Electrodiagnostic Approach to Patients with Suspected Radiculopathy

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CHAPTER 8 Electrodiagnostic Approach to Patients with Suspected Radiculopathy

Timothy R. Dillingham

INTRODUCTION

Cervical and lumbosacral radiculopathies are conditions involving a pathological process affecting the spinal nerve root. Commonly, this is a herniated nucleus pulposus that anatomically compresses a nerve root within the spinal canal. Another common etiology for radiculopathy is spinal stenosis resulting from a combination of degenerative spondylosis, ligament hypertrophy, and spondylolisthesis. Inflammatory radiculitis is another pathophysiological process that can cause radicular pain and/or radiculopathy. It is important to remember, however, that other more ominous processes such as malignancy and infection can present with the same symptoms and signs of radiculopathy as the more common causes.

This chapter deals with the clinical approach used in an electrodiagnostic laboratory to evaluate a person with neck pain, lumbar spine pain, or limb symptoms which are suggestive of radiculopathy. The indications for referring for testing as well as the limitations of testing are discussed to give a greater understanding of this important diagnostic procedure. This is not intended to be a basic chapter dealing with how to perform electrodiagnostic studies.

Given the extensive differential diagnosis for limb and spine symptoms, it is important for electrodiagnosticians to develop a conceptual framework for evaluating these referrals with a standard focused history and physical examination and a tailored electrodiagnostic approach. Accurately identifying radiculopathy by electrodiagnosis whenever possible, provides valuable information that helps guide treatment and minimizes other invasive and expensive diagnostic and therapeutic procedures.

SPINE AND NERVE ROOT ANATOMY: DEVIATIONS FROM THE EXPECTED

Spinal anatomy is discussed in detail in Chapters 46 and 80 by Russell Gilchrist and will not be emphasized here. From an electrodiagnostic perspective, however, there are several specific anatomical issues that merit further discussion.

At all levels the dorsal root ganglion (DRG) lies in the intervertebral foramen. This anatomical arrangement has implications for clinical electrodiagnosis of radiculopathy, namely that sensory nerve action potentials (SNAPs) are preserved in most radiculopathies as the nerve root is affected proximal to the DRG.

Regarding the cervical nerve roots and the brachial plexus, there are many anatomic variations. Perneczky ¹ described an anatomic study of 40 cadavers. In all cases, there were deviations from accepted cervical root and brachial plexus anatomy. Levin, Maggiano, and Wilbourn² examined the pattern of abnormalities on electromyography (EMG) in 50 cases of surgically proven cervical root lesions. A range of needle EMG patterns was found with EMG demonstrating less specificity for the C6 root level, but more specificity and consistent patterns for C8, C7, and C5 radiculopathies. In subjects with C6 radiculopathies, half the patients showed findings similar to those with C5 radiculopathies and the other half demonstrated C7 patterns. This surgical group was more severely affected than patients who do not require surgical interventions, and this pattern may not hold for less symptomatic patients.

In the lumbar spinal region dorsal and ventral roots exit the spinal cord at about the T11–L1 boney level and travel in the lumbar canal as a group of nerve roots in the dural sac. This is termed the ‘horse’s tail’ or cauda equina. This poses challenges and limitations to the EMG examination. A destructive intramedullary (spinal cord) lesion at T11 can produce EMG findings in muscles innervated by any of the lumbosacral nerve roots and manifest the precise findings on needle EMG as those seen with a herniated nucleus pulposus at any of the lumbar disc levels. For this reason, the electromyographer cannot precisely determine the anatomic location of a lumbar intraspinal lesion producing distal muscle EMG findings in the lower limbs. The needle EMG examination can only identify the root or roots that are physiologically involved, but not the precise anatomic site of pathology in the lumbar spinal canal. This is an important limitation requiring correlation with imaging findings to determine which anatomic location is most likely the offending site. This can be difficult in elderly persons with foraminal stenosis as well as moderate central canal stenosis at more than one site.

In a prospective study of 100 patients with lumbosacral radiculopathy who underwent lumbar laminectomy, EMG precisely identified the involved root level 84% of the time.³ Needle EMG failed to accurately identify the compressed root in 16%. However, at least half of the failures were attributable to anomalies of innervation. Another component to this study involved stimulating the nerve roots intraoperatively with simultaneous recording of muscle activity in the lower limb using surface electrodes. These investigators demonstrated variations in root innervation, such as the L5 root innervating the soleus and medial gastrocnemius, in 16% of a sample of 50 patients. Most subjects demonstrated dual innervation for most muscles.³

These findings underscore the limitations of precise localization for root lesions with EMG. The electrodiagnostician should maintain an appreciation of these anatomic variations to better convey the level of certainty with respect to diagnostic conclusions.

PHYSICAL EXAMINATION

The electrodiagnostic examination is an extension of the standard clinical examination. The history and physical examination are vital initial steps in determining what conditions may be causing the presenting symptoms. Most radiculopathies present with symptoms in one limb. Multiple radiculopathies such as are seen in cervical spinal stenosis or lumbar stenosis may cause symptoms in more than one limb. A focused neuromuscular examination that assesses strength, reflexes, and sensation in the affected limb and the contralateral limb is important, providing a framework for electrodiagnostic assessment.

An algorithmic approach to utilizing physical examination and symptom information to tailor the electrodiagnostic evaluation is shown in Figure 8.1. In this approach, symptoms and physical examination signs create a conceptual framework for approaching these sometimes daunting problems. Admittedly, there are many exceptions to this approach with considerable overlap in medical disorders which might fall within multiple categories. Radiculopathies and entrapment neuropathies are examples of such conditions with a variety of clinical presentations and physical examination findings, such that they are included in both focal symptom categories with and without sensory loss. In the case of a person with lumbosacral radiculopathy, a positive straight leg raise test may be noted in the absence of motor, reflex, or sensory changes. Conditions such as myopathies and polyneuropathies better fit this algorithmic approach, given that symptoms and physical examination signs are more specific. Figure 8.1 also contains musculoskeletal disorders and denotes how they fall into this conceptual framework. The electrodiagnostician must be willing to modify the electrodiagnostic examination in response to nerve conduction and EMG findings and adjust the focus of the examination in light of new information.

Fig. 8.1 Algorithmic approach to structuring the electrodiagnostic examination based upon physical examination signs and the location of the patient’s symptoms. Focal symptoms refer to single limb symptoms whereas generalized symptoms are present when the patient complains of symptoms affecting more than one limb.

The implications of symptoms and signs on electrodiagnostic findings were investigated by Lauder and colleagues for suspected cervical and lumbosacral radiculopathies.^4,⁵ Even though physical examination findings were better at predicting who would have a radiculopathy, many patients with normal examinations had abnormal electrodiagnostic studies, indicating that clinicians should not curtail electrodiagnostic testing simply because the physical examination is normal. For lower limb symptoms, loss of a reflex or weakness dramatically increased the likelihood of having a radiculopathy by EMG. Losing the Achilles reflex, for instance, resulted in an odds ratio of 8.4 (p<0.01), in other words eight times the likelihood of having a radiculopathy by EMG with this physical examination finding compared to someone without loss of this reflex.⁴ Similar findings were noted for upper limb symptoms; if a reflex was lost or weakness was noted the likelihood of having a cervical radiculopathy confirmed by EMG was many times greater.⁵ Combinations of findings, particularly weakness plus sensory loss or plus reflex changes, resulted in a ninefold greater likelihood of cervical radiculopathy and two to three times greater likelihood of lumbosacral radiculopathy.^4,⁵

The American Association of Neuromuscular and Electrodiagnostic Medicine Guidelines for Radiculopathy Evaluation

The American Association of Neuromuscular and Electrodiagnostic Medicine (AANEM) guidelines recommend that for an optimal evaluation of a patient with suspected radiculopathy, a needle EMG screen of a sufficient number of muscles and at least one motor and one sensory nerve conduction study should be performed in the involved limb.⁶ The nerve conduction studies are necessary to exclude polyneuropathy. The sufficiency of the EMG screen and a recommended number of muscles is discussed in detail below. An EMG study is considered diagnostic for a radiculopathy if EMG abnormalities are found in two or more muscles innervated by the same nerve root, and different peripheral nerves, yet muscles innervated by adjacent nerve roots are normal.⁷ This assumes, of course, that other generalized conditions such as polyneuropathy are not present.

It is often necessary to study bilateral limbs, particularly if a single limb shows EMG findings suggestive of radiculopathy and the patient has symptoms in both the studied and the contralateral limb. If bilateral limbs are involved, the electrodiagnostician should have a low threshold for studying selected muscles in an upper limb (if the lower limbs are abnormal on EMG) or a lower limb (if both upper limbs are abnormal) to exclude a generalized process such as polyneuropathy or motor neuron disease. Likewise, additional nerve conduction studies are appropriate to exclude other suspected conditions and the electrodiagnostician should have a low threshold for expanding the study.

H-REFLEXES, F-WAVES, AND NERVE CONDUCTION

Nerve conduction studies, H-reflexes, and F-waves are not very useful for confirming radiculopathy. They are useful, however, to exclude polyneuropathy or mononeuropathies.

H-reflexes

H-reflexes have commonly been used to determine whether a radiculopathy demonstrates S1 involvement.⁷ It is a monosynaptic reflex that is an S1-mediated response and can differentiate to some extent L5 from S1 radiculopathy. Many researchers have evaluated their sensitivity and specificity with respect to lumbosacral radiculopathies and generally found a range of sensitivities from 32% to 88%.⁷^–¹² However, many of these studies suffered from lack of a control group, imprecise inclusion criteria, or small sample sizes.

Marin et al.¹² prospectively examined the H-reflex and the extensor digitorum brevis reflex in 53 normals, 17 patients with L5, and 18 patients with S1 radiculopathy. Patients included in the study had all of the following: (1) radiating low back pain into the leg, (2) reduced sensation or weakness or positive straight leg raise test, and (3) either EMG evidence of radiculopathy or structural causes of radiculopathy on magnetic resonance imaging (MRI) or computed tomography (CT) imaging. The maximal (2 SD) value for the H-reflex side-to-side latency difference was 1.8 ms as derived from the normal group. They analyzed the sensitivity of the H-reflex for side-to-side differences greater than 1.8 ms or a unilaterally absent H-reflex on the affected side. The H-reflex only demonstrated a 50% sensitivity for S1 radiculopathy, 6% for L5 radiculopathy, but had a 91% specificity. Amplitudes were not assessed in this study. These results suggest that the H-reflex has a low sensitivity for S1 root level involvement.

H-reflexes may be useful to identify subtle S1 radiculopathy, yet there are a number of shortcomings related to these responses. They can be normal with radiculopathies ¹² and, because they are mediated over such a long physiological pathway, they can be abnormal due to polyneuropathy, sciatic neuropathy, or plexopathy.⁷ They are most useful in the assessment for polyneuropathy.

In order to interpret a latency or amplitude value and render a judgment as to the probability that it is abnormal, precise population-based normative values encompassing a large age range of normal subjects must be available for comparison of these nerve conduction findings. Falco et al.¹³ demonstrated in a group of healthy elderly subjects (60–88 years old) that the tibial H-reflex was present and recorded bilaterally in 92%. Most elderly are expected to have normal H-reflex studies and, when abnormalities are found in these persons, the electrodiagnostician should critically evaluate these findings and the clinical scenario before attributing H-reflex abnormalities to the aging process.

