What is Ground Glass Opacity & Why Is It Seen In COVID-19 Scans
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
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
- human coronaviruses
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