# Skin and Soft Tissue Infections: Abscess, Cellulitis, and Necrotizing Fasciitis

## Introduction

Skin and soft tissue infections (SSTIs) account for over 14 million ED visits annually and represent a spectrum from simple superficial infections to life-threatening deep-space emergencies. The emergency physician must differentiate purulent from non-purulent infections, identify the red flags for necrotizing fasciitis, and understand the evolving landscape of antimicrobial resistance, particularly community-associated MRSA (CA-MRSA).

## Classification

SSTIs are broadly classified into three categories. Purulent infections include abscesses, furuncles, and carbuncles, and are predominantly associated with Staphylococcus aureus, including MRSA. Non-purulent infections include cellulitis and erysipelas, which are predominantly caused by beta-hemolytic streptococci (Group A Streptococcus). Necrotizing SSTIs include necrotizing fasciitis, Fournier gangrene, and gas gangrene, which may be polymicrobial or monomicrobial and involve deep fascial planes.

## Abscess

### Pathophysiology and Microbiology

An abscess is a walled-off collection of pus within the dermis or deeper tissues. Community-associated MRSA (the USA300 strain) is the predominant pathogen in community-acquired abscesses in the United States. Risk factors include IV drug use, prior MRSA infection, close contacts with MRSA, incarceration, and contact sports.

### Clinical Features

The classic presentation is a tender, fluctuant, erythematous nodule that may have surrounding cellulitis. Point-of-care ultrasound is highly accurate for distinguishing abscess from cellulitis and identifying the extent of the collection, with a sensitivity of 96 percent and specificity of 83 percent. On ultrasound, an abscess appears as an anechoic or hypoechoic fluid collection with posterior acoustic enhancement and may contain internal echoes or septations.

### Treatment

Incision and drainage (I&D) is the definitive treatment, and antibiotics alone are insufficient. The technique involves local anesthesia, a stab incision over the point of maximal fluctuance, expression of purulent material, breaking up loculations with a hemostat, irrigating the cavity, and loosely packing with iodine-impregnated gauze or placing a loop drain. The loop drain technique passes a vessel loop or Penrose drain through the cavity via two stab incisions, tied loosely, and is associated with less pain and equivalent outcomes compared to traditional packing.

Adjunctive antibiotics after I&D are recommended by the IDSA for abscesses associated with extensive surrounding cellulitis, systemic signs of infection, immunosuppression, or failure of I&D alone. The landmark study by Talan et al. (2016) demonstrated that TMP-SMX after I&D reduced treatment failure. When antibiotics are prescribed, TMP-SMX 1 to 2 DS tablets orally twice daily or doxycycline 100 mg orally twice daily for 7 to 10 days are the standard choices.

<image>Step-by-step illustration of incision and drainage technique for a cutaneous abscess, showing local anesthetic injection, stab incision, expression of purulent material, loculation breakdown with hemostat, irrigation, and loose packing with iodoform gauze strip</image>

## Cellulitis

### Pathophysiology and Microbiology

Cellulitis is an acute spreading infection of the dermis and subcutaneous tissue without a drainable collection. It is predominantly caused by beta-hemolytic streptococci (Groups A, B, C, and G), with S. aureus being less common in non-purulent cellulitis. Risk factors include a skin break (tinea pedis is the most common portal of entry for lower extremity cellulitis), lymphedema, obesity, prior cellulitis, and peripheral vascular disease.

### Clinical Features

The presentation is an expanding area of erythema, warmth, tenderness, and edema without a focal collection. Erysipelas is a form of cellulitis involving the upper dermis, characterized by well-demarcated raised borders and caused by Group A Streptococcus, commonly affecting the face and lower extremities. The differential diagnosis includes deep vein thrombosis (unilateral leg swelling with risk factors), contact dermatitis, and venous stasis dermatitis.

### Treatment

Mild non-purulent cellulitis is treated with oral antibiotics targeting streptococci, such as cephalexin 500 mg orally four times daily or dicloxacillin 500 mg orally four times daily for 5 to 7 days. If MRSA is a concern due to purulence or prior MRSA history, TMP-SMX or doxycycline should be added. Moderate to severe cellulitis requires IV antibiotics, with cefazolin 2 grams IV every 8 hours for streptococcal coverage or vancomycin 15 to 20 mg/kg IV every 12 hours if MRSA is a concern. The borders of erythema should be marked with a skin marker to monitor for progression. The affected extremity should be elevated, and underlying tinea pedis should be treated.

## Necrotizing Fasciitis

### Classification

| Type | Microbiology | Typical Patient | Key Features |
|------|-------------|----------------|--------------|
| Type I (polymicrobial) | Mixed aerobic + anaerobic | Diabetic, post-surgical, perineal (Fournier) | Most common type; synergistic infection |
| Type II (monomicrobial) | Group A Streptococcus or CA-MRSA | Young, healthy individuals | Associated with toxic shock syndrome |
| Type III (gas gangrene) | Clostridium perfringens | Post-trauma or post-surgical | Rapid gas production; crepitus |

Type I (polymicrobial) involves mixed aerobic and anaerobic organisms and is the most common type, typically occurring in diabetic patients, post-surgical patients, and in the perineum (Fournier gangrene). Type II (monomicrobial) is most commonly caused by Group A Streptococcus or CA-MRSA, can occur in young healthy individuals, and is associated with toxic shock syndrome. Type III (gas gangrene) is caused by Clostridium perfringens, occurs in the setting of trauma or surgery, and is characterized by rapid gas production.

