Ground-glass opacification/opacity (GGO) is a descriptive term referring to an area of increased attenuation in the lung on computed tomography (CT) with preserved bronchial and vascular markings. It is a non-specific sign with a wide aetiology including infection, chronic interstitial disease and acute alveolar disease.

Ground glass opacification is also used in chest radiography to refer to a region of hazy lung radiopacity, often fairly diffuse, in which the edges of the pulmonary vessels may be difficult to appreciate.

Ground-glass opacities have a broad aetiology:

• normal expiration
  • on expiratory acquisitions, which can be detected if the posterior membranous wall of the trachea is flattened or bowed inwards
• partial filling of air spaces
• partial collapse of alveoli
• interstitial thickening
• inflammation
• oedema
• fibrosis
• lepidic proliferation of neoplasm

There are four broad causes for pathologies that exhibit ground-glass opacity on CT scans, these being;

Broadly speaking, the differential for ground-glass opacification can be split into ⁵:

• infectious processes (opportunistic vs non-opportunistic)
• chronic interstitial diseases
• acute alveolar diseases
• other causes

Infections

Opportunistic

• pneumocystis pneumonia (PCP/PJP)
• cytomegalovirus (CMV) pneumonia
• herpes simplex virus (HSV) pneumonia
• respiratory syncytial virus (RSV) bronchiolitis: type of infectious bronchiolitis
• other infectious causes

Non-opportunistic

• viral pneumonia
  • human coronaviruses
    • COVID-19
    • Middle East respiratory syndrome coronavirus (MERS-CoV) infection
    • severe acute respiratory syndrome (SARS)
  • herpesviridae

Chronic interstitial diseases

• eosinophilic pneumonias: ground-glass opacification can be seen in many of the eosinophilic pneumonias but is most commonly seen in:
  • simple pulmonary eosinophilia (SPE): nodules with a GGO halo
  • idiopathic hypereosinophilic syndrome (IHS): nodules with a GGO halo
  • acute eosinophilic pneumonia (AEP): bilateral patchy areas of GGO with interlobular septal thickening
  • eosinophilic drug reactions: peripheral airspace consolidation and GGO
• idiopathic interstitial pneumonias
  • non-specific interstitial pneumonia: GGO with linear or reticular markings, micronodules, consolidation, and microcystic honeycombing
  • usual interstitial pneumonia (UIP): focal GGO with macrocystic honeycombing, reticular opacities, traction bronchiectasis, and architectural distortion
  • cryptogenic organising pneumonia (COP): formerly bronchiolitis obliterans with organising pneumonia (BOOP); GGO with airspace consolidation and mild bronchial dilatation
  • exudative phase of acute interstitial pneumonia (AIP): diffuse lung consolidation with GGO
  • respiratory bronchiolitis-associated interstitial lung disease (RB-ILD): patchy GGO centrilobular nodules and bronchial wall thickening
  • desquamative interstitial pneumonia (DIP): GGO with linear or reticular opacities
  • lymphoid interstitial pneumonia (LIP): GGO often in association with perivascular cystic lesions, septal thickening, and centrilobular nodules
• sarcoidosis (pulmonary manifestations of sarcoidosis)

Acute alveolar disease

• alveolar oedema or pulmonary oedema
  • cardiogenic pulmonary oedema
  • adult respiratory distress syndrome (ARDS)
  • other causes of non-cardiogenic pulmonary oedema
• hypersensitivity pneumonitis: especially acute and subacute forms

Other causes

• neoplastic processes with a lepidic proliferation pattern
  • atypical adenomatous hyperplasia
  • localised adenocarcinoma
  • adenocarcinoma in situ or minimally invasive (formerly bronchoalveolar cell carcinoma)
• drug toxicity

Rare causes

• focal interstitial fibrosis: a non-neoplastic entity with a nodular ground-glass opacity that does not change over a long period of time; can be mistaken for a neoplastic process
• aspergillosis: a nodule with surrounding ground-glass opacity (CT halo sign) is rare except in severely immunocompromised patients
• thoracic endometriosis
• traumatic lung injury (pulmonary contusion)
• poisoning e.g. acute/subacute phase of paraquat poisoning
• pulmonary cryptococcus infection: solitary or multiple pulmonary nodules with or without peripheral GGO
• granulomatosis with polyangiitis
• Henoch-Schönlein purpura