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Imaging Diagnosis

It is unlikely that the initial diagnosis will be made by imaging, except in the occasional patient with a localized mass who is otherwise asymptomatic. Ultrasonography, computed tomography (CT), and magnetic resonance imaging (MRI) may all show muscle abnormalities before there is a clear clinical diagnosis.

In the early stages, ultrasonography will show the soft tissue hypoechoic edema and may delineate the infection, which usually extends up to the fascia (Figs. 30.4, 30.5). Thin septa may be seen on some scans. Sometimes soft tissue radiography (Fig. 30.6) can also demonstrate blurring of tissue planes, but more important will be the recognition of any bone changes. Close to the soft tissue abscess there may be bone reabsorption and osteopenia. Later, a periosteal reaction will develop, usually lamellar in pattern but sometimes of the spiculated type. This can be mistaken for a malignant bone tumor. In chronic abscesses, which persist for some months, the underlying bone may become dense and sclerotic, particularly in the pelvic bones when there is a persistent psoas abscess (Fig. 30.7). In a minority of patients, there will be frank osteomyelitis in the region of the abscess.

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Fig. 30.4A,B. Ultrasound scans of two African patients in Zimbabwe, with pyomyositic abscesses in the abdominal muscles. A Patient aged 24 years, with an abscess in the rectus abdominis muscle. B Patient aged 3 years, with an abscess in the obliquus abdominis muscle.

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Fig. 30.5A-E. Ultrasonography in pyomyositis. These scans demonstrate diffuse interstitial edema in (A) the gluteal muscle, (B) the quadriceps femoris muscles, and (C) the paraspinal muscles. In D there is abscess formation in the abdominal wall and in E a similar abscess in the chest wall. All these patients were scanned in Uganda. (Courtesy of Dr. L. Belli et al. and Skeletal Radiol, 1992).

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Fig. 30.6A-C. Tropical myositis abscesses in the pectoral muscle (A), the right buttock (B), and the upper arm (C). All occurred in African patients in Zimbabwe.

Fig. 30.7. Dense sclerosis in the right iliac bone resulting from a chronic abscess in the right psoas muscle. This patient was an African from Zimbabwe. The initial clinical diagnosis was "chronic" appendicitis.

All patients with pyomyositis should have an initial radiograph of the chest, and chest radiography should be repeated after an interval, since lung abscess and septic pericarditis are possible complications. Elevation of the right side of the diaphragm may be the first indication of a metastatic liver abscess. Ultrasonography (or CT) can be used to localize such hepatic or psoas abscesses and exclude any peritoneal spread. CT or MRI can be used to identify the cerebral or spinal abscesses which are known complications of pyomyositis.

Ultrasonography has an important role in the exact localization of an abscess, and in guided needle aspiration when there is any clinical doubt. CT or MRI will also demonstrate the abscess and show the extent of the infection, but for practical purposes ultrasonography will provide sufficient information to guide treatment for most patients.

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