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Skin and Soft Tissue Infections 

Skin and Soft Tissue Infections
Skin and Soft Tissue Infections

Emily Shuman

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date: 05 July 2022

  1. A. Introduction

    1. a. The severity of skin and soft tissue infections can vary from insignificant to life-threatening and is determined by both the causative organism and the depth of tissue involvement.

    2. b. Cellulitis and animal bites are common reasons for emergency department visits and hospital stays.

    3. c. Pyomyositis and necrotizing soft tissue infections are two clinical entities that may be difficult to distinguish from cellulitis but require different therapies. Necrotizing soft tissue infections are associated with a high mortality rate and are often not diagnosed in a timely fashion.

  2. B. Cellulitis

    1. a. Definition. Cellulitis is an infection of the dermal and subcutaneous tissues, including the deep dermis and fat.

    2. b. Clinical manifestations. Patients are often febrile and complain of pain at the affected site. On physical examination, erythema, warmth, tenderness, and edema are notable.

    3. c. Microbiology. The most common etiologic agents are Staphylococcus aureus and streptococcal species, most commonly group A Streptococcus. In specific settings, other organisms may cause cellulitis, including gram-negative bacilli and anaerobes.

    4. d. Approach to the patient. Several historical features may provide important clues about the etiology of disease and may lead the physician to broaden empiric therapy or to watch for serious manifestations.

      1. i. Has any trauma occurred in the involved area? Injections (e.g., diabetic patients requiring insulin, intravenous drug users) and skin tears can provide routes of entry for bacteria.

      2. ii. Has the involved area been exposed to soil, fresh water, or salt water? Soil exposure may increase the risk for anaerobes or gram-negative infections. Water exposure increases the risk of infection with organisms such as Aeromonas hydrophila or Vibrio vulnificus.

      3. iii. Does the patient have any comorbid conditions? Underlying diabetes or neoplasm may increase the risk of group B Streptococcus infection. Patients with cirrhosis are at increased risk for V. vulnificus infection after salt-water exposure or consumption of raw shellfish.

      4. iv. Is the cellulitis adjacent to a surgical wound or a diabetic foot ulcer? Both these circumstances increase the risk of infection with gram-negative organisms or resistant gram-positive organisms (e.g., methicillin-resistant S. aureus [MRSA]).

      5. v. Did the cellulitis start in an area of an animal bite? (See C., below.)

      6. vi. Does the patient’s pain seem out of proportion to the examination? Is the area of cellulitis spreading over minutes to hours? Is there crepitus, or are there bullous lesions? A positive answer to any of these questions should lead the physician to consider more serious diagnoses that require more aggressive intervention (see E., below).

      7. vii. Is the border of the lesion raised and well demarcated? Is the involved area significantly edematous and indurated? The presence of these findings likely indicates erysipelas, which is actually a more superficial infection than cellulitis. Erysipelas is almost always caused by group A Streptococcus.

      8. viii. Is there an abscess? The presence of an abscess may be indicative of infection with community-acquired MRSA (caMRSA). The classic presentation of caMRSA is a cutaneous lesion that is often referred to by patients as a “spider bite,” with development of surrounding cellulitis and abscess formation.

    5. e. Treatment

      1. i. In the absence of several of the risk factors discussed later (see B. d.) (e.g., animal bite, surgical wound, presence of underlying diabetes mellitus or malignancy, contamination of the site with soil or environmental water), narrow coverage with an agent that covers streptococcal and staphylococcal species is preferred. Reasonable intravenous choices include nafcillin and cefazolin; good oral choices include cephalexin and dicloxacillin.

      2. ii. If cellulitis is associated with a surgical wound, vancomycin may be a more appropriate initial empiric antibiotic choice. Gram-negative coverage should be considered and tailored to the appropriate organisms based on historical exposures.

      3. iii. If caMRSA is suspected, vancomycin is an appropriate intravenous choice. Good oral choices include linezolid, tetracycline, trimethoprim-sulfamethoxazole, or clindamycin. If an abscess is present, drainage is indicated.

  3. C. Animal Bites

    1. a. Epidemiology. Animal bites are a common reason for emergency department visits. Patients often present acutely, within hours of the bite, or later after cellulitis or deeper infections have developed.

    2. b. Microbiology. Infections due to bites from dogs and cats most often are caused by S. aureus, streptococcal species, gram-positive anaerobes, and Pasteurella multocida. Less common organisms include Capnocytophaga canimorsus, which can cause fulminant disease in patients with liver disease or splenectomy; Francisella tularensis (tularemia); and rabies virus.

    3. c. Approach to the patient

      1. i. Determine the history of the bite

        1. 1. What type of animal was it? Was the bite unprovoked? The first responsibility of the treating physician is to determine whether the patient needs rabies prophylaxis. Rabies is uncommon in the United States, but it is fatal without appropriate prophylaxis. Bats and raccoons are known to harbor the rabies virus. Domesticated dogs are not a common source of the virus, but an unprovoked attack should lead to observation of the animal and consideration of prophylaxis.

        2. 2. How rapidly have symptoms developed after the bite? Rapidly progressive cellulitis should lead to consideration of P. multocida infection.

