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Diagnostic Imaging Review

Necrotizing fasciitis

Ragaisis, Taryn MS, PA-C; Breunig, Michael MS, PA-C

Author Information
Journal of the American Academy of Physician Assistants: September 2020 - Volume 33 - Issue 9 - p 50-52
doi: 10.1097/01.JAA.0000695004.69613.73
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CASE

A 70-year-old woman hospitalized for concurrent heart failure and an exacerbation of chronic obstructive pulmonary disease (COPD) developed left groin pain on hospital day 4.

History

The patient presented to the ED with a chief complaint of dyspnea and chest pain. A review of systems was positive for productive cough and decreased exercise capacity. She was diagnosed with acute hypercapnic respiratory failure, secondary to concurrent heart failure and COPD exacerbation. She was admitted to the ICU for further treatment, including bilevel positive airway pressure ventilation, diuresis, inhaled bronchodilators, antibiotics, and systemic corticosteroids.

She was transferred out of the ICU on hospital day 3. On hospital day 4, she complained of new-onset pain and redness of her left upper thigh and groin. The patient denied fever or chills.

She had a history of multiple hospital admissions with hypercapnic respiratory failure, heart failure with preserved ejection fraction of 68%, pulmonary hypertension with overlap severe obstructive sleep apnea, and type 2 diabetes. She smoked about three cigarettes a day.

Physical examination

The patient's vital signs were BP, 123/58 mm Hg; heart rate, 108 beats/minute; respirations, 16; oral temperature 36.8° C (98.2° F); and SpO2, 91% on 5 L/min of supplemental oxygen via nasal cannula. She was alert and oriented, responding appropriately to questions, and in no acute distress. Her oral mucosa appeared dry. Lungs were clear to auscultation bilaterally. Abdomen was nontender to palpation with normoactive bowel sounds throughout. No pedal edema was noted. The left groin and upper anterior thigh were diffusely edematous, widely erythematous and tender to light palpation. The erythema was a deep red color. No discrete area of fluctuance was noted; however, the patient did not tolerate deep palpation. No drainage, superficial skin sloughing or bullae were apparent.

Diagnostic testing

Laboratory results were significant for a hemoglobin of 11.2 g/dL (normal range, 11.6 to 15 g/dL), leukocytes of 26,800 cells/mm3 (normal range, 3,400 to 9,600 cells/mm3), platelets of 251,000 cells/mm3 (normal range, 157,000 to 371,000 cells/mm3), sodium of 133 mmol/L (normal range, 135 to 145 mmol/L), serum creatinine of 0.49 mg/dL (normal range, 0.59 to 1.04 mg/dL), glucose of 129 mg/dL (normal range, 70 to 140 mg/dL), lactate of 2.1 mmol/L (normal range, 0.5 to 2.2 mmol/L), and C-reactive protein of 195.8 mg/L (normal range, less than 5 mg/L).

Given the patient's physical examination findings and hemodynamic stability, the immediate concern was for a skin and soft-tissue infection (SSTI). An abscess was the leading differential diagnosis, followed closely by cellulitis. Ultrasound was ordered to assess for underlying abscess. Soft-tissue ultrasound of the left groin in the area of redness demonstrated subcutaneous air in the superficial soft tissues. No definite fluid collection was noted; however, shadowing from the subcutaneous gas could potentially obscure findings of a fluid collection (Figure 1). The ultrasound findings of subcutaneous gas were highly concerning for necrotizing soft-tissue infection. Because of the high level of concern with these findings, broad-spectrum IV antibiotics including clindamycin, piperacillin/tazobactam, and vancomycin were initiated, along with fluid resuscitation. A stat general surgery consult and CT scan of the pelvis were ordered for further evaluation.

FIGURE 1.
FIGURE 1.:
Skin and epidermis are the first isoechoic lines seen superiorly (white arrow). The hypoechoic layers directly inferior make up the subcutaneous tissue. The hyperechoic band (yellow arrow) in the subcutaneous layer with posterior “dirty” acoustic shadowing underneath (red arrow) delineates the presence of air. This shadowing obstructs visualization of the deep tissue.

CT of the pelvis with IV contrast demonstrated subcutaneous edema extending from the left groin along the anterior and medial aspect of the upper left thigh with a large amount of subcutaneous gas. Mild haziness of the left adductor musculature, skin thickening along the anterior medial aspect of the left thigh, and prominent nodes in the left groin were noted (Figure 2). Findings were consistent with necrotizing infectious process of the left groin and left upper thigh.

FIGURE 2.
FIGURE 2.:
From superficial to deep, there is subcutaneous emphysema (white arrow) with evident subfascial fluid tracking along the superficial fascial layers causing diffuse thickening of the anteromedial left thigh. This fluid continues to track even to the deep intermuscular fascia about the left adductor muscle (yellow arrow).

General surgery recommended emergency operative management. After completion of the CT, the patient was promptly taken to the OR for skin and soft-tissue excision and fasciotomy of the left thigh. Postoperatively, the patient did well. Repeat irrigation and debridement was completed the following day. She was discharged to a skilled nursing facility on hospital day 11 with ongoing antibiotics, local wound care, and close outpatient follow-up.

