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.

PURPOSE OF ELECTRODIAGNOSTIC TESTING

Electrodiagnostic testing is expensive and uncomfortable for patients and so it is important to understand why it is performed and the expected outcomes. Electrodiagnostic testing serves several important purposes:

(1) It effectively excludes other conditions that mimic radiculopathy such as polyneuropathy or entrapment neuropathy. Haig and colleagues ²⁴ demonstrated that the referring diagnostic impression is often altered with electrodiagnostic testing.

(2) Electrodiagnostic testing can, to some extent, suggest severity or extent of the disorder beyond clinical symptoms. Involvement of other extremities can be delineated or the involvement of multiple roots may be demonstrated, such as in the case of lumbosacral spinal stenosis.

(3) There is utility in solidifying a diagnosis. An unequivocal radiculopathy on EMG in an elderly patient with non-specific or mild lumbar spondylosis or stenosis on MRI reduces diagnostic uncertainty and identifies avenues of management such as lumbar steroid injections or decompression surgery in certain situations.

(4) Outcome prediction may be possible. If surgical intervention is planned for a lumbosacral radiculopathy, a positive EMG preoperatively improves the likelihood of a successful outcome postoperatively (see below). This is an area that deserves more research attention.

USEFULNESS OF ELECTRODIAGNOSTIC TESTING

The value of any test depends upon the a priori certainty of the diagnosis in question and this principle applies to electrodiagnostic testing. For a condition or diagnosis for which there is great certainty before additional testing, the results of the subsequent tests are of limited value. This concept, diminishing returns on the road to diagnostic certainty, is important.

For instance, in a patient with acute-onset sciatica while lifting, L5 muscle weakness, a positive straight leg raise, and an MRI showing a large extruded L4–5 nucleus pulposus, an EMG test will be of limited value in confirming the diagnosis of radiculopathy.

In contrast, an elderly diabetic patient with sciatica, limited physical examination findings, and equivocal or age-related MRI changes presents an unclear picture. In this case electrodiagnostic testing is of high value, placing in perspective the imaging findings and excluding diabetic polyneuropathy as a confounding condition.

ELECTROMYOGRAPHY AND DIAGNOSTIC SENSITIVITIES

The need for EMG, particularly in relationship to imaging of the spine, has been recently highlighted.²⁵ Needle EMG is particularly helpful in view of the fact that the false-positive rates for MRI of the lumbar spine are high, with 27% of normals having a disc protrusion.²⁶ For the cervical spine, the false-positive rate for MRI is much lower, with 19% of subjects demonstrating an abnormality, but only 10% showing a herniated or bulging disc.²⁷ Radiculopathies can occur without structural findings on MRI, and likewise without EMG findings. The EMG only evaluates motor axonal loss or motor axon conduction block and for these reasons a radiculopathy effecting the sensory root will not yield abnormalities by EMG. If the rate of denervation is balanced by reinnervation in the muscle, then spontaneous activity is less likely to occur.

The sensitivity of EMG for cervical and lumbosacral radiculopathies has been examined in a number of studies. The results of some of these studies are displayed in Table 8.1, which lists the ‘gold standards’ against which these EMG findings were compared. Studies using a clinical standard may reflect a less severe group, whereas those using a surgical confirmation may indicate a more severely involved group. The sensitivity for EMG is unimpressive, ranging from 49% to 92% in these studies. Electromyography is not a sensitive test, yet likely has higher specificity. The issue of specificity and its value in electrodiagnosis was underscored by Robinson.²⁵ It is apparent that EMG is not a very good screening test. In terms of screening tests, MRI is better for identifying subtle structural abnormalities, with EMG to assess their clinical relevance and to exclude other disorders.

Table 8.1 Selected studies evaluating the sensitivity of EMG relative to various ‘gold standards’

PARASPINAL MUSCLE EXAMINATION

Several reports suggested high rates of false-positive fibrillations in lumbar paraspinal muscles.^28,²⁹ Dumitru, Diaz, and King³⁰ conducted a well-designed study to examine whether or not fibrillations are found in the lumbar paraspinal muscles of normal asymptomatic volunteers. These investigators examined lumbosacral paraspinal muscles and intrinsic foot muscles with monopolar EMG and recorded potentials for analysis. Regular firing rate was required in order for classification as fibrillation potentials. They found many irregularly firing potentials with similar waveform characteristics as fibrillations and positive sharp waves (PSW). By excluding these irregularly firing potentials (atypical endplate spikes) they found much lower false-positive paraspinal fibrillation prevalences than other investigators. Only 4% of these normal subjects had lumbar fibrillations or PSW potentials by EMG testing. The investigators felt that the higher prevalences of spontaneous activity reported by other investigators^28,²⁹ were due to not fully appreciating the similarity between innervated and denervated spontaneous single muscle fiber discharges. This well-designed quantitative study underscores the need to assess both firing rate and rhythm as well as discharge morphology when evaluating for fibrillations and positive waves in the lumbar paraspinal muscles. Electrodiagnosticians should take care not to overcall fibrillations in lumbosacral paraspinal muscles by mistaking irregularly firing endplate spikes for fibrillations.

Paraspinal muscles may be abnormal in patients with spinal cancers ³¹^–³³ or amyotrophic lateral sclerosis,³⁴ and following spinal surgery³⁵ or lumbar puncture.³⁶ In fact, fibrillations can be found years after lumbar laminectomy.³⁵ The absence of paraspinal muscle fibrillations in such patients is helpful, but finding fibrillations in someone after laminectomy is of uncertain relevance as these fibrillations may be residual from the previous muscle damage or relatively new denervation.

Investigations over the last decade have provided insights into better quantification and examination of lumbosacral paraspinal muscles. The lumbar paraspinal muscle examination has been refined through investigations that used a grading scale for the findings.³⁷^–⁴⁰ The ‘mini PM’ score provides a quantitative means of deriving the degree of paraspinal muscle denervation.⁴⁰ It distinguishes normal findings from persons with radiculopathy. This novel and quantitative technique may prove useful to identify subtle radiculopathies or spinal stenosis with greater precision.

Cervical and lumbar paraspinal muscles should only be examined for insertional activity and spontaneous activity while at rest. Recruitment findings and motor unit morphology for these muscles has not been established and consequently we do not know for sure what constitutes normal. Examiners should not overcall radiculopathies based upon ‘reduced recruitment’ or ‘increased polyphasicity’ in the paraspinal muscles. Paraspinal muscles either show spontaneous activity and therefore localize the lesion to the root level or they do not. There is considerable overlap in paraspinal muscles with single roots innervating fibers above and below their anatomic levels. For this reason, the level of radiculopathy cannot be delineated by paraspinal EMG alone, but rather is based upon the root level that best explains the distribution of muscles demonstrating fibrillations.

IDENTIFICATION AS A SEPARATE CONCEPT FROM SENSITIVITY

Electrodiagnostic testing is uncomfortable and expensive. Because electrodiagnosis is a composite assessment composed of various tests, a fundamental question is; when has the point of diminishing returns been reached? Some radiculopathies cannot be confirmed by needle EMG, even though the signs and symptoms along with imaging results suggest that radiculopathy is present. A screening EMG study involves determining whether or not a radiculopathy, if present, can be confirmed by EMG. If the radiculopathy cannot be confirmed, then presumably no number of muscles can identify the radiculopathy. If it can be confirmed, then the screen should identify this possibility with a high probability. The process of identification can be conceptualized as a conditional probability: given that a radiculopathy can be confirmed by needle EMG, what is the minimum number of muscles which must be examined in order to confidently recognize or exclude this possibility? This is a fundamentally different concept from sensitivity. It involves understanding and defining the limitations of a composite test (group of muscles).

HOW MANY AND WHICH MUSCLES TO STUDY

The concept of a screening EMG encompasses identifying the possibility of an electrodiagnostically confirmable radiculopathy. If one of the muscles in the screen is abnormal, the screen must be expanded to exclude other diagnoses, and to fully delineate the radiculopathy level. Because of the screening nature of the EMG exam, electrodiagnosticians with experience should look for more subtle signs of denervation and, if present in the screening muscles, then expand the study to determine if these findings are limited to a single myotome or peripheral nerve distribution. If they are limited to a single muscle, the clinical significance is uncertain.

The cervical radiculopathy screen

Dillingham et al.⁴¹ conducted a prospective multicenter study evaluating patients referred to participating electrodiagnostic laboratories with suspected cervical radiculopathy. A standard set of muscles were examined by needle EMG for all patients. Those with electrodiagnostically confirmed cervical radiculopathies, based upon EMG findings, were selected for analysis. The EMG findings in this prospective study also encompassed other neuropathic findings: (1) positive sharp waves, (2) fibrillation potentials, (3)complex repetitive discharges (CRD), (4) high-amplitude, long-duration motor unit action potentials, (5) increased polyphasic motor unit action potentials, or (6) reduced recruitment. There were 101 patients with electrodiagnostically confirmed cervical radiculopathies representing all cervical root levels. When paraspinal muscles were one of the screening muscles, five-muscle screens identified 90–98% of radiculopathies, six-muscle screens identified 94–99%, and seven-muscle screens identified 96–100% (Tables 8.2 and 8.3). When paraspinal muscles were not part of the screen, eight distal limb muscles recognized 92–95% of radiculopathies. Six-muscle screens, including paraspinal muscles, yielded consistently high identification rates, and studying additional muscles lead to only marginal increases in identification. Individual screens useful to the electromyographer are listed in Tables 8.2 and 8.3. In some instances a particular muscle cannot be studied due to wounds, skin grafts, dressings, or infections. In such cases the electromyographer can use an alternative screen with equally high identification. These findings were consistent with those derived from a large retrospective study.⁴²

Table 8.2 Five-muscle screen identifications of patients with cervical radiculopathies

Muscle screen	Neuropathic	Spontaneous activity
Without paraspinals
Deltoid, APB, FCU	92%	65%
Triceps, PT
Biceps, triceps	85%	54%
EDC, FCR, FDI
Deltoid, triceps	84%	58%
EDC, FDI, FCR
Biceps, triceps	91%	60%
PT, APB, FCU
With paraspinals
Deltoid, triceps, PT	98%	80%
APB, PSM
Biceps, triceps, EDC	95%	73%
FDI, PSM
Deltoid, EDC, FDI	90%	73%
PSM, FCU
Biceps, FCR, APB	95%	77%
PT, PSM

The screen detected the patient with cervical radiculopathy if any muscle in the screen was one of the muscles which were abnormal for that patient.

Neuropathic findings for nonparaspinal muscles included positive waves, fibrillations, increased polyphasic potentials, neuropathic recruitment, increased insertional activity, CRDs, or large amplitude/long duration motor unit action potentials.

For paraspinal muscles the neuropathic category included fibrillations, increased insertional activity, positive waves, or CRDs. Spontaneous activity referred only to fibrillations or positive sharp waves.

APB, abductor pollicis brevis; FCU, flexor carpi ulnaris; FCR, flexor carpi radialis; PSM, cervical paraspinal muscles; FDI, first dorsal interosseous; PT, pronator teres, supra-supraspinatus, infra-infraspinatus; EDC, extensor digitorum communis.

Adapted with permission, Dillingham et al.⁴¹

Table 8.3 Six-muscle screen identifications of the patients with cervical radiculopathies

Muscle Screen	Neuropathic	Spontaneous Activity
Without paraspinals
Deltoid, APB, FCU	93%	66%
Triceps, PT, FCR
Biceps, triceps, FCU	87%	55%
EDC, FCR, FDI
Deltoid, triceps	89%	64%
EDC, FDI, FCR, PT
Biceps, triceps, EDC	94%	64%
PT, APB, FCU
With paraspinals
Deltoid, triceps, PT	99%	83%
APB, EDC, PSM
Biceps, triceps, EDC	96%	75%
FDI, FCU, PSM
Deltoid, EDC, FDI	94%	77%
PSM, FCU, triceps
Biceps, FCR, APB	98%	79%
PT, PSM, triceps

Muscle abbreviations, identification criteria, and definitions are described in Table 8.2.

