Necrotizing fasciitis overview

# Overview
Necrotizing fasciitis or fasciitis necroticans, commonly known as flesh-eating bacteria, is a rare infection of the deeper layers of skin and subcutaneous tissues, easily spreading across the fascial plane within the subcutaneous tissue. Many types of bacteria can cause necrotizing fasciitis (eg. Group A streptococcus, Vibrio vulnificus, Clostridium perfringens, Bacteroides fragilis), of which Group A streptococcus (also known as Streptococcus pyogenes) is the most common cause. It is severe inflammation of the muscle sheath that leads to necrosis of subcutaneous tissue and adjacent fascia.

# Historical Perspective
Narcotizing fasciitis was first described by Hippocrates in the fifth century B.C. as the complication of erysipelas. During the Civil War, necrotizing fasciitis was described as "hospital gangrene" by Confederate Army surgeon Joseph Jones. In 1924, Frank L. Meleney reported a series of 20 patients as having hemolytic streptococcal gangrene, later called Meleney's gangrene. Necrotizing fasciitis of perineum was described in 1883 by the French physician Jean Alfred Fournier.

# Classification
Necrotizing fasciitis may be classified according to International Classification of Diseases-10 (ICD-10) into M72.6 Necrotizing fasciitis. Based on microbiological findings, necrotizing fasciitis may be classified into four types: Type I, Type II, Type III, and Type IV. Necrotizing fasciitis is further classified based on anatomic location and severity of symptoms.

# Pathophysiology
The pathophysiology of necrotizing fasciitis is common to all types, but the speed of development and associated clinical features differs depending on the causative organisms. Following transmission, the bacteria uses the entry site to invade the fascial planes which causes the wide spread necrosis of superficial fascia, deep fascia, subcutaneous fat, nerves, arteries, and veins. Necrotizing fasciitis can be a serious complication of omphalitis in the neonate. The pathogenesis of necrotizing fasciitis is the result of bacterial and host factors. The exact pathogenesis of type 1 necrotizing fasciitis is not fully understood but polymicrobial species work synergistically to enhance the spread of infection. Group A streptococcus is the most common causative agent of type 2 necrotizing fasciitis. Bacterial virulence factors, exotoxins, superantigens and host immune system plays a major role in the pathogenesis of type II necrotizing fasciitis. Recurrent necrotizing fasciitis is caused by MRSA. On gross pathology, the characteristic findings of necrotizing fasciitis include subcutaneous emphysema, skin sloughing, bulae and necrosis. Inflammatory changes are seen on microscopic histopathology.

# Causes
The causative organisms vary depending on the type of necrotizing fasciitis: Type I (polymicrobial), Type II (monomicrobial), Type III (Gram negative monomicrobial, including marine related organisms) and Type IV (fungal).

# Differentiating Necrotizing Fasciitis from Other Diseases
Necrotizing fasciitis must be differentiated from other diseases that cause erythema, pain, edema and necrosis of soft tissues such as sunburn, cellulitis, erysipelas, diabetic myonecrosis and vasculitis.

# Epidemiology and Demographics
The incidence of necrotizing fasciitis in adults is 0.40 cases per 100,000 people/year and the incidence in children is higher at 0.08 cases per 100,000 people/year.The overall mortality rate in the United states from 2003- 2013 was 4.8/1,000,000 per year. Patients from all age groups can develop necrotizing fasciitis but slightly more common among >50 years age and effects men and women equally.The incidence rate of necrotizing fasciitis is high in black, Hispanic, and American Indian individuals compared to Whites and low in Asian individuals.

# Risk Factors
Common risk factors in the development of necrotizing fasciitis include trauma, alcoholism, diabetes, intravenous drug abuse, immunosupression and burns.

# Screening
According to the U.S. Preventive Service Task Force (USPSTF), there is insufficient evidence to recommend routine screening for necrotizing fasciitis.