DISCUSSION

Necrotizing fasciitis is a deep SSTI in which bacteria metabolize enzymes and toxins that precipitate poor microcirculation, leading to ischemia and cell death along the fascial layer.1 Because of reduced water solubility, toxins can accrue and produce subcutaneous gas and crepitus in the soft tissue.2 The most common symptoms associated with necrotizing fasciitis are fever (67%), local swelling (78%), and pain or tenderness (68%).3 Often, clinicians have relied on the Laboratory Risk Indicator for Necrotizing Fasciitis (LRINEC) score to help guide a diagnosis of necrotizing fasciitis over cellulitis or abscess. However, this score has a poor sensitivity and cannot be used to rule out the diagnosis of necrotizing fasciitis.4 Since 2010, between 700 and 1,200 cases are reported each year in the United States, which is thought to be a gross underestimate of true incidence.5 Although rare, the morbidity and mortality associated with necrotizing fasciitis is significant, with an estimate of one in three patients dying despite treatment for the condition.5 Precipitating events include mechanisms that compromise the protective skin barrier, such as penetrating injury, trauma, surgery, and minor invasive procedures; however, necrotizing fasciitis also can be idiopathic.2 A small retrospective case study analysis found that age (OR 1.12; 95% CI 1.012-1.236; P = .028), diabetes (OR 62; 95% CI 5.479-701.53; P = .001), and affected surface area (OR 20.8; 95% CI 2.41-179.78; P = .006) were risk factors associated with mortality.6

Necrotizing fasciitis is associated with high morbidity and mortality and also prolongs hospital length of stay and cost of care. A multivariate analysis conducted in Florida assessed the length of stay and cost of necrotizing fasciitis in 87 hospitals and found that the median length of stay was 13 days (range, 1 to 172 days) and the median total cost of therapy was $54,544 (minimum $5,760, maximum $550,687).7 This did not include the cost of rehabilitation facilities and outpatient appointments postdischarge.7

Necrotizing fasciitis is a time-imperative diagnosis requiring emergency treatment. Definitive management is surgical debridement of the necrotic tissue and irrigation. Historically, the diagnosis is suspected during clinical assessment; intraoperative findings (including necrosis of the fascia, loss of fascial integrity, and pus with a “dish water” appearance) and microbiology and histopathology results are confirmatory.1,8 When examining the usefulness of imaging in a patient with suspected deep SSTI, be careful that it does not delay definitive management. For patients with necrotizing fasciitis, a delay beyond 12 hours in diagnosis or treatment can be fatal, and a delay of even a few hours can increase the risk of death.9 However, diagnosing necrotizing fasciitis based solely on clinical findings is incorrect in up to 64% of cases.8

From a clinical perspective, using ultrasound when suspecting an SSTI is practical, predominantly to differentiate between an abscess and cellulitis.10 Ultrasound used in this context is a quick diagnostic tool that allows for dynamic assessment in real time.11 Not only does it spare the patient from ionizing radiation, but it also can be cost-effective.12 Dissecting gas along fascial planes is a specific sign of necrotizing fasciitis and visualized on ultrasound as hyperechoic lines, with comet-tail artifact and posterior “dirty” acoustic shadowing. However, dissecting gas is only reportedly seen on plain radiograph in 24.8% to 55% of patients and on CT in 55% of patients with necrotizing fasciitis.13 If subcutaneous gas is absent, ultrasound can identify fascial irregularities, abnormal fluid collections along fascial planes, and diffuse fascial thickening compared with the contralateral unaffected side.13 These are specific signs that differentiate necrotizing fasciitis from cellulitis.13 The mnemonic STAFF (subcutaneous thickening, air, and fascial fluid) has been proposed to describe ultrasonographic findings of necrotizing fasciitis.9 In a small, single-center study of patients with clinically suspected necrotizing fasciitis, ultrasound was shown to have a sensitivity of 88.2%, with a specificity of 93.3%, positive predictive value of 83.3%, and negative predictive value of 95.4%; however, significant study limitations were noted.14 Given the small sample size, single-center study, sole sonographer operating the ultrasound, and high incidence rate of necrotizing fasciitis in the study, generalizability is limited, and thus the role of ultrasound in the evaluation of necrotizing fasciitis is yet to be determined. Due to the general lack of evidence, ultrasound is not recommended as a diagnostic imaging modality for necrotizing fasciitis at this time. Also, in patients with subcutaneous air, ultrasound waves are unable to penetrate into deeper tissues, limiting its ability to assess the extent of the infection or aid in surgical planning. Obtaining an ultrasound of a presumed SSTI also may be limited by body habitus and patient comfort. However, in practice, observing these findings on ultrasound when evaluating patients for SSTIs should prompt a strong consideration for necrotizing fasciitis, supporting the decision to obtain a CT or emergency surgical consultation.

For many years, CT and MRI have been used as the main adjuncts to surgical intervention for diagnosing necrotizing fasciitis. MRI is 90% to 100% sensitive and 50% to 85% specific for necrotizing fasciitis.15 MRI is not recommended to aid in the diagnosis of necrotizing fasciitis because it is not readily available in an emergency setting, requires a significant amount of time for imaging, is generally unavailable in medical centers, and has poor specificity for necrotizing fasciitis.4 In comparison, CT has a sensitivity of 80% to 100% and reported specificity of 80% to 91% for necrotizing fasciitis. Two parameters visualized by CT significantly associated with necrotizing fasciitis are involvement of the fascia and lack of fascial enhancement.16 In a small retrospective analysis, CT with IV contrast was able to discriminate findings of necrotizing fasciitis from other musculoskeletal infections, including nonnecrotizing fasciitis.16 A CT image demonstrating fascial gas has a sensitivity of 88.5% and specificity of 93.3% for necrotizing soft-tissue infection.4 Given the wider availability of CT, in addition to its imaging detail, it is the most-used imaging modality for necrotizing fasciitis and is the current standard of care.

CONCLUSION

Ultrasonography findings of necrotizing fasciitis include subcutaneous gas, thickening of the fascia, and abnormal fluid collection along fascial planes. However, given the current lack of evidence, ultrasound is not recommended for evaluating patients with suspected necrotizing fasciitis, and CT is the initial imaging modality of choice.

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