What does ground glass change in the lungs indicate?

Ground Glass Changes in the Lungs: Definition and Clinical Significance

Ground glass opacity (GGO) represents a hazy increase in lung density on CT imaging where bronchial and vascular structures remain visible, indicating either active inflammation (alveolitis) that may respond to treatment, or early fibrosis requiring different management. 1

Radiographic Definition

Ground glass opacity appears as diffuse homogeneous increased lung density on high-resolution CT (HRCT) with preserved visibility of vessels and bronchial walls—this distinguishes it from consolidation where these structures become obscured. 1, 2

What GGO Indicates Pathologically

The clinical significance depends critically on associated findings:

Isolated GGO (Without Fibrotic Features)

• Represents inflammatory cells in the alveolar septum or alveolar lumen (alveolitis). 1
• This pattern suggests potentially reversible disease that may respond to treatment. 2
• Common causes include active inflammation, partial alveolar filling, or increased capillary blood volume. 3

GGO With Fibrotic Features

• When GGO occurs with reticular lines and traction bronchiectasis/bronchiolectasis, it indicates established lung fibrosis. 1
• This pattern may progress to reticular opacity or honeycombing on follow-up. 1
• The extent of GGO correlates with physiologic impairment and helps predict treatment response. 1

Most Common Causes by Clinical Setting

Inpatient Setting

Hydrostatic pulmonary edema is the dominant cause, accounting for 75% of cases. 4

• Presents with hazy opacities, Kerley lines, and often pleural effusions. 5
• The combination of septal thickening and pleural effusions has 91% specificity for hydrostatic edema. 4
• Diffuse infection accounts for approximately 7% of inpatient cases. 4

Outpatient Setting

Interstitial lung diseases account for 49% of cases, with hypersensitivity pneumonitis (HP) and connective tissue disease-related ILD being most common. 4

• Hypersensitivity pneumonitis: Small, poorly defined centrilobular nodules with widespread GGO; the "three-density pattern" (hypoattenuating, normal, and hyperattenuating lobules) is highly specific. 1, 6
• Nonspecific interstitial pneumonia (NSIP): GGO without basal or peripheral predominance. 1, 5
• Desquamative interstitial pneumonitis (DIP): Extensive GGO, often in smokers. 1, 5
• Respiratory bronchiolitis-ILD: Ground-glass opacity with centrilobular nodules in smokers. 7, 5

Immunocompromised Patients

• Hydrostatic edema remains most common (46%), but diffuse infection accounts for 24% of cases. 4
• In bone marrow suppression, 80% of GGO cases are due to opportunistic viral infection. 4
• Pneumocystis pneumonia presents with diffuse bilateral perihilar infiltrates and patchy GGO with peripheral sparing. 1

Acute/Subacute Presentations

Organizing Pneumonia

• Patchy consolidation or GGO in predominantly peripheral or peribronchovascular distribution. 1
• The "reversed halo sign" (GGO surrounded by consolidation ring) may suggest this diagnosis. 1

Acute Exacerbation of Idiopathic Pulmonary Fibrosis

• New bilateral ground-glass opacities and/or consolidation superimposed on pre-existing reticular pattern or honeycombing. 7
• Represents diffuse alveolar damage (DAD) pattern with 50% or greater mortality. 7

Drug-Induced and Immune Checkpoint Inhibitor Pneumonitis

• New or progressive pulmonary infiltrates with ground glass changes, typically bilateral but may be asymmetric. 7
• Drug withdrawal is the mainstay of treatment; grade 2 or higher requires corticosteroids with minimum 4-6 week taper. 7

COVID-19 Pneumonia

• Multiple, patchy, sub-segmental ground-glass densities with subpleural distribution. 1
• "Paving stone-like" appearance from GGO with interlobular septal thickening is characteristic. 1

Critical Diagnostic Pitfalls

Extensive GGO (>30% of lung involvement) argues strongly against idiopathic pulmonary fibrosis and should prompt consideration of alternative diagnoses like NSIP, HP, or DIP. 1, 6

• Expiratory imaging can create pseudo-GGO appearance; ensure inspiratory scans are obtained. 4
• In pulmonary veno-occlusive disease, GGO with mosaic pattern is coupled by thickening of interlobular septa, distinguishing it from chronic thromboembolic disease. 7
• Centrilobular distribution of GGO is more specific for pulmonary veno-occlusive disease than panlobular distribution. 7

Diagnostic Approach

CT imaging is the modality of choice—more reliable than chest radiographs for identifying GGO. 7

• Obtain HRCT with both inspiratory and expiratory views to assess for mosaic attenuation and air-trapping. 6
• Evaluate distribution pattern: diffuse, patchy, peripheral, basal, centrilobular, or subpleural. 5
• Assess for associated findings: reticular opacities, honeycombing, traction bronchiectasis, septal thickening, pleural effusions, or lymphadenopathy. 7, 5
• Consider bronchoscopy with bronchoalveolar lavage when infection or HP is suspected; BAL lymphocyte differential >20% supports HP diagnosis. 6