Utilization of CT scan for diagnosis of NF is supported only by small studies that which focus on images using IV contrast enhancement. The most common CT finding is focal subcutaneous fat infiltration and fascial thickening [
1]. These findings lack specificity and can be seen with non-necrotizing infections and even non-inflammatory conditions [
2]. MRI is considered to be the current gold standard imaging testing, with a sensitivity of 93% in detecting NF [
3]. This imaging modality, however, can be time consuming to obtain in a rapidly fatal disease. With regard to US, there are emerging research and approaches for the diagnosis of necrotizing soft tissue infections as there have been multiple case reports that demonstrated accurate diagnosis of NF with US where as CT and MR only demonstrated changes consistent with cellulitis [
4]. The STAFF exam provides a simple and memorable acronym for identifiable signs of necrotizing soft tissue infection (NSTI) on US—subcutaneous thickening, air, and fascial fluid [
5]. With regard to fascial fluid amount, the best cut-off to diagnose NF is reported as 2 mm of fluid accumulation, which has a sensitivity of 75% and a specificity of 70.2% [
6]. However, data with US is limited as well, and a negative US exam, much like a negative CT scan or LRINEC (laboratory risk indicator for necrotizing fasciitis) score, does not exclude the diagnosis [
5]. US is reported to have a sensitivity of 88.2%, specificity of 93.3%, a positive predicative value of 83.3%, a negative predictive value of 95.4% and an accuracy of 91.9% in the diagnosis of NF [
7]. Surgical exploration remains the gold standard for both diagnosis and treatment and was appropriately performed in this patient case [
8]. The time to successful surgical debridement is considered the single most important variable influencing mortality in this patient population [
9]. What is unique about our case goes beyond using US for diagnosis, but also using this modality to help identify the muscle compartment to be explored during surgery. If the US results were utilized in this case to aid the site of surgical exploration, the ultimate outcome of this patient may have been different.
With regard to the surgical decisions on areas of debridement for NF, examination of surgical reference textbooks reveals vague allusions to “immediate and extensive surgical debridement”, but without any specific technique mentioned [
10,
11]. A surgical review article on NF states to make an incision over any “overtly necrotic area”, and when this is not evident, to make an incision over the area “deemed to be the center of the disease process”, without any further elaboration [
12]. This lack of standardization is particularly concerning in early NSTIs when the skin may be spared and not exhibit any impressive or obvious appearance of underlying pathology. Our patient in this case is an example of this, as her skin changes were misleading in the operating room (her entire left lower extremity was mottled) and led to two negative biopsies. The specific location of the infection was correctly identified on bedside ultrasound and but not on CT. Unfortunately, the compartment identified on ultrasound was not explored during the initial operation.
The utility of US to help guide surgical debridement with regard to the extent or timeliness has yet to be defined in the literature. In this particular case described below, US would have been helpful in guiding the surgeons toward a specific anatomic location for their surgical incision and drainage. Other than the STAFF exam, there does not appear to be any unified or systematic process for evaluating a limb or area for underlying NF when the area involved is either extensive or uncertain. In a review of the US literature, we found a routine approach to soft tissue ultrasound that could be applied by trained physicians to systematically evaluate an extremity. The same principles could be applied to other areas of the body as well. Primarily, scanning an unaffected area/contralateral extremity, as well as over areas that are concerning for pathology [
13] should be considered. Scanning in multiple planes is also recommended for localization and providing as much information as possible [
13]. We propose combining both approaches to create a protocolized US to help identify optimal areas for fascial exploration—sonographic exploration for fascial exploration (SEFE) (Table
1).
Table 1Explanation of the steps to perform a SEFE examination
Step 1: Scan all fascial compartments (such as anterior, lateral, superficial posterior and deep posterior in the lower extremity) even if skin changes are not present |
Step 2: Do you have BOTH diffuse subcutaneous thickening AND fascial fluid > 2 mm present? = If so, this is diagnostic for NF |
Step 3: Additionally look for supporting, but not mandated findings such as subcutaneous air or abnormal architecture of the muscle tissue |
Step 4: Mark area of US findings on patient skin and consult surgery for exploration |