The Scope of the EMG Examination

Published on 03/03/2015 by admin

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Chapter 1 The Scope of the EMG Examination

Electromyography (EMG) is a term that was first coined by Weddell et al in 1943 to describe the clinical application of needle electrode examination of skeletal muscles. Since then, and at least in North America, the nomenclature “EMG” or “clinical EMG” has been used by physicians to refer to the electrophysiologic examination of peripheral nerve and muscle that include the nerve conduction studies (NCS) as well as the needle evaluation of muscles. These terms continue to cause confusion that hinders communication among physicians and healthcare workers. Some physicians refer to the study as EMG/NCS, reserving the name EMG solely to the needle EMG evaluation and adding the term NCS to reflect the nerve conduction studies separately. Others have used the title needle EMG or needle electrode examination to reflect the needle evaluation of muscles, while keeping the term EMG to describe the entire evaluation of nerve and muscle. More recently, a nonspecific term, the “electrodiagnostic (EDX) examination,” has gained popularity to serve as an umbrella covering both the needle EMG and NCS. Other nomenclature used worldwide includes the electrophysiologic examination, which may be confused with the cardiac electrophysiological studies, and the electroneuromyographic (ENMG) examination which is the most accurate description of the study, yet unfortunately not widely used. Finally, physicians performing and interpreting these studies are called electromyographers (EMGers), electrodiagnosticians, or EDX consultants.

Regardless, the designations, EDX, EMG, clinical EMG, or ENMG examinations are best used interchangeably to reflect the entire electrophysiological study of nerve and muscle (NCS and needle EMG), while the terms “needle EMG” or needle electrode examination should be reserved for the specific testing which involves needle electrode evaluation of muscle. This author uses the terms EMG examination and EDX examination interchangeably, and refers to the needle examination of muscle as needle EMG.

The EDX examination comprises a group of tests that are usually complementary to each other and often nec-essary to diagnose or exclude a neuromuscular problem (Table 1-1). These include principally the nerve conduction studies (NCS), that are sensory, motor, or mixed, and the needle EMG, sometimes referred as “conventional” or “routine” needle EMG to distinguish this test from other needle EMG studies including single fiber EMG and quantitative EMG. Also, “concentric” or “monopolar” needle EMG is sometimes utilized to reflect the type of needle electrode used. In addition to the two main components of the EMG examination, three late responses are often incorporated with the NCSs and have become an integral part of the NCSs. These include the F waves also referred to as F responses, the H reflexes also known as H responses, and the blink reflexes. Two specialized tests are often added to the routine EDX study mainly in patients with suspected neuromuscular junction disorders. These include the repetitive nerve stimulations and the single fiber EMG. Finally, a group of specialized studies that require special expertise as well as sophisticated equipment and software, used as a clinical and research tool in the assessment of the microenvironment of the motor unit, include motor unit action potential (MUAP) morphology analysis, MUAP turns and amplitudes analysis, macro EMG, motor unit number estimate (MUNE), and near-nerve recording studies.

Table 1-1 The Spectrum of Clinical Electromyography (Electrodiagnosis)

THE REFERRAL PROCESS TO THE EMG LABORATORY

Patients are referred to the EMG laboratory for EDX studies following a clinical assessment by a physician who suspects a disorder of the peripheral nervous system. For example, a patient with intermittent hand paresthesias and positive Phalen’s signs may be referred to the EMG laboratory to evaluate a possible carpal tunnel syndrome. The background and specialty of the referring physician plays a significant role in the planning and execution of the EDX study. In the experience of this author, this usually follows one of these three scenarios:

1. The referring physician is also the EDX consultant (electromyographer). In other words, the patient is examined first by the EDX consultant (usually a neurologist or physiatrist) who performs and interprets the EDX study. The advantage of this situation is that the neurological examination is often thorough and the differential diagnosis is limited. Hence, the selection of NCSs and the choice of sampled muscles on needle EMG are well guided by the neurological findings. Though this situation is ideal, it is not common or practical in a busy EMG laboratory. Also with this approach, the electromyographer may encounter one or two pitfalls. The first is that he/she may perform a very limited and suboptimal study and become excessively biased by the clinical information, resulting in a significant number of diagnostic errors. The second hazard is that the EDX consultants may change the interpretation of similar findings among different studies to suit and support the clinical diagnosis. For example, a diabetic patient with denervation of quadriceps, iliacus, thigh adductors, and lumbar paraspinal muscles may be diagnosed in the EMG laboratory as consistent with lumbar radiculopathy or diabetic amyotrophy depending on the temporal course of the symptoms, pain characteristics, status of diabetic control, or findings on imaging of the spine.

2. The referring physician is well versed with the anatomy and disorders of the peripheral nervous system and the EDX examination. The physician is often a neurologist or physiatrist, but occasionally a neurosurgeon or an orthopedist. In this situation, the referral information includes brief, yet focused, clinical information, and a limited differential diagnosis. In these situations, the EDX consultant performs an EDX study on the symptomatic limb(s) to confirm or exclude the suspected diagnosis or, sometimes, makes an alternative diagnosis which may have not been considered by the referring physician.

3. The referring physician is not well versed with disorders of the peripheral nervous system. Often, the referral working diagnoses in these patients are vague, nonspecific, or extensive. Since the EDX study has limitations related to patient discomfort, expense, and time constraints, a directed neurological history and a brief neurological examination is often mandatory before planning and executing the EDX study. Unfortunately, contacting the referring physician to extract more specific information is often not fruitful.

THE EMG LABORATORY PROCEDURES

Testing an Adult

Patients referred to the EMG laboratory should have a referral form completed by the referring physician with relevant clinical information and preferably a pertinent neurological differential diagnosis (Figure 1-1). Referring physicians should also describe the EDX study to their adult patients, particularly in regard to the discomfort associated with it, without creating unnecessary heightened anxiety. If unclear about the technical details of the procedure, they should encourage their patients to contact the EMG laboratory prior to the test date, to get a verbal or written description of the procedure. Such written descriptions should be widely available in all referring physicians’ offices (Table 1-2).