# Natural History, Complications, and Prognosis
If left untreated, the acute inflammatory changes spread quickly, accompanied by high fever and extreme weakness leading to necrosis of soft tissue. Common complications of necrotizing fasciitis include limb loss, sepsis, toxic shock syndrome, disseminated intravascular coagulation (DIC). Depending on the extent of the necrotizing fasciitis at the time of diagnosis, the prognosis may vary. The prognostic factors associated with necrotizing fasciitis include diabetes mellitus, acute renal failure, admission serum creatinine >2mg/dl and admission white blood cells >30,000 cells mm3.

# Diagnosis

## Diagnostic Criteria
LRINEC is a diagnostic scoring system used to distinguish necrotizing fasciitis from other soft tissue infections.

## History and Symptoms
During early stages, the symptoms of necrotizing fasciitis are non-specific. Symptoms include fever, nausea, and fatigue.

## Physical Examination
Physical examination of patients with necrotizing fasciitis is usually remarkable for local soft tissue signs such as warmth, tenderness beyond margins of erythema, swelling, erythema with ill defined margins, blistering/bullae, skin discoloration, foul discharge (greyish or brown discharge), fluctuance, crepitus, skin sloughing or necrosis, absence of lymphangitis or lymphadenopathy (lymphangitis is rarely observed in necrotizing fasciitis patients), sensory and motor deficits (e.g. localized anesthesia). Finger probe test is useful in the diagnosis of necrotizing fasciitis.

## Laboratory Findings
Laboratory findings consistent with the diagnosis of necrotizing fasciitis include positive blood and tissue culture, elevated inflammatory markers, leukocytosis and elevated serum creatinine.

## Imaging Findings

### Electrocardiogram
There are no electrocardiogram findings associated with necrotizing fasciitis. Electrocardiogram may be helpful in the diagnosis and management of complications of necrotizing fasciitis.

### X Ray
On x ray of affected area, necrotizing fasciitis is characterized by subcutaneous gas or soft tissue swelling (specific x-ray finding) and increased soft tissue thickness and opacity. Chest x-ray findings associated with necrotizing fasciitis include early changes of fluid overload and adult (acute) respiratory distress syndrome (ARDS).

### CT
On CT scan, necrotizing fasciitis is characterized by subcutaneous emphysema, asymmetrical fascial thickening associated with fat stranding, edema and abscesses.

### MRI
On MRI, necrotizing fasciitis is characterized by features such as loss of muscle texture and high signal intensity compatible with intramuscular hemorrhage in T1 weighted image. On T2-weighted images, subcutaneous and intramuscular edema in a reticulated pattern and subfascial and interfascial crescentic fluid collection are seen.

### Ultrasound
Ultrasound is more useful in children (with raising incidence after primary varicella infection). On ultrasound, necrotizing fasciitis is characterized by distorted and thickened fascial planes with turbid fluid accumulation in the fascial layers, subcutaneous edema and soft tissue gas.

## Biopsy
On frozen section biopsy, histologic criteria for diagnosis of necrotizing fasciitis include necrosis of superficial fascia, fibrinous thrombi of arteries and veins, polymorphonuclear infiltration of the dermis and fascia, and presence of microorganisms within the destroyed fascia and dermis.

# Treatment

## Medical Therapy
Necrotizing fasciitis is a medical and surgical emergency. The mainstay of therapy for necrotizing fasciitis includes surgical exploration and debridement along with antimicrobial therapy. Initial pharmacologic therapy often includes a combination of intravenous antibiotics including penicillin, vancomycin, and/or clindamycin.

## Surgery
Surgery is the mainstay of treatment for necrotizing fasciitis. Immediate surgical referral remains the only method of reducing mortality and morbidity.

## Primary Prevention
Effective measures for the primary prevention of necrotizing fasciitis include prevention of trauma/breaks in skin integrity, treatment of underlying infections, hand washing, proper wound care and proper management of underlying co-morbidities.

## Secondary Prevention
Secondary prevention strategies following necrotizing fasciitis include early diagnosis and prompt treatment with either antibiotics or surgery. This strategy prevents or slows the progression and complications of the disease.