Making sense of EMG studies

"What You Should Know"



 

Nervous System
 


 


Making Sense of EMG Studies

Many technicaI factors can affect the test

Juan L. Joy, M.D.

 
 

Introduction

Clinical electromyography (EMG) consists of nerve conduction studies (NCS) and needle electromyography (needle EMG). In the strict sense of the word, EMG only refers to the needle electrode examination of muscles, however is has traditionally been used to refer to both NCS and needle EMG. EMC is a very powerful diagnostic modality for the evaluating the peripheral nervous system which, in competent hands, can provide invaluable information that may not be obtainable with any other diagnostic test. It can also complement the information obtained from other sources, such as imaging studies, for optimal localization of a lesion.

More so than most electrodiagnostic tests, EMG is extremely dependent on the skill of the examiner. There are many technical factors which can affect the test and produce erroneous results if not recognized5,6,8. It is also important to note that EMC is not a standard test like electrocardiography. There are dozens of muscles and nerves that can be tested, and it is the electromyographer's job to tailor the examination to the patient's particular clinical problem. What is tested is as important as how competently it is done.

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How does it work?

Peripheral nerves transmit electrical impulses and can be simplistically thought of as extremely efficient conductive wires. For NCS studies, a noninvasive stimulator applies brief electrical impulses to a peripheral nerve transcutaneously, the nerve then transmits the impulse and a response is recorded by electrodes at some distance away. The time it takes for the stimulus to reach the recording electrodes (latency) can be accurately measured and a velocity of transmission calculated. Both motor and sensory nerves can be examined. Healthy nerves will transmit the electrical impulse faster than diseased ones.

Needle EMG does not introduce any electrical stimulation, instead it records the intrinsic electrical activity of skeletal muscle fibers. The needle is quite slim (about a 25 gauge) and produces minor discomfort which most patients can tolerate. Needle EMG findings suggestive of denervation include:

  1. fibrillations,
  2. positive sharp waves, and
  3. giant motor unit potentials (MUP).

Abnormalities pointing to a myopathy include small short-duration MUP. Since the root and peripheral nerve supply to the muscles is known, needle EMG can be used to localize peripheral nerve or root lesions by noting which muscles show denervation changes.

For most conditions, NCS and needle EMG complement each other and are performed at the same session.

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Common conditions referred for EMG - Table 1

Common Conditions Referred For EMG
CONDITION EMG YIELD EMG FEATURES COMMENT
CTS
91-98%
+ median n. TL,
+ median SCV,
nl ulnar NCS
Most common entrapment neuropathy
Ulnar Neuropathy (elbow)
73-91%
slow MCV across elbow, CB Wrist entrapment is less frequent
Polyneuropathy
>80%
diffusely slow Diabetes most common etiology in USA
Radiculopathy
80-100%
denervation in paraspinals and myotome EMC not positive before 7-20 days
Myopathy
varies*
small-short MUP Polymyositis is most common
*The yield of EMC varies with the etiology and severity of the myopathy3.
ABBREVIATIONS: n. = nerve, TL = terminal latency, NCS = nerve conduction studies, SCV = sensory conduction velocity, MCV = motor conduction velocity CB = conduction block, MUP = motor unit potentials

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Carpal Tunnel Syndrome

CTS is the single most common referring diagnosis in our EMG laboratory and by far the most common entrapment neuropathy2,9. Electrophysiological features include:

  1. prolongation of the median nerve motor latency upon stimulation at the wrist ("terminal" or "distal" latency),
  2. normal median motor conduction velocity in the forearm,
  3. slow median sensory conduction velocity across the wrist,
  4. normal motor and sensory NCS of the ulnar nerve (not involved in CTS). NCS are positive in 91-98% of patients with clinical CTS8.

Although false negatives do occur, they are so infrequent that the diagnosis should be reconsidered. An asymptomatic contralateral CTS can be seen in 32% of patient 8.

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Ulnar Neuropathy

The second most frequent entrapment neuropathy is ulnar neuropathy at the elbow2,9. Entrapment at the wrist is much less frequent. An useful clinical tip in making a distinction is that when the lesion is at the wrist, only the sensation in the palm is affected, whereas with a lesion at the elbow both the palm and the dorsum of the hand are numb. This is so because the ulnar nerve branch which supplies sensation for the dorsum of the hand (dorsal ulnar cutaneous nerve) leaves the main trunk of the ulnar nerve at the forearm so it would not be involved in lesions at the wrist.

The cardinal EMG findings in elbow involvement are:

  1. slowing of ulnar motor conduction velocity across the elbow,
  2. conduction block* across the elbow segment and,
  3. needle EMG evidence of denervation in ulnar-innevated muscles.

Electrodiagnostic studies are positive in 73-91% of cases8.

*Conduction block is defined as a 50% or greater decrease in amplitude of the motor potential upon proximal stimulation and is a classical sign of focal demyelination8.

