Tropical pyomyositis

Tropical pyomyositis is a rare condition characterised by primary muscle abscesses arising within large skeletal muscles (such as the large muscles of the limbs and trunk). Tropical pyomyositis occurs in humid tropical regions of Central Africa and Central America, and also in parts of the Amazon rainforest and southeast provinces of Brazil.

Children and young adults, who are otherwise healthy, are most commonly affected. Staphylococcus aureus is the bacteria responsible in around 90% of cases. Group A streptococcus accounts for another 1-5% of cases and a range of other bacteria account for the remainder. The exact cause of the disease is unknown. Some reported cases have been associated with previous blunt muscle trauma, nutritional deficiencies, and viral and parasitic infections.

Clinical features

The clinical features of tropical pyomyositis can be divided into 3 distinct phases.

• Initially there is 1 to 2 weeks of diffuse (widespread) pain, which may or may not be accompanied by fever. Most commonly the large muscles of the trunk, legs, or upper arms are involved. Single or multiple muscle groups may be involved.
• In the second phase, a mass develops which progressively grows and becomes hard and painful over a further 1 to 2 weeks. The area is tender to the touch with a wooden consistency. As the abscess forms, high fevers and systemic symptoms (such as nausea, vomiting, and malaise) develop.
• If the abscess remains untreated, the third stage develops. The abscess may extend into an adjacent bone or joint, or septicaemia (blood poisoning) may develop. Septicaemia can result in septic shock, kidney failure, and metastatic (distant spread of) abscesses. In around 1.8% of cases the illness is fatal.

Diagnosis

The diagnosis of tropical pyomyositis can be difficult because the condition is rare, and the classical features of an abscess can be hidden by the tense overlying muscles.

• Blood cultures may be positive for Staphylococcus aureus in 30% of patients.
• Ultrasound scans, CT, and MRI can help with early diagnosis.
• Pus aspirated from the abscess can be viewed under a microscope or cultured in the laboratory to check for the presence and type of bacteria. However in 15-30% of cases, the pus may be sterile.

Treatment

The abscess is surgically drained and appropriate antibiotics are given.