Impact of underlying diseases on the clinical characteristics and outcome of primary pyomyositis.

Background and Purpose: Primary pyomyositis is increasingly recognized in non-tropical areas, its incidence seeming to mirror the increase in immunocompromised populations. In this study, we sought to analyze the differences in clinical characteristics, causative organisms, treatment and outcome between pyomyositis patients with and without underlying diseases.
Methods: Thirty five patients with a diagnosis of primary pyomyositis seen in our hospital between July 1989 and July 2006 were enrolled. Descriptive information concerning age, gender, clinical features, underlying comorbid diseases, results of blood tests, blood culture, muscle or pus culture, disease severity and clinical stages at the time of diagnosis, therapy, and outcome were collected by review of medical charts.
Results: Of the 23 cases with underlying diseases, the mean age was 47.8 years (range, 24 to 79 years). Of the 12 patients without underlying diseases, the mean age was 26.2 years (range, 2 to 72 years). The lower extremities was the most common site of involvement. Staphylococcus aureus was the most frequent causative organism. Gram-negative organisms were isolated in 30.4% of patients with underlying diseases and in none of the patients without underlying diseases (p=0.07). Positive blood culture was significantly more common in patients with underlying diseases than in patients without underlying diseases (52.2% vs 8.3%, p=0.013). The mortality rate was higher in patients with underlying diseases than in patients without underlying diseases (39.1% vs 0.0%, p=0.015). White blood cell count (p=0.017), Acute Physiology and Chronic Health Evaluation (APACHE) II score (p<0.001), recurrence (p=0.004), and presence of underlying diseases (p=0.015) were significant prognostic factors for mortality by univariate analysis. APACHE II score (odds ratio, 1.57; 95% confidence interval, 1.13 to 2.20; p=0.008) was found to be a significant independent risk factor for mortality in multivariate logistic regression analysis. For prediction of mortality, the best cut-off point in APACHE II score was 16 (sensitivity, 77.8%; specificity, 92.3%; accuracy, 88.6%).
Conclusions: Patients with primary pyomyositis should be treated with appropriate broad-spectrum antibiotics and be monitored closely for complications. This study found that patients who suffered from primary pyomyositis with underlying diseases had a higher rate of Gram-negative bacterial infections, bacteremia and mortality. The APACHE II score at diagnosis was found to be an independent prognostic factor for mortality.