Skin and Soft-Tissue Infections (Case 47)
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.
Skin and soft-tissue abscess
Diabetic foot infection
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.
• 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.
• 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.
• 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
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A breach in the skin may be microscopic or obvious. Pathogenic organisms that either are part of normal skin flora or are colonizing the skin, or organisms introduced from an exogenous source, may then be introduced and cause infection. Multiplication of organisms in the skin and upper dermis elicit an inflammatory response that leads to the cardinal clinical features of cellulitis. The infection may spread via the lymphatics to the bloodstream, resulting in concomitant bacteremia. S. pyogenes is a normal part of the skin flora and is the most common cause of cellulitis. Although not considered part of the normal flora, S. aureus may colonize the skin in individuals who use needles such as injection drug users, diabetics, and hemodialysis patients. Other streptococci (e.g., group C, G, or F) may cause cellulitis. The incidence of infections due to Streptococcus agalactiae (group B streptococcus) is increasing among nonpregnant adults; immunosuppression, diabetes, chronic liver or kidney disease, malignancy, and older age are risk factors. Gram-negative organisms are an uncommon cause of cellulitis, although they may occur in wound infections that are associated with health care. S. aureus and S. pyogenes are potential pathogens when cellulitis occurs following a bite; other pathogens that must be considered are organisms that compose the oral flora of the biter. Pasteurella multocida is associated with cat and dog bites, Capnocytophaga canimorsus is associated with dog bites, and Eikenella corrodens is associated with human bites. In addition, oral anaerobes must always be considered in the microbiology of bite wounds.
As discussed previously, pain, swelling, erythema, and warmth are the typical clinical features of cellulitis. Patients may have signs of systemic toxicity including fever and chills.
A diagnosis of cellulitis can be made based on the presentation and clinical findings; a microbiologic diagnosis cannot be confirmed unless there is purulent drainage from a wound, abscess formation, or bacteremia.
Semisynthetic penicillins (nafcillin, dicloxacillin) and first-generation cephalosporins (cephalexin, cefazolin) are first-line choices for the treatment of cellulitis; clindamycin is a reasonable alternative for penicillin-allergic patients. If cellulitis occurs as a health care–associated infection, or if there is a concern regarding the possibility of MRSA, agents with activity against this pathogen such as vancomycin will need to be utilized. SSTI that occurs after a bite should be treated with a β-lactam–β-lactamase inhibitor combination like amoxicillin–clavulanate or ampicillin–sulbactam to cover all the potential pathogens that can cause bite wound infections. There is no standard duration of therapy; antibiotics should be discontinued once there is significant improvement of the edema and erythema. See Cecil Essentials 101.
Erysipelas is caused by S. pyogenes (group A streptococcus). Similar to that outlined above for cellulitis, erysipelas is a more superficial form of cellulitis that exhibits prominent involvement of the lymphatics.
Erysipelas presents as a bright-red or salmon-colored area with a sharp demarcation, raised above the surrounding skin. Erysipelas is more common in older adults and typically occurs on the face (usually the cheek) or the lower extremity. Because of the prominent lymphatic involvement, the skin may have a “peau d’orange” appearance.
The diagnosis is made on the basis of the clinical presentation and examination findings.
Classic erysipelas can be treated with oral penicillin or amoxicillin. Sometimes there may be uncertainty regarding whether or not a lesion represents erysipelas or cellulitis; in these cases, antibiotics that also provide coverage for S. aureus (dicloxacillin, cephalexin) should be chosen. See Cecil Essentials 101.
Skin and Soft-Tissue Abscess
Any infection of the skin and soft tissue may form an area of induration and suppuration that leads to a localized collection of pus. Both S. aureus and S. pyogenes (along with other streptococci) can cause skin abscess. In the last several years there has been a dramatic increase in skin and soft-tissue infections due to CA-MRSA. These isolates are clearly distinct from health care–associated MRSA infections, and many belong to a single clone, the USA 300 clone. Although CA-MRSA organisms are resistant to β-lactam antibiotics, unlike health care–associated MRSA, they are susceptible to many other antimicrobials. Skin abscesses that occur in injection drug users and those that complicate diabetic foot infections are frequently polymicrobial, including the Enterobacteriaceae and anaerobes in addition to S. aureus and streptococci.
