As mentioned, NSTI are a group of diseases with high mortality and morbidity, even when the correct diagnosis and treatment is made on time. Unfortunately, this is not an easy diagnosis, as the initial manifestations can be mistaken with more frequent and less severe diseases like cellulitis, erysipelas, diabetes mellitus decompensation, gastroenteritis and others [6]. More specific clinical signs, like large hemorrhagic bullae, skin necrosis, fluctuance, crepitus, and sensory and motor deficits are late signs of this condition. Clinical criteria have been developed to increase the accuracy of the clinical evaluation, namely the LRINEC Score [12], but the sensitivity and specificity are still not ideal [13]. Gas on plain radiographs was only seen in 16.9% in one series [14]. POCUS can improve diagnose accuracy for NSTI when used in combination with clinical evaluation as it is increasingly available, fast and can be performed at the bedside. One study estimated a sensitivity of 88.2% and specificity of 93.3% for the diagnosis on patients with clinically suspected NSTI [15]. The main findings of NSTI by ultrasound are reflected in this case and can be summarized in [15,16,17,18]: loss of the normal tissue architecture to a “cobblestone” appearance, with irregularity and thickening of the fascia, abnormal fluid collections along the fascia, seen as hypoechogenic zones, and, in more advanced cases, the presence subcutaneous air, defined by hyperechogenic foci with a posterior dirty acoustic shadowing. Even in the absence of crepitus at physical examination or plain radiographs, POCUS can show evidence of gas in the soft tissue, indicative of advanced disease and a marker of worse prognosis. The presence of a thickened fascia can make it difficult to differentiate the underlying structures. Yet, there is always the possibility of comparing with another similar unaffected structure, usually the other limb. Ultrasound can also be helpful to guide fluid drainage if a collection is present and rule out deep vein thrombosis.
Contrary to plain radiographs and CT, ultrasound has no ionizing radiation and is also relatively cheap. These features allow the clinician to repeatedly monitor the evolution of the patient, searching for possible local or systemic complications.
Distinguishing NSTI from cellulitis (Fig. 5), the main alternative diagnosis, or other causes of soft-tissue edema using POCUS is not always possible, especially if it involves the deeper fascia, as a thickened fascia can appear in both cases [19]. Some ultrasound findings, including an irregularity of the fascia and an abnormal fluid collection along the fascial plane can help distinguish between the NSFI and cellulitis [16]. However, this evaluation is more difficult, strengthening the importance of taking the information obtained by POCUS together with the clinical evaluation and laboratory results.
Evaluation of deep structures can be difficult because of the hyperechogenic inflamed subcutaneous tissue that reflects most of the ultrasound beams. Also, as with other applications of POCUS, it is a user-dependent technique that requires some practice before being correctly used.
CT and MRI are still the most sensitive methods of diagnosis. CT is the most used imaging method to make the diagnosis, with a higher spacial resolution than ultrasound that allows the evaluation of deeper structures not accessible to ultrasound. It has a sensitivity of 80% in detecting NSTI [20]. MRI is the gold standard for the non-invasive diagnosis of NSTI, with sensitivity and specificity varying according the used criteria (presence of gas has a 100% specificity while extensive involvement of the intermuscular fasciae has a 100% sensitivity) [21]. However, these exams are less available, more time-consuming and expensive. The patient must also be transferred to the radiology department, which can lead to delays in the diagnosis and initiation of the therapy and lack of appropriate monitoring.
In this case, combining a high suggestive clinical evaluation and the documentation of the traditional signs observed in NSTI by POCUS, including the presence of gas in the tissue, allowed for a faster initiation of appropriate therapy and prompt surgical debridement, without having to transport the patient to a time-consuming exam like CT or MRI that could otherwise delay the diagnosis and the treatment. This case also shows complete remission of the ultrasound signs 1 month after the event.