The lumbosacral radiculopathy screen

A similar prospective multicenter study was conducted at five institutions by Dillingham et al.⁴³ Patients referred to participating electrodiagnostic laboratories with suspected lumbosacral radiculopathy were recruited and a standard set of muscles examined by needle EMG. Patients with electrodiagnostically confirmed lumbosacral radiculopathies, based upon EMG findings, were selected for analysis. As described above for the prospective cervical study, neuropathic findings were analyzed along with spontaneous activity. There were 102 patients with lumbosacral radiculopathies representing all lumbosacral root levels. When paraspinal muscles were one of the screening muscles, four-muscle screens identified 88–97%, five-muscle screens identified 94–98%, and six-muscle screens 98–100% (Tables 8.4–8.6). When paraspinal muscles were not part of the screen, identification rates were lower for all screens and eight distal muscles were necessary to identify 90%. If only four muscles can be tested due to limited patient tolerance, as seen in Table 8.4, and if one of these muscles are the paraspinals, few electrodiagnostically confirmable radiculopathies will be missed. A large retrospective study noted consistent findings, concluding that five muscles identified most electrodiagnostically confirmable radiculopathies.⁴⁴

Table 8.4 Four-muscle screen identifications of patients with lumbosacral radiculopathies

Screen	Neuropathic	Spontaneous Activity
Four muscles without paraspinals
ATIB, PTIB, MGAS, RFEM	85%	75%
VMED, TFL, LGAS, PTIB	75%	58%
VLAT, SHBF, LGAS, ADD	52%	35%
ADD, TFL, MGAS, PTIB	80%	67%
Four muscles with paraspinals
ATIB, PTIB, MGAS, PSM	97%	90%
VMED, LGAS, PTIB, PSM	91%	81%
VLAT, TFL, LGAS, PSM	88%	77%
ADD, MGAS, PTIB, PSM	94%	86%

The screen identified the patient if any muscle in the screen was abnormal for that patient. The muscle either demonstrated neuropathic findings or spontaneous activity.

For paraspinal muscles the neuropathic category included fibrillations, increased insertional activity, positive waves, or CRDs.

PSM, lumbosacral paraspinal muscles; PTIB, posterior tibialis; ATIB, anterior tibialis; MGAS, medial gastrocnemius, LGAS, lateral gastrocnemius, TFL, tensor fascia lata, SHBF, short head biceps femoris; VMED, vastus medialis; VLAT, vastus lateralis; RFEM, rectus femoris; ADD, adductor longus.

Adapted from Dillingham, et al.⁴³, with permission.

Table 8.5 Five-muscle screen identifications of patients with lumbosacral radiculopathies

Screen	Neuropathic	Spontaneous activity
Five muscles without paraspinals
ATIB, PTIB, MGAS, RFEM, SHBF	88%	77%
VMED, TFL, LGAS, PTIB, ADD	76%	59%
VLAT, SHBF, LGAS, ADD, TFL	68%	50%
ADD, TFL, MGAS, PTIB, ATIB	86%	78%
Five muscles with paraspinals
ATIB, PTIB, MGAS, PSM, VMED	98%	91%
VMED, LGAS, PTIB, PSM, SHBF	97%	84%
VLAT, TFL, LGAS, PSM, ATIB	97%	86%
ADD, MGAS, PTIB, PSM, VLAT	94%	86%

Abbreviations and definitions of muscle abnormalities are the same as in Table 8.4.

Table 8.6 Six-muscle screen identifications of patients with lumbosacral radiculopathies

Screen	Neuropathic	Spontaneous Activity
Six muscles without paraspinals
ATIB, PTIB, MGAS, RFEM, SHBF, LGAS	89%	78%
VMED, TFL, LGAS, PTIB, ADD, MGAS	83%	70%
VLAT, SHBF, LGAS, ADD, TFL, PTIB	79%	62%
ADD, TFL, MGAS, PTIB, ATIB, LGAS	88%	79%
Six muscles with paraspinals
ATIB, PTIB, MGAS, PSM, VMED, TFL	99%	93%
VMED, LGAS, PTIB, PSM, SHBF, MGAS	99%	87%
VLAT, TFL, LGAS, PSM, ATIB, SHBF	98%	87%
ADD, MGAS, PTIB, PSM, VLAT, SHBF	99%	89%
VMED, ATIB, PTIB, PSM, SHBF, MGAS	100%	92%
VMED, TFL, LGAS, PSM, ATIB, PTIB	99%	91%
ADD, MGAS, PTIB, PSM, ATIB, SHBF

Abbreviations and definitions of muscle abnormalities are the same as in Table 8.4.