        3. 3. Was the bite a cat bite and was it near a joint? Cat bites can leave deep puncture wounds, which appear unimpressive but can inoculate organisms deep into a joint or tendon sheath, leading to septic arthritis, osteomyelitis, or tenosynovitis. Examination should be performed carefully to consider these complications, and patients with cat bites should be followed closely and treated aggressively.

      2. ii. Culture the wound if possible.

    4. d. Treatment. In non–penicillin-allergic patients, the treatment of choice is ampicillin/sulbactam or amoxicillin/clavulanate. Ciprofloxacin plus clindamycin may be considered in the penicillin-allergic patient.

  4. D. Pyomyositis

    1. a. Definition. Pyomyositis is a bacterial infection of the muscle, which can mimic cellulitis.

    2. b. Epidemiology. In the United States, pyomyositis occurs most often in immunocompromised patients (e.g., patients with diabetes or cirrhosis, individuals taking corticosteroids) and those with local trauma (e.g., intravenous drug users).

    3. c. Microbiology. The most common etiology is S. aureus. Streptococci and gram-negative bacilli are much less common.

    4. d. Approach to the patient. Typically, the onset is subacute. Pyomyositis progresses to a febrile illness with pain, erythema, and fluctuance at the involved site.

      1. i. Pyomyositis should be considered in the differential diagnosis in patients with the following conditions:

        1. 1. Risk factors (e.g., patients receiving systemic corticosteroids, presence of diabetes mellitus or cirrhosis, intravenous drug use, HIV infection).

        2. 2. Slowly resolving cellulitis despite usual treatment.

        3. 3. “Cellulitis” in a less typical location (e.g., thigh) or with more induration or fluctuance than expected.

      2. ii. Magnetic resonance imaging (MRI) or computed tomography (CT) of the affected area can help make the diagnosis.

    5. e. Treatment. Adequate therapy requires both antistaphylococcal antimicrobial therapy (e.g., nafcillin) and adequate drainage (e.g., percutaneous aspiration or open surgical drainage).

  5. E. Necrotizing Soft Tissue Infections

    1. a. Definition. Necrotizing soft tissue infection is a necrotizing cellulitis that involves the superficial fascia and subcutaneous tissues.

      Hot Key

      Necrotizing soft tissue infections are a clinical emergency and have a high mortality rate, even with optimal therapy. Differentiation of a necrotizing soft tissue infection from cellulitis in the early stages requires an astute clinician and close follow-up.

    2. b. Epidemiology. Patients at increased risk include those with diabetes, alcoholism, immunosuppressed status, or peripheral vascular disease.

    3. c. Clinical manifestations

      1. i. There is often a preexisting history of trauma, including recent surgery, but the insult may be minor (e.g., a small cut or site of injection).

      2. ii. The most common involved areas are the extremities, perineum, and abdominal wall.

      3. iii. Spread of infection is rapid. The initial stages are notable for pain out of proportion to the examination, with minimal or absent skin changes. Over hours to days, loss of sensation can occur due to tissue ischemia and nerve destruction. Erythema, edema, cyanosis, and bullous formation may develop. Crepitus (subcutaneous gas) is an ominous sign. Patients appear ill, with high fever, and become hemodynamically unstable.

    4. d. Microbiology. Two primary types of necrotizing soft tissue infection have been described. The first is polymicrobial and may involve gram-positive and gram-negative organisms, including anaerobes. The second is caused by group A Streptococcus.

    5. e. Approach to the patient

      1. i. Diagnosis of this condition in the early stages is difficult. Serial clinical examinations are necessary in patients who appear to have cellulitis with atypical features (too much pain, too high a fever, bullous lesions, hemodynamic instability) and in those with underlying immunosuppressive conditions (diabetes, alcohol abuse, end-stage renal disease) or recent trauma. Marking the involved area with a pen is helpful to gauge progression over time.

      2. ii. CT or MRI may be helpful but should be reserved for stable patients. In acutely ill patients, diagnosis should be confirmed in the operating room.

        Hot Key

        If a diagnosis of necrotizing soft tissue infection is being considered, it is never too early to ask for a surgical evaluation.

    6. f. Treatment. Adequate therapy combines urgent surgical debridement with antimicrobial therapy. Blood cultures and surgical wound cultures can help guide therapy. Antimicrobial therapy should be broad and include anaerobic coverage until the etiologic organisms are identified. Many clinicians favor including clindamycin in the regimen because its mechanism of action (i.e., protein synthesis inhibition) leads to inhibition of bacterial toxin synthesis.

Suggested Further Readings

Bisno AL, Stevens DL. Streptococcal infections of skin and soft tissues. N Engl J Med 1996;334:240–6. (Classic Article.)Find this resource:

Moran GJ, Krishnadasan A, Mower WR, et al. Effect of cephalexin plus trimethoprim-sulfamethoxazole vs cephalexin alone on clinical cure of uncomplicated cellulitis: a randomized clinical trial. JAMA 2017;317:2088–96.Find this resource:

Raff AB, Kroshinsky D. Cellulitis: a review. JAMA 2016;316:325–37.Find this resource:

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 Infectious Diseases Society of America. Clin Infect Dis 2014;59:e10–e52.Find this resource:

Stryjewski ME, Chambers HF. Skin and soft-tissue infections caused by community-acquired methicillin-resistant Staphylococcus aureus. Clin Infect Dis 2008;46:S368–S77.Find this resource: