EMG & Common Surgical Conditions of the Upper Limbs

EMG is a common diagnostic procedure for evaluation of suspected surgical nerve disorders affecting the upper limbs. The most common of these conditions are carpal tunnel syndrome (CTS), cervical radiculopathy, and ulnar neuropathy at the elbow (UNE). This article briefly reviews how EMG findings help guide surgical decision making.

CTS

CTS is the most common focal neuropathy worldwide and carpal tunnel release among the most performed surgeries.¹ CTS symptoms are the most common reason for referral for EMG. The chief role of EMG in CTS is diagnostic. In addition to confirming or ruling out a median neuropathy at the wrist, EMG assesses the electrophysiologic severity of the neuropathy and evaluates for mimics and coexisting disorders (eg, polyneuropathy, cervical radiculopathy, or ulnar neuropathy).

A prospective study of 255 people with upper extremity nerve complaints evaluated the impact of EMG on the final diagnosis.² Standardized histories, clinical examinations, and EMGs were performed in all cases by a single physician. EMG substantially altered the clinical impression in 42%, confirmed a diagnosis in 37%, and did not clarify findings in 21%.

The sensitivity and specificity of specific nerve conductions studies (NCS) in CTS has also been studied.³ The median sensory antidromic study, which is the most used median sensory NCS, has a sensitivity of 0.65 and specificity of 0.98 for CTS. Therefore, more sensitive NCS must be used, or we will miss one-third of the cases of CTS in the EMG lab. The more sensitive NCS compare the latencies of the median nerve across the carpal tunnel vs either the ulnar or the radial nerve outside the carpal tunnel over the same distance (Table).

The combined sensory index (CSI) is the sum of the latency differences of the:

1. orthodromic median and ulnar palmar mixed nerve action potential comparison studies at 8 cm,

2. antidromic median and ulnar sensory nerve action potential (SNAP) comparison studies to the fourth digit at 14 cm, and

3. antidromic median and radial SNAP comparison studies to the first digit at 10 cm.

CSI of 1.0 msec is a positive finding, and a retrospective study of 272 cases showed a correlation of CSI and surgical outcome in which 71% of persons with CSI 2.5-4.6 msec had complete resolution of pain and paresthesia with carpal tunnel release.⁴

Moderately severe positive NCS findings are among the variables that predict good outcome of carpal tunnel release surgery in a multivariate predictive model developed by evaluation of 3,000 cases and then tested prospectively in 885 individuals.¹ Correlation with areas under a receiver operating characteristic curve of 0.7 was found. Other variables in this model that predicted good surgical outcome were night waking, a family history of CTS, a good response to corticosteroid injection, and female gender. Factors associated with poor outcomes were greater functional impairment, diabetes, hypertension, and surgery on the dominant hand. The authors acknowledge other studies have found significant discrepancies in the literature about how the severity of positive NCS findings correlate with good surgical outcomes. Some studies have found no correlation between NCS severity and surgical outcomes, some have shown a direct relation between NCS severity and outcomes, whereas others have found an inverse relationship between NCS severity and surgical outcomes.

The question of whether CTS may be too severe to benefit from surgery has been raised. A small prospective study assessed 10 people with extreme CTS (ie, no recordable standard median motor and sensory NCS) before and after carpal tunnel release by standardized questionnaire and NCS.⁵ Preoperatively, all participants reported severe nocturnal pain. Postoperatively, all reported improvement in paresthesias and hypesthesia. At 6 months following surgery, 9 reported freedom from pain. Hand dexterity improved in most, but thenar atrophy did not improve by 6 months. NCS 6 months after the surgery showed that compound muscle action potentials (CMAPs) reappeared in 8 participants and SNAPs reappeared in 5. The NCS parameters that reappeared were all low amplitude with prolonged latencies.

A limitation of this study is that it only included people with idiopathic CTS and excluded those with comorbid conditions (eg, diabetes, polyneuropathy, and endocrinopathies). Another small retrospective study assessed outcomes in 12 people who had carpal tunnel release for “end-stage” disease (ie, preoperative NCS showed no recordable standard median CMAP or SNAP).⁶ None had pain greater than 1 out of 10 at follow up. The study was not prospective, however, and did not report preoperative pain. Satisfaction was reported in 83%, who said they would choose carpal tunnel release again. The 2 participants who were unsatisfied complained of sensory deficits that improved but did not resolve after surgery and then worsened later. Based on these findings, it is reasonable to refer carefully selected patients with very severe or extreme CTS on NCS criteria for surgical evaluation if they have severe nocturnal pain. These individuals should be counseled, however, that sensorimotor deficits may persist after surgery and the most likely symptoms to improve are pain and nocturnal awakening.