In patients with upper limb symptoms suggestive of cervical radiculopathy, H-reflexes and F-waves are not useful in diagnosis but rather help exclude polyneuropathy as an underlying cause of symptoms. One study by Miller and colleagues ¹⁴ examined the H-reflexes in the upper limb in a set of patients defined by a combination of clinical criteria (no imaging or EMG studies) as having definite or probable cervical radiculopathy. They tested the H-reflex for the FCR, the ECR, the APB, and the biceps heteronymous reflex. The later reflex is derived by stimulating the median nerve in the cubital fossa and recording over the biceps brachii muscle, averaging 40–100 trials. These reflex studies had a 72% sensitivity overall for the group with 100% for the subset of patients with definite cervical radiculopathy. In contrast, needle EMG demonstrated 90% sensitivity for the definite group. Although these findings suggest a possible role for these upper limb H-reflexes, they are highly specialized, time consuming, and difficult to consistently elicit. They may have a role in sensory radiculopathies where needle EMG will not be positive and imaging findings are equivocal. Further studies are necessary to clarify whether the findings of Miller et al.¹⁴ can be duplicated at other centers.

F-waves

F-waves are late responses involving the motor axons and axonal pool at the spinal cord level. They can be assessed and classified by using the minimal latency, mean latency, and chronodispersion or scatter.⁷ As in the case of H-reflexes, they demonstrate low sensitivities and are not specific for radiculopathy; rather, they are a better screen for polyneuropathy. Published sensitivities range from 13% to 69%; however, these studies suffer from many of the same shortcomings that are found in the H-reflex studies.^8,^15,¹⁶

London and England ¹⁷ reported two cases of persons with neurogenic claudication from lumbosacral spinal stenosis. They demonstrated that the F-wave responses could be reversibly changed after 15 minutes of ambulation, which provoked symptoms. This suggested an ischemia-induced conduction block in proximal motor neurons. A larger-scale study of this type might find a use for F-waves in the identification of lumbosacral spinal stenosis and assist with the delineation of neurogenic from vascular claudication.

Motor and sensory nerve conduction studies

Standard motor and sensory nerve conduction studies are not helpful in identifying a cervical or lumbosacral radiculopathy. However, they should be performed to screen for polyneuropathy and exclude common entrapment neuropathies if the patient’s symptoms could be explained by a focal entrapment. Remember that based upon the anatomy of the dorsal root ganglion, sensory responses should be normal in most radiculopathies. If they are absent, this should raise suspicion for another diagnosis such as polyneuropathy, plexopathy, or mononeuropathy.

Plexopathies often pose a diagnostic challenge as they are similar to radiculopathies in symptoms and signs. In order to distinguish plexopathy from radiculopathy, sensory responses which are accessible in a limb should be tested. In plexopathy, they are likely to be reduced in amplitude, whereas in radiculopathy they are generally normal. If substantial axonal loss has occurred at the root level, the compound muscle action potential recorded in muscles innervated by that root may be reduced in both plexopathies and radiculopathies. This is usually when severe axonal loss has occurred such as with cauda equina lesions or penetrating trauma that severely injures a nerve root. The distal motor latencies and conduction velocities are usually preserved as they reflect the fastest conducting nerve fibers.⁷

SOMATOSENSORY EVOKED POTENTIALS, DERMATOMAL SOMATOSENSORY EVOKED POTENTIALS, AND MAGNETIC EVOKED POTENTIALS

The AANEM guidelines recently examined the literature and concluded that somatosensory evoked potentials (SEPs) may be useful for cervical spondylosis with cord compression. Likewise, in lumbosacral spinal stenosis, dermatomal somatosensory evoked potentials (DSEPs) may be useful in defining levels of deficits.⁶ These tests are not necessary for electrodiagnostic testing for persons with suspected radiculopathies and their usefulness is limited to special circumstances. These tests are not recommended for the routine evaluation of persons with suspected radiculopathy.

