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CHAPTER 19

Pyomyositis

John Chinda
Emmanuel A. Ameh
Lohfa B. Chirdan

Introduction Complement levels were normal. This prompted the proposition that
Pyomyositis is a primary acute bacterial infection of skeletal muscles defects of opsonising and complement fixing IgM antibodies against
associated with abscess formation. The use of the term “tropical pyo- microorganisms are implicated in the aetiology of pyomyositis, at least
myositis” should be restricted to primary muscle abscess arising de in adults.
novo. It should not be used to describe intermuscular abscess; abscess Human immunodeficiency virus (HIV) infection may have led to an
extending into muscles from adjoining tissues, such as bone or subcu- increasing incidence of pyomyositis in areas with a high prevalence of
taneous tissues; or abscess secondary to septicaemia. HIV infection; 14,15 this is now thought to be an important predisposing
Pyomyositis is predominantly experienced in the tropics and factor in the aetiopathogenesis of pyomyositis.
relatively low-income countries, but it can also occur in temperate and Although the classical presentation is with muscle abscess, the
developed countries. Zur first described the condition in 1885 as an hallmark of the disease is not an abscess but finding myositis in a
1
endemic disease in the tropics; since then, there have been reports from biopsy specimen of involved muscle.
tropical as well as temperate regions. 2–11 In the early stages of pyomyositis, muscles show oedematous
Demographics separation of fibres, followed by patchy myocytolysis, progressing to
complete disintegration. The fibres are surrounded by lymphocytes
Pyomyositis is common among children in the tropics, accounting for and plasma cells. Muscles fibres may heal without abscess formation
1–4% of all hospital admissions in some tropical countries 10,11 and 1 per or degenerate, progressing to suppuration with bacteria and
2
3,000 paediatric admissions in Southern Texas. In sub-Saharan Africa, polymorphonuclear leucocytes.
70% of affected children are younger than 10 years of age and both
sexes are equally affected. In another large report from sub-Saharan Clinical Presentation
3
Africa, 36% of all affected patients were children. The large muscles of the lower limbs and trunk are particularly prone to
Although cases are seen throughout the year, maximum incidence involvement, but small muscles, such as those of the orbit may rarely be
2
has been noted during the rainy and wet monsoon season in India. 9 involved. Commonly affected muscle groups are shown in Figure 19.1.
Microbiology
Staphylococcus aureus is the most common primary causative patho-
gen. It is seen in up to 90% of cases in tropical areas and 75% of cases in
9,11
temperate countries. Group A streptococci account for another 1–5%
of cases. Several other microorganisms implicated include streptococ-
cus groups B, C, and G, Pneumococcus, Salmonella, Escherichia coli,
Neisseria, Haemophilus, Aeromonas, Serratia, Yersinia, Pseudomonas,
Klebsiella, Citrobacter, Fusobacterium, and Mycobacterium.
In tropical regions, pus cultures are sterile in 15–30% of cases
9
and 90–95% of patients also have sterile blood cultures, due largely
to use of antibiotics before presentation. Blood cultures are positive in
20–30% of cases in temperate regions. Better microbiological culture
techniques in the temperate regions may account for this difference.
Pathogenesis and Pathology
The precise pathogenesis of pyomyositis remains obscure. It is believed
that staphylococcal bacteraemia and muscle damage are prerequisites
for the clinical scenario. Skeletal muscle tissue is known to be intrinsi-
cally resistant to bacterial infection under normal circumstances, but
it has been shown experimentally that if normal muscle is damaged,
it becomes vulnerable to haematogenous invasion by bacteria, with
subsequent abscess formation. 12
A number of conditions predispose to skeletal muscle damage.
These include trauma, nutritional deficiencies, immunosuppression,
parasitic infestations, viral infections, and intravenous drug abuse.
In one report involving adults with pyomyositis, serum
13
immunoglobin M (IgM) level was found to be significantly lower, Source: Drawing of child taken from http://images.medscape.com/pi/features/ald/fb/fbc-ap.pdf.
Figure 19.1: Distribution of pyomyositis at 35 sites in African children.
and mean levels of IgG and IgA significantly higher, than in controls.