Pyomyositis is a bacterial infection of the muscle, which can form abscesses and spread locally. It is relatively uncommon in temperate climates, often has an insidious onset, and can be difficult to diagnose. It is even more challenging in the paediatric population where the history may not be clear, examination is inconsistent, and blood tests are non-specific.
It was initially described in 1885 as a disease predominating tropical areas (hence also known as tropical myositis or myositis tropicans). It differs from myositis in that it is always precipitated by bacteria and is confined to distinct areas in muscles.
Pyomyositis can occur in any age group but typically affects young adults and children. In a North American case review of 98 patients, 37% of patients were children less than 16 years old. Another American review of 246 patients showed that 33% of patients were children. There is also a male predominance, with male:female ratios observed from 1.5:1 to 3:1.
Pyomyositis is relatively uncommon in temperate climates. In the tropics, the incidence of hospital admission for this varies from 1-4 percent. There is little data to indicate a local incidence rate in New Zealand, however a 9 year study in Victoria, Australia estimated an incidence of 0.5 cases per 100,000 person-years.
Recently, the incidence of disease appears to be increasing, with a retrospective cohort study in Australia showing an increase from 2 cases to 8.7 cases per 10,000 ED admissions over a 10 year period. The reasons for this are unclear.
Pyomyositis is often precipitated by trauma, with other predisposing factors including immunocompromise, chronic illness, IV drug use, and concurrent infection. The role of trauma is supported by Miyake’s work in the early 1900’s where the experimental conditions were identified that gave rise to abscess formation in staphylococcal bacteraemia. When healthy rabbits were inoculated with IV boluses of staphylococcus, they occasionally developed small abscesses in the abdominal viscera, but never in skeletal muscle. When specific muscles were damaged by mechanical pinching or electricity, small abscesses developed within 2 days to 28 days following inoculation in some of the damaged muscles in nearly half of the rabbits, with no abscess formation in undamaged skeletal muscle. In addition, there have been a number of case reports of patients suffering from pyomyositis soon after exercise. This suggests that the disrupted muscle architecture is at risk of promoting abscess formation during a period of bacteraemia.
This theory is also supported by a study from the British Army which showed that two-thirds of soldiers with pyomyositis had trauma at the affected site, and that the incidence of the disease increased as training intensity increased.
Up to 75% of reported cases are in immunocompromised patients, with the incidence of pyomyositis in those with human immunodeficiency virus (HIV) up to 31%.
In a review of 246 non-immunocompromised cases since 1981 in America, 48% of patients had at least one underlying disease. The most common medical condition was diabetes mellitus, followed by malignancy, a rheumatologic disorder, liver cirrhosis, renal failure, then lung disease. This review also observed that the location of the infection was unifocal in 81% of cases. The thigh was the most common site of involvement, followed by the psoas muscle, then the upper extremities.
People with underlying medical conditions have a higher mortality, with mortality of up to 8% compared to 1% in those without underlying medical conditions in the literature review above. A case series of over 200 cases in Nigeria showed overall mortality of 0.89%.
Staphylococcus aureus is the most common causative organism, with studies suggesting it causes up to 90 percent of cases in tropical areas, and up to 75 percent in temperate regions. The next most common cause is Streptococcus pyogenes, accounting for 1-5 percent of cases. Other organisms reported include non-group A streptococci, pneumococci, and gram-negative enteric bacilli such as Escherichia coli. Gram-negative and anaerobic organisms are more common in immunocompromised patients.
Pyomyositis can be difficult to diagnose due to its relative rarity in temperate environments. In children, it is particularly challenging due to insidious, non-specific signs and symptoms, and a range of differential diagnoses including muscle contusion/sprain, fracture, transient synovitis, spondyloarthropathy, inflammatory arthritis, malignancy, thrombosis, and other infection such as osteomyelitis or septic arthritis. This often results in delayed diagnosis. Studies report a mean delay of 6-10 days from the onset of symptoms to diagnosis. Non-accidental injury should always be considered.
Classical clinical features of pyomyositis include pain, stiffness, swelling, erythema, tenderness and fever. However, many of these are not present on presentation, and fever, if present at all, is cyclical in nature due to the bacteraemia. The first symptom is usually pain, with fever not becoming common until later stages when a collection has formed. As Chiedozi explains, the natural history of pyomyositis includes 3 stages: invasive, purulent and late stage. The invasive stage begins with the insidious onset of dull, cramping pain in a large muscle, followed one week later by oedema, worsening pain and low grade fever. The examination reveals little in the way of inflammation with minimal swelling or fluctuance. A “wooden or hard rubber” feel on palpation may be appreciated. In Chiedozi’s series of 205 cases of pyomyositis in Nigeria, only 2 percent of the patients presented at this stage. Patients presenting between 10 and 21 days from the onset of symptoms are said to be in the purulent stage. They often display marked oedema and tenderness of the muscle, associated with fever and other signs of inflammation. More than 90 percent of Chiedozi’s case series presented in the purulent stage. The late stage is characterised by a systemically unwell patient, who may look toxic. Obvious fluctuance, tenderness, high fever, septicaemia and coma may occur.
A detailed history and examination should be taken, along with a full musculoskeletal examination, given that around 15-40% of cases will have multifocal infection. A cardiorespiratory examination should not be omitted, as a number of cases of pyomyositis have been complicated by pyopericarditis and empyema.
Laboratory findings are non-specific, with 50-60% of patients displaying a leucocytosis. Blood cultures are positive in 25-30% of patients in temperate regions, with organism identification usually from abscess culture.