Dillingham and Dasher ⁴⁵ re-analyzed data from a study published by Knutsson almost 40 years earlier.⁴⁶ In this detailed study, 206 patients with sciatica underwent lumbar surgical exploration. All subjects underwent standard EMG by the author (Knutsson) with a standard set of 14 muscles using concentric needles. The examiner was blinded to other test results and physical examination findings. In addition to the EMG and surgical information, myelogram and physical examination data were derived. In this contemporary re-analysis, screens of four muscles with one being the PSM yielded an identification rate of 100%, a 92% sensitivity with respect to the intraoperative anatomical nerve root compressions, and an 89% sensitivity with respect to the clinical inclusion criteria.⁴⁵ This study, using data from four decades ago, confirmed that four-muscle screening examinations provide high identification. These findings are consistent with contemporary work showing that screens with relatively few muscles (six) are optimal.

As described above, these research efforts were undertaken to refine and streamline the EMG examination. The strongest studies, contemporary prospective multicenter investigations, provide the best estimates for a sufficient number of muscles.^41,⁴³ In summary, for both cervical and lumbosacral radiculopathy screens the optimal number of muscles appears to be six muscles, including the paraspinal muscles and muscles that represent all root level innervations. When paraspinal muscles are not reliable, then eight nonparaspinal muscles must be examined. Another way to think of this:

‘To minimize harm, six in the leg and six in the arm’

LUMBAR SPINAL STENOSIS

With the aging population in the United States and the increasing prevalence of lumbar spinal stenosis that occurs in the elderly, this condition takes on greater public health significance. In fact, an entire edition of the Physical Medicine and Rehabilitation Clinics of North America was recently dedicated to this complex topic.⁴⁷ There are few studies involving spinal stenosis and electromyography. For lumbosacral spinal stenosis, Hall and colleagues⁴⁸ showed that 92% of persons with imaging-confirmed stenosis had a positive EMG. They also underscored the fact that 46% of persons with a positive EMG study did not demonstrate paraspinal muscle abnormalities, only distal muscle findings. In 76%, the EMG showed bilateral myotomal involvement.⁴⁸ These results suggest that in such patients, distal limb findings may be the most prominent and electromyographers should not expect fibrillations in lumbosacral paraspinal muscles.

In the United States, diabetes is on the increase, with increasing prevalence and incidence.⁴⁹ Diabetes often confounds accurate diagnosis of radiculopathy and spinal stenosis.^50,⁵¹ Inaccurate recognition of sensory polyneuropathy, diabetic amyotrophy, or mononeuropathy can lead to unnecessary surgical interventions. In a recent prospective study by Adamova and colleagues,⁵⁰ the value of electrodiagnostic testing was assessed. There were three groups; one group composed of 29 persons with imaging confirmed clinically mild lumbar spinal stenosis, 24 subjects with both diabetes and polyneuropathy, and 25 healthy age-matched volunteers served as control subjects. In this well-designed study, sural sensory amplitudes distinguished the diabetic polyneuropathy group (an amplitude of 4.2 microvolts or less was found in 47% of diabetic patients and only 17% of stenosis patients). The ulnar F-wave was prolonged in polyneuropathy patients and not lumbar stenosis patients and the radial SNAP was similarly reduced in the group with polyneuropathy.⁵⁰ These findings underscore the value of performing sensory testing in the involved extremity as well as an upper limb to fully recognize diabetic polyneuropathy when present and differentiate this condition from lumbar spinal stenosis or radiculopathy.

LIMITATIONS OF THE EMG SCREEN

These cervical and lumbosacral muscle screens should not substitute for a clinical evaluation and differential diagnosis formulation by the electrodiagnostic consultant. Rather, information from investigations described above allows the electrodiagnostician to streamline the EMG evaluation and make better-informed clinical decisions regarding the probability of missing an electrodiagnostically confirmable radiculopathy when a given set of muscles are studied. Performing a focused history and physical examination is essential, and these screens should not supplant such clinical assessment or a more detailed electrodiagnostic study when circumstances dictate.