EMG is not needed for diagnosis of all cases of CTS. Some cases can be diagnosed confidently by history and clinical examination alone. There are also people with CTS who have negative EMG findings. After all, the sensitivity of NCS for CTS is 0.85, not 1. A prospective study of 94 people with a clinical diagnosis of CTS, half with NCS-verified median neuropathy at the wrist and half without, before and after carpal tunnel release evaluated whether the presence of positive NCS findings correlated with surgical outcome.⁷ Both groups had similar improvement in symptoms and outcome measured by the Quick Disabilities of the Arm Shoulder and Hand (QuickDASH) scale. Those with positive preoperative NCS findings, however, reported significantly higher satisfaction with surgery as measured by a battery of multiple-choice questions.

NCS and the presence of a worker’s compensation claim are associated with outcomes after carpal tunnel release. A retrospective study of 93 workers randomly selected from a postoperative database found a positive association between the severity of NCS findings and good surgical outcomes.⁸ Notably, participants had relatively mild NCS findings. Another retrospective study of 131 cases found positive NCS findings and preoperative worker’s compensation claim were associated with outcomes after reoperation for CTS.⁹ This study included participants with idiopathic CTS and CTS associated with other medical comorbidities. A 50% or more improvement in symptoms with reoperation was reported by 65% of participants and 11% had subsequent operations. Positive preoperative EMG findings were present in 82%. The mean duration between first and second surgery was 2 to 3 years (range 1.5-18 years). Incomplete section of the flexor retinaculum was present in 40% at reoperation. Those who had positive preoperative NCS findings had better final symptom severity and functional outcome scores than those with negative NCS findings. Participants who had positive NCS findings and no worker’s compensation claims had the best outcomes, whereas those with negative NCS findings and a worker’s compensation claim had the worst outcomes. Participants with sensory symptoms in the ulnar nerve distribution also had worse outcomes.

In summary, EMG provides valuable information for management of suspected CTS. NCS can confirm the diagnosis with a high specificity (0.95) and reasonably high sensitivity (0.85). Moderate severity NCS findings and the CSI are electrodiagnostic features that can help select patients for carpal tunnel release but will not capture everyone who could benefit. The presence and severity of NCS findings help predict surgical outcomes but must be considered in the context of other factors that also affect outcomes (eg, severe nocturnal pain, worker’s compensation claim, greater functional impairment, diabetes, hypertension, and surgery on the dominant hand). The other major benefit of NCS in the evaluation of CTS is to identify coexisting pathologies (eg, polyneuropathy or ulnar neuropathy). This additional diagnostic information is important for patient counseling because symptoms due to the polyneuropathy or ulnar neuropathy will not improve with surgery for CTS. The importance of needle EMG in the evaluation of CTS is to assess for conditions other than CTS (eg, cervical radiculopathy, brachial plexopathy or motor neuron disease) that may be causing or contributing to symptoms.

Cervical Radiculopathy

Cervical radiculopathy is a common indication for EMG, although it is not always needed if the diagnosis is clear by history, clinical examination, and imaging. When diagnosis is not entirely clear, EMG can assist management decisions by providing electrophysiologic evidence supporting diagnosis and localizing the level of a radiculopathy. EMG also identifies conditions that can have overlapping signs and symptoms with cervical radiculopathy (eg, brachial plexopathy, CTS, or cubital tunnel syndrome). EMG findings can also help predict progression to overt myelopathy in individuals who have imaging evidence of spondylitic cervical cord compression and without current clinical evidence of myelopathy.

