Neoplastic Brachial Plexopathy Workup

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• A general lab survey, such as a complete metabolic panel, complete blood count, and urinalysis, usually has been obtained by the primary physician and can be used to screen for signs of neoplasm. (More involved studies exist, but they are beyond the scope of this text/specialty.) If a patient is referred for what might be thought of as simple shoulder pain and turns out to have constitutional symptoms (eg, fevers, weight loss, malaise) and signs of brachial plexopathy, other lab tests certainly may be of value. Examples include a carcinoembryonic antigen (CEA) in a patient with prior colon cancer, a prostate specific antigen (PSA) in a patient with prior prostate cancer, or serum protein electrophoresis (SPEP) in a patient with back pain and pain at rest.

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• Plain radiographs of the chest, shoulder, and cervical spine should be obtained to look for neoplastic changes.

• MRI of the plexus has a high sensitivity (some studies have reported 100%) in detecting neoplastic involvement of the brachial plexus; however, both CT scan and MRI can present difficulty detecting infiltrating neoplasms and distinguishing them from radiation fibrosis. A well-defined mass lesion on CT scans or MRIs is more suggestive of NBP than RBP. ^[10]

• Bone scan may be helpful in finding evidence of metastases.

• Myelography may be more successful at finding epidural metastasis extruding through neural foramina (so-called dumbbell tumor).

• Positron emission tomography may be useful in identifying metastasis in or near the brachial plexus; FDG-PET identifies tumors not clearly imaged by CT or MR scanning in some patients.

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• The conventional diagnostic procedures for differentiating brachial plexopathy are nerve conduction and electromyography studies. Advances in MRI techniques have shown promise as well.

• In general, electrodiagnostic examination is used to localize a lesion, characterize its pathology, and establish prognosis and facilitate a treatment plan. The key question is whether the limb weakness is due to axonotmetic (dead) or neurapraxic (functionally blocked) axons.

• Nerve conduction studies (NCS) may reveal axon loss that often is so severe that lower trunk-mediated sensory potentials (eg, ulnar, medial antebrachial) are absent, and motor responses (median and ulnar) are of low amplitude or absent. Needle examination often reveals motor unit potential loss and spontaneous activity. For axonotmetic lesions, the longer regrowth distance (especially for the lower plexus, where reinnervation must reach the hand) indicates a less favorable prognosis. Neurapraxic lesions are observed less commonly in NBP than RBP, but, barring progression of the lesion/mass, function resumes relatively rapidly, and the prognosis is excellent.

• Comparison of the amplitude of a peripheral evoked response after 7 days with that of the contralateral side can provide an estimate of the degree of injury/recovery. The number of voluntary motor units on needle examination is also a guide to outcome. Peak muscle strength can be regained even with 50% axon loss, although this gain is at the expense of muscle endurance. Remarkably, in muscles where high force or sustained use is not needed, a 90% axon loss might be tolerated, but a year's recovery time might be required. Myokymia is common is RBP, occurring in 50-70% of patients, but it is rare in NBP (4%). Despite the potential utility of electrodiagnostic studies, limitations exist. When pain is the only initial symptom, EMG results may be within the reference range, although the results almost always reveal abnormalities by the time permanent motor and/or sensory deficits are present.

• Somatosensory evoked potentials (SSEP) are noninvasive tests that may be used to assess sensory impairments associated with NBP.

The histopathology for secondary neoplasms most often involves adenocarcinoma, squamous cell carcinoma, or, less often, large cell carcinoma. Of the primary neoplasms, schwannomas are encapsulated tumors, and neurofibromas are unencapsulated tumors believed to arise from the near fascicles.