If one of the six muscles studied in the screen is positive, there is the possibility of confirming electrodiagnostically that a radiculopathy is present. In this case, the examiner must study additional muscles to determine the radiculopathy level and to exclude a mononeuropathy. If the findings are found in only a single muscle, they remain inconclusive and of uncertain clinical relevance. If none of the six muscles are abnormal, the examiner can be confident of not missing the opportunity to confirm by EMG that a radiculopathy is present, and can curtail the painful needle examination. The patient may still have a radiculopathy, but other tests such as MRI will be necessary to confirm this clinical suspicion. This logic is illustrated in Figure 8.2.

Fig. 8.2 Implications of a positive EMG screening evaluation.

It is important to remember that the EMG screens for cervical and lumbosacral radiculopathies were validated in a group of patients with limb symptoms suggestive of radiculopathies. These screens will not provide sufficient screening power if a brachial plexopathy is present or if a focal mononeuropathy such as a suprascapular neuropathy is the cause of the patient’s symptoms. The electrodiagnostician should always perform EMG on weak muscles to increase the diagnostic yield. These screens do not sufficiently screen for myopathies or motor neuron disease. It is incumbent upon the electrodiagnostician to formulate a differential diagnosis and methodically evaluate for the likely diagnostic possibilities, further refining the examination as data are acquired.

SYMPTOM DURATION AND THE PROBABILITY OF FIBRILLATIONS

In the past, a well-defined temporal course of events was thought to occur with radiculopathies despite the absence of studies supporting such a relationship. It was a commonly held notion that in acute lumbosacral radiculopathies, the paraspinal muscles denervated first, followed by distal muscles, and that reinnervation started with paraspinal muscles and then the distal muscles. This paradigm was recently addressed with a series of investigations.⁵²^–⁵⁵ For both lumbosacral and cervical radiculopathies, symptom duration had no significant relationship to the probability of finding spontaneous activity in paraspinal or limb muscles.

There is no evidence to support a relationship between the duration of a patient’s symptoms and the probability of finding fibrillations in paraspinal or limb muscles. This simplistic explanation, although widely quoted in the older literature, does not explain the complex pathophysiology of radiculopathies. Electrodiagnosticians should not invoke this relationship to explain the absence or presence of fibrillations in a particular muscle.

IMPLICATIONS OF AN ELECTRODIAGNOSTICALLY CONFIRMED RADICULOPATHY

It is important that the electrodiagnostician not forget that EMG does not indicate the exact cause of the symptoms, only that motor axonal loss is taking place. A spine tumor, herniated disc, bony spinal stenosis, chemical radiculitis, or severe spondylolisthesis can all yield the same EMG findings. This underscores the need to image the spine with MRI to assess for significant structural causes of electrodiagnostically confirmed radiculopathy. A negative EMG test should not curtail obtaining an MRI if clinical suspicion for radiculopathy is high. Given the low sensitivities of needle EMG, it is not an optimal screening test, but rather a confirmatory test.

There are few studies that examine outcomes and the usefulness of electrodiagnosis in predicting treatment success, the exception being surgical outcomes for lumbar discectomy in patients with herniated nucleus pulposus. Tullberg et al.⁵⁶ evaluated 20 patients with lumbosacral radicular syndromes who underwent unilevel surgery for disc herniations. They evaluated these patients before surgery and 1 year later with lower limb EMG, nerve conduction studies, F-waves, and SEPs. They showed that the electrodiagnostic findings did not correlate with the level defined by CT for 15 patients. However, those patients in whom electrodiagnostic testing preoperatively was normal were significantly more likely to have a poor surgical outcome (p<0.01). In spite of the fact that the sample size in this study was small, the significant correlation of a normal electrodiagnostic study with poor surgical outcome suggests that this may be a true relationship.

Spengler and Freeman ⁵⁷ described an objective approach to the assessment of patients preoperatively for laminectomy and discectomy for lumbosacral radiculopathy. Spengler et al.⁵⁸ confirmed and underscored these previous findings regarding objective methods to assess the probability of surgical success preoperatively. In this preoperative screening evaluation, the EMG findings were combined with imaging, clinical, and psychological assessments. The EMG findings figured prominently (one-quarter of the scale): those patients with positive EMGs were more likely to have a better surgical outcome. This was particularly true when the EMG findings correlated with the spinal imaging findings, in a person without psychological or dysfunctional personality issues.