What is a ground glass opacity (GGO) in the lung?

Ground Glass Opacity in the Lung: Definition and Clinical Significance

Ground glass opacity (GGO) is a radiological finding on high-resolution CT (HRCT) characterized by a hazy increase in lung density with preservation of bronchial and vascular margins, unlike consolidation where these structures are obscured. 1

Radiological Characteristics

• GGO appears as areas of increased attenuation in the lung where underlying vessels and bronchial structures remain visible 2, 3
• Can be patchy or diffuse in distribution 1
• May occur alone or in combination with other radiological findings such as:
  • Reticular lines
  • Traction bronchiectasis
  • Honeycombing
  • Consolidation 2

Clinical Significance

Diagnostic Value

• GGO is a significant radiological finding that often indicates active and potentially treatable lung disease 2
• The extent of GGO on CT is an important predictor of survival in certain conditions like idiopathic pulmonary fibrosis (IPF) 2, 4
• When GGO appears with reticular lines and traction bronchiectasis, it typically indicates lung fibrosis 2

Common Etiologies

GGO can be associated with various conditions:

1. Interstitial Lung Diseases:

   • Idiopathic pulmonary fibrosis (IPF)
   • Nonspecific interstitial pneumonia (NSIP)
   • Hypersensitivity pneumonitis
   • Desquamative interstitial pneumonitis (DIP) 2

2. Infectious Processes:

   • Pneumocystis pneumonia
   • COVID-19 pneumonia (typically bilateral, peripheral, patchy) 4
   • Various viral, bacterial, or fungal pneumonias 1

3. Inflammatory/Immune Conditions:

   • Cryptogenic organizing pneumonia (COP)
   • Sarcoidosis 1

4. Neoplastic Processes:

   • Early lung adenocarcinoma (particularly persistent focal GGOs) 5
   • Atypical adenomatous hyperplasia 4

5. Other Causes:

   • Pulmonary edema
   • Alveolar proteinosis
   • Drug-induced lung injury
   • Radiation pneumonitis 1

Diagnostic Approach

When GGO is identified on imaging:

• Evaluate the distribution pattern (peripheral, central, upper or lower lung predominance)
• Assess for associated findings (reticular lines, honeycombing, traction bronchiectasis)
• Consider the clinical context and patient history 2, 4

Important Considerations

• Extensive GGO (>30% of lung involvement) should prompt consideration of diagnoses other than IPF, particularly desquamative interstitial pneumonitis 2
• GGO without basal or peripheral predominance may suggest hypersensitivity pneumonitis, respiratory bronchiolitis-ILD, idiopathic BOOP, or nonspecific interstitial pneumonia 2
• Persistent focal GGOs may represent early adenocarcinoma and require appropriate follow-up 4, 5

Management Implications

• In IPF, ground glass opacity may be associated with alveolar inflammation or fibrotic thickening of alveolar septa 2
• Some studies suggest that ground glass attenuation may predict physiologic improvement after steroid treatment in certain conditions 2
• Areas of GGO may progress to reticular opacity or honeycombing on follow-up evaluation 2
• For persistent GGO nodules, management approaches include serial CT monitoring and consideration of surgical resection for lesions showing growth or developing solid components 4, 6

Pitfalls in Interpretation

• GGO must be distinguished from mosaic patterns of lung attenuation caused by primary vascular diseases or airway abnormalities 3
• Dependent atelectasis can mimic GGO and should be confirmed with prone inspiratory views 2
• Missing early malignancy in persistent focal GGOs can have serious consequences 4

Ground glass opacity is a significant radiological finding that warrants thorough evaluation and appropriate follow-up to determine its etiology and guide management decisions.