A soft-tissue abscess may form in an area of cellulitis. An abscess should be suspected if there is an area of more intense edema or fluctuance, or if the cellulitis is failing to respond to appropriate antibiotic therapy. Frequently development of an abscess is apparent on examination; occasionally CT scan or MRI is required to evaluate for an abscess in the deeper soft tissues that may not be readily apparent clinically. CA-MRSA SSTIs most commonly present as multiple abscesses, although a single abscess may occur. Patients will frequently report that they have “spider bites” or give a history of a household member with “spider bites.” Outbreaks of CA-MRSA skin infection have been described among prisoners, men who have sex with men, and athletes (wrestlers, football players); however, many patients do not have a specific risk factor for CA-MRSA infection, so the diagnosis must be suspected based on the clinical presentation.
As discussed above, a skin and soft-tissue abscess can frequently be diagnosed based on the clinical examination. In situations where abscess is suspected clinically but is not clearly present on exam, CT or MRI can confirm the presence of a collection of pus. Abscess drainage should always be cultured to confirm the microbiologic diagnosis and direct appropriate antibiotic therapy. A Gram stain can provide rapid initial information regarding the microbiology of the infection.
Surgical drainage is as important as antimicrobial therapy in the management of skin and soft-tissue abscess. Sometimes surgical drainage may be all that is required for abscesses that are small (<5 cm in diameter) and not associated with a significant area of surrounding cellulitis. When an abscess occurs in the setting of typical cellulitis, initial antibiotic coverage directed toward S. aureus and streptococci should be sufficient pending further information from cultures. Trimethoprim-sulfamethoxazole (TMP-SMX) has emerged as the treatment of choice for CA-MRSA infections; clindamycin and doxycycline are alternatives. Abscesses in injection drug users and those that complicate diabetic foot infections (discussed below) will require broader spectrum coverage to include treatment for gram-negative organisms and anaerobes. See Cecil Essentials 101.
Necrotizing fasciitis is an SSTI that has spread beyond the dermis to the deeper subcutaneous tissues and fascia. Necrotizing fasciitis type I is a polymicrobial infection that can include gram-positive, gram-negative, and anaerobic organisms; type II is caused by S. pyogenes. The elaboration of toxins (e.g., streptococcal pyrogenic exotoxin) is important in the pathogenesis of this rapidly progressive and fulminant infection.
Necrotizing fasciitis must be suspected when a patient with cellulitis has a rapidly spreading infection, especially when associated with signs of severe systemic toxicity or severe pain that seems out of proportion to the clinical findings. Rapidly forming bullae, ecchymosis, cutaneous anesthesia, and evidence of skin necrosis may occur; palpable crepitance is strongly suggestive of necrotizing fasciitis. Occasionally, patients may present just with severe pain in the involved area and signs of systemic toxicity.
The possibility of necrotizing fasciitis is a medical and surgical emergency, and surgical consultation should be obtained promptly. The diagnosis can be suggested by findings on imaging with CT or MRI, but surgical evaluation should not be delayed to obtain these studies; the diagnosis can only be confirmed with certainty at the time of surgery. Blood cultures should always be obtained in patients with suspected necrotizing fasciitis.
Patients with suspected necrotizing fasciitis must be managed initially with broad-spectrum antibiotics to ensure coverage of gram-positive bacteria (including MRSA), gram-negative bacteria, and anaerobes (e.g., vancomycin or daptomycin, with either piperacillin-tazobactam or a carbapenem). Clindamycin should also be used in the regimen for patients with group A streptoccal necrotizing fasciitis, as it will inhibit the production of toxin, which is important in the pathogenesis of this infection. Many experts recommend the use of IV immunoglobulin to treat the accompanying streptococcal toxic shock–like syndrome. Extensive surgical debridement is frequently required; antibiotic therapy can then be adjusted based on the results of tissue cultures obtained at the time of surgery. See Cecil Essentials 101.
