Presence of deep fascial fluid in imaging necrotizing fasciitis: Importance of this feature in diagnosing necrotizing fasciitis without imaging manifestations of soft tissue gas

I read with great interest the important original contribution by Lin et al., “The relationship between fluid accumulation in ultrasonography, diagnosis, and prognosis of patients with necrotizing fasciitis” (Lin et al. 2019), where the authors profile a retrospective cohort of 48 patients with clinically suspected necrotizing fasciitis (NF) in the upper or lower extremities who received an ultrasound examination of suspected limb with NF performed by three emergency physicians. Their study details that the degree of deep fascial fluid accumulation can help diagnose and determine prognostic effects of NF. Certainly, imaging may have a role in diagnosing and defining the anatomic extent of NF, an important consideration for the surgeon to plan their operative approach (Ballard et al. 2018). It is important to note that while ultrasound does offer a dynamic examination and may be more readily available compared to computed tomography (CT) (the case in Lin et al.’s experience), this is not the case for all institutions, including very high volume emergency departments where CT can be obtained rapidly (which may be faster than an ultrasound examination from the present author’s institutional dynamics and experience). However, in comparing Lin et al.’s experience to prior NF CT cohorts (Ballard et al. 2018; McGillicuddy et al. 2011) there are a number of different important features that ultrasound can provide in imaging NF.

A CT scoring system for NF has been described for distinguishing NF from other soft tissue or fascial infections. In the original cohort (McGillicuddy et al. 2011), 305 patients underwent CT for evaluation of NF and 44 patients had surgically confirmed NF, including 11 patients with extremity NF. The following factors with weighted scores were described for distinguishing NF from other soft tissue infections: presence of air - 6 points, muscle/fascial edema – 5 points, and fluid tracking, lymphadenopathy, and subcutaneous edema accounting for 3, 2, and 1 point(s) respectively; the suggested threshold score for NF is >6 points. Future ultrasound cohorts of NF could benefit from adopting this scoring system, Lin et al.’s work does an excellent job of quantifying fascial fluid and the presence of it tracking along fascial planes can easily be interrogated sonographically. The authors importantly and correctly distinguish the presence of subcutaneous edema, which in prior CT series is present in nearly every patient (Ballard et al. 2018; McGillicuddy et al. 2011), and deep fascial fluid accumulation. These diagnostic criteria, and if they would be used in ultrasound profiling of NF, are particularly important in diagnosing NF without imaging manifestations of soft tissue gas. A contrast of soft tissue gas incidence is accounted for when comparing Lin et al.’s ultrasound cohort with only 3/48 (6%) patients compared to CT cohorts using the NF CT scoring system that range from 44–95% (Ballard et al. 2018; McGillicuddy et al. 2011). Both Lin et al.’s sonographic cohort and prior CT cohorts represent the important contribution of deep fascial fluid as an imaging manifestation of NF. The authors chose to exclude non-extremity NF, which would include Fournier gangrene. This would be an interesting cohort for future studies in which CT has shown value in defining the extent of fascial involvement and confirming the often clinically suspected NF diagnosis (Ballard et al. 2018).

In Lin et al.’s study, there are several limitations and discrepancies compared to prior CT cohorts of NF which should be addressed in future ultrasound studies of NF. The authors provide the lower (71%) and upper (29%) extremity involvement in their cases but do not provide any more detail about the anatomic locations of these NF sites or if there were associated wounds or ulcers (Lin et al. 2019). Particularly, it would be of interest to know if anatomic sites with greater subcutaneous fat, such as the thigh had different ultrasound characteristics compared to NF associated with the foot which presumably may have been more frequently associated with ulcers. The authors provide there were three patients with sonographic manifestations of subcutaneous emphysema, which presumably manifested as diffuse hyperechoic foci with posterior acoustic shadowing. Their imaging incidence of only 3/48 (6%) patients with imaging appearance of soft tissue gas is much lower compared to higher incidence in CT series (44–95%) (Ballard et al. 2018; McGillicuddy et al. 2011). Lin et al. suggest in their introduction that CT or magnetic resonance imaging may indicate a negative result after a suggestive ultrasound examination, but it is unclear if the authors routinely or selectively obtained different imaging modalities after ultrasound examinations. For future studies using ultrasound to profile suspected NF, it would be worthwhile to compare sonographic findings to subsequently obtained CT examinations.