Ground-glass opacity (GGO) is a term used by radiologists to describe a specific finding on a Computed Tomography (CT) scan of the lungs. It is not a medical diagnosis, but rather a descriptive observation indicating an abnormality within the lung tissue. This hazy appearance suggests that something is partially filling the air sacs or thickening the walls of the tiny air passages. GGO means the lung tissue is denser than normal, but not so dense that it completely blocks the view of underlying structures.
What Ground-Glass Opacity Looks Like
This finding gets its name because its appearance on the CT image resembles frosted or ground glass, where light is scattered but not entirely obscured. On a normal CT scan, healthy, air-filled lungs appear black, but GGO shows up as a hazy gray area of increased density. The defining characteristic of GGO is that the outlines of the pulmonary blood vessels and the walls of the bronchi (airways) remain visible through the haze.
This finding differentiates GGO from “consolidation,” where air in the lung is completely replaced by fluid or tissue, making underlying structures invisible. The physical mechanism behind GGO involves only a partial displacement of air within the alveoli, the tiny air sacs where gas exchange occurs. This partial filling can be caused by fluid, pus, blood, inflammatory cells, or an increase in the thickness of the alveolar walls themselves. The appearance of GGO indicates that the lung parenchyma—the functional lung tissue—is affected, but air still remains within the alveoli. Inflammation that causes the alveolar walls to swell, such as in certain interstitial lung diseases, also results in this hazy pattern.
Primary Categories of GGO Causes
GGO causes can be grouped into three main categories: infectious, inflammatory/interstitial, and neoplastic (cancerous). The specific pattern and distribution of the GGO on the scan often provide initial clues about the likely cause. For example, a widespread or diffuse pattern across both lungs often suggests a systemic or acute process, while a localized, focal pattern raises different concerns.
Infectious Causes
GGO is a common finding in many types of pneumonia because the infection triggers an inflammatory response that partially fills the air sacs. Viral infections are particularly known for producing GGO, including common causes like influenza, respiratory syncytial virus (RSV), and the SARS-CoV-2 virus responsible for COVID-19. In COVID-19 patients, GGO often presents in the periphery of both lungs, sometimes mixed with areas of consolidation. Certain atypical bacterial and fungal infections also manifest as GGO.
For example, Pneumocystis jirovecii pneumonia (PJP), which typically affects individuals with weakened immune systems, often results in extensive GGO, frequently centered around the middle of the lungs. The rapid onset of symptoms combined with a diffuse GGO pattern helps distinguish acute infections from more chronic conditions.
Inflammatory and Interstitial Causes
Non-infectious causes of GGO relate to inflammation or scarring within the lung tissue. Hypersensitivity pneumonitis, an allergic reaction to inhaled substances like mold or chemicals, can cause GGO, sometimes predominantly in the upper or central lung zones. Drug-induced lung injury, where medications trigger an inflammatory response in the lungs, is another significant cause. Pulmonary edema, which is fluid accumulation often due to congestive heart failure, is a frequent cause of GGO. Other causes include diffuse alveolar hemorrhage, where small blood vessels leak blood into the alveoli, seen in certain autoimmune conditions.
Neoplastic and Pre-Cancerous Causes
When GGO appears as a localized nodule, it is often a sign of early-stage lung cancer, particularly a type called adenocarcinoma. These focal GGOs represent a non-invasive or minimally invasive form of cancer, such as atypical adenomatous hyperplasia (a precursor lesion) or adenocarcinoma in situ. These lesions tend to grow along the existing alveolar walls without destroying the underlying structure. These focal lesions can be “pure GGOs,” meaning they are entirely hazy, or “part-solid GGOs,” which contain a small, denser solid component within the haze.
The presence of a solid component significantly increases the suspicion of a more invasive cancer. Because these lesions can grow slowly, they often require a different management approach than rapidly growing infections.
Interpreting a GGO Finding
Interpreting ground-glass opacity depends heavily on the patient’s overall context, including symptoms, medical history, and laboratory results. A GGO found in a patient with a fever and cough is managed differently than one found incidentally during a screening CT scan in an otherwise healthy person. The initial step is often to determine if the GGO is acute and likely transient, or chronic and persistent.
Many GGOs, particularly those caused by acute infections or inflammation, will resolve on a follow-up CT scan after a few weeks or months of appropriate treatment. If the cause is suspected to be benign and transient, a period of observation is recommended. Conversely, GGOs that persist or slowly increase in size are approached with greater suspicion for malignancy.
For persistent or focal GGOs, medical guidelines recommend surveillance with serial CT scans to monitor for change over time. An increase in the size of the nodule or the development of a solid component within a GGO is a strong sign of progression toward invasive cancer. If growth is confirmed, a biopsy or surgical removal may be necessary to establish a definitive diagnosis and treatment plan.