What is the definition of invasive aspergillosis?

Definition of Invasive Aspergillosis

Invasive aspergillosis (IA) is a severe fungal infection caused by Aspergillus species (predominantly Aspergillus fumigatus) that invades tissue through angioinvasion or direct extension, occurring primarily in immunocompromised patients with defects in neutrophil or T-cell function. 1, 2

Core Pathophysiologic Features

The defining characteristic of IA is tissue invasion by Aspergillus hyphae, which distinguishes it from colonization or allergic forms of aspergillosis. 3 The infection occurs when:

• Inhaled Aspergillus conidia germinate into hyphae that invade tissue, most commonly the sinopulmonary tract, reflecting inhalation as the principal portal of entry 3
• Angioinvasion leads to thrombosis, hemorrhage, and tissue necrosis, which accounts for the characteristic radiographic findings and high mortality 4
• Impaired phagocytic function prevents clearance of conidia and hyphae, as phagocytosis is the main host defense mechanism 4

High-Risk Populations

IA predominantly affects specific immunocompromised groups, with risk stratified by the type and severity of immune defect 1:

Classic High-Risk Groups

• Prolonged and severe neutropenia (particularly >10 days with absolute neutrophil count <500 cells/μL), especially in patients with acute leukemia, myelodysplastic syndrome, or aplastic anemia 1, 3
• Allogeneic hematopoietic stem cell transplant recipients, with three distinct risk periods: neutropenia post-conditioning, acute graft-versus-host disease treatment, and chronic graft-versus-host disease treatment 1
• Solid organ transplant recipients, particularly lung transplant patients with prior Aspergillus colonization, and liver transplant recipients with high MELD scores or fulminant hepatic failure 1
• Chronic granulomatous disease due to defective neutrophil oxidative killing 1, 3
• Advanced AIDS (typically CD4 count <50 cells/μL with additional risk factors like neutropenia or corticosteroid use) 1

Emerging High-Risk Groups

• Critically ill patients with severe viral pneumonia (influenza, COVID-19), where the term COVID-19-associated pulmonary aspergillosis (CAPA) has been specifically defined 5, 2
• Decompensated liver cirrhosis, particularly alcoholic hepatitis treated with corticosteroids, with 53% of cases diagnosed only post-mortem 1
• Severe COPD exacerbations requiring corticosteroids, with mortality exceeding 70% in most series 1, 6
• Intensive care unit patients with acute respiratory distress syndrome, severe bacterial infection, or burns, occurring at 4-6 per 1000 ICU admissions 1

Diagnostic Criteria Framework

The diagnosis requires integration of host factors, clinical presentation, and mycological evidence 1:

EORTC/MSG Criteria Components

• Host factors: Documented immunocompromising condition as described above 1
• Clinical criteria: Compatible signs/symptoms with radiographic evidence of infection 1
• Mycological criteria: Positive culture, microscopy, galactomannan antigen, or PCR from sterile or non-sterile sites 1

Important Diagnostic Caveats

• EORTC/MSG criteria detect only 50% of IA cases in HIV-infected patients and other non-hematological populations 1
• Angioinvasive radiographic presentation (halo sign, air crescent sign) is uncommon in non-hematological patients, where alveolar infiltrates predominate 1, 4
• Airway-invasive presentation occurs in 37% of heart transplant recipients and is associated with delayed diagnosis and worse outcomes 1
• Isolation of Aspergillus from respiratory cultures has much lower positive predictive value in non-hematological patients, requiring caution to prevent over-diagnosis 1

Alternative Criteria for ICU Patients

• AspICU criteria have been developed specifically for critically ill patients, as traditional EORTC/MSG criteria perform poorly in this population 5
• CAPA consensus criteria provide specific definitions for COVID-19-associated aspergillosis 5

Clinical Presentation Patterns

The most common manifestations include 1:

• Pulmonary IA: Fever unresponsive to antibiotics, pleuritic chest pain, hemoptysis, and progressive respiratory failure
• Sino-orbital IA: Sinusitis with orbital or intracranial extension in neutropenic patients
• Disseminated IA: Hematogenous spread to brain, skin, or other organs
• Tracheobronchial IA: Ulcerative or pseudomembranous airway disease, particularly in lung transplant recipients

Timing Considerations

• Cases with symptom onset ≥7 days after hospital admission are more likely nosocomial 1
• Surveillance using EORTC/MSG criteria every 3 months is reasonable for leukemia and transplant centers due to paucity of culture-confirmed cases 1