What is ground glass infiltration?

Ground Glass Infiltration: Definition and Clinical Significance

Ground glass opacity (GGO) is a radiographic finding characterized by a hazy increase in lung density with preservation of bronchial and vascular margins, representing partial filling of air spaces, inflammatory or fibrotic interstitial thickening, or increased capillary blood volume. 1, 2

Radiographic Characteristics

• GGO appears as a diffuse homogeneous increase in lung density on high-resolution computed tomography (HRCT), where vessels and bronchial walls remain visible (unlike consolidation, where these structures are obscured) 1, 2
• When GGO occurs with reticular lines and traction bronchiectasis/bronchiolectasis, it indicates lung fibrosis 1
• Isolated GGO without fibrotic features is usually associated with inflammatory cells in the alveolar septum or alveolar lumen (alveolitis) 1

Clinical Significance

• GGO is a nonspecific finding but always significant as it often indicates active and potentially treatable disease 3
• The extent of GGO on CT correlates with physiologic impairment and can help predict response to treatment 1
• GGO may progress to reticular opacity or honeycombing on follow-up evaluation in fibrotic processes 1
• Areas of GGO may regress with treatment in certain conditions like desquamative interstitial pneumonitis (DIP) 1

Common Causes of Ground Glass Opacities

Interstitial Lung Diseases

• Idiopathic pulmonary fibrosis (IPF) - though extensive GGO (>30% of lung involvement) should prompt consideration of diagnoses other than IPF 1, 4
• Nonspecific interstitial pneumonia (NSIP) - typically presents with GGOs without basal or peripheral predominance 1, 4
• Desquamative interstitial pneumonitis (DIP) - characterized by extensive GGOs 1, 4
• Hypersensitivity pneumonitis - may show GGOs with centrilobular nodules and mosaic attenuation 1

Infectious Causes

• Pneumocystis pneumonia - typically presents with diffuse bilateral perihilar infiltrates, patchy areas of GGO with peripheral sparing 1
• Viral pneumonias 5
• Mycobacterial infections - including nontuberculous mycobacterial hypersensitivity-like lung disease 1

Other Causes

• Pulmonary edema - presents with hazy opacities and Kerley lines 4
• Alveolar hemorrhage - bilateral patchy GGOs in middle and lower lung zones 4
• Drug-induced lung disease 4, 5
• Radiation pneumonitis - typically shows GGOs within the radiation portal 4
• Alveolar proteinosis 3, 5

Diagnostic Approach

• Evaluate distribution pattern (diffuse, patchy, peripheral, basal) 4, 5
• Note associated findings such as reticular opacities, honeycombing, traction bronchiectasis, and centrilobular nodules 4
• Consider specific patterns:
  • "Three-density pattern" (hypoattenuating, normal, and hyperattenuating lobules in close proximity) is highly specific for fibrotic hypersensitivity pneumonitis 1
  • "Reversed halo sign" (area of GGO surrounded by a ring of consolidation) may suggest fungal pneumonia, particularly due to mucormycosis 1
  • Mosaic attenuation (coexisting areas of varying attenuation) may indicate small airway disease, particularly in hypersensitivity pneumonitis 1

Clinical Correlation

• Duration of symptoms is important in narrowing differential diagnosis 6
• Presence or absence of lung fibrosis, especially honeycombing, helps distinguish acute from chronic processes 6
• Patient's smoking history may point to specific etiologies 6
• Immune status and preexisting medical conditions are vital considerations 5

Ground glass opacities represent a significant radiographic finding that warrants thorough evaluation. While nonspecific, their pattern, distribution, and associated findings can help narrow the differential diagnosis and guide appropriate management.