Staphylococcus aureus pyomyositis reporting is increasing in temperate (nontropical) climates and among immunocompetent hosts.1 Methicillin-resistant S. aureus (MRSA) pyomyositis in particular has also been reported. An alarming series of 4 cases within 6 months of lower extremity pyomyositis in Washington, DC, due to MRSA was reported by Ruiz et al.2 Methicillin-resistant S. aureus pyomyositis occurring without commonly accepted risk factors is particularly concerning, given the relatively high mortality of classic tropical pyomyositis and the rising incidence of MRSA infections. With the ability of MRSA to retain this type of virulence, we may see an increased incidence in nontropical countries and among immunocompetent individuals. We report a unique presentation in Northwest Texas of chest wall MRSA pyomyositis which highlights a previously unreported phenomenon of local invasion resulting in empyema thoracis.
A 49-year-old mother of 8 presented to our institution in Amarillo, Texas, with 2 days of increasing left flank pain aggravated with movement. Symptoms began as a "muscle spasm" while lifting a heavy object. There was no associated fever, cough, pleurisy, dysuria, hematuria, or pain in the spine, ribs, and abdomen. A month prior she had periumbilical "boils" that resolved spontaneously. Two of her 3 children living at home had recent incisions of skin abscesses. She was febrile (102°F) with tachycardia. Her pain and discomfort were apparent, but she did not otherwise appear ill. Findings included poor dentition, a left heart systolic flow murmur, left lung base crackles, and left flank tenderness (not localized to spine or ribs). No abnormalities were seen in her abdominal examination, and hip flexion could be performed without back pain. There were also no abnormalities seen in her skin examination except for 2 small periumbilical scars. Leukocyte count on admission was 12.6 × 103/mL. There were no abnormalities seen in her urine analysis, stool guaiac, and chest film. Computed tomographic (CT) scan ruled out perinephric, spleen, uterine, and psoas muscle infection. The key CT finding (Fig. 1) was asymmetric enlargement of the left latissimus dorsi and serratus anterior consistent with muscle inflammation. Also noted were subtle pleural surface irregularities without obvious effusion. Soft-tissue needle aspiration of the tender area did not reveal pus. Within 24 hours, MRSA was isolated from admission blood cultures. Treatment with intravenously administered vancomycin was started.
Several days after admission, despite improvement of fever, leukocytosis, and flow murmur, she developed new-onset pleurisy in the same area as prior back pain. Chest CT revealed a new left pleural effusion and adjacent subsegmental atelectasis. Thoracentesis revealed a culture-negative exudate. An unrevealing further search for occult infection included transthoracic echocardiography (for endocarditis), abdominal ultrasound, cervical cultures, and tagged white cell scan. Magnetic resonance imaging and CT studies showed no signs of a perispinal, vertebral body, psoas muscle, or rib infection. At no time did the patient have a cellulitis in the area of muscle tenderness.
Extensive organizing loculations of the pleural space developed (despite 1 week of antibiotics) requiring thoracoscopic surgical decortication. Community-acquired MRSA (CA-MRSA) was isolated from the pleural fluid. The isolate was resistant to ampicillin, penicillin, and erythromycin. It was susceptible to gentamicin, levofloxacin, rifampin, tetracycline, trimethoprim- sulfamethoxazole, and vancomycin (minimum inhibitory concentration ≤2). The patient ultimately recovered completely.
Pyomyositis is a primary subacute pyogenic infection of striated muscles without penetrating trauma or spread from a contiguous septic focus. It presents as abscesses, carbuncles, or infected sinuses lying deep in skeletal muscles. It is a very common infection in the tropical countries, first described by Scriba in 1885.3 The first case reported in the United States was in 1971.4 The incidence of pyomyositis is still unknown. In nontropical countries, pyomyositis is mostly seen with conditions that affect the host defenses such as intravenous drug abuse, HIV, diabetes mellitus, connective tissue diseases, or malignancy.5,6 The pathogenesis of pyomyositis is still not well understood. Early diagnosis of pyomyositis is missed most of the time because of nonspecific signs and symptoms. It is important to note that pyomyositis can be difficult to diagnose because there are often no skin findings (cellulitis), and muscle tenderness can be hard to differentiate from deeper tissue tenderness. Diagnosis may also be difficult due to the relative rarity of pyomyositis in nontropical settings. Prior occult bacteremia could be the starting point for pyomyositis. The causative organism is S. aureus in 70% of the cases.7
The prevalence of MRSA is increasing in hospitals and in the community. Community-acquired MRSA constitutes 8% to 20% of MRSA cases. The threat of CA-MRSA is increasing worldwide. Community-acquired MRSA seems to be more virulent than similar resistant strains found in the hospitals. It can infect healthy young people without risk factors. Hospital-acquired MRSA is generally isolated 2 or more days after hospitalization, surgery, or dialysis. Residence in a long-term care facility within 1 year before admission is a risk factor. The presence of a percutaneous medical device (eg, central line, tracheostomy tube, or gastrostomy tube) is also a risk factor. Prior isolation of MRSA is an additional risk factor for hospital-acquired MRSA.8
The first case of pyomyositis specifically with MRSA was reported from Singapore in 1996.9 About 12 more cases have been reported since then, of which 9 were community acquired. Methicillin-resistant S. aureus pyomyositis is most often associated with bacteremia. Multilocalized nontropical pyomyositis has been reported; however, in most cases, the infection was restricted to skeletal muscle.10
Pulmonary abscess or pleural thickening is seen occasionally with metastatic tropical pyomyositis.11 We believe that this is the first report of CA-MRSA nontropical pyomyositis leading directly to empyema. The radiograph sequence (Figs. 1 and 2) demonstrates muscle infection (pyomyositis) preceding direct pleural space invasion. Although in our case the pleural infection may be an atypical metastasis from the pyomyositis, we believe it is more likely that the empyema represents direct invasion of bacteria from the pyomyositis to the pleural space. This hypothesis is supported by the fact that the patient did not develop any evidence of metastatic infection elsewhere.
The diagnosis of pyomyositis could be difficult in early stages of the disease in nonendemic areas; however, the disease has to be considered in patients presenting with fever and muscle pain. This case was unusual because CA-MRSA pyomyositis was complicated by direct pleural space invasion resulting in empyema thoracis. With the rising incidence of virulent CA-MRSA, we are likely to find more nontropical pyomyositis with unusual patterns of invasion.
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