli are also opacified due to edema, the thickened septal lines will not be visible against their background.
Septal thickening is not strictly specific to pulmonary edema alone. For differential diagnosis, it should be kept in mind that they also become visible in the development of fibrosis or malignant infiltration (e.g., lymphangitis carcinomatosa — lymphangitis carcinomatosa).
The "peribronchial cuffing" sign
The "peribronchial cuffing" sign (peribronchial cuffing) is an extremely useful additional sign of interstitial edema. It is assessed primarily on plain chest radiographs. The essence of the sign is that the airway wall, which normally appears thin and sharply defined, pathologically thickens and becomes indistinct.
Briefly: answers to topical questions
Which diseases can cause a ground-glass opacity sign on CT?
The ground-glass opacity sign on CT can be caused by various interstitial, alveolar, and mixed pathological processes.
- Fibrotic non-specific interstitial pneumonia — in patients with systemic connective tissue diseases.
- Acute intra-alveolar hemorrhage — manifests as extensive homogeneous opacification.
- Pulmonary edema — causes mild ground-glass opacity.
- Acute respiratory distress syndrome (ARDS) — in combination with consolidation.
- Alveolar microlithiasis (Alveolar Microlithiasis) — widespread opacification.
- Lymphocytic interstitial pneumonia (Lymphocytic Interstitial Pneumonitis) — focal opacities.
The sign is also characteristic of acute eosinophilic pneumonia, alveolar proteinosis, diffuse pulmonary hemorrhage, and a number of infections.
What types of Kerley lines exist?
There are two main types of Kerley lines, which represent thickened lung septa.
| Characteristic | Kerley A lines (Kerley A lines) | Kerley B lines (Kerley B lines) |
|---|---|---|
| Anatomy | Thickened deep septa | Thickened interlobular septa |
| Dimensions | Length up to 80–100 mm | Width 1–2 mm, length 30–60 mm |
| Localization | Inner two-thirds of the lung, directed medially to the hilum | Subpleural regions, perpendicular to the pleural surface |
| Shape | Sometimes curved | Linear opacities |
Which infectious diseases typically manifest with a ground-glass opacity sign?
The ground-glass opacity sign on CT is typically manifested by various viral, bacterial, and fungal infections.
- Cytomegalovirus pneumonia (Cytomegalovirus Pneumonia) — diffuse and focal opacities.
- Pneumocystis pneumonia (Pneumocystis Pneumonia) — perihilar opacities.
- Mycoplasma infection (Mycoplasma) — indistinct rounded opacities.
- Influenza and parainfluenza viruses (Influenza and parainfluenza viruses) — including avian influenza.
- Middle East respiratory syndrome (Middle East Respiratory Syndrome, MERS) — diffuse opacification.
- Human metapneumovirus (Human metapneumovirus, hMPV) — focal areas.
The ground-glass "halo" sign around nodules is found in zoonotic pneumonias, mucormycosis (Mucormycosis), and varicella-zoster virus infection (Varicella-zoster virus).
To distinguish ground-glass opacity from consolidation, remember the rule: "ground-glass opacity" is transparent — vessels and bronchial walls are visible through it. Consolidation is "concrete": the lung tissue becomes so dense that the vessels and bronchial walls within it are completely obscured.
Class and exam questions
What does the appearance of traction bronchiectasis on CT indicate?
The presence of subsegmental bronchi that do not taper toward the periphery indicates microscopic lung tissue fibrosis, the size of which is too small to be directly visualized on CT.
Why might Kerley lines disappear in severe pulmonary edema?
Thickened and edematous septal lines cease to be visible if the adjacent alveoli also fill with fluid and become opacified on the scan.
Is interlobular septal thickening a 100% sign of heart failure?
No, this sign is not strictly specific. Septa also thicken in fibrotic processes and malignant infiltration, such as lymphangitis carcinomatosa.
What else to study on this topic
- Acute respiratory distress syndrome (ARDS): gravitationally dependent consolidation
- Traction bronchiectasis and fibrotic nonspecific interstitial pneumonia
- Carcinomatous lymphangitis (lymphangitis carcinomatosa)
- Topography and imaging of Kerley A and B lines
- "Peribronchial cuffing" sign on a plain chest radiograph
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