Ground-glass opacity (GGO) is a term used by medical professionals to describe a specific finding on computed tomography (CT) scans of the lungs. It is a radiological pattern indicating a subtle alteration in lung tissue density. GGO is not a disease itself, but a visual sign pointing to underlying changes within the lungs that require further investigation.
Understanding Ground-Glass Opacity
On a CT scan, healthy lung tissue appears dark because it is primarily filled with air. Ground-glass opacity, however, presents as a hazy, gray area of increased density within the lung. Despite this increased density, the underlying blood vessels and bronchial structures remain visible through the affected area, distinguishing it from a more opaque consolidation where these structures would be obscured. This hazy appearance is likened to frosted or sand-blasted glass, which gives the finding its name.
The presence of GGO indicates that the lung’s air spaces are partially filled with substances like fluid, pus, or cells. It can also signify a thickening of the alveolar walls, which are the tiny air sacs where gas exchange occurs, or the interstitium, the tissue surrounding these sacs. This partial filling or thickening increases the tissue’s density, making it appear lighter on the scan. GGO can be diffuse, appearing in multiple areas, or localized to a specific region of the lung.
Infectious Causes
Infections are a common reason for ground-glass opacity to appear in the lungs, as they often cause inflammation and fluid accumulation. Viral infections, such as influenza, respiratory syncytial virus (RSV), and COVID-19 (SARS-CoV-2), frequently induce GGO. COVID-19 often presents as bilateral opacities at the lung periphery, with over 80% of patients showing GGOs on CT scans.
Certain bacterial infections, particularly atypical pneumonias like Mycoplasma pneumoniae, can also manifest as GGO, unlike typical bacterial pneumonias that often lead to more solid consolidations. Fungal infections, such as Pneumocystis pneumonia, are another infectious cause where GGO is a recognized finding. In these cases, the body’s immune response leads to inflammation and fluid in the alveoli.
Non-Infectious Inflammatory Causes
Beyond infections, ground-glass opacity can stem from various non-infectious inflammatory conditions. Acute interstitial pneumonia (AIP), a rapidly progressive lung disease, often presents with GGO and can resemble acute respiratory distress syndrome (ARDS). AIP involves widespread damage to lung tissue, leading to fluid and cellular buildup.
Hypersensitivity pneumonitis, an inflammatory reaction to inhaled allergens, commonly features GGO, sometimes alongside small nodules. Acute respiratory distress syndrome (ARDS), a severe lung injury, is characterized by extensive GGO and consolidation due to widespread inflammation and fluid leakage. Autoimmune diseases, including lupus and rheumatoid arthritis with lung involvement, can also cause GGO due to inflammation and fluid in the lung tissue.
Other Medical Conditions
Ground-glass opacity can also be a sign of other diverse medical conditions. Pulmonary edema, fluid accumulation in the lungs often due to heart failure, is a frequent cause of GGO, as increased pressure forces fluid into the air sacs.
Drug-induced lung injury, an adverse reaction to certain medications, can cause inflammation and result in GGO. Diffuse alveolar hemorrhage, involving bleeding into the lung’s air sacs, also presents with GGO due to blood filling the alveoli. Additionally, some chronic interstitial lung diseases, which cause scarring and inflammation, may show GGO in their early stages. Certain types of lung cancer, particularly early-stage adenocarcinomas, can also appear as GGO, sometimes as a nodule.
Diagnosis and Significance
Ground-glass opacity is primarily identified through computed tomography (CT) scans of the lungs. While CT reveals GGO, it does not provide a definitive diagnosis of the underlying cause, as it is a non-specific finding.
To pinpoint the specific cause, further diagnostic tests are often needed. These may include blood tests, sputum cultures, or more invasive procedures like bronchoscopy with bronchoalveolar lavage or lung biopsy. The significance and treatment of GGO depend entirely on its underlying cause. Some GGOs might resolve on their own, while others indicate serious diseases requiring prompt medical intervention.