Myositis, Pyomyositis, and Necrotizing Fasciitis

Staphylococcus aureus

Although S. aureus myositis is uncommon, the incidence has increased among adults and children in recent years, largely due to the emergence of CA-MRSA. ^, In the US, the USA300 clone of CA-MRSA accounts for most infections, although USA300 clones of methicillin-susceptible S. aureus are similarly challenging. It appears that virulence has evolved in the USA300 clonal type through the increased expression of core genome-encoded virulence determinants, such as the two-component exotoxins (e.g., LukAB, LukDE, α-hemolysin, phenol-soluble modulins) and acquisition of the phage-encoded Panton-Valentine leukocidin (PVL) genes. Each of these toxins impact disease progression in animal models of USA300 infection. The basis of virulence in other CA-MRSA epidemics is less well understood.

Streptococcus pyogenes

S. pyogenes (i.e., group A Streptococcus [GAS]) can cause myositis. The most severe form is necrotizing myositis (i.e., streptococcal myonecrosis or spontaneous streptococcal gangrenous myositis).

Cases typically occur among adult men and young adults. The disease usually occurs spontaneously without a history of penetrating trauma or underlying immunosuppression. The portal of entry often is unknown. Some cases begin with a sore throat, suggesting that pharyngitis might have led to bacteremia and seeding of the muscle, but most cases occur without an antecedent illness.

The clinical presentation includes an initial prodromal stage with flu-like symptoms, which may include rash and myalgia. This progresses to intense local muscle pain that is disproportionate to clinical findings and to local tense swelling and fever. The course can evolve rapidly over several hours ( Fig. 74.1 ).

Clostridium Species

Clostridial gas gangrene (i.e., myonecrosis) most commonly is caused by Clostridium perfringens, C. novyi, C. histolyticum, and C. septicum . C. perfringens is the most frequent cause of trauma-associated gas gangrene. Infection occurs in a variety of settings: traumatic wounds with soil contamination, surgery involving the bowel or biliary system, septic abortions, vascular disease with arterial insufficiency, and injection of medications (e.g., epinephrine) or illicit drugs (e.g., heroin). Common inciting events include contamination of the site with Clostridium spp. (which exist in the soil and as part of the gastrointestinal [GI] flora of humans) and devitalized tissue. A foreign body is also a risk factor. In contrast to traumatic gas gangrene, spontaneous gangrene is principally associated with the more aerotolerant C. septicum and occurs predominantly in patients with underlying GI conditions, including occult colon cancer, bowel infarction, or neutropenic enterocolitis.

C. septicum infections occur in children with three major predispositions: neutrophil dysfunction (e.g., malignancies, congenital or cyclic neutropenia, aplastic anemia), bowel ischemia (e.g., hemolytic uremic syndrome, intussusception), and trauma. Malignancy underlies more than 50% of cases; acute myelocytic and lymphocytic leukemias are the most common. Cyclic and congenital neutropenia is identified in 20% of the published pediatric cases.

Clinical features associated with C. septicum infections in children include fever, vomiting, diarrhea, hematochezia, anorexia, and acute abdomen or distention . Despite treatment with surgery and parenteral antibiotics, mortality rates for children with C. septicum infections remain higher than 50%.

Presenting symptoms of clostridial myonecrosis include intense pain, edema, and a sweet-smelling discharge that occur several hours to a few days after injury. The wound is initially pale but can evolve to a bronze color with hemorrhagic bullae. Classic findings include gas in the tissues detected by gas bubbles emitted from the wound or seen on radiographic films. Although gas often is considered to be essential for a diagnosis of gas gangrene, the absence of crepitus or gas on examination should not deter consideration of this diagnosis. Failure of the muscle to contract on stimulation and lack of bleeding of the wound during operation are characteristic findings.

Typical laboratory findings include leukocytosis and a hemolytic anemia due to clostridial α-toxin. Bacteremia occurs in 15% of cases. Gram stain evaluation of the wound exudate usually shows a lack of neutrophils and an abundance of gram-positive bacilli with blunt ends.