The Halo Effect in Invasive Fungi Disease

In this video, Dr Varun Mehra will be talking about the clinical signs and symptoms of invasive fungal disease and ‘The Halo effect’.

‘The Halo sign’ is a characteristic pattern often found when taking CT chest images of Invasive Pulmonary Aspergillosis (IPA) patients, and is defined by a ground-glass opacity encircling a pulmonary nodule or mass. This pattern is caused by an inflammatory necrotic mass with a surrounding oedema and neutrophilic infiltrates.

During a study by Kami M et al. (2002), it was found that the ‘Halo’ was only found in hospitalised patients with haemotological malignancy and neutropenia with IPA, with 13 out of 17 patients showing this pattern on CT chest scans. In patients without IPA, the ‘Halo’ pattern was absent. This shows how useful the ‘Halo’ sign can be in diagnosing IPA. However, the ‘Halo’ sign can be found in other diseases such as Coccidioidomycosis, Kaposi’s Sarcoma, and Metastatic Angiosarcoma, indicating that this a low sensitivity diagnosis tool, and you should consider additional clinical features and other diagnostic tests to hone down the diagnosis.

Other variations of the ‘Halo’ sign include ‘Air Crescent’ signs which can also present themselves in patients with Tuberculosis, Haemoptysis, Neutropenia, and refractory pyrexia. ‘Air Crescent’ usually appears later in the evolution of the IPA lung lesion, with the recovery of the neutrophil count. In the correct context however, it could be argued to be more likely to be a fungal disease.

Current methods in imaging include CT scans, PET scans, and CTPA scans. CT scans are the most common and standard imaging technique for detecting pulmonary infection, though they are time and operator dependent, cannot differentiate between Aspergillus and other pathogenic fungi, and have limited specificity and predictive value. PET scans are more sensitive, though it is still difficult to differentiate from pure inflammation or true infection.

CTPA is a type of CT scan that can identify obstructions in blood vessels. Obstructions in the contrast through the consolidation lesion shown in the scan could suggest thrombosis and necrosis of the blood vessels, indicating a high likelihood of fungal disease. This technique is sensitive to fungal disease. CTPA is still operator and centre dependent however since other centres have not been able to replicate successful results and a highly trained radiologist is required to do these scans.

The guidelines of definitions of invasive fungal disease were revised in 2019 to be based on three criteria.

• Host factors may put the host more at risk of invasive fungal disease.
• Typical clinical, radiological, and serological features can give you indications that this is an invasive fungal disease.
• Mycology. Using additional biomarkers to improve the sensitivity of radiological tests such as Galactomannan and PCRs.