Laboratory Studies
Electrodiagnosis has become a mainstay in the diagnostic evaluation of brachial plexopathies. Electrodiagnostic tests provide physiologic data about the continuity of pathways and of lesion type and severity. Serial testing is helpful to determine prognosis. [18, 19]
While positive waves and fibrillations (which indicate axonal injury) do not appear for several weeks after injury, sensory nerve action potentials (SNAPs) can be useful within days of injury to distinguish a presynaptic lesion from a postsynaptic lesion. With postsynaptic lesions, SNAPs are absent, whereas they are present with presynaptic ganglionic lesions. [20]
Somatosensory evoked potentials (SSEPs) are also useful to assess proximal lesions, such as root avulsions. [20, 21, 22]
Imaging Studies
Radiography
Many peripheral nerve injuries can be associated with other soft-tissue or bone injuries that can be detected at radiography.
Radiographs of the injury site help to identify fractures or foreign bodies. For example, fractures of the cervical spine are frequently associated with brachial plexus injuries.
In cases of phrenic nerve paralysis, chest radiographs demonstrate unilateral elevation of the diaphragm.
Midhumeral fractures are associated with radial nerve injuries, and midforearm fractures of the ulna or radius are associated with median or ulnar nerve injuries, respectively.
To rule out bony and ligamentous injuries, all patients with axillary nerve injury should initially undergo radiography of the shoulder and cervical spine.
MRI and CT scanning
Computed tomography (CT) scanning can be used in the investigation of occult fractures that are not depicted on plain radiographs. With myelography, CT scanning can be used to demonstrate root avulsion. [23, 24, 25]
The resolution of the fine anatomic detail of soft tissue is better with magnetic resonance imaging (MRI) than with CT scanning.
Conventional MRI is used to visualize normal and abnormal peripheral nerve structures. [2] Moreover, in a study by West and colleagues, MRI depicted signal intensity changes in denervated muscle as early as 4 days after clinical symptoms developed. [26] With short-tau inversion recovery (STIR) techniques, signal intensity changes in the thenar muscles were demonstrated on MRI scans of 100% of the patients with clinically advanced carpal tunnel syndrome.
With neurapraxic nerve injuries, [22] the signal intensity in the innervated muscles remains normal on STIR or T2-weighted images. Therefore, after a peripheral nerve injury, early MRI of the muscle can be useful in distinguishing a neurapraxic injury from more severe axonotmesis or neurotmesis.
Because CT scanning and traditional MRI techniques have inherent limitations in their resolution and distinction of peripheral nerves from the surrounding structures, magnetic resonance neurography (MRN) has been developed.
MRN can depict normal and abnormal peripheral nerves in various regions of the body. The injured peripheral nerve can be assessed by orienting the images along the course of the damaged nerve. For example, the loss of signal intensity on T2-weighted images indicates damage to the myelin sheath.
In addition, loss of water content in denervated nerves of the deep muscles can be assessed with MRN when needle electromyography (EMG) is difficult to perform. [2]
The predictive value of MRN in the diagnosis of peripheral nerve trauma has not yet been reliably established.
A retrospective study by Brogan et al found through MRI evaluation that among 280 adults with traumatic brachial plexus injuries, 23 of them (8.2%) had a concomitant full-thickness rotator cuff tear. The investigators therefore proposed that the rotator cuff be imaged in patients with traumatic brachial plexopathy when treatment options are being assessed. [27]
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Other Tests
Clinical threshold testing can be used to evaluate sensory function in peripheral nerves. These tests can be used to determine the level of stimulus necessary to elicit a response.
Semmes-Weinstein monofilaments are fine filaments that exert a discrete amount of pressure on the fingertips. They are used to perform threshold testing. Vibratory senses can be assessed by means of clinical threshold testing with low (30 Hz) to high (256 Hz) frequencies.
Histologic Findings
At light microscopy, nerves injured with epineurectomy or a crush mechanism have widespread fiber degeneration and myelin debris in the subperineurial region. The centrofascicular areas are relatively preserved compared with the subperineurial regions. The central vessels are preserved mostly within the centrofascicular area of the injured nerve. The thickness of myelin in the axons is decreased after injury, and the internodal length becomes more variable compared with its length before injury. A loss of cross-sectional area without a loss in the muscle fiber count begins within 1 week of denervation.