### Clinical Features -- Red Flags

Pain out of proportion to the examination findings is the most sensitive early finding. Other concerning features include rapidly spreading erythema that extends beyond marked borders, crepitus on palpation from subcutaneous gas, skin changes such as dusky discoloration, hemorrhagic bullae, and dermal gangrene, and systemic toxicity with high fever, tachycardia, hypotension, and altered mental status. Late or "hard" signs include anesthesia of the overlying skin from cutaneous nerve destruction, gray or necrotic tissue, and frank tissue sloughing.

### Diagnostic Aids

The LRINEC score (Laboratory Risk Indicator for Necrotizing Fasciitis) incorporates WBC, hemoglobin, sodium, glucose, creatinine, and CRP, with a score of 6 or higher suggesting necrotizing fasciitis. However, its sensitivity is limited and it should not replace clinical judgment. CT with contrast can demonstrate fascial thickening, fat stranding, and fascial gas and is more sensitive than plain films for gas detection. MRI is the most sensitive modality for fascial involvement but should not delay surgical consultation. The "finger test" is a bedside exploration under local anesthesia in which lack of bleeding, dishwater gray fluid, and loss of tissue resistance to blunt dissection are diagnostic.

<image>Cross-sectional anatomical diagram comparing the depth of infection in cellulitis (dermis and subcutaneous tissue), abscess (walled-off collection in subcutaneous tissue), and necrotizing fasciitis (infection spreading along the fascial plane between subcutaneous fat and muscle), with labels identifying each tissue layer</image>

### Treatment

Emergent surgical debridement is the definitive treatment, and mortality increases with each hour of delay. Broad-spectrum antibiotics should include vancomycin plus piperacillin-tazobactam plus clindamycin, with clindamycin included specifically to inhibit toxin production by Group A Streptococcus. An alternative regimen is meropenem plus vancomycin plus clindamycin. Aggressive IV fluid resuscitation and vasopressors are used as needed. Surgical consultation should not be delayed for imaging studies. If clinical suspicion is high, the patient should go directly to the operating room. Re-exploration and serial debridements are typically necessary. Mortality ranges from 20 to 40 percent even with appropriate treatment and approaches 100 percent without surgery.

## Special Populations and Sites

Periorbital (preseptal) cellulitis is an infection anterior to the orbital septum that is treated with oral antibiotics. It must be distinguished from orbital (postseptal) cellulitis, which presents with proptosis, ophthalmoplegia, and pain with eye movement and requires CT imaging and IV antibiotics. Diabetic foot infections are polymicrobial and should be classified by severity using IDSA guidelines; mild infections may be managed as outpatients, while moderate to severe infections require IV antibiotics and often surgical debridement. Injection drug users are at risk for unusual organisms (Pseudomonas, anaerobes, Clostridium) and have higher rates of deep-space infections and necrotizing fasciitis.

<image>Clinical photograph-style illustration showing three stages of necrotizing fasciitis progression: early stage with erythema and pain out of proportion, intermediate stage with hemorrhagic bullae and dusky skin discoloration, and late stage with frank skin necrosis and crepitus</image>

## Clinical Pearls

Incision and drainage is the cornerstone of abscess treatment, and antibiotics alone will not resolve a drainable collection. Pain out of proportion to examination findings is the earliest and most reliable clinical indicator of necrotizing fasciitis. Cellulitis without purulence is predominantly streptococcal, and MRSA coverage is not routinely needed. Ultrasound should be used routinely to differentiate abscess from cellulitis and guide drainage. When necrotizing fasciitis is suspected, surgery should be consulted immediately, and diagnostic imaging should never delay operative intervention.

## References

1. Stevens DL, Bisno AL, Chambers HF, et al. Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the IDSA. *Clin Infect Dis*. 2014;59(2):e10-e52.
2. Talan DA, Mower WR, Krishnadasan A, et al. Trimethoprim-sulfamethoxazole versus placebo for uncomplicated skin abscess. *N Engl J Med*. 2016;374(9):823-832.
3. Wong CH, Khin LW, Heng KS, et al. The LRINEC (Laboratory Risk Indicator for Necrotizing Fasciitis) score: a tool for distinguishing necrotizing fasciitis from other soft tissue infections. *Crit Care Med*. 2004;32(7):1535-1541.
4. Tayal VS, Hasan N, Norton HJ, et al. The effect of soft-tissue ultrasound on the management of cellulitis in the emergency department. *Acad Emerg Med*. 2006;13(4):384-388.