What is the role of electrodiagnostic studies in the evaluation of cervical disc disease?

Updated: Apr 16, 2020
  • Author: Michael B Furman, MD, MS; Chief Editor: Dean H Hommer, MD  more...
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Answer

Electrodiagnostic studies continue to be standard for evaluating neurologic function of the cervical spine. Advantages of these tests include limited expense and low morbidity.

Nerve conduction studies (NCSs) and electromyography (EMG) studies provide physiologic assessment of cervical nerve root and peripheral nerve function.

Needle EMG can detect acute, subacute, and chronic radicular features if motor nerve fiber pathology exists.

A diagnosis of radiculopathy is apparent when needle EMG reveals abnormal spontaneous potentials and/or certain changes in motor unit action potentials, in 2 or more muscles innervated by the same nerve root but by different peripheral nerves. Ideally, EMG abnormalities also should be demonstrated in the paraspinal muscles to confirm the diagnosis of radiculopathy.

In a study by Dillingham et al, cervical radiculopathy may be identified as much as 100% of the time using preset muscle screens. If positive findings are found in 1-2 muscle(s) in such screen, this result is positive. [31]

When paraspinal muscles were one of the screening muscles, 5 muscle screens identified 90-98% of radiculopathies, 6 muscle screens identified 94-99%, and 7 muscle screens identified 96-100%. When paraspinal muscles were not part of the screen, 8 distal limb muscles recognized 92-95% of radiculopathies. An 8 muscle screen that excludes the cervical paraspinal muscles is a valuable tool to help diagnose radiculopathy in those patients with prior history of cervical spinal laminectomy.

A compound motor action potential amplitude drop of 50% or more indicates significant axonal loss. This assessment is made via NCS of motor axons.

NCS/EMG is especially helpful in differentiating cervical radiculopathy from confounding neuropathic conditions (eg, ulnar nerve entrapment, carpal tunnel syndrome, peripheral neuropathy, plexopathy).

Unfortunately, cervical radiculopathies involving exclusively sensory axons (ie, without involvement of motor axons) rarely are detected by electrodiagnostic studies, which is a shortcoming of this diagnostic modality. In addition, routine motor NCSs do not evaluate the C6 and C7 nerve roots, which are most commonly involved, or the levels above.

Unlike needle EMG (which involves intramuscular evaluation and is a well-accepted diagnostic test), surface EMG generally is not considered to have an accepted role in the diagnosis of radiculopathy.

Somatosensory evoked potentials (SEPs) evaluate sensory conduction peripherally and centrally.

Lower limb SEPs involving tibial and peroneal nerves, which assess spinal cord conduction, are more sensitive in diagnosing myelopathy than are upper limb median and ulnar SEPs.

Dermatomal evoked potentials have been used to detect cervical radiculopathy but are of questionable value.


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