Electromyographic (EMG) examination demonstrating marked segmental compromise of the posterior primary ramus distal to the spinal root with relative sparing of the anterior ramus may be the earliest objective evidence of paraspinal muscle metastasis. Antecedent studies are often initially normal, failing to disclose the underlying cause of back pain. Although paraspinal muscle metastasis has been histopathologically demonstrated at postmortem, attempts to image the suspected malignancy with computed tomography have been unsuccessful because the tumor in muscle remains isodense. This study reports the use of magnetic resonance imaging (MRI) to substantiate the existence of EMG-suspected paraspinal muscle metastasis. An EMG pattern of segmental posterior primary ramus denervation is not pathognomonic of metastasis. A confirmatory MRI, however, does permit earlier treatment with palliative radiation therapy.
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