Orthopaedic Knowledge Update®: Musculoskeletal Infection 2 Print + Ebook (AAOS - American Academy of Orthopaedic Surgeons)

Chapter 27: Necrotizing Fasciitis and Other Complicated Skin and Soft-Tissue Infections

not yield pus. When treatment is delayed, stage 2 occurs greater than 1 to 3 weeks after initial symptom onset and is suppurative. It is characterized by fever, muscle tenderness, and edema. A deep abscess may be present, and percu taneous aspiration of the affected muscle usually yields purulent material. A 2021 review determined that stage 3 is characterized by symptoms of systemic infection and toxicity with frank fluctuation in the affected muscle. 60 Another 2021 review reported that at this stage, various complications of S aureus bacteremia can occur, includ ing osteomyelitis, septic arthritis, septic shock, infectious endocarditis, septic emboli, pneumonia, pericarditis, brain abscess, and acute kidney failure. 61 A 2020 review reported that MRI is the most useful imaging technique for diagnosing pyomyositis, although other modalities could also be used for diagnosis such as ultrasonography or CT. 62 Because the pathogenesis of pyomyositis involves bacteremia, two sets of blood cul tures should be obtained, especially in patients presenting with systemic symptoms or ectopic sites of complications. A 2020 review reported that ultrasonography and CT are also useful for percutaneous drainage or aspiration of purulent sites to identify a pathogen with antibiotic susceptibility screening, particularly in patients who have negative blood cultures. 63 Ideally, if the patient’s condi tion permits, imaging with specimen collection will be performed before systemic antibiotics are initiated. Stage 1 pyomyositis can be managed with parenteral antimicrobial agents alone and without percutaneous or open surgical drainage. The choice of initial empiric drug depends on the patient’s immune status. 31,63 If the patient is immunocompetent, then gram-positive cocci ( S aureus [including MRSA] and beta-hemolytic streptococci) should be covered, and the preferred therapy is vancomycin. If the patient is immunocompromised, broad coverage is needed against gram-positive, gram-negative, and anaer obic bacteria; the preferred therapy is a combination of vancomycin and a beta-lactam/beta-lactamase inhibitor (such as ampicillin-sulbactam, piperacillin-tazobactam, ticarcillin-clavulanate). 31,63 Duration of parenteral ther apy is typically 3 to 4 weeks, and it should be adjusted based on clinical and radiographic improvement as well as microbiologic diagnosis. For example, if the patient has extensive, multifocal, or poorly drained infection, longer courses of therapy may be warranted. In addition, duration of therapy should be tailored if complicated bacteremia is present (eg, endocarditis or osteomyelitis). In addition to a course of antibiotics, stages 2 and 3 of pyomyositis require image-guided percutaneous aspiration or surgical drainage and débridement depend ing on the abscess size, the depth of muscle involve ment, and the presence of necrosis. An overview from 2020 determined that antimicrobial therapy should be

administered in a timely manner, especially in patients with symptoms of systemic infections (stage 3). 64

ANIMAL BITES Infected animal bite wounds should be suspected with findings of fever, tenderness, erythema, swelling, warmth, purulent drainage, and lymphangitis 24 hours following dog bites and 12 hours following cat bites. In 2019, a retrospective study reported that younger patients typi cally have bites involving the head and neck as opposed to older patients whose upper extremities are usually affected. 65 A 2018 review reported that although cat bites are less common and have less damaging effect than dog bites, they can cause small puncture wounds that are often more difficult to manage with débridement. 66 It is important to note that cat bites can have a benign outward appearance and still put the patient at a greater risk for deep soft-tissue infections including abscesses, osteomyelitis, and septic arthritis/tenosynovitis. Animal bite wound infections are usually polymicro bial caused by animal oral flora, human skin flora, and environmental organisms, with Pasteurella spp. being the most common organism isolated from both cat and dog bites. 66 Other pathogens, including aerobes (such as Staphylococcus spp. and Streptococcus spp.) and anaer obes (such as Porphyromonas spp., Bacteroides spp., and Fusobacterium spp.), are often identified and may lead to abscess formation. Pasteurella multocida tends to cause a more severe and rapidly spreading cellulitis that can lead to osteomyelitis. A 2021 report indicated that cat scratch fever caused by Bartonella henselae is another potential pathogen and is usually self-limited, but may rarely present with lymphadenitis, osteomyelitis, or prolonged fever. 67

Section 6: Bone, Joint, and Soft-Tissue Infections

Prompt identification of the causative pathogen is essential, especially in a patient with immunocompromis ing conditions, asplenia, alcohol addiction, and cirrhosis and even sometimes in healthy individuals. Patients with infected animal bites should have aerobic and anaerobic bacterial wound cultures obtained and blood cultures drawn, especially in patients with immunocompromis ing conditions or signs of systemic infection. Because Capnocytophaga canimorsus , which is more common after dog bites, is difficult to isolate and identify on microbiologic media due to its slow growth, polymerase chain reaction is the gold standard for its bacteriologic diagnosis. 68 A retrospective study from 2020 determined that serologic assays should be obtained if Bartonella spp. are of concern. Radiography is recommended ini tially to detect debris or any residual teeth from the ani mal bite; however, MRI is the preferred modality for infection detection, especially osteomyelitis. 69 Copyright © Wolters Kluwer, Inc. Unauthorized reproduction of the content is prohibited. 2023

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Orthopaedic Knowledge Update ® : Musculoskeletal Infection 2