While not specific to pyomyositis, a Finnish prospective trial of 265 children admitted to hospital, with bone or joint infections, attempted to identify the predictive value of certain laboratory tests in diagnosing bacteraemia. It assessed the predictive value of white blood cell count, serum alkaline phosphatase, ESR and CRP. It found that none of the markers were able to stratify the risk of bacteraemia in children with an acute bone or joint infection. It did find that CRP was the only marker that was significantly higher in bacteraemic patients, and therefore could be used to predict bacteraemia, or used to distinguish between true positive cultures and contaminated cultures.
There are no studies that have assessed a clinical prediction algorithm for the diagnosis of pyomyositis.
Radiological imaging is important for diagnosing pyomyositis. Ultrasound is a relatively accessible, non-invasive, radiation sparing modality that can diagnose an abscess. The typical findings are of a bulky muscle with abnormal echotexture and a hypoechoic focal lesion, occasionally with internal debris and air bubbles.
When ultrasound is inconclusive, computed tomography (CT) or magnetic resonance imaging (MRI) is indicated. MRI is the investigation of choice as it provides the highest definition imaging, and when used with contrast, is the most sensitive and specific imaging for pyomyositis. Pyomyositis is characterised on MRI by abnormal signal intensity within a skeletal muscle with irregular margins and an infiltrative appearance which shows a homogeneous and diffuse enhancement after gadolinium administration, without a defined mass effect.
When there is no focus on examination, but clinical suspicion is high, or if there are concerns of multifocal disease, gadolinium-based nuclear scintigraphy is effective at localising the infection.
Plain X-ray has little role in the diagnosis of pyomyositis. If performed, a soft tissue mass or wide fascial planes may be seen. However, it may be useful if investigating for fracture, malignancy, or osteomyelitis.
Stage 1 (invasive) pyomyositis, with the diffuse inflammatory process, may be treated with oral antibiotics alone. Most patients present in stage 2 (purulent) or stage 3 (late) disease, where abscess formation has occurred, and will require IV antibiotics plus drainage.
Initial treatment should begin with empiric antibiotics. A broad spectrum antibiotic with Staphylococcus aureus and Streptococcal cover is appropriate. Locally, high dose IV flucloxacillin is the empiric antibiotic of choice. This will vary between geographic locations as it is dependent on the local microbe prevalence and susceptibilities. Patients with known or suspected methicillin-resistant Staphylococcus aureus (MRSA) must have vancomycin or clindamycin treatment considered. In immunocompromised patients, such as those with HIV, gram negative organism cover should be included.
The use of CT or ultrasound guided percutaneous catheter drainage has been shown to be effective and assists with both treatment and diagnosis. It may spare the patient a general anaesthetic, allow a smaller incision, drain locations that would otherwise be a challenging surgical approach, and allow faster post-operative mobilisation.
In extensive infections, or if there is the possibility of concomitant osteomyelitis or septic arthritis, surgical exploration and drainage might be preferable.
Irrespective of the procedure performed, antibiotic therapy should not be delayed for the sake of obtaining a culture if the patient is toxic.
There is no consensus on duration of antibiotic therapy, as there is little data demonstrating recurrence or treatment failure. In general, antibiotic treatment is recommended for 2-4 weeks, with IV therapy until clinical improvement is noted. Some case series describe treatment up to 6 weeks.
Occasionally, penicillin-based antibiotics are ineffective when treating streptococcal pyomyositis. This is postulated to be due to the “Eagle effect”. Harry Eagle observed that penicillin had reduced antibacterial effect on older abscesses in mouse models and questioned the role of bacterial numbers. It has since been established that penicillin has a reduced efficacy when used in high bacterial loads. It is thought that when treating established streptococcal infections, the organisms have reached a steady state of growth and the colony growth rate has slowed. Penicillin operates by inhibiting cell wall synthesis; therefore has reduced bactericidal activity when bacterial replication slows. In this situation, clindamycin, should be initiated. Clindamycin interferes with protein synthesis at the ribosomal level and its efficacy is not affected by the growth phase of the bacterial colony.
Following successful treatment, there is very little residual deformity and minimal loss of function, even if there has been extensive muscle damage. Physical therapy and rehabilitation might be necessary in severe cases, or in those who have required bed rest. Studies have shown that a short term immobilisation (less than 14 days) results in a reduction of 5-10% in the muscle mass of vastus lateralis. Even if there is no deformity or loss of function, patients may benefit from a rehabilitation program due to deconditioning associated with muscle disuse.
- Pyomyositis is a bacterial infection of skeletal muscle which, if left untreated, forms abscesses and can lead to toxaemia. It was historically a disease of the tropics but is increasing in incidence in temperate climates.
- The insidious nature of the disease can lead to delayed diagnosis, especially in children. Clinical features include pain and fever, with localised swelling and erythema. It most often affects the lower limb. A high level of suspicion is required, especially early in the course of the illness where fever may not be present.
- Risk factors for pyomyositis include trauma, immunodeficiency, chronic illness, IV drug use and concurrent infection.
- The most common causative organism is Staphylococcus aureus, followed by Streptococcus pyogenes.
- MRI is the most useful diagnostic tool but is aided by blood work and bacterial identification from blood or aspirate cultures.
- Treatment may be sufficient with antibiotic therapy alone. However, most cases present with some abscess formation (stage 2 or stage 3) and therefore also require drainage.
- Initial antibiotic therapy should target Staphylococcus aureus and Streptococcus pyogenes. For immunocompromised patients, gram negative organism cover should be instituted. In those patients who are very unwell, or those who do not appear to be responding to a beta-lactam antibiotic, clindamycin should be considered due to the “Eagle effect”.
- Treatment duration is not well established, but a period of 2-4 weeks is generally accepted.
- Rehabilitation and physical therapy should be considered in those with deformity, loss of function, or where bed rest has been required.
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