Pyomyositis is an acute bacterial infection of skeletal muscles with accumulation of pus in the intra-muscular area. It usually occurs in the lower limbs and trunk spontaneously or following penetrating wounds, vascular insufficiency, trauma or injection. Poor nutrition, immune deficiency, hot climate and intense muscle activity are highly associated factors. The most common causative agent is Staphylococcus aureus. Streptococci can also be detected in acute form.
Predisposing factors include:
- Nutritional deficiencies
- Viral infections
- Parasitic infections
- Diabetes mellitus
- Liver cirrhosis
- Renal insufficiency
- Organ transplantation
- Treatment with immunosuppressive drugs.
- local: exquisite muscle tenderness, oedema giving muscles “woody” texture on palpation.
- systemic: regional adenopathy and fever.
- pyomyositis of the psoas muscle: patient keeps hip flexed and experiences pain on hip extension. If the abscess is on the right side, the clinical signs are the same as for appendicitis with pain in the right iliac fossa.
Pyomyositis has three clinical stages
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.
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.
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.
- Immediate intravenous antibiotics before surgery
- Surgical drainage of all abscess
- Excision of all necrotic muscles
- Supportive care