A neutropenic patient (ANC <100/µL) on 3 weeks of broad-spectrum antibiotics develops fever unresponsive to antibiotics, new pulmonary infiltrates and a lung CT showing a 'halo sign' (ground glass density surrounding a nodule). Serum galactomannan ELISA is positive at index >0.5. Which pathological process creates the halo sign?
- A Bacterial pneumonia with surrounding reactive alveolar oedema
- B Cryptococcal infection with a mucoid halo from capsular polysaccharide accumulation
- C Invasive pulmonary aspergillosis: central infarction of the nodule with surrounding haemorrhagic halo from angioinvasion and bleeding into air spaces ✓
- D Viral pneumonitis with peribronchovascular inflammation
Explanation
The CT halo sign in neutropenic patients is the radiological hallmark of early invasive pulmonary aspergillosis (IPA). Aspergillus hyphae invade blood vessel walls (angioinvasion), causing thrombosis and infarction of the central nodule, with surrounding haemorrhage into alveolar spaces appearing as a 'halo' of ground-glass opacity. As neutrophil recovery occurs, the halo disappears and may be replaced by a 'crescent sign' (air within cavitating nodule). Serum galactomannan (1,5-beta-galactofuranose residues of Aspergillus cell wall) ELISA is used for early non-invasive diagnosis in high-risk patients; specificity is highest in haematology patients.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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Written and medically reviewed by the StethoPrep medical team.