Understanding GGO

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.

Causes

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

Differential Diagnosis

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
  • pneumocystis pneumonia (PCP/PJP)
  • cytomegalovirus (CMV) pneumonia
  • herpes simplex virus (HSV) pneumonia
  • respiratory syncytial virus (RSV) bronchiolitis: type of infectious bronchiolitis
  • other infectious causes
  • viral pneumonia
    • human coronaviruses
      • COVID-19
      • Middle East respiratory syndrome coronavirus (MERS-CoV) infection
      • severe acute respiratory syndrome (SARS)
    • herpesviridae
  • 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)
  • 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
  • neoplastic processes with a lepidic proliferation pattern
    • atypical adenomatous hyperplasia
    • localised adenocarcinoma
    • adenocarcinoma in situ or minimally invasive (formerly bronchoalveolar cell carcinoma)
  • drug toxicity
  • 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