A retrospective study of 50 cases with surgically proven single-level cervical radiculopathy between C5 and C8 compared radiculopathy level at surgery with the pattern of positive findings on preoperative EMG.¹⁰ People with polyradiculopathy, myelopathy, and previous surgery were excluded from this study. The distribution of needle EMG fibrillation potentials was used as the primary electrodiagnostic factor correlating electrophysiologic localization with surgical findings. Muscles showing chronic denervation without fibrillation potentials on EMG were not included in the analysis. In 56% there was a C7 radiculopathy and positive needle EMG findings at the triceps brachii, flexor carpi radialis, anconeus, and/or pronator teres muscles. The triceps had the highest sensitivity for C7 radiculopathy at 100%, and the pronator teres had the lowest sensitivity at 60%. Among the 14% with C5 radiculopathy, there were positive needle EMG findings at the supinator, infraspinatus, deltoid, brachioradialis and/or biceps brachii muscles. In 18% with C6 radiculopathy there were EMG patterns typical of C7 or C5 radiculopathy with or without involvement of the pronator teres and/or triceps. In 12% with C8 radiculopathy, there were positive EMG findings in the extensor indicis proprius, first dorsal interosseous, and abductor digiti minimi muscles. Two-thirds with C8 radiculopathy had positive EMG findings in the flexor pollicis longus and one-half had positive EMG findings in the abductor pollicis brevis. Needle EMG of the cervical paraspinals was positive in only 47% of cases overall, and the sensitivity was even lower at C7 at 31%.

EMG may help guide surgical decision making in cervical spondylitic cord compression without overt symptoms or signs of myelopathy. Analysis of clinical, demographic, radiologic, and neurophysiologic data in 66 consecutive cases predicted progression to symptomatic cervical myelopathy.¹¹ In all cases, there were symptoms of cervical radiculopathy or moderate-to-severe axial neck pain with admission to neuro-logy and cervical spine MRI findings showing structural spinal cord compression with or without T2 or T1 signal changes within the cord. All individuals had pain that could be controlled conservatively, and none had unequivocal signs of myelopathy. Participants had a detailed clinical examination every 6 months for 2 to 4 years. Progression to myelopathy occurred in 20, and features found to predict progression with statistical significance were clinical signs of cervical radiculo-pathy, EMG findings of an anterior horn cell lesion, and abnormal somatosensory evoked potentials (SSEPs). EMG findings were differentiated as “radicular” or “anterior horn cell lesion” based on whether the findings affected 1 or more than 1 myotome, respectively. A multivariate logistic model correctly classified 90% of cases as progression to cervical spondylitic myelopathy or no progression with a 5% annual rate of progression. Male gender (P=.038), clinical signs of radiculopathy at entry (P<.001), abnormal SSEP (P=.016), and EMG denervation affecting more than 1 myotome (P<.001) predicted progression to clinical myelopathy. Notably, male gender was correlated with radiculopathy (P<0.001) and thus excluded from the multivariate analysis. The final prediction model included symptomatic cervical radiculopathy (odds ratio [OR]=40), positive EMG (OR=13), and abnormal SSEP (OR=6). Although the model correctly predicted progression in 90% of cases, this was retrospective analysis of cases used to generate the model rather than prospective verification of new cases.

In summary, the pattern of muscles with needle EMG fibrillation potentials in single-level cervical radiculopathy correlates with the surgically identified level of nerve root compression. The combination of muscles showing fibrillation potentials in C5, C7, and C8 radiculopathies represented distinct groups. In C6 radiculopathy there were identical EMG fibrillation patterns as in C5 or C7 radiculopathy. Symptomatic radiculopathy and denervation on needle EMG are strong predictors of progression to overt clinical myelopathy with imaging showing evidence of cervical spondylitic cord compression and without clinical evidence of an overt myelopathy.

NCS are also helpful in the diagnosis of cervical radiculopathy. In pure radiculopathy, SNAP findings should be within the normal range if the lesion is proximal to the dorsal root ganglion, which is typically located within the neural foramen. NCS can identify coexisting or alternative diagnoses. For example, a person with thumb numbness may have a C6 radiculopathy, median neuropathy, or both, and someone with numbness of the little finger may have a C8 radiculopathy, ulnar neuropathy, or both. Confirming cervical radiculopathy diagnosis or providing an alternative diagnosis that explains the symptoms both have the potential to change management. Thus, both NCS and needle EMG can provide significant complementary data in diagnosis of suspected cervical radiculopathy

UNE

UNE is the second most common entrapment neuro-pathy and a common indication for EMG referral. The differential diagnosis of UNE is broad and includes C8/T1 radiculopathy, cervical myelopathy, or lower trunk brachial plexopathy. Both NCS and needle EMG aid in confirming diagnosis of clinically suspected UNE or providing evidence of coexisting or alternative pathology.

