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Cellulitis
is acute bacterial infection of the skin and subcutaneous tissue
most often caused by streptococci or staphylococci. Symptoms and signs
are pain, rapidly spreading erythema, and edema; fever may occur,
and regional lymph nodes may enlarge. Diagnosis is by appearance;
cultures are sometimes helpful but awaiting these results should
not delay empiric therapy. Treatment is with antibiotics. Prognosis
is excellent with timely treatment.
Etiology
Cellulitis is most often caused by group A β-hemolytic streptococci (eg, Streptococcus pyogenes) or Staphylococcus aureus. Streptococci cause diffuse, rapidly spreading infection because enzymes produced by the organism ( streptokinase , DNase, hyaluronidase) break down cellular components that would otherwise contain and localize the inflammation. Staphylococcal cellulitis is typically more localized and usually occurs with an open wound or cutaneous abscess.
In the last few years, methicillin-resistant S.
aureus (MRSA) has become more common in the community. Historically, MRSA was typically confined to patients who were exposed to the organism in a hospital or nursing facility. MRSA infection should now be considered in patients with community-acquired cellulitis, particularly in those with cellulitis that is recurrent or unresponsive to monotherapy.
Less common causes are group B streptococci (eg, S. agalactiae) in older patients with diabetes; gram-negative bacilli (eg, Haemophilus influenzae) in children; and Pseudomonas aeruginosa in patients with diabetes or neutropenia, hot tub or spa users, and hospitalized patients. Animal bites may cause cellulitis, with Pasteurella multocida from cats and Capnocytophaga sp from dogs. Immersion injuries in fresh water may result in cellulitis caused by Aeromonas hydrophila; in warm salt water, by Vibrio vulnificus.
Risk factors include skin abnormalities (eg, trauma, ulceration, fungal infection, other skin barrier compromise due to preexisting skin disease), which are common in patients with chronic venous insufficiency or lymphedema. Scars from saphenous vein removal for cardiac or vascular surgery are common sites for recurrent cellulitis, especially if tinea pedis is present. Frequently, no predisposing condition or site of entry is evident.
Symptoms and Signs
Infection is most common in the lower extremities. Cellulitis is typically unilateral; stasis dermatitis closely mimics cellulitis but is usually bilateral. The major findings are local erythema and tenderness, frequently with lymphangitis and regional lymphadenopathy. The skin is hot, red, and edematous, often with surface appearance resembling the skin of an orange (peau d'orange). The borders are usually indistinct, except in erysipelas (a type of cellulitis with sharply demarcated margins—see Bacterial Skin Infections: Erysipelas). Petechiae are common; large areas of ecchymosis are rare. Vesicles and bullae may develop and rupture, occasionally with necrosis of the involved skin. Cellulitis may mimic deep venous thrombosis but can often be differentiated by one or more features (see Table 1: Bacterial Skin Infections: Differentiating Cellulitis and Deep Venous Thrombosis ). Fever, chills, tachycardia, headache, hypotension, and delirium may precede cutaneous findings by several hours, but many patients do not appear ill. Leukocytosis is common.
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Table 1
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Differentiating Cellulitis
and Deep Venous Thrombosis
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Feature
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Cellulitis
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Deep Venous Thrombosis
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Skin temperature
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Hot
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Normal or cool
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Skin color
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Red
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Normal or cyanotic
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Skin surface
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Peau d'orange
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Smooth
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Lymphangitis and regional lymphadenopathy
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Frequent
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Nonexistent
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Diagnosis
Diagnosis is by examination. Skin and (when present) wound cultures are generally not indicated because they rarely identify the infecting organism. Blood cultures are useful in immunocompromised patients to detect or rule out bacteremia. Culture of involved tissue may be required in immunocompromised patients if they are not responding to empiric therapy or if blood cultures do not isolate an organism.
Prognosis
Most cellulitis resolves quickly with antibiotic therapy. Local abscesses occasionally form, requiring incision and drainage. Serious but rare complications include severe necrotizing subcutaneous infection (see Bacterial Skin Infections: Necrotizing Subcutaneous Infection) and bacteremia with metastatic foci of infection.
Recurrences in the same area are common, sometimes causing serious damage to the lymphatics, chronic lymphatic obstruction, and lymphedema.
Treatment
Treatment is with antibiotics. For most patients, empiric treatment effective against both group A streptococci and S. aureus is used. Oral therapy is usually adequate with dicloxacillin 250 mg or cephalexin 500 mg po qid for mild infections. Levofloxacin 500 mg po once/day or moxifloxacin 400 mg po once/day works well for patients who are unlikely to adhere to multiple daily dosing schedules. For more serious infections, oxacillin or nafcillin 1 g is given IV q 6 h. Use of initial empiric therapy against MRSA is not typically advised unless there is compelling clinical evidence (eg, contact with a documented case or outbreak; high culture-documented prevalence in a practice area). For penicillin-allergic patients or those with suspected or confirmed MRSA infection, vancomycin 1 g IV q 12 h is the drug of choice (see also Gram-Positive Cocci: Treatment). Linezolid is another option for the treatment of MRSA at a dose of 600 mg IV or po q 12 h for 10 to 14 days. Teicoplanin has a mechanism of action similar to vancomycin . It is commonly used outside the USA to treat MRSA; the usual dose is 6 mg/kg IV q 12 h for 2 doses, followed by 6 mg/kg (or 3 mg/kg) IV or IM once/day. Immobilization and elevation of the affected area help reduce edema; cool, wet dressings relieve local discomfort.
Cellulitis in a neutropenic patient requires empiric antipseudomonal antibiotics (eg, tobramycin 1.5 mg/kg IV q 8 h and piperacillin 3 g IV q 4 h) until blood culture results are available. Penicillin is the drug of choice for P. multocida, an aminoglycoside (eg, gentamicin ) is effective against A. hydrophila, and tetracycline is preferred for V. vulnificus.
Recurrent leg cellulitis is prevented by treating concomitant tinea pedis, which often eliminates the source of bacteria residing in the inflamed, macerated tissue. If such therapy is unsuccessful or not indicated, recurrent cellulitis can sometimes be prevented by benzathine penicillin 1.2 million units IM monthly or penicillin V or erythromycin 250 mg po qid for 1 wk/mo. If these regimens prove unsuccessful, tissue culture may be required.
Last full review/revision October 2007 by A. Damian Dhar, MD
Content last modified February 2008
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