DSEPs can document physiological evidence of multiple or single root involvement in lumbosacral spinal stenosis and may be useful in the case where spinal canal narrowing is minimal and the patient has symptoms. This testing also complements standard needle EMG. Snowden et al.¹⁸ found that for single and multilevel lumbosacral spinal stenosis, DSEPs revealed 78% sensitivity relative to spinal imaging. In this well-designed prospective study, DSEP criteria as well as inclusion criteria were precisely defined. The predictive value for a positive test was 93%.

Yiannikas, Shahani, and Young ¹⁹ demonstrated that SEPs may be useful for cervical myelopathy. In this study of 10 patients with clinical signs of myelopathy, all 10 had abnormal peroneal SEPs and seven had abnormal median SEPs.

Maertens de Noordhout et al.²⁰ examined motor and SEPs in 55 persons with unequivocal signs and symptoms of cervical spinal myelopathy. In this group 87% showed gait disturbances, and 82% showed hyperreflexia. MRI was not the diagnostic standard as these authors felt that MRI was prone to overdiagnosis; rather, metrizamide myelography showed unequivocal signs of cervical cord compression for all of these patients. Magnetic stimulation of the cortex was performed and the responses measured with surface electrodes. In these subjects 89% demonstrated abnormalities in magnetic evoked potential (MEP) to the first dorsal interosseus muscle and 93% had one MEP abnormality. At least one SEP abnormality was noted in 73%.

Tavy et al.²¹ examined whether MEPs or SEPs assisted in identifying persons with radiological evidence of cervical cord compression but who were without clinical markers for myelopathy. All patients had clinical symptoms of cervical radiculopathy, but not myelopathy. In this group, MEPs were normal in 92% and SEPs were normal in 96%. These investigators concluded that MEPs and SEPs are normal in most cases of persons with asymptomatic cervical stenosis. This indicates that abnormal MEPs and SEPs are likely to be true-positive findings and not false positives related to mild asymptomatic cord compression. It is important to remember that cervical spondylosis is a process that causes a continuum of problems including both radiculopathy and myelopathy.

The inherent variability and difficulty in determinations of what constitutes normal SEPs prompted investigation. Dumitru and colleagues ²² examined the variations in latencies with SEPs. In 29 normal subjects, they examined the ipsilateral intertrial variations, arithmetic mean side-to-side differences, and maximum potential side-to-side differences with stimulation of the superficial peroneal sensory nerve, sural nerve, and L5 and S1 dermatomes with respect to P1 and N1 latencies and peak-to-peak amplitudes. Considerable ipsilateral intertrial variation was observed and side-to-side comparisons revealed a further increase in this inherent variation regarding the above measured parameters. They suggested an additional parameter with which to evaluate SEPs: the maximum side-to-side latency difference.

Dumitru and colleagues,²³ in a study involving persons with unilateral and unilevel L5 and S1 radiculopathies, evaluated DSEPs and segmental SEPs. History, physical examination, imaging studies, and electrodiagnostic medicine evaluations clearly defined patients with unilateral/unilevel L5 or S1 nerve root compromise. Regression equation analysis for cortical P1 latencies evaluating age and height based on comparable patient and control reference populations revealed segmental and dermatomal sensitivities for L5 radiculopathies to be 70% and 50%, respectively, at 90% confidence intervals. Similar sensitivities were obtained for 2 standard deviation mean cortical P1 latencies. Side-to-side cortical P1 latency difference data revealed segmental and dermatomal sensitivities for S1 radiculopathies to be 50% and 10%, respectively, at 2 standard deviations. These investigators questioned the clinical utility of both segmental and dermatomal SEPs in the evaluation of patients with suspected unilateral/unilevel L5 and S1 nerve root compromise, finding little utility for these tests in persons with single-level lumbosacral radiculopathy.