From the Guidelines
Magnetic Resonance Imaging (MRI) is the best imaging modality for diagnosing necrotizing fasciitis when available and if the patient is stable enough for the study. MRI offers superior soft tissue contrast that can clearly demonstrate fascial fluid collections, gas formation, and tissue necrosis characteristic of necrotizing fasciitis, as noted in the 2018 WSES/SIS-E consensus conference recommendations 1. It can detect subtle changes in the deep fascial planes and identify the extent of infection before clinical signs become apparent. The findings on MRI that are suggestive of necrotizing fasciitis include thick abnormal signal intensity on fat-suppressed T2-weighted images, low signal intensity in the deep fascia on fat-suppressed T2-weighted images, and extensive involvement of the deep fascia 1.
However, in emergency situations where rapid diagnosis is critical, Computed Tomography (CT) with intravenous contrast is often used instead due to its wider availability and faster acquisition time. CT can identify gas in soft tissues, fascial thickening, and fluid collections, with a sensitivity of 100% and specificity of 81% in identifying necrotizing soft tissue infections, as reported in a case series study 1. Plain radiographs may show subcutaneous gas but have limited sensitivity, and should not be used to rule out necrotizing infection, as recommended by the 2018 WSES/SIS-E consensus conference 1.
Ultrasound can be useful as a bedside screening tool to detect fascial thickening and fluid collections, particularly when other imaging modalities are unavailable, with a sensitivity of 88.2% and specificity of 93.3% in a prospective observational study of 62 patients with clinically suspected necrotizing fasciitis 1. It's essential to note that imaging should never delay surgical intervention when necrotizing fasciitis is strongly suspected clinically, as early surgical debridement remains the definitive treatment and is essential for improving patient outcomes, as emphasized in the 2014 practice guidelines for the diagnosis and management of skin and soft tissue infections by the Infectious Diseases Society of America 1.
From the Research
Imaging Modalities for Necrotizing Fasciitis
The diagnosis of necrotizing fasciitis can be facilitated by various imaging modalities, each with its own strengths and limitations. The choice of imaging modality depends on several factors, including the severity of the condition, the location of the infection, and the availability of resources.
Characteristics of Imaging Modalities
- Magnetic Resonance Imaging (MRI): Considered the gold standard for evaluating necrotizing fasciitis, MRI offers high sensitivity, specificity, and soft tissue contrast 2. It can accurately detect fascial necrosis and define the extent of involvement 3.
- Computed Tomography (CT): CT scans can help identify thickening of the deep fasciae, fluid accumulation, and reactive hyperemia, but these findings lack specificity 4. CT can be useful in evaluating the extent of disease and possible complications, especially in areas difficult to study with ultrasound 5.
- Ultrasound: While ultrasound can provide useful information, its effectiveness is limited by the infiltration of the hypodermis, which can block ultrasound transmission 4. However, point-of-care ultrasound (POCUS) has shown promise in identifying necrotizing fasciitis with high sensitivity and specificity 6.
- Radiography: Radiography can detect gas within the necrotized fasciae, but this finding is not always present 4.
Diagnostic Accuracy and Limitations
- The diagnostic accuracy of imaging modalities can be affected by various factors, including the severity of the condition, the location of the infection, and the experience of the interpreter 4, 5.
- Imaging studies should not delay emergency surgical treatment in patients with established necrotizing fasciitis 4.
- A combination of imaging modalities, laboratory tests, and clinical evaluation can provide a more accurate diagnosis and guide proper treatment 5, 6.