# Diagnostic Radiology/Musculoskeletal Imaging/Infection/Necrotizing fasciitis

Necrotizing fasciitis is a rare, often fulminant, rapidly progressive infectious process primarily involving the fascia and subcutaneous tissue. The pathophysiology involves the rapid spread of infection along the fascial planes, fascial necrosis and thrombosis of the subcutaneous blood vessels, leading to cutaneous gangrene.
Approximately 500 to 1500 cases reported annually in the US. The most often associated comorbities are diabetes mellitus and peripheral vascular disease. It is most often peripheral, involving the lower limb.
Delay to diagnosis is one of the most prominent predictors of mortality (along with diabetes). Prognosis for necrotizing fasciitis depends heavily on early recongnition and determination of the extent of necrosis in the preoperative planning.
Clinical studies are lacking that compare different imaging modalities to the gold standard of fasciotomy. These series are typically small and only look at a handful of cases.

## Clinical Findings

Patients usually present with the triad of exquisite pain (often out of proportion to physical findings), swelling and fever. Tenderness, erythema, and warm skin are commonly the only signs of early disease. Distinction between uncomplicated infectious cellulitis or fasciitis, and necrotizing fasciitis is important. The former entities are treated with high-dose antibiotics without the need for surgical debridement. Necrotizing fasciitis is a surgical emergency.
Classification schemes describe the syndromes associated with necrotizing fasciitis and have overlapping features
* Type I: mixed infection of anaerobes plus facultative species such as streptococci or Enterobacteriaceae. Acute, rapidly developing infection of the deep fascia, marked pain, tenderness, swelling, and often crepitus.
* Type II: infection with group A streptococci. Acute infection, often accompanied by toxic shock syndrome, rapid progression of marked edema to violaceous bullae and necrosis of subcutaneous tissue, absence of crepitus.

## Radiologic Findings
### Plain Films

Plain films: Gas on plain films is thought to be an inconsistent sign, seen is well less than half of cases.
Fig. 1 56 year old man with acute leukemia and necrotizing fasciitis.
51 year old woman with history of IV drug abuse, now presenting with necrotizing fasciitis
Fig. 2 51 year old woman with history of IV drug abuse, now presenting with necrotizing fasciitis.
Image above looks like postmortem of neurofibromatosis not necrotizing fasciitis. Might want to check your data saved under "NF"

### MRI

MRI: In identifying necrotizing fasciitis on MRI, findings include involvement of the deep fascia and demonstration of fascial necrosis.
* Fascial inflammation or fluid accumulation is demonstrated by low intensity on T1 and high intensity on T2.
* Fascial necrosis is evidenced by the lack of contrast enhancement of the involved area upon the administration of gadolinium.
MRI also plays a role in determining the extent of fascial involvement to aid in the preoperative assessment. However, Arlan et al. point out that the findings in MRI can be nonspecific and most preoperative decisions should be based on clinical status.
T1W and T2W MR of the lower extremity showing fascial thickening and fluid accumulation between the subcutaneous tissues and fascial layer (respectively) in this patient with necrotizing fasciitis.
Fig. 3 T1W and T2W MR of the lower extremity showing fascial thickening and fluid accumulation between the subcutaneous tissues and fascial layer (respectively) in this patient with necrotizing fasciitis.

### CT

Contrast enhanced CT can help define the extent of disease and possible complications, especially in sites that are difficult to study. However, large clinical studies correlating CT findings with pathologic findings are lacking.

### Ultrasound

Some believe there is limited use for ultrasound in the diagnosis of necrotizing fasciitis. Its main limitation being when the anatomic sites are difficult to ascertain (too deep) by US. However, in their series, Yen et al. found ultrasound to have a sensitivity of 88% and a specificity of 93% (positive predictive value of 83%). Their criteria included diffuse thickening of the subcutaneous tissue accompanied by a fluid accumulation more than 4 mm in depth along the fascial layer, when compared to the normal limb.

### Summary

Although one can see findings of necrotizing fasciitis on practically any imaging modality, none of these findings are absolutely specific. The optimal diagnostic approach consists of pooling all of the sometimes contradictory clinical and imaging data and making the best estimate from that. When in doubt, one should approach these challenging patients with a high index of suspicion and err on the side of patient safety. In other words, if there is sufficient likelihood that a patient has necrotizing fasciitis (your mileage may vary), surgical intervention should proceed as soon as possible.

## Treatment

Fasciotomy: Obviously the gold standard. Timely intervention in those cases where clinical suspicion for necrotizing fasciitis is high is imperative. Fasciotomy allows both for confirmation of the diagnosis with histology, but also rapid iniation of treatment. Typical findings include gray discoloration of the fascia and resistance to blunt dissection.

## References

* Necrotizing fasciitis by Teran Colen, M.D.
* Arslan A., et al. Necrotizing fasciitis: unreliable MRI findings in the preoperative diagnosis. Eur J Rad. 36 (2000) 139-143.
* Brothers TE, Tagge DU, Stutley JE, Conway WF, Del Schutte H Jr, Byrne TK. Magnetic resonance imaging differentiates between necrotizing and non-necrotizing fasciitis of the lower extremity. J Am Coll Surg. 1998 Oct;187(4):416-21.
* Hoadley DJ, Mark EJ. Case records of the Massachusetts General Hospital. Weekly Clinicopathological exercises. Case 28-2002. A 35-year-old long-term traveler with a rapidly progressive soft-tissue infection. N Engl J Med. 2002 Sep 12;347(11):831-7.
* Revelon G, Rahmouni A, Jazaerli N, Godeau B, Chosidow O, Authier J, Mathieu D, Roujeau JC, Vasile N. Acute swelling of the limbs: magnetic resonance pictorial review of fascial and muscle signal changes. Eur J Radiol. 1999 Apr;30(1):11-21.
* Swartz MN. Cellulitis. N Engl J Med. 2004 Feb 26;350(9):904-12.
* Yen et al. Ultrasonographic Screening of Clinically-suspected Necrotizing Fasciitis. Acad Emerg Med. Dec 2002., Vol 9, No. 12. 1448-1451.