Taking the Mystery Out of an EMG/NCS Report – Part 2

In Part One of this article, I discussed the basics of nerve conduction studies and waveform analysis. In Part Two, I will cover principles of needle electromyography (EMG), specific diseases and injuries that relate to hand therapy, and how EMG/NCS findings relate to outcomes for certain injuries and surgeries of the upper extremities. 

 The needle EMG examination is performed “on the fly” requiring in-depth knowledge of nerve and muscle physiology and how they relate to both normal and abnormal electrical activity in the muscle. While the muscle fibers belonging to a single motor unit can be distributed over 5-10 mm or more, the needle records activated motor units up to only about 1-2 mm from the recording tip. This means that only a small portion of the muscle is actually evaluated during the needle EMG portion of the test. Needle EMG studies can be broken down into two basic components: 1) the muscle at rest and, 2) voluntary muscle contraction. 

Insertional and Spontaneous Activity

Evaluation of the muscle at rest can be divided into two additional categories. The first component is insertional activity. Insertional activity deals with what happens immediately following insertion of the needle into the muscle and minimally advancing it with short, quick bursts. In normal muscle, there is a brief increase in electrical potentials lasting <300 ms before returning to normal baseline silence denoting normal muscle membrane stability. In the case of damaged muscles or damaged nerves innervating the muscle, the normal membrane stability may become unstable and insertional activity will be increased lasting >300-500 ms in duration. In muscles that have become fibrotic or have fatty deposits, such as is often noted in chronic issues, the insertional activity may be decreased (<300 ms or absent). The second evaluation component of muscle at rest is known as spontaneous activity. In normal muscle the electrical activity returns to a silent baseline immediately after the insertional activity. In muscle that has lost innervation, the electrical activity may show membrane instability and is most often noted in the form of positive sharp waves and fibrillation potentials.

Check out this video.

There is some debate as to the actual origin of these waveforms or if they are even separate and distinct entities, but regardless, they are significant of muscle membrane instability found in both myopathic (muscular dystrophies, myotonias, etc.) and neuropathic (carpal tunnel syndrome, polyneuropathies, radiculopathies, etc.) diseases. Additionally, fasciculation potentials are another frequent form of spontaneous activity and although they may be found in normal muscle as well as many different diseases, they are most commonly related to motor neuron diseases such as amyotrophic lateral sclerosis (ALS). Complex repetitive discharges are another common abnormality noted in muscles at rest. These consist of a spontaneously firing group of action potentials creating various loud and bizarre sounds. Watch this video for more! Complex repetitive discharges are most commonly associated with chronic conditions. Several other types of spontaneous activity such as myotonic discharges and myokymic discharges exist and may relate to other specific types of diseases that will not be discussed in this article. 

Voluntary Muscle Contraction 

Voluntary muscle contraction generates a motor unit action potential (MUAP) which is a summated recording of the firing of all muscle fibers in a specific motor unit (nerve fiber and all the muscle fibers it innervates). Each muscle is made up of thousands of motor units. Parameters for evaluation include amplitude, duration, phasicity, recruitment, and interference pattern.  

Amplitude is measured from the peak of the negative deflection to the peak of the positive deflection. Normal amplitude for Type I fibers is between 300 V to 1000 V and for Type II fibers between 1000 V and 5000 V. Duration is measured from the initial deflection from the baseline to the final return to baseline. Normal duration is less than 12 ms.  

Phasicity is a measure of the number of times the waveform crosses the baseline plus one. If the waveform crosses the baseline two times it is said to be triphasic which is the most common form of a MUAP. MUAPs with 5 or greater phases is said to be polyphasic. Polyphasic potentials have two different ways in which they are created and depending on their morphology may represent two completely different aspects of nerve injury. When an injury to nerve fibers occurs, it may cause the corresponding muscle fibers to lose their connection to the nerve (for the purposes of our discussion we will call this process denervation). In response to the denervation, healthy nearby nerve fibers may send out sprouts to re-innervate the denervated muscle fibers in a process called collateral sprouting. The newly innervated fibers “fire” out of sync with the other muscle fibers in the same motor unit which results in increased phases of the MUAP. These polyphasics typically exhibit a normal amplitude with increased waveform duration. As this renovated motor unit matures, the number of phases will decrease; however, the amplitude of the MUAP will increase as a sign of a chronic or old injury. Nerve fibers that have undergone Wallerian degeneration will try to regenerate and find the muscle fibers they previously innervated. As they reinnervate and take over control of those muscle fibers, these immature motor units “fire” out of sync creating polyphasicity and may demonstrate a small amplitude as they go about collecting their rightful motor fibers. 

