Presentation
Patient presents with 3 days of excruciating, progressively worsening right lower extremity pain. Notes a concomitant "rash" in his groin spreading down the ipsilateral thigh.
Patient Data
Cine loops 1 and 2: ultrasound transducer transverse/longitudinal anteromedial thigh over erythema visualised on physical exam
- loss of superficial (dermis, subcutaneous fat) architectural organisation with diffusely increased thickness, hazy increase in echogenicity
- musculature obscured by overlying irregularly dispersed hyperechoic foci with indistinct posterior acoustic shadowing most consistent with fascial gas/air
- intermittent visualisation of heterogeneous, primarily hypoechoic collections in the far-field suggestive of fluid collections adjacent to the deep fascia
Cine loop 3: unaffected contralateral extremity
Extensive soft tissue emphysema in the lower extremity, including throughout the adductor and posterior compartment muscles with relative sparing of the anterior compartment.
Air tracks from the level of the hip joint at its cephalad-most extent through the posterior aspect of the proximal calf musculature and extends beyond the caudad field of view. There was no drainable rim enhancing collections visualised.
A crescentic fluid collection is most prominently visualised apposed to the medial aspect of investing fascia of the posterior compartment with interdigitating locules of air. Fat stranding is visualised throughout subcutaneous fat, most prominently on the posterior/medial aspect of the extremity.
There were no visualised osseous destructive changes in the femur, left hemipelvis, proximal tibia, or proximal fibula.
Case Discussion
Necrotising fasciitis is a rapidly progressive skin and soft tissue infection characterised by invasion of deep fascial layers causing diffuse microvascular occlusion and necrosis 1.
By aetiology, it may be broadly categorised into two groups; type I (polymicrobial), which is typically associated with medical comorbidities (e.g. diabetes, immunosuppression, peripheral vascular disease) and type II (monomicrobial, typically streptococcus/staphylococcus species), which may occur in the absence of any risk factors 2.
Surgery and trauma are common precipitants. Presenting signs and symptoms may include swelling, erythema, and pain, which may progress to cutaneous necrosis, bullae formation, and palpable crepitus 3.
While ultimately a clinical diagnosis, imaging plays a crucial role in timely diagnosis and defining the extent of disease. Radiography may be normal in early disease or show nonspecific soft tissue swelling; more advanced disease may demonstrate air tracking along fascial planes.
CT is highly sensitive in detecting the presence of subcutaneous emphysema and can define the local extent of disease as well as complications pertinent to surgical management. Supportive features include fascial oedema, gas tracking along fascial planes, and increased dermal thickness 4.
Point of care ultrasound recognition of features of necrotising fasciitis may augment timely diagnosis. Sonographic features described include 5:
- irregular, echogenic collections with "dirty" posterior acoustic shadowing
- oedematous subcutaneous fat with loss of typically organised echotexture
- thickened fascial layers
- fluid collections abutting the deep fascia
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