Ulnar motor NCS can localize the lesion by identifying focal demyelination at the elbow (eg, partial conduction block or slowing of conduction velocity across the elbow but not within the forearm). Ulnar motor inching studies can also be used to increase sensitivity and provide additional localizing information. Ulnar NCS can show reduced amplitudes from axonal loss, which, when mild, sometimes requires contralateral studies to identify. Reduced amplitude ulnar SNAP identification is helpful to confirm the postganglionic location of the lesion in an ulnar neuropathy. A low amplitude ulnar SNAP should not be seen in an isolated C8 radiculopathy, which is preganglionic and should therefore have a normal ulnar SNAP. If motor NCS do not localize the ulnar neuropathy to the elbow, needle EMG can identify the most proximal motor branch of the ulnar nerve showing denervation. Localization by needle EMG, however, has the limitation that it can only narrow the location of the lesion to at or proximal to the branch to the most proximal ulnar innervated muscle showing denervation.

When performing ulnar motor NCS, the position of the upper extremity affects results because the ulnar nerve has significant slack at the elbow to accommodate movement.¹² This means that when the elbow is extended, the ulnar nerve has more redundancy and travels a longer course. Because the measured distance of the ulnar nerve with the elbow extended is shorter than the actual distance of the nerve, conduction velocity measurements will be artifactually slow. When the elbow is flexed at 90 to 135 degrees, the cut-off value for slowing across the elbow relative to within the forearm is 10 to 11 m/sec or less. With the elbow extended, the cut-off for focal slowing increases to 25 to 30 m/sec. Therefore, ulnar motor NCS across the elbow must be tested with the elbow flexed between 90 and 135 degrees or there will be frequent false positives that could lead to inappropriate surgeries.

Conduction block is another feature of UNE, although there is not complete agreement on the definition of ulnar nerve partial conduction block at the elbow. A drop in CMAP amplitude across the elbow greater than 10% or 20%, depending on the source, has been proposed as an electrodiagnostic cutoff.

Martin-Gruber anastomosis, a benign variant not thought to cause symptoms, rarely mimics ulnar conduction block across the elbow. A prospective study of 73 people who had ulnar nerve transposition for a clinical diagnosis of cubital tunnel syndrome showed an inverse relation between good outcome and the drop in ulnar CMAP amplitude between the above- and below-elbow stimulation sites.¹³ Lower NCS findings, more severe symptom and severity scores, and female gender were associated with good outcomes in a multivariant analysis.

Another retrospective study of 193 people with UNE found that the conduction block of the ulnar first dorsal interosseous CMAP across the elbow combined with a normal distal ulnar abductor digiti minimi CMAP amplitude was highly predictive of complete resolution of symptoms after surgery.¹⁴ Complete resolution of symptoms occurred in 86% of patients with this combination. In contrast, only 7% of those with a low amplitude CMAP and no conduction block had complete resolution of their symptoms. The difference in findings between the 2 studies may be explained by more strict inclusion criteria in the latter study, which only included people with EMG findings of definite UNE, less than 2 years of symptoms, and no alternative or confounding diagnoses. In contrast, the first study included individuals with EMG who had no findings of ulnar neuropathy and ulnar neuropathy that did not meet electro-diagnostic criterion for localization to the elbow.

In a small study of 19 people with NCS demonstrating UNE, all had improvement in almost all the ulnar NCS values after surgery.¹⁵ Preserved ulnar sensory amplitudes were predictive of symptom resolution after surgery.

NCS and needle EMG can both aid in the management of ulnar neuropathy at the elbow by confirming the diagnosis, excluding mimics, and helping to predict surgical outcomes.

Conclusion

EMG can be helpful in the diagnosis and management of common neurogenic conditions affecting the upper extremity. Electrophysiologic findings can assist surgical decision making by confirming the diagnosis, excluding mimics, and helping to predict outcome. EMG findings, however, are not the only determinant in management and must be considered in the context of the presenting symptoms, clinical examination, and demographic factors when deciding whether to refer for surgical consultation in order to achieve the best outcomes.