Pyomyositis: causes, symptoms, diagnosis, treatment, and prognosis

Pyomyositis, also known as tropical pyomyositis, is a primary bacterial infection involving the striated muscle.


Causes

Pyomyositis can be caused by local soft tissue infection uncontrolled, as well as systemic disease, low body resistance, and acute bacterial infection. Infections tend to occur in large muscles of lower back and lower extremities, most infections are caused by staphylococci, and few are caused by streptococcus.

Pyomyositis is less common, but can occur in immunocompromised patients, especially AIDS patients. In many tropical regions, the disease can often occur in children and adults, especially malnourished individuals. Uncontrolled diabetic patients are also susceptible to this disease.

Pathogenesis

It is believed that the pathogenesis of pyomyositis is hematogenous dissemination and may be caused by bacteremia. The most common sites of onset are the quadriceps femoris muscle, gluteal muscles, shoulder muscles, and upper limb muscles. Multiple abscesses are present in about 40% patients.


Signs and Symptoms

The lesions are common in the femoral and gluteal muscles. Sciatic pain, limited motion, and fever are present.

Local lesions often present with redness, swelling, hot, throbbing pain, and tenderness in clinical practices.

The duration of the disease can be divided into 3 stages:

Stage I (1 - 2 weeks)

Symptoms include myalgia, erythema, swelling, and edema in the skin, without fever.

Stage II (2 - 3 weeks)

Localized induration, pain, and enlarged mass are present.

Stage III

Exacerbated local pain and suppuration can involve adjacent joints and bones, and about 1.8% of patients can develop sepsis, leading to shock and death.


Laboratory examinations

Blood routine examination: white blood cells, especially neutrophils, are significantly elevated.

Blood biochemical examination: erythrocyte sedimentation rate is increased significantly.

Bacterial culture examination with puncture or drainage: negative results are present in the early stage, and staphylococcus aureus can be seen in the culture with pus from the lesions. Occasionally, streptococcus pyogenes or colibacillus can be found in the culture.

Blood culture test: if with sepsis, positive results are visible in blood culture.

Other examinations

X-ray examination

Swollen muscles and increased density in the lesions can be seen, intermuscular spaces are blurred or absent, and clear boundary is absent even in localized swelling.

Computed tomography (CT) examination

Diffusely enlarged, poorly demarcated, lesioned muscles with declined and uneven density can be seen. When the abscess is formed, homogeneous low-density fluid accumulation can be observed, and thin separations may be present. Contrast enhanced scan reveals inflamed tissue and abscess edge enhancement, no enhancement in the central low-density area, and enhancement in separations. If with cellulitis, pachydermia, stripped subcutaneous fat, unclear intervals between fat and fascia, and subcutaneous venectasia can be visible. Gas shadows may be present in the lesion. If adjacent bones are involved, osteomyelitis and bone destruction can be seen.

Magnetic resonance imaging (MRI) examination

In T1WI, high signal in the enlarged lesioned muscles, low signal in the abscess, and high signal on the edge of the abscess are visible. In T2WI, high signal in the lesion, low signal on the edge of the abscess can be seen. In contrast enhanced scan, significant enhancement in the surrounding of the abscess is present. Low signal in T1WI and diffuse high signal in T2WI in the lesion neighboring cellulitis is visible. Obvious high signal in the lesion can be visible with fat suppression technique. In addition, hydrarthrosis is visible in the adjacent joint of the affected muscle, and the effusion is not interconnected to the abscess.


Histopathology

Localized or diffuse tissue inflammation, congestion, edema, exudation, and abscess formation can be seen.


Diagnosis

On the basis of clinical manifestations and results of laboratory and other examinations, definitive diagnosis can be provided.


Treatment

In the non-suppurative phase, antibiotics are effective against pyomyositis. Before the bacterial culture results are reported, antibiotics can be selected according to experience. Most are caused by staphylococcus aureus, so that penicillinase resistant penicillins can be administered. After the results of the bacterial culture are provided, sensitive antibiotics can be selected.

Active treatment of primary diseases such as diabetes and AIDS is necessary.

Incision and drainage may be required if with abscess.

Prognosis

With active treatment, good prognosis is present.