Foot ulcers may occur in patients with diabetes mellitus as a result of unrecognized trauma due to sensory neuropathy. These infections are generally considered to be polymicrobial, including staphylococci, streptococci, anaerobes, and Enterobacteriaceae; long-standing ulcers that have been treated with previous courses of antimicrobial therapy for secondary infection may be colonized with Pseudomonas and other non–lactose-fermenting gram-negative pathogens. The pathogenesis of these ulcers also involves vascular insufficiency due to peripheral vascular disease. Nonhealing ulcers become colonized with bacteria; infection of the ulcer base may occur, or the ulcer may serve as a portal of entry for bacteria resulting in cellulitis of the surrounding intact skin and soft tissues. As the ulcer base deepens, contiguous spread to the underlying bone may occur, resulting in osteomyelitis. Polymorphonuclear cell dysfunction in the setting of poorly controlled blood glucose also contributes to the pathogenesis of infection.
As described above, patients may present with a malodorous, draining, infected ulcer with or without concomitant cellulitis and osteomyelitis.
Ulcers must be carefully inspected; after cleansing and debridement of the ulcer base, a sterile cotton-tipped swab can be utilized to determine if the ulcer extends to the bone. The positive predictive value for osteomyelitis if the ulcer extends to bone is >90%, although the negative predictive value is only ~50%, so further investigation with plain films or MRI will be required to exclude a diagnosis of osteomyelitis. Cultures may then be obtained from the ulcer base to guide microbiologic therapy; if there is evidence of osteomyelitis, long-term antibiotic therapy will be required. Thus, the confirmation of a microbiologic diagnosis with cultures obtained from a bone biopsy is essential.
For patients with diabetic foot infections accompanied by signs of systemic infection, initial empirical antibiotics will have to be broad spectrum so as to cover the diversity of pathogens that can cause these infections, including gram-positive, gram-negative, and anaerobic organisms. β-lactam–β-lactamase inhibitor combinations such as ampicillin-sulbactam or piperacillin-tazobactam are reasonable initial choices, with further decisions regarding continued therapy guided by the results of properly obtained cultures. Patients with nonhealing ulcers and evidence of underlying osteomyelitis who do not have evidence of cellulitis involving the surrounding skin and soft tissues, and no clinical signs of systemic toxicity, should not receive empirical antibiotic therapy; therapy should be chosen on the basis of microbiologic results from a bone biopsy. The importance of a multidisciplinary approach to the management of diabetic foot infections cannot be overemphasized, including optimal control of the blood glucose, assessment of the vascular supply with evaluation for surgical revascularization, if possible, and adequate surgical debridement of infected tissue and bone, in addition to appropriate antibiotics. See Cecil Essentials 101.
a. Vibrio vulnificus: Hemorrhagic bullous cellulitis can occur as a result of infection with V. vulnificus after seawater exposure (either ingestion of oysters or cutaneous exposure); this pathogen may cause severe sepsis in patients with cirrhosis, iron overload, hemolytic anemia, chronic kidney disease, malignancy, or other immunocompromised states.
b. Pseudomonas species: Colonization can cause folliculitis after exposure to hot tubs or infections (including osteomyelitis) of puncture wounds through sneakers.
c. Aeromonas hydrophila: SSTIs with this organism are often accompanied by myonecrosis after freshwater exposure, or exposure to medicinal leeches.
d. Clostridium septicum: This organism can cause gas gangrene in patients with colonic lesions or neutropenia; other Clostridia species have been associated with necrotizing fasciitis in users of “black tar” heroin.
e. Cryptococcus neoformans: This organism can be involved in cellulitis in patients with defects of cell-mediated immunity, such as transplant patients.
f. Atypical mycobacteria: Mycobacterium marinum can cause cellulitis following exposure to seawater or fish tanks; rapidly growing mycobacteria (M. chelonae, M. fortuitum, M. abscessus) can cause SSTIs in both immunocompetent and immunocompromised individuals, most often after traumatic injury to the skin.
g. Sporotrichosis: Papular, erythematous lesions can develop along the lymphatics in rose gardeners, florists, farmers, and others with traumatic injuries who are exposed to soil, due to the fungal pathogen Sporothrix schenckii; the atypical mycobacteria described above can also present with a “sporotrichoid” appearance.