 Recruitment of motor units during voluntary contraction is dependent upon the amount of force generated. When evaluating motor units for recruitment the initial goal is to instigate the firing of a single motor unit within the recording area of the needle. The firing rate (recruitment frequency) should be less than 12 Hz (fires per second) before an additional motor unit is recruited. In the case of reduced available motor units, the firing rate of the single motor unit will be higher than normal before the additional motor units jump in to help out. This is known as reduced recruitment. Interference pattern (IP) is a measure of the “fullness” of the screen with activated motor units for moderate to full contraction of the muscle. Although IP can be a measure of overall recruitment of muscle fibers, it is not of good diagnostic value since it can be controlled by the effort of the patient. Decreased IP in conjunction with other significant abnormal findings is helpful. If decreased IP is the only abnormal finding then it is not useful in the interpretation of abnormalities. 

Abnormal Needle EMG Patterns 

Abnormal needle EMG findings are usually noted in myotomal or nerve patterns. For example, a herniated cervical disc between C6 and C7 causes foramenal narrowing and compression of the exiting C7 nerve root. Abnormal needle EMG findings may be noted in the triceps (radial nerve, C6-C8 root), anconeus (radial nerve, C7-8 root), flexor carpi radialis (median nerve, C6-C8 root), lower cervical paraspinals (dorsal rami, C7-T1 roots), etc. Note that the findings should be found in muscles innervated by more than one specific nerve if the injury is at the nerve root level. In the case of injury to a specific nerve, abnormal activity may be found in muscles innervated by multiple nerve roots but only in those muscles innervated by a specific nerve. For example, pronator teres syndrome might have abnormal findings in the flexor carpi radialis (median nerve, C6-C8 roots), palmaris longus (median nerve, C7-T1 roots) and abductor pollicis brevis (median nerve, C8-T1 roots). EMG findings in the extensor indices (radial nerve, C7-8 root), and abductor digiti minimi (ulnar nerve, C8-T1) would be normal. Any muscles innervated by the injured nerve proximal to the site of injury would also be normal. 

Putting It All Together 

Now that you understand nerve conduction and EMG findings, you can relate them to specific diseases. Let’s look at a couple case studies to put it all together. 

 Case Study 1: 

Patient is a 44-year-old female with a 2-year history of insidious onset intermittent numbness and tingling in the right 1st-4th digits. She reports the symptoms are worse at night and shaking her hands helps to relieve the symptoms. The patient also reports pain in the wrists and in the medial elbows bilaterally. She denies any neck problems or weakness in the upper extremities. Abnormal physical exam findings reveal positive Tinel’s test bilaterally at the wrists and elbows. Otherwise, strength, reflexes, and sensation are WNL.  

Nerve conduction studies show prolonged right median antidromic sensory distal latencies recorded at digits 2(D2) (4.8 ms) and 3(D3) (5.1 ms) with preferential slowing across the wrist to palm segments (2.9 ms D2, 3.2 ms D3) as compared with the palm to digit segments (1.9 ms D2, 1.9 ms D3). SNAP amplitudes are decreased at the wrist especially as compared to the palm. The right median motor distal latency is prolonged (4.8 ms) with decreased CMAP amplitudes (3.8 mV). Nerve conduction velocities are WNL across the forearm. Ulnar nerve studies are WNL. What is the impression based on these findings?  

