Author(s): Todd R. Lefkowitz, MD
Originally published: September 10, 2015 Last updated: July 24, 2020
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OVERVIEW AND DESCRIPTION

Definition of the assessment/treatment procedure(s)

The electrodiagnostic (EDX) assessment of cervical, thoracic, and lumbosacral radiculopathies will be reviewed. Approximately 60% – 90% of all radiculopathies affect the lumbosacral nerve roots with the vast majority of those affecting the L5 and S1 roots1-3. Cervical radiculopathies likely account for only 5%-10% of all spinal nerve root disorders affecting the C7, C6, C8 and C5 nerve roots in decreasing frequency2-3. Thoracic radiculopathies occur, but are rare; representing only 0.15% – 4% of symptomatic disc herniations of the spine4. They have also been reported in a subset of patients with diabetic polyradiculoneuropathy5.

EDX techniques including nerve conduction studies (NCS) and needle electromyography (NEE) are routinely used in the clinical evaluation of patients with suspected radicular lesions. Given the lack of a universal “gold standard” for evaluating radiculopathies, studies that establish physiological dysfunction of nerve roots continue to be useful5. 

NCS are needed to rule-out the presence of other conditions which may produce similar signs and symptoms as radiculopathies (ie, peripheral entrapment neuropathies, plexopathies, or peripheral polyneuropathy). As the pathology is at the root level, sensory NCS are expected to be normal given the proximal location of the dorsal root ganglion (DRG) within the neural foramen; however, abnormal superficial peroneal and sural sensory nerve action potentials (SNAPs) have been reported with L5 and S1 root lesions, respectively, which may occur with a far lateral disc herniation that impinges the DRG within the foramen5. Normal SNAPs argue for a radiculopathy versus a more peripheral lesion in patients with radiating extremity pain and/or paresthesias (ie, an L5 or C8 radiculopathy rather than a peroneal or ulnar neuropathy, respectively).

Compound muscle action potential (CMAP) amplitudes may be reduced by severe axonal lesions affecting the primary nerve root innervation of the muscles used for pick-up (ie, extensor digitorum brevus for L5 radiculopathies) during routine NCS. In cervical radiculopathies, however, distal CMAP amplitudes are not helpful in lesions outside a C8-T1 myotomal distribution as the more commonly affected nerve roots in the cervical spine (ie, C6, C7) do not innervate the muscles commonly used for pick-up in the upper limb (ie, abductor pollicis brevis and abductor digiti minimi). Since non-cranial peripheral nerves are composed of multiple nerve root contributions, CMAP amplitudes are generally unaffected by a single nerve root lesion.

The value of F waves in the evaluation of radiculopathies remains limited in clinical practice. F waves are often normal in less severe lesions, but may abnormal in polyradiculopathies, plexopathies or peripheral nerve lesions6. As noted above, since non-cranial peripheral nerves are composed of multiple nerve root contributions, F waves are likely to be normal in a single radicular lesion. Despite reports that various F-wave parameters (ie, minimal latency, maximal latency, chronodispersion) may improve the diagnostic yield for cervical radiculopathy when combined with needle EMG, abnormal F-wave findings cannot be used to localize a lesion to a specific cervical nerve root level7.  This fact plus the supramaximal stimulation required to elicit F-waves, contributes to the low use of these studies to diagnose cervical radiculopathy.

H-reflexes can be helpful in the electrodiagnosis of S1 radiculopathies. Asymmetrical prolongation or absence of H reflexes may be present in S1 nerve root injuries with a reported sensitivity of 50% and a specificity of 91%8. It should be noted that H-reflexes become abnormal as soon as compression occurs and may remain abnormal indefinitely, making this test more useful than the other late responses9. The upper extremity H-reflex study (mediated through C6/C7 levels through the median nerve recorded over the flexor carpi radialis muscle) has been shown to add utility to the electrodiagnosis of cervical radiculopathy, especially in patients whose clinical symptoms are not clear and needle EMG is normal10.

Somatosensory evoked potentials (SEPs) have the theoretical advantage of evaluating afferent nerve fiber dysfunction in radiculopathies. Specifically, dermatomal SEPs should be useful as they assess the sensory fibers of a single nerve root. Based on the available evidence, however, SEPs are still considered investigational for acute cervical and lumbosacral radiculopathies.  They may be useful in patients with suspected cervical myelopathy or other central causes of neurological symptoms (ie, multiple sclerosis)5,11.

The NEE remains the most widely accepted method for confirming the presence of a radiculopathy affecting the motor axons1. Ideally, six muscles including the paraspinals should be screened for both cervical and lumbar radiculopathies as sensitivities of the NEE are relatively poor12. In patients with cervical radiculopathy, sensitivities ranged from 50%-72% while sensitivities for lumbosacral radiculopathy ranged from 49% – 86%12-13. In many patients, however, axonal loss is not detected as overlapping innervations and varying degrees of axonal injury likely obscure the detection of these lesions. In other circumstances with a negative NEE, a demyelinating process with or without conduction block or a purely sensory lesion affecting the DRG must be considered5.

Ideally, the EDX examination should coincide with the development of denervation potentials (ie, positive sharp waves and fibrillations) in the axial and limb muscles. Abnormal spontaneous electrical activity usually first appears in the paraspinal muscles approximately 7-10 days following injury, followed by the proximal and then distal limb muscles several weeks later3. Reinnervation follows this same proximal-to-distal pattern. As such, the NEE should be performed approximately 3-4 weeks post-injury.

NEE abnormalities found in two or more muscles innervated by the same nerve root but by different peripheral nerves confirms the presence of a radiculopathy2. However, paraspinal fibrillations and/or positive sharp waves may be the only EDX abnormality found in about 20% of patients with lumbosacral root injuries and in about 40% of patients with cervical root injuries14-15.  In contrast to most limb muscles, abnormal spontaneous electrical activity may be seen in the cervical and lumbar paraspinal muscles of asymptomatic, older individuals16-18. There are those who contest these finding arguing the close similarity in appearance of atypical appearing endplate spikes with spontaneous activity19. It therefore remains contentious to make the diagnosis of radiculopathy when abnormal spontaneous electrical activity is limited to the paravertebral muscles alone. Fibrillations and positive sharp waves may also remain chronically present after posterior laminectomies.

The density of fibrillation potentials depends upon the innervation ratio (ie, ratio between the total number of extrafusal muscle fibers to the number of innervating motor axons) of the muscle being sampled20. The innervation ratio may be quite high in larger muscles such as the gastrocnemius where one motor axon innervates roughly 1,934 muscle fibers20. Therefore, losing a small percentage of motor axons to root injury will generate a relatively large number of fibrillating muscle fibers. It should be noted, however, that fibrillations and positive sharp waves will disappear with time due to motor unit reinnervation or loss of muscle membrane reactivity5.

The NEE in patients with long-standing cervical or lumbar radiculopathy may detect changes consistent with chronic neurogenic injury. Relying solely though on motor unit action potential (MUAP) morphology to diagnose radiculopathy is not recommended as there are rarely enough motor axons injured in a particular muscle to produce discernible alterations in recruitment and, even rarer, are they found in a definable myotomal distribution5. Moreover, the time constraints and sampling size imposed by the motor unit analysis limits the evaluation of motor unit duration, amplitude, and polyphasia necessary to adequately characterize a chronic radiculopathy5. If reinnervation cannot keep pace with ongoing axon loss, both low- and high-voltage fibrillation potentials may be seen suggesting an acute-on-chronic radiculopathy. This situation can also occur when there is a chronic denervating disease or when there are more than one chronologically distinct denervation injuries. In contrast to neurotemetic lesions such as radial nerve palsies resulting from gunshot or stab wounds where the EDX examination can help prognosticate return of motor function, no such information can be reliably inferred from the EDX examination of patients with cervical or lumbosacral radiculopathies. The inherent difficulty correlating fibrillation potential density with the severity of nerve root injury and the lack of available distal CMAP amplitudes makes prognostication difficult. Furthermore, the multisegmental innervation of most peripheral nerves helps preserve the distal CMAP amplitude even with severe injury to one nerve root. Identifying those patients with demyelinating or purely sensory lesions is also limited as the NEE is expected to be normal with the possible exception of reduced recruitment.