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Peripheral Neuropathy

In this group of conditions the peripheral nerves are affected in a diffuse fashion and more or less symmetrically. This is distinguished from entrapment neuropathies where only one nerve is affected. In the USA the most common etiologies are diabetes mellitus (by far the most frequent), alcohol abuse, and uremia. In third world countries, leprosy is one of the leading causes8.

EMG features include:

  1. diffuse slowing of nerve conduction velocity in sensory and/or motor nerves, usually worse in the lower limbs,
  2. sensory nerves are usually affected earlier than motor ones,
  3. needle EMG evidence of denervation in distal muscles is not always present and is not essential for the diagnosis. NCS are diagnostic in 80 % of cases8.

Apart from establishing the diagnosis, NCS can also be helpful in suggesting the etiology of the neuropathy. Peripheral neuropathies are classified as "axonal" or "demyelinating", depending on which part of the nerve is most affected by the disease process. The great majority of polyneuropathies are of the axonal type. Only a handful are demyelinating, so finding NCS features of demyelination really narrows down the differential diagnosis6,8. Examples of demyelinating peripheral neuropathies include: Guillain-Barre syndrome, chronic demyelinating neuropathy, and polyneuropathy associated with gammopathy.

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Cervical and Lumber Radiculopathy

This the second common referral diagnosis (after CTS). Needle EMG is essential to establish the diagnosis. NCS are usually normal in radiculopathies, but are required to exclude concurrent peripheral neuropathy or mononeuropathy which may affect the needle EMG examination.

    EMG features include:
  1. needle EMG evidence of denervation involving paraspinal muscles* and,
  2. denervation involving limb muscles of the same root innervation (myotome). The yield is at least 80%8 and in some series as high as 100%4.

*Paraspinal muscles receive innervation directly from the nerve root, so evidence of denervation in these muscles localizes the lesion to the nerve root6.

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Myopathy

In adults, polymyositis is the most common cause of acquired myopathy3. Needle EMG is essential for diagnosis. NCS are performed for the same reasons as in radiculopathies.

Small-short polyphasic motor unit potentials are the EMG hallmark6.

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Suggestions on how to make more effective referrals

As mentioned above, EMG examination can be performed in many nerves and muscles and must be tailored to the particular clinical situation. Be as specific as you can in your referral diagnosis. In cases of radiculopathy, if a specific root level is suspected, mention it. For entrapment neuropathies, include the nerve and the suspected site of involvement (wrist, elbow, etc.). A short pertinent clinical history is also very helpful. Important conditions to mention are diabetes mellitus, alcoholism, uremia, cancer, and collagen vascular diseases. In general, any severe systemic disease is worth mentioning. It is best to request both NCS and needle EMG since they usually complement each other.

The time of onset of symptoms is very important since needle EMC evidence of denervation takes at least 7-10 days to develop following nerve injury 7. Thus, a negative needle EMG examination done within 10 days of the onset of symptoms should be repeated later. A two weeks interval is recommended to be certain that enough time has passed for needle EMG findings to be detectable.

The following is a example of a short but powerful referring history: 50 year-old previously healthy female to rule out a right C6 radiculopathy. Symptoms started 2 months ago.

Both NCS and needle EMG are only mildly uncomfortable to most patients. No special preparation is required for the test, except to avoid applying oils or lotions on the skin as it interferes with electrode adherence.

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Clinical Correlation

As with any other ancillary medical test, clinical correlation is crucial. Ask yourself, does the EMG diagnosis makes sense with the clinical picture? An inadequately performed or interpreted EMG can become a nightmare and lead to unnecessary procedures and even surgery1. If the EMG diagnosis is confusing, discuss it with the electromyographer or request a formal neurological or neurosurgical consultation. As with everything else in medicine, common sense is the key.

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References

  1. Brown WF, Dellon AL, Campbell WW: Electrodiagnosis in the management of focal neuropathies: The "WOC" syndrome. Muscle Nerve 17:1336-1342, 1994.
  2. Entrapment Neuropathies. Dawson DM, Hallet M, Millender LH, eds. Little Brown and Co., Boston/Toronto, 1983.
  3. Myology. Engel AG, Banker BQ, eds. McCraw-Hill, New York, 1986.
  4. Joy JL, Lopez-Mendez A, Moreland LW, Alarcon CS, Oh SJ: Electrophysiologic findings in patients with radiographically proven spinal stenosis. Muscle Nerve 12:766-767, 1989.
  5. Kimura J: Principles and pitfalls of nerve conduction studies. Ann Neurol 16:415-429,1984.
  6. Electrodiagnosis in Diseases of Nerve and Muscle. Kimura J, ed. F.A. Davis Co., Philadelphia, 1989.
  7. Oh SJ. Eiectromyography in peripheral nerve injuries. Med Times 108:94-100, 1980.
  8. Clinical Electromyography: Nerve Conduction Studies. Oh SJ, ed. Williams & Wilkins, Baltimore, 1993.
  9. Focal Peripheral Neuropathies. Stewart JD, ed. Elsevier, New York, 1987.

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