Traumatic Peripheral Nerve Lesions Workup
- Author: Neil R Holland, MBBS, MBA, FAAN; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE more...
Magnetic resonance neurography (MRN) uses a high resolution fast spin echo T2 imaging technique to demonstrate abnormal signal within nerve trunks at sites of major nerve injuries and compression. This technique can be particularly useful for identifying nerve injuries, such as piriformis syndrome and brachial plexus injuries, at sites inaccessible to conventional nerve conduction studies.
The same technique can be used to demonstrate pseudomeningoceles in the cervical spine from traumatic nerve root avulsions.[2, 3]
A carefully planned electrodiagnostic study is critical for determining the completeness and pathophysiology of all nerve injuries.
The completeness of a nerve injury can be determined any time after the injury. The presence of voluntary motor unit potentials on needle electromyography (EMG) examination of a clinically paralyzed muscle always indicates that the nerve injury, at least the branch or fascicle supplying that individual muscle, is partial.
In general, sensory responses are affected earlier and more severely than motor responses in peripheral nerve injuries. A reduction in sensory response amplitude of 50% or more, compared to the other (unaffected) side, is the most sensitive indication of peripheral nerve injury. Normal sensory responses are seen with nerve root injuries, even from clinically anesthetic regions, because the injured nerve segment is proximal to the dorsal root ganglion.
The physician performing the EMG must be fully cognizant of the time course of wallerian degeneration when performing nerve conduction studies to differentiate demyelination from axonal loss. The dissociation between the rates of degeneration of motor and sensory fibers can be a particular source of problem for the novice. A nerve conduction study performed 3-7 days after a peripheral nerve injury may show low-amplitude evoked compound muscle action potential (CMAP) with normal amplitude sensory nerve action potential (SNAP), a pattern usually interpreted as nerve root injury/avulsion.
Needle EMG findings correlate poorly with the degree of axonal loss. Denervation potentials do not appear for as long as 21 days after the nerve injury; the delay depends on the distance between the nerve injury and affected muscle. Moreover, the density of denervation potentials cannot be extrapolated to indicate the severity of axonal loss. Denervation potentials should be absent even 21 days after a pure demyelinating injury. However, most nerve injuries are mixed, and even predominantly demyelinating lesions suffer some secondary loss, often resulting in surprisingly profuse denervation potentials.
The amplitude of distal evoked CMAP and SNAP responses yields the maximum information regarding the degree of axonal loss that has occurred in motor and sensory fibers 10 or more days after a nerve injury. Evoked amplitudes must be compared to either a baseline study (immediately after the injury) or to the response evoked on the contralateral (normal) side. Adequate assessment of nerve injuries may necessitate the use of nonconventional nerve conduction studies.
The motor nerves used conventionally in conduction studies of the upper extremity, the median and ulnar, are both derived from the lower cord and medial trunk of the brachial plexus. A musculocutaneous motor nerve conduction study is required to assess the degree of axonal loss in cases of upper trunk plexus injuries.
The presence of a relatively preserved distal CMAP response amplitude in a paralyzed muscle more than 7-10 days after a nerve injury always should suggest more proximal conduction block. In most cases, the conduction block will be determined readily by comparing evoked CMAP response amplitudes from stimulation proximal and distal to the injury site.
In some instances, however, the conduction block may be too proximal to be demonstrated reliably by conventional motor nerve conduction studies (eg, conduction block at the nerve root level). In these instances, F-wave responses may be absent despite the presence of more normal distal evoked CMAP responses. Additionally, somatosensory-evoked potential (SEP) testing and/or nerve root stimulation may be used to demonstrate proximal conduction block even at the nerve root level.
In summary, a carefully planned and executed electrodiagnostic study is paramount in the evaluation of nerve injuries. Needle EMG can demonstrate whether the injury is complete or incomplete at any time after injury. Nerve conduction studies are required to differentiate demyelination from axon loss; they yield the maximal information in this regard approximately 10 days after the injury. Nerve conduction studies should be bilateral to allow side-to-side comparisons of amplitude. Some types of injuries may necessitate the use of unconventional studies to adequately assess the degree of axon loss to each individual nerve branch or fascicle.
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