Necrotizing fasciitis

Necrotising fasciitis refers to a rapidly progressive and often fatal infection of soft-tissue fascia deep to the skin but superficial to the muscles.

Epidemiology

Necrotising fasciitis is relatively rare, although its prevalence is thought to be rising due to an increase in the number of immunocompromised patients with HIV infection, diabetes mellitus, cancer, alcoholism, vascular insufficiency, and organ transplants. It can also occur after trauma or around foreign bodies in surgical wounds ⁷.

Pathology

There are at least two recognised forms:

• the most common type is a polymicrobial infection with both aerobic and anaerobic organisms such as Clostridium, Proteus, Escherichia coli, Bacteroides, and Enterobacteriaceae: this form is often seeded from underlying infections such as diverticulitis
• the second form of the disease is caused by a single organism: most commonly group A streptococci, the “flesh-eating bacteria,” and is seen in approximately 10-15% of cases ^3,4; toxic shock syndrome may complicate this latter form

Location

While it can affect any part of the body, 50% of cases involve the lower extremities, other common areas include the upper extremities, the perineum, truncal areas, and the submandibular region ^4,12.

Subtypes

• Fournier gangrene: necrotising fasciitis of the perineum

Radiographic features

It should always be noted that no imaging modality can reliably exclude underlying necrotising fasciitis in the absence of soft tissue gas and negative study should not preclude obtaining a tissue biopsy. Also, in cases with very high clinical suspicion imaging should not delay surgical intervention. Hence, imaging plays a very limited role in the diagnosis and management of necrotising fasciitis.  

Plain film

The common plain radiographic findings non-specific an often similar to those of cellulitis, with increased soft-tissue thickness and opacity. Radiographs can be normal until the advanced stages of infection and necrosis. The characteristic finding of gas in the soft tissues is seen in only a minority of cases.

CT

The sensitivity of CT is 80%, but the specificity is low ¹². CT classically tends to show soft-tissue gas associated with fluid collections within the deep fascia, although this finding is inconstant. 

Other non-specific findings include:

• asymmetrical fascial thickening associated with fat stranding
• oedema extending into the inter-muscular septa and the muscle
• thickening of one or both of the superficial and deep fascial layers

Although fascial fluid collections are typically non-focal, abscesses may be seen. 

Postcontrast CT 

Some authors describe the diffuse enhancement of fascia with contrast ^3,8. If there is no enhancement of the fascia, this can be a finding that can help differentiate from a non-necrotising fasciitis ⁷.

Ultrasound

Ultrasound may be more useful in children ^3,4 (with a rising incidence after primary varicella infection ¹¹). Sonographic findings include distorted and thickened fascial planes with turbid fluid accumulation in the fascial layers and subcutaneous oedema. Sonographic assessment may, however, be limited by soft-tissue gas, although if identified, this finding may be of diagnostic benefit.

MRI

MRI is considered the modality of choice in the investigation of necrotising fasciitis with a sensitivity of 93% ¹². T1 and T2 sequences are imperative to assess both the anatomy involved and detect fascial thickening ¹².

• T1: usually has subtle abnormality with loss of muscle texture and may show high signal intensity compatible with intramuscular haemorrhage ¹⁰
• T2: usually shows subcutaneous and intramuscular oedema in a reticulated pattern as well as subfascial and interfascial crescentic fluid collections ¹⁰

Treatment and prognosis

Mortality rates can range between 25-75% ⁵. Treatment is usually with a prompt surgical fasciotomy with debridement of the necrotic tissue.

Differential diagnosis

Necrotising fasciitis is rare, consider ^12,13

• nonnecrotizing fasciitis
• ischemic myonecrosis
• dermatomyositis
• graft vs host disease

For gas within soft tissues consider

• subcutaneous emphysema from a non-infective cause: has an entirely different clinical presentation

History and etymology

It was first described by Joseph Jones, an American army surgeon during the American civil war ³.

See also

• subcutaneous abscess