Etymology

• Etymology: Derived from the Greek word “halos,” meaning “ring” or “circle.” In radiology, it refers to a radiological finding characterized by a central lesion surrounded by a “halo” of ground-glass attenuation.
• AKA: None specific, but often described in association with infectious, neoplastic, or inflammatory conditions.
• What is it?
  The halo sign is a radiologic feature seen on computed tomography (CT) of the chest. It describes a central, nodular opacity surrounded by a ring of ground-glass attenuation, indicating a range of pathological processes.
• Caused by:
  1. Infection:
     • Invasive fungal infections (e.g., Aspergillus species, mucormycosis) causing angioinvasion and hemorrhage.
     • Tuberculosis and septic emboli.
  2. Neoplasm:
     • Angioinvasive pulmonary aspergillosis.
     • Leukemia or lymphoma with secondary fungal infections.
  3. Inflammation/Immune:
     • Granulomatosis with polyangiitis (Wegener’s granulomatosis).
     • Rheumatoid nodules or organizing pneumonia.
  4. Other:
     • Airway hemorrhage or vasculitis.
     • Pulmonary metastases (e.g., angiosarcoma, choriocarcinoma).
• Resulting in:
  • A central nodular lesion with perilesional ground-glass opacity.
  • Pathological findings consistent with alveolar hemorrhage, inflammation, or neoplastic invasion.
• Structural Changes:
  • Central lesion (often necrotic or hemorrhagic) surrounded by affected alveoli exhibiting ground-glass opacities due to hemorrhage, edema, or inflammatory infiltrates.
• Pathophysiology:
  • The central lesion may represent neoplastic growth, infectious angioinvasion, or granulomatous inflammation.
  • The ground-glass halo reflects perilesional processes such as hemorrhage, alveolar damage, or inflammatory cell infiltration.
• Pathology:
  • Gross: A central nodular area with surrounding hemorrhage or inflammation.
  • Microscopic: Central necrosis, fungal hyphae (if infectious), or inflammatory infiltrates with adjacent alveolar hemorrhage.
• Diagnosis:
  • CT imaging is critical for detecting the halo sign.
  • Often requires further clinical and laboratory correlation (e.g., biopsy, cultures) for definitive diagnosis.
• Clinical:
  • Symptoms depend on the underlying condition but may include fever, cough, hemoptysis, and dyspnea in infectious or inflammatory cases.
• Radiology:
  • CXR: Often nonspecific; the lesion may be undetectable or appear as a nonspecific opacity.
  • CT:
    • Parts: Central nodular lesion with peripheral halo.
    • Size: Variable, depending on the underlying pathology.
    • Shape: Round to irregular nodules with a surrounding ring.
    • Position: Can occur in any lung zone.
    • Character: Nodular opacity with surrounding ground-glass attenuation.
    • Associated Findings: Additional nodules, cavitation, or pleural effusions (depending on the etiology).
  • MRI: Rarely used but can delineate soft tissue involvement.
  • PET-CT: Can detect metabolic activity of lesions, useful in neoplastic or inflammatory conditions.
• Pulmonary Function Tests (PFTs):
  • Typically normal unless underlying lung pathology affects ventilation or diffusion.
• Management:
  • Treat the underlying cause:
    • Antifungal therapy for angioinvasive fungal infections.
    • Antibiotics for bacterial infections.
    • Immunosuppressive therapy for inflammatory conditions.
  • Monitor progression with repeat imaging to assess treatment response.
• Recommendations:
  • Early recognition of the halo sign on CT in immunocompromised patients can prompt life-saving antifungal treatment.
  • Tissue biopsy or bronchoalveolar lavage is often required to confirm etiology.
• Key Points and Pearls:
  • The halo sign is a nonspecific finding but is highly suggestive of angioinvasive fungal infections in immunocompromised patients.
  • Other causes include vasculitis, neoplasms, or hemorrhagic metastases.
  • Ground-glass attenuation reflects perilesional alveolar hemorrhage or inflammation, aiding in the differential diagnosis.
  • Rapid diagnosis and treatment of the underlying condition are essential to improve outcomes.