RELEVANCE TO CLINICAL PRACTICE

EDX testing is an extension of the physician’s history taking and physical examination. Perhaps the best rationale to support its use is establishing whether nerve root injury is present and which roots are affected. In that context, performing EDX studies when root injury has previously been established or disproved through complimentary investigations to the patient’s physical examination such as magnetic resonance imaging (MRI) or computed tomography (CT) myelography is unwarranted.  EDX testing is also useful in situations of suspected acute-on-chronic disease, such as differentiating an acute radiculopathy from an underlying chronic radicular process as may be seen in spinal stenosis. It is also helpful in distinguishing acute radiculopathies from other underlying neurological conditions such as diabetic peripheral neuropathy which can often obscure acute radicular clinical findings.

EDX testing can help clarify a patient’s diagnosis when imaging studies are equivocal. MRI scans of the lumbar spine are known to have high false-positive rates with one study reporting 27% of normal subjects having a disc protrusion21.  The false-positive rate of cervical spine MRI scans is lower with only 10% of asymptomatic subjects reported having a herniated or bulging disc22.

As surgical success rates vary, EDX testing can provide objective data to help identify patients who may benefit from surgical decompression. The NEE, for example, was reported to correlate well with nerve root injury at surgery in 78.6% of patients based on findings of fibrillations and positive sharp waves in limb muscles in one early study23. A later study compared the outcome of patients with lumbar radiculopathy from disc herniation treated surgically to those patients with lumbar radiculopathy treated conservatively but found the initial EMG had no prognostic ability to predict outcome at five years24. For those patients considering epidural steroid injections (ESI) as treatment for painful lumbar radiculopathy, a recent study found that those with EMG-positive radiculopathy had better pain scores and functional improvement after ESI than those with EMG-negative radiculopathy25. In a small study of patients with cervical radiculopathy, those with positive pre-operative EMGs fared better after discectomy and fusion than patients with normal EMGs26.

Complications arising from EDX testing are exceedingly rare in the general population. Imaging studies performed on patients after undergoing NEE demonstrated an aggregated risk of bleeding of approximately 1% for all muscles with no significantly higher incidence seen in patients on systemic anticoagulation27. As such, there is no compelling reason to defer the NEE in patients suspected of having a radiculopathy who are taking commonly prescribed anticoagulants. Patients on novel oral anticoagulants were found to have minimal risk of clinically relevant hemorrhagic complications, not significantly different from those patients on warfarin in a 2019 study28. Concern over puncturing the pleural and abdominal cavities, especially in obese patients, limits the use of the NEE of the intercostals and anterior abdominal wall muscles in diagnosing thoracic radiculopathies. Ultrasound with Doppler imaging may be used to help avoid blood vessels when localizing deep muscles targeted for NEE.

Routine EDX testing should not pose a safety hazard to patients with cardiac pacemakers or defibrillators, but NCS should be avoided in patients with external cardiac pacemakers as the conductive lead threaded into the heart is electrically sensitive29. In patients with significant anxiety disorders, skin infections, grafts, or varicosities, proceeding with the EDX examination remains at the discretion of the examining physician.

The ability of patients to provide informed consent underlies most medical procedures and tests, including EDX testing. Hospitalized patients, however, especially those in intensive care settings, may not be able to provide informed consent. In these cases, family members may be approached for consent if the requested testing is thought to clarify the patient’s diagnosis or change management.

Guidelines for ethical behavior in EDX medicine have been developed by the American Association of Neuromuscular and Electrodiagnostic Medicine (AANEM)30. Central to the EDX consultation is the physician’s fiduciary duty to safeguard the patient’s physical, psychological, and emotional interests as it relates to the prescribed testing. The EDX examination should be planned to minimize patient discomfort by stimulating the least number of nerves and needling the fewest muscles necessary to confirm or refute the presence of a radiculopathy. Should the patient experience significant discomfort, the examination should be terminated and the patient advised that no lasting harm is expected. As patients have a tendency to overestimate the benefits of medical intervention and underestimate its harms, shared decision making in EDX medicine, especially as it relates to the diagnosis of cervical and lumbar radiculopathies, may help reduce the number of unnecessary tests and the procedures resulting from their use31.

References

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Original Version of the Topic

Todd R. Lefkowitz, MD, Richard Kim, MD. Electrodiagnosis of Radiculopathies (Cervical, Thoracic, and Lumbar). Originally published:09/10/2015

Previous Revision(s) of the Topic

N/A

Author Disclosure

Todd R. Lefkowitz, MD
Nothing to Disclose