What is tropical pyomyositis?
Tropical pyomyositis is a rare condition characterised by primary muscle abscesses arising within large skeletal muscles. It was first described by Scriba in 1885.
Who gets tropical pyomyositis?
Tropical pyomyositis most commonly affects children and young adults, who are otherwise healthy. Most are 10–40 years of age, with a male to female ratio of 1.5:1. It can follow:
- Blunt muscle trauma
- Intravenous drug use
- Intramuscular injections.
Predisposing factors include:
- Nutritional deficiencies
- Viral infections
- Parasitic infections
- Diabetes mellitus
- Malignancy
- Liver cirrhosis
- Renal insufficiency
- Organ transplantation
- Treatment with immunosuppressive drugs.
Human immunodeficiency virus (HIV) has been found to be a strong risk factor for pyomyositis, due to immune compromise, primary HIV myopathy, antiretroviral therapy, and staphylococcal carriage.
What causes tropical pyomyositis?
- Staphylococcus aureus is responsible in around 90% of cases.
- Group A streptococcus accounts for another 1–5% of cases
- Other bacteria cultured from pyomyositis include Group B, C and G streptococcus, pneumococcus, haemophilus spp., and Gram negative bacilli. Recently, community-associated (CA)–methicillin-resistant S. aureus (MRSA) has been noted.
What are the clinical features of tropical pyomyositis?
Tropical pyomyositis can present with a single or multiple abscesses. The most commonly affected muscles are abdominal, spinal and gluteal muscles, quadriceps, pectoralis major, serratus anterior, biceps, iliopsoas and gastrocnemius.
Tropical pyomyositis has 3 distinct stages.
Invasive stage
Initially, there are 1–2 weeks of diffuse (widespread) pain, which may or may not be accompanied by fever. Since infection is deep in the muscle, overlying erythema is usually not seen. It can mimic a haematoma, thrombophlebitis, fasciitis and osteomyelitis.
Suppurative stage
In the second phase, a mass develops which progressively grows and becomes hard and painful over a further 1–2 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.
Late stage
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, metastatic (distant spread of) abscesses and death.
How is tropical pyomyositis diagnosed?
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. Laboratory findings can be nonspecific with leucocytosis and elevated ESR/C-reactive protein.
- Blood cultures may be positive for S. aureus in 30% of patients.
- Muscle enzymes (aldolase, creatine phosphokinase, aminotransferase, and lactic dehygrogenase) are usually normal or slightly raised.
- Ultrasound scan shows hypoechoic areas with an increase in muscle mass and collection of fluid.
- CT scan shows low attenuation with loss of muscle planes and a surrounding rim of contrast enhancement.
- MRI shows hyperintense rim on T1 weighted images on gadolinium DTPA scan with peripheral enhancement.
- 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.
What is the treatment for tropical pyomyositis?
Surgical drainage of abscess: primary wound closure and vacuum drainage promotes better healing.
Appropriate antibiotics are given. Penicillin β-lactamase resistant penicillin (flucloxacillin) and vancomycin are the most commonly used drugs. Broad spectrum empirical antibiotics are needed for patients with HIV infection or other immunosuppressive state.
Treatment is continued till wound is clear of infection and the patient afebrile for 7–10 days.
What is the outcome for tropical pyomyositis?
If diagnosed early, the disease is curable, but delay in diagnosis often leads to a prolonged stay in hospital. Aggressive therapy may be needed.
A higher rate of Gram-negative bacterial infections, bacteremia and mortality is seen in in patients with pyomysoitis when they suffer from an underlying systemic illness.
Mortality is reported to be 10% in temperate regions.