An Educational Overview of Electrodiagnostic (EMG/NCS) Study


Electrodiagnostic Study Consultation
Normal & Abnormal Electromyographic (EMG) Findings
Normal & Abnormal Nerve Conduction Study (NCS) Findings

Electrodiagnostic Study Consultation:

Your doctor or health care provider has requested an Electrodiagnostic: EMG/NCS study, which commonly includes two components:  EMG (Electromyogram), and NCS (Nerve Conduction Study).  The test is performed by physicians with specialized training. It is an extension of the clinical presentation and physician examination, and helps to determine if your problem affects the nerves and muscles in your body, and the reason(s) for functional impairment and loss. This test is often requested when there are complaints of numbness and/or tingling, burning pain, and/or weakness.  The following explanation and educational materials included will help you to understand the EMG/NCS procedure, and allow you to ask questions about the test from the staff and/or physician.  All the physicians in our group performing these studies have advanced training in the evaluation and treatment of muscle and nerve function, and American Association of Neuromuscular and Electrodiagnostic Medicine (AANEM) Board CertifiedWe are honored and very proud indeed to announce that in September, 2011, our EDX laboratory was awarded national  Accreditation § – Exemplary status * by AANEM on its first submission.  At the time, we are 1 in less than 20 EDX Laboratories Accredited Nationwide with Exemplary status and the First Multi-physician EDX laboratory in Washington State awarded this Prestigious distinction.

EMG/NCV Study Video

4 minute 11 seconds

Made available by Electrodiagnosis & Musculoskeletal Associates

Electrodiagnosis Logo

who is turn indebted to Drasko Simovic, M. D., director of EMG Laboratory, for creating, and making publically available this superb educational video which provides a neurophysiological overview regarding the importance of EMG/NCV study.

Please arrive 10 – 15 minutes early for registration, or bring the completed downloaded paperwork with youClick here for Electrodiagnostic: EMG/NCS or SEP Consultation Forms.  No special preparation is required, except to keep the skin free of any body lotion on the day of the examination. It is always helpful to bring a list of all your medications, but you do not need to limit food or medications except if you are being evaluated for Myasthenia Gravis. If you have had: Bone, CT, or MRI scans, and/or X-Rays or other specialized tests of relevance, then bring copies of the reports with you if available.  
The examinations are done while you are lying either on your back, side or abdomen.  It takes about 30 – 60 minutes for a one to two extremity examination.  Everything possible has been done with this test to keep any pain that you may experience to a minimum, but you have the right to stop the procedure at any time.  In excess of 4500 of these examinations are done each year in our office, and all of our Electrodiagnostic physician specialists are highly trained with additional Board Certification held by the American Board of Neuromuscular and Electrodiagnostic Medicine/AANEM. 
Please let us know if you:

  1. Are on “blood thinners” such as Coumadin/Warfarin, or others.
  2. Have a bleeding disorder such as Hemophilia.
  3. Have a Pacemaker/Defibrillator unit (cardiac implant).
  4. Have a deep brain stimulator.
  5. Are allergic to Latex or any other topical product. 

If you have special needs, please let us know . . . Thank you for your cooperation
The results are sent to your referring physician or health provider, and he/she considers the results to make the best decision about the next step in your treatment. 

After the test is completed, you may carry on your normal daily routine. The electrodiagnostic physician interprets the results of your study, and completes a detailed report which is sent to the referring physician or health provider.  He/she will consider the results and make the best decision about the next step in your evaluation and/or treatment best suited for your problem.

Electrodiagnostic Study is a procedure which commonly includes two components:  EMG (Electromyogram), and NCS (Nerve Conduction Study).  For those with implanted Deep Brain Stimulators, the unit will need to be temporarily turned off prior to Electrodiagnostic testing.  For those with Cardiac Pacemaker-Defibrillators, having detailed information about your particular unit is important.

During an EMG study, the physician analyzes the electrical activity in muscles by inserting a fine pin electrode. Electrode insertion may cause mild temporary discomfort. The physician can determine whether the muscle, and the nerve supplying that muscle, is working properly by analyzing the firing pattern of muscle fibers on a video monitor and listening over a loudspeaker. Evaluation of both the resting muscle membrane as well as the recruitment pattern from the muscle fibers yields valuable information. Only brand new, laboratory sealed and sterilized disposable needles are used for every patient which are discarded after use to prevent transmission of infection including AIDS or hepatitis. 

EMG is a recording of electrical activity from a small portion of a muscle supplied by a specific nerve, and is used to confirm a clinical diagnosis of a nerve or muscle disease or injury, and provides invaluable information helping to determine the severity, activity, location and type of problem you are having.  A muscle is always studied both at rest, as well as during a light contraction.  Our equipment is able to accurately measure the electrical activity, and with the doctors expertise, the patterns can be analyzed & interpreted.  A never before or again used tiny sterilized pin electrode is utilized that is inserted into a few of your muscles.  The wave patterns &sounds will be displayed on our equipment, and recorded if indicated.  You will feel the prick as the pin electrode goes through the skin followed by a milder deeper aching sensation.  It is important to relax your muscles during the test.  With relaxation, the pin is frequently not felt after going through the skin.  When the examination is over, very little discomfort remains for most.  There is a possibility for bruising but only a very remote possibility for infection, among other rare complications.  At times a quickly evaporating cold spray is used to provide slight topical numbing.  After the study, you should be able to resume your daily routine. 

During NCS study, the physician tapes small electrodes on the skin and applies a brief electric stimulus to different portions of motor and sensory nerves. Nerve stimulation causes a tingling sensation. The physician evaluates the electrical responses of the nerve or muscle being tested and can determine if the nerve impulse is (a) conducted normally, (b) at a slow speed, or (c) not transmitted at all.

NCS determines how well an electrical impulse travels through your nerves.  We create a series of mild electrical shocks at different places along the course of a nerve while recording the responses.  The shock is usually not painful and more of a surprise, although but can cause the muscle being tested to twitch or contract.

Normal & Abnormal Electromyographic (EMG) Findings

It is very important that every muscle evaluated by needle EMG, assesses both the resting muscle membrane, and also the active recruitment pattern. 

The Resting Muscle Membrane

A normal muscle at rest is electrically silent as demonstrated below. When the needle is inserted or adjusted slightly, there is a brief spurt of electrical activity, followed by a very quick return to electrical silence absent any spontaneous activity.

Normal resting muscle membrane

When there is muscle and/or nerve irritation, or a metabolic abnormality, the muscle membrane becomes hyper-excitable and results in increased insertional activity. The discharges are sometimes referred to as