These findings suggest a compromise of the right median nerve across the carpal tunnel segment caused by injury to the myelin as suggested by the preferential slowing of the nerve conduction across the wrist and loss of axons as suggested by the decrease in amplitudes.  

In two separate studies published by Jeremy Bland, FRCP 1, 2, he developed a severity grading scale based on EMG/NCS findings for carpal tunnel syndrome and linked the severity to probable outcomes post-surgery. Bland found that patients rated mild, moderate, and severe had very good outcomes and patient satisfaction while those graded normal, very mild, very severe, and extremely severe had poor outcomes.

Case Study 2: 

 The patient is a 37-year-old male with progressive neck pain radiating from the left upper extremity to the mid-forearm. He reports a decreasing ability to perform push-ups with the left upper extremity. The patient reports numbness and tingling in the left 1st– 3rd digits. He also reports decreased CROM with extension and left rotation. Abnormal physical exam findings include MMT 4-/5 in the left deltoid and biceps and absent biceps reflex on the left. The sensation is decreased to light touch and pinprick over the left C6 dermatome. 

Nerve conduction studies are within normal limits for bilateral median, ulnar, and superficial radial nerves. The left deltoid, biceps, infraspinatus, and cervical paraspinal muscles showed positive sharp waves and fibrillation potentials with decreased recruitment and interference pattern in the left biceps. What is the impression based on these findings? 

Increased muscle membrane instability was noted in the left C5-6 myotomal pattern as well as in the cervical paraspinal muscles innervated by the dorsal rami. These findings are consistent with a left C5-6 radiculopathy. 

Conclusions

EMG/NCS is an important tool performed by both physical therapists and physicians and is useful in the diagnosis of many neuromuscular diseases encountered by hand therapists (brachial plexopathies, cubital tunnel, radial neuropathies, carpal tunnel syndrome, radiculopathies, etc.). These diseases are often difficult to accurately diagnose based on clinical signs and symptoms alone. Appropriate use of EMG/NCS can help to focus on appropriate treatments, predict outcomes, and avoid unnecessary surgeries. Understanding the EMG/NCS report will help the therapist to apply appropriate treatment and set suitable goals, all in an effort to provide the best care available and improve the lives of our patients. 

 References 

1. Bland JDP. A neurophysiological grading scale for carpal tunnel syndrome. Muscle Nerve 2000;23:1280-1283.
2. Bland JDP. Do nerve conduction studies predict the outcome of carpal tunnel decompression? Muscle Nerve 2001;24:935-940. 

Additional Reading 

1. Liu FJ, et al. Prognostic value of magnetic resonance imaging combined with electromyography in the surgical management of cervical spondylotic myelopathy. Exper and Ther Med 2013;5:1214-1218.
2. Pawar S, et al. The study of diagnostic efficacy of nerve conduction study parameters in cervical radiculopathy. J Clin Diag Research 2013;7(12):2680-2682.
3. Bsteh G, et al. Prognosis and prognostic factors in non-traumatic acute-onset compressive mononeuropathies – radial and peroneal mononeuropathies. Eur J Neurol 2013;20:981-985. 
4. Nicotra A, et al. Cervical radiculopathy: discrepancy or concordance between electromyography and magnetic resonance imaging? Br J Neurosurg 2011;25(6):789-790. 
5. Coster S, et al. Diagnostic value of history, physical examination and needle electromyography in diagnosing lumbosacral radiculopathy. J Neurol 2010;257:332-337. 
6. Derr J, et al. Predicting recovery after fibular nerve injury: which electrodiagnostic features are most useful? Am J Phys Med Rehabil 2009;88:547-553. 
7. Alrawi MF, et al. The value of neurophysiological and imaging studies in predicting outcome in the surgical treatment of cervical radiculopathy. Eur Spine J 2007;16:495-500. 
8. Malikowski T et al. Prognostic values of electrodiagnostic studies in traumatic radial neuropathy. Muscle Nerve 2007;36:364-367. 
9. Nardin RA, et al. Electromyography and magnetic resonance imaging in the evaluation of radiculopathy. Muscle Nerve 1999;22:151-155.