Pyomyositis or Muscle Abscess – Causes and Treatment

Pyomyositis is characterized by suppuration within skeletal muscles, manifesting as single or multiple abscesses. It is also known as tropical pyomyositis as it affected tropical people more commonly. Pyomyositis is common in tropical regions but is rare in temperate climates but showing rise and association with HIV infection or other immunosuppressive conditions, including diabetes mellitus, cancer, connective-tissue diseases, and cirrhosis.

The term pyomyositis or tropical pyomyositis should be restricted to primary muscle abscess arising de novo and not for intermuscular abscess,  abscess  extending into muscles from adjoining tissues or abscess secondary to septicemia.

A Gluteal Muscle Abscess

Staphylococcus aureus is the most common organism. Commonly involved muscles are quadriceps, glutei, pectoralis major, serratus anterior, biceps, iliopsoas, gastrocnemius, abdominal and spinal muscles.

Other terms for this are  myositis tropicans and  tropical skeletal muscle abscess, and tropical myositis.

The classical presentation is with muscle abscess, the hallmark of the disease is not an abscess but finding myositis on a biopsy specimen of involved muscle. Therefore, some authors prefer the term myositis instead of pyomyositis.

The disease is seen in all age groups, although young males are the most susceptible group. Maximum incidence is seen at 10–40 years of age with a male to female ratio of 1.5:1.

Causes of Pyomyositis

• Staphylococcus aureus
  • Most common
• Group A streptococcus accounts for another 1%–5% of cases
• Uncommon Organisms
  • Streptococcus (groups B, C, G)
  • Pneumococcus
  • Neisseria
  • Haemophilus
  • Aeromonas
  • Serratia
  • Yersinia
  • Pseudomonas
  • Klebsiella
  • Escherichia.
• Rare Organisms
  • Salmonella
  • Citrobacter
  •  Fusobacterium
  •  Anaerobes
  • Mycobacterium

Pathogenesis of Pyomyositis

Skeletal muscle tissue is resistant to bacterial infections under normal circumstances. Sequestration of iron by myoglobin, an essential nutritional requirement of proliferating bacteria is considered responsible for this property.

Sequestration of iron thus results in slower growth of bacteria and helps preventing infection.

Experimental studies have failed to produce abscesses even in cases of direct inoculation.

But the damaged muscle is  susceptible to hematogenous invasion by bacteria with subsequent abscess formation.

Following occurrences are considered to be causing the damage which might make muscles susceptible.

• Blunt trauma or vigorous
• Nutritional deficiencies
• Viral and parasitic infections

It has also been suggested that an abnormality of the immune system may be the underlying cause in many cases.

Intravenous drug abuse impairs cellular and humoral immunity and is a risk factor.

Tropical pyomyositis is also increasingly documented in persons infected with HIV.

Mechanisms include muscle damage caused by HIV infection per se, zidovudine therapy, infections caused by parasites and mycobacteria, and impaired host defences including dysfunction of helper T-cells and granulocytes.

Other immunosuppressive conditions, including diabetes mellitus, cancer, connective-tissue diseases, and cirrhosis also increase the risk.

Stages of Pyomyositis

Invasive stage

• Duration of symptoms <10 days
• Pain in affected muscle
• Low-grade fever
• Wooden or hard stiffness on palpation of muscle
• Mild leucocytosis
• Needle aspiration negative for pus
• The invasive stage may resolve itself, mimicking fibromyalgia or may progress to next stage of suppuration.

Suppurative Stage

• Duration of symptoms 10–21 days
• Oedema
• Marked tenderness of affected muscle
• Pyrexia
• Leucocytosis
• Needle aspiration yields pus

Late or Septic

• Duration of symptoms >21 days
• Fluctuant swelling in muscle
• High-grade fever
• Severely ill
• Septicaemia
• Leucocytosis
• Needle aspiration yields pus

Clinical Presentation

In invasive stage, the presenting symptoms are  fever and  painful firm swelling.  There could be erythema over the swelling. But erythema is absent in deep seated infections.

The swelling is tender and woody in consistency.

In suppurative stage [1-3 weeks], abscess forms in the muscle. There would be high grade fever and other systemic symptoms. This is the usual stage when patient seeks the consultation.

Most cases present at this time. The classical signs of abscess, fluctuation and erythema, may be lacking because the overlying muscle is tense. Needle aspiration at this stage yields pus. Regional lymph nodes are not involved.

If the abscess remains untreated [>3 weeks], spread of infection occurs which may lead to bacteremia, followed by septicaemia, septic shock, acute renal failure, and metastatic abscesses.

Ralery patients may present with toxic shock syndrome or pyrexia of unknown origin. The patients may present with pressure effects of the abscess. Depending on the site it could be acute abdomen, compression of the spinal cord or compartment syndrome.

In younger children, the complain may be of refusal to use the affected limb. For example, there could be inability or refusal to walk.

Diagnosis of Pyomyositis

Aspiration of pus from the muscle or muscle biopsy with culture and tissue staining in cases of absent abscesses is the gold standard for diagnosis (pus may be sterile in 15%–30% of cases).

Muscle biopsy also helps to exclude osteosarcoma, polymyositis, trypanosomiasis, toxoplasmosis, cysticercosis, and trichinosis.

Lab Studies

Blood Investigations

Routine lab tests reveal anemia, leucocytosis, raised erythrocyte sedimentation rate, and C-reactive protein.

Blood cultures have low positivity.

Serum levels of muscle enzymes—that is, aldolase, creatine phosphokinase, aminotransferase, and lactic dehydrogenase—are normal or slightly raised.

Tests for immunodeficient states

• HIV
• Diabetes
• Rheumatological disorders
• Malignancies should be carried out, along with immunoglobulin levels.

Histopathology

• Edematous separation of fibres
• Patchy myocytolysis
• Lymphocytes and plasma cells.
• Suppuration with bacteria and polymorphonuclear leucocytes.

Needle Aspiration

The swelling should  be aspirated with a large-bore needle (not smaller than 18 gauge) to

confirm the presence of pus. The aspirated pus is usually yellowish in color but may be brownish or blood-stained.

Microbiological Examination

The aspirated [ and/or drained pus]  should be subjected to gram staining, aerobic and aneorbic culture and drug sensitivity.

A biopsy of the abscess wall and/or muscle taken at time of open drainage should also be cultured.

Imaging

Electromyography

Short duration, low amplitude polyphasic potentials

 Ultrasonography

First imaging investigation usually

Characteristic features are

• Muscle swelling;
• Hypoechoic areas in the muscle belly
• Heterogenous hypoechoic areas
• Hyperechoic areas.

X-ray

X-ray of the affected part esp limb should always be done to rule out acute osteomyelitis.

Otherwise , x-rays would be normal. However it should be remembered that x-ray changes in osteomyelitis may take about 10 days to appear and in early period, the x-ray may be normal in spite of osteomyelitis.

Chest x-ray may be done if there are chest symptoms present due to complications like pneuomonia etc.

MRI

MRI is the modality of choice for accurate assessment of the muscle involvement.

Muscle edema [high T2 signal], fluid collection with peripheral contrast enhancement and diffuse muscle enlargement are . MRI is especially useful in deeply seated muscles.

MRI with gadolinium enhancement can increase the confidence of identifying or excluding the presence of  abscess but this may give high-dose irradiation to the child.

CT

Enlargement and decreased attenuation of the affected muscle with effacement and stranding of surrounding fat planes.

If an abscess is present it will appear as an intramuscular fluid collection with peripheral rim-enhancement.

Differential Diagnoses

• Pyrexia of unknown origin
• Muscle contusion
• Septic arthritis
• Osteomyelitis
• Cellulitis
• Muscle hematoma
• Deep vein thrombosis
• Muscle rupture or muscle strain
• Osteosarcoma of muscle
• Trichinosis
• Leptospirosis
• Cysticercosis
• Toxoplasmosis

Treatment of Pyomyositis

Pyomyositis demands aggressive management. This includes immediate administration of broad spectrum antibiotics.

Diffuse myositis without abscess may respond to an antimicrobial agent alone but abscess may develop eventually requiring drainage. Following this, the abscess cavity must be prevented from  premature closure by packing and  daily dressing.

First generation cephalosporin along with aminoglycosides are commonly used empirical antibiotics and once culture sensitivity report is available, the antibiotics are changed accordingly.

For HIV infection and immunosuppressed patients clindamycin may be added.

The treatment should be continued till wound is clean, the leucocyte count becomes normal, and the patient is afebrile for 7–10 days.

The affected part, especially limbs are splinted for rest.

Patients may be anaemic, particularly those presenting late. Any  severe anaemia may require  correction by blood transfusion. Patients who are malnourished will require some form of nutritional support  and rehabilitation.

REFERENCES

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