Skin and Soft-Tissue Infections (Case 47)

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Skin and Soft-Tissue Infections (Case 47)

Patricia D. Brown MD

Case: A 68-year-old woman presents with progressive pain and swelling of her right lower extremity. Pain and swelling began in the dorsum of the foot 2 days ago, and she noted that the skin appeared red and felt hot. The pain, swelling, and redness now extend almost to the knee. She complains of feeling feverish but denies chills. There is no history of recent trauma to the foot. She denies any history of chronic lower extremity edema. She has a history of hypertension and type 2 diabetes mellitus, with normal fasting blood glucose measurements at home and a normal hemoglobin A1C (HgbA1C) on her last clinic visit 2 months ago, but her blood glucose reading was 350 mg/dL this morning. Her medications include metformin, glyburide, and lisinopril. Physical examination is remarkable for a temperature of 38.7°C with a pulse of 110 beats per minute (bpm), a blood pressure of 145/80 mm Hg, and a normal respiratory rate. Examination of the extremities reveals tense edema of the right lower extremity extending from the dorsum of the foot to the knee. This area is markedly erythematous and tender with increased local temperature. There is no evidence of bullae or blisters, no evidence of any breaks in the skin, and no palpable crepitance or fluctuance. Careful inspection of the right thigh reveals faintly erythematous linear streaks extending from above the knee to the groin region on the inner aspect of the thigh; tender adenopathy is palpated in the right inguinal region. Examination of the left lower extremity reveals no abnormalities, and the remainder of the examination is unremarkable.

Differential Diagnosis

Cellulitis

Skin and soft-tissue abscess

Diabetic foot infection

Erysipelas

Necrotizing fasciitis

 

Speaking Intelligently

Although there is a very broad differential diagnosis for swelling of the lower extremities, the clinical presentation of acute-onset unilateral swelling (tumor), accompanied by the classic findings of erythema (rubor), warmth (calor), and pain (dolor) is characteristic of a skin and soft-tissue infection (SSTI), most commonly cellulitis. While deep venous thrombosis (DVT) should most certainly be considered in a patient who presents with unilateral swelling of the upper or lower extremity (and DVT may present with erythema, warmth, and even fever), the physical findings described in this case, along with the absence of risk factors for DVT, would make the diagnosis highly unlikely. Streptococcus pyogenes is the most common cause of cellulitis, followed by Staphylococcus aureus. A dramatic increase in SSTIs due to community-associated strains of methicillin-resistant S. aureus (CA-MRSA) has occurred in the United States in the last several years; however, these patients most commonly present with multiple skin abscesses.

PATIENT CARE

Clinical Thinking

• The history and examination must focus on determining the extent of the infection, possible predisposing factors, and features that might provide clues to the microbiology of the infection.

• SSTIs must generally be treated on an empirical basis, since unless a patient has evidence of abscess formation or very severe infection that results in bacteremia or necrotizing fasciitis, a microbiologic diagnosis is rarely confirmed.

• An overall assessment of severity of illness will determine whether the patient can be managed as an outpatient with oral antibiotics or will require admission to the hospital for IV therapy.

History

• Focus on predisposing factors for cellulitis, risk factors for specific pathogens, and symptoms that may suggest serious infection.

• Risk factors for SSTI include any history of break in the skin that could serve as a portal of entry for bacteria such as trauma or insect bite, or any preexisting skin lesion such as venous stasis ulcers or ulcers due to arterial insufficiency.

• A common and often overlooked portal of entry in patients with cellulitis of the lower extremity is tinea pedis, leading to small cracks in the skin between the toes.

• Patients with chronic lower extremity edema, those who have undergone saphenous vein harvest for coronary artery bypass grafting, and those who have lymphedema secondary to pelvic surgery or radiation therapy are also predisposed to SSTI.

• A previous episode of severe cellulitis may lead to scarring of the lymphatics and predispose the patient to recurrent episodes of cellulitis.

• Recurrent cellulitis of the upper extremity in women who have undergone mastectomy with extensive lymph node dissection is seen less frequently now that breast-conserving surgery and limited lymph node dissection are more commonly performed.

• Any history of environmental exposures that may suggest uncommon pathogens should be noted; a patient with SSTI presenting as multiple skin abscesses, suggestive of CA-MRSA, should be questioned about exposure to others with SSTI.

• SSTIs that occur following bite wounds have a microbiology that includes pathogens that are part of the normal oral flora of the animal (or person) that bit the patient.

• Symptoms that would suggest a more severe infection such as necrotizing fasciitis include severe pain that is out of proportion to the clinical findings and a history of a rapidly spreading infection.

• SSTI due to either S. aureus or S. pyogenes may also predispose the patient to toxic shock syndrome.

Physical Examination

• Review the vital signs; although fever is not unexpected, hypotension is suggestive of sepsis and/or necrotizing fasciitis.

• Perform a careful inspection of the involved area, noting the presence of ulcers, abscesses, blisters, or bullae, and looking for the presence of lymphangitis, which causes linear erythematous streaks along the course of the lymphatic drainage.

• Note the presence of regional lymphadenopathy. Carefully palpate the area of cellulitis; crepitance (palpable gas in the tissues) suggests necrotizing fasciitis, as does diminished sensation or the presence of large bullae, ecchymosis, or skin necrosis. Areas of fluctuance suggest an underlying abscess.

• Note the presence of tinea pedis in patients with lower extremity cellulitis, as treatment may prevent recurrent infections.

• Examine the other extremity, and note the presence of chronic edema or evidence of venous or arterial insufficiency.

• Many clinicians will use a pen to mark the extent of erythema to help discern if the infection is rapidly spreading. Careful documentation of the extent of erythema is particularly important in settings where the patient will be referred to another physician, who will then need to determine the response to therapy. However, it is not uncommon for the extent of cellulitis to worsen within the first 24 hours of appropriate treatment.

Tests for Consideration

• A microbiologic diagnosis is usually not confirmed in a patient with cellulitis.

• If purulent drainage is noted from a wound or abscesses are present, one should submit material for Gram stain and culture to confirm a specific microbiologic diagnosis and guide antibiotic therapy.

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• When cellulitis occurs in the setting of a chronic ulcer, clinicians may be tempted to send swabs from the ulcer for culture; however, organisms present in a chronic ulcer may be colonizers and not reflective of the pathogen causing the surrounding SSTI.

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• Patients with severity of illness sufficient to warrant hospital admission should have blood cultures; a recent study found that up to one third of patients hospitalized with cellulitis who had an underlying comorbidity such as diabetes mellitus, peripheral vascular disease, or congestive heart failure had concomitant bacteremia.

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IMAGING CONSIDERATIONS

→ If the history is suggestive, obtain a plain radiograph to exclude the possibility of a foreign body. Obtain plain films with diabetics or other patients with sensory neuropathy who have cellulitis of the foot to exclude the possibility of a foreign body.

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→ Plain radiographs can also reveal evidence of underlying osteomyelitis in patients with a chronic ulcer complicated by infection.

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→ CT or MRI scan: May be needed if there is a concern that cellulitis has been complicated by formation of a deeper abscess.

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→ While imaging with CT or MRI may reveal findings supportive of the diagnosis of necrotizing fasciitis, it must be emphasized that the possibility of necrotizing fasciitis is an emergency that requires urgent surgical evaluation; this evaluation should not be delayed while sending the patient for imaging studies.
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