Clinical context:

In the Emergency Radiology department, the primary reason for consultation in cases of soft tissue infection is to rule out the possibility of necrotising infection when clinical suspicion arises. To this end, imaging studies—typically a CT scan—are requested to confirm the suspicion. Before proceeding with the study, it is essential to ask a series of questions (Figure 1).

The LRINEC score (Laboratory Risk Indicator for Necrotizing Fasciitis) uses readily accessible laboratory parameters and does not consider clinical findings. This tool allows for the detection of necrotizing soft tissue infections at early stages, as laboratory parameters are usually the first to change during these phases, while clinical findings may not be pronounced (Table 1).

Another advantage is its ability to stratify patients with uncertain findings into risk groups (low, intermediate, or high).

• A score of ≥6 should increase suspicion of necrotizing soft tissue infection, often warranting an imaging study.
• A score of ≥8 is highly suggestive of this disease. This evaluation helps reduce the time to diagnosis of the infection.

After addressing the questions outlined earlier, it is crucial to consider an action plan to facilitate the management of the suspected condition, whether or not imaging is required. Below is an example protocol based on the risk group established using the LRINEC score (Figure 2):

• With LRINEC <6, the most recommended course of action is administering appropriate treatment alongside serial measurements of LRINEC values.
• With LRINEC between 6 and 7, the treatment is the same as for the lower-risk group, but it also requires evaluation by Plastic Surgery. In these cases, imaging is often necessary to assess the extent of the disease, the presence of drainable collections, and signs of necrotizing soft tissue infection. In this risk group, imaging findings may be nonspecific, and clinical presentation guides management.
• With LRINEC ≥8, evaluation by Plastic Surgery is essential. Imaging studies may not be feasible, and early surgical intervention by the surgical team is often required due to the patient's clinical condition, as the presentation in this group tends to be highly evident.

In all cases, if there is instability and no other cause justifying the clinical presentation, an urgent CT scan should be performed.

Nomenclature:

The skin is the most superficial organ, consisting of two layers—epidermis and dermis—that cannot be distinguished on imaging. Therefore, infectious pathologies involving these structures will not be visible. Beneath the skin lies the hypodermis or subcutaneous tissue, a fibroadipose layer of variable thickness that is highly vascularized (Figure 3).

Fascia terminology can be confusing, so it is crucial to clarify a few points.

Anatomists classify fascia into superficial and deep:

• Some consider the superficial fascia to include the entire hypodermis, while others define it as a layer of loose fibrous tissue, though there is no clear consensus regarding its location. It may correspond to the deep dermis, the aforementioned hypodermis, or a layer situated near the deep fascia, but separated from it.
• The deep fascia is a much denser layer of collagen with an external peripheral muscular component and an internal intermuscular component (Figure 4).

Once the various structures that may be involved are identified, and given that the evaluation of the superficial fascia exceeds our current imaging resolution, it is useful to define several concepts:

• Cellulitis (non-necrotizing cellulitis): Infection primarily limited to the subcutaneous tissue and, therefore, the superficial fascia.
• Necrotizing fasciitis: Infection of the deep fascia, with or without involvement of adjacent structures.
• Non-necrotizing fasciitis: This term is reserved for cases where fascia involvement is not of infectious origin, such as paraneoplastic fasciitis, eosinophilic fasciitis, nodular fasciitis, and proliferative fasciitis.
• Myositis: Inflammation of the muscles, which can have various etiologies. In cases of bacterial myositis, abscess formation (pyomyositis) may occur as a complication.

Imaging Protocol

The imaging protocol in the emergency setting is summarized in Table 2.

Necrotizing soft tissue infection:

Necrotizing fasciitis, now more appropriately termed necrotizing soft tissue infection (NSTI), is an acute process with a rapidly progressive course. The extent of the condition varies (it typically involves a considerable area), and prompt therapeutic intervention is crucial to ensure patient survival, as progression to sepsis and multiorgan failure can occur within hours (Figure 5).

The causes of NSTI are diverse, and there are several predisposing factors that increase its likelihood. These factors are also present in other soft tissue infections (Figure 6). In nearly 80% of cases, the infection arises from a loss of skin integrity, leading to bacterial colonization and subsequent infection.

Necrotizing soft tissue infections are classified based on whether they are caused by multiple microorganisms or a single causative agent:

• Type 1 (Polymicrobial): Common in elderly/immunocompromised patients; affects the trunk and perineum; often involves gas formation; both aerobic and anaerobic microorganisms are involved (e.g., Bacteroides spp., Clostridium spp., Peptostreptococcus).
• Type 2 (Monomicrobial): Caused by Group A Streptococcus or MRSA; occurs in any age group and in healthy individuals
• Type 3: includes highly lethal infections (e.g., Vibrio vulnificus, Aeromonas hydrophila), even with optimal treatment.

Pathophisiology:

There are two scenarios in the development of necrotizing infection, which will answer the question of whether or not there is an external entry point. These are summarized in Figure 7 and Figure 8.

Clinical Presentation

• Pain disproportionate to the lesion is a hallmark feature, often mimicking a muscle tear. Some patients (e.g., diabetic neuropathy) may experience minimal disconfort.
• Early signs include erythema and warmth rapidly spreading to other areas, edema and tension beyond the affected area. Patients with no known entry point may not exhibit cutaneous manifestations until the later stages of the disease.
• Cutaneous symptoms are accompanied by systemic signs of shock (tachycardia, hypotension, altered consciousness), as well as fever.

Any patient with sudden-onset pain, with or without an identifiable entry point, should be evaluated for NSTI.

Imaging Findings

These are summarized in Table 3.

Special forms of NSTI:

Fournier's gangrene

Fournier’s gangrene is a necrotizing soft tissue infection of the perineal, genital, and/or perianal regions. While some cases are idiopathic, common causes include skin ulcers, urinary tract infections, and gastrointestinal diseases like colorectal cancer or diverticulitis.

The infection is bilateral in 60% of cases, with multiple fascial compartments involved in 70%. It predominantly affects men, though cases in women are increasing. Diabetes mellitus is a key risk factor, along with other immunosuppressive conditions.

Imaging findings include fat stranding, fascial thickening, and gas bubbles dissecting muscle planes (Figure 9). Assessing levator ani muscle involvement and determining the infection’s extent and underlying cause (e.g., fistula or abscess) are crucial for surgical planning.

Meleney's synergistic gangrene

Meleney's gangrene is a life-threatening infection caused by Staphylococci and microaerophilic Streptococci. It spreads rapidly, causing necrosis of abdominal skin and subcutaneous tissue, sometimes reaching the muscle. This rare necrotizing infection of the anterior abdominal wall typically appears two weeks after minor trauma or surgery.

Imaging findings align with NSTI (Figure 10). Early aggressive debridement, broad-spectrum antibiotics, and optional negative pressure wound therapy aid wound healing.