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Differential Diagnosis

Tropical pyomyositis may be mistaken for many pathological processes. It must be differentiated from an abscess caused by guinea worm; this is usually extramuscular and displaces the muscles instead of destroying them, as in pyomyositis. Tuberculous or mycotic infections usually develop sinuses early and are much less painful, with less local tenderness and edema. When the abscess involves the muscles of the abdominal wall or pelvis the differential diagnosis includes an appendiceal abscess, psoas abscess arising from the spine, or even a strangulated hernia or splenic abscess. A metastatic abscess in the liver may be mistaken for an amebic abscess. The general toxicity of the patient has led to the erroneous diagnosis of typhoid fever or hepatitis. Metastatic cerebral abscesses have been mistaken for cerebrovascular accidents, and abscesses within the spine may clinically resemble spinal cord injury, tuberculosis, or tumor. MRI, CT, or ultrasonography can be very valuable in making an accurate differential diagnosis.

There are two conditions which might be confused with pyomyositis, or may perhaps be variants of the same disease. Focal myositis is an inflammatory lesion of skeletal muscle. There are now over 30 recorded cases, occurring in both sexes and from 7 to 67 years of age. The patient complains of a slightly painful lump, most commonly in the lower limb but also occurring in the arm, back, abdominal muscles, neck, face, and even the tongue. The mass enlarges over some months and is clinically diagnosed as a neoplasm. There are few systemic changes and there are no abnormal biochemical, hematological, or other laboratory results. At surgery the muscles are pale and swollen, and rubbery or fibrotic to feel. Histologically, there is inflammatory myopathy and a lymphocytic infiltrate, with areas of necrosis and regeneration. Unlike pyomyositis, focal myositis undergoes spontaneous regression, which in a small number of cases is not complete and may require surgery. CT has shown that the myositis causes quite extensive disruption, with diffuse fatty infiltration. Also unlike pyomyositis, focal myositis is not multifocal and there are no systemic symptoms. The CT or ultrasound appearances of focal myositis might be mistaken for an aggressive lipoma or liposarcoma, or for pyomyositis in the early stages. The clinical symptoms will aid the differential diagnosis.

"Muscle necrosis" is a similar condition clinically but with a specific history; it is the result of lying on one limb for a long period, often following a drug overdose. The clinical diagnosis is usually thrombophlebitis, but there is no response to anticoagulation therapy. Venography does not show any thrombosis, but may show displacement of the major veins by the palpable, tender, edematous mass in the muscle. In one case, femoral arteriography showed a highly vascular mass, without bone involvement. A soft tissue neoplasm was suspected, but surgery showed a hemorrhagic-fibrotic mass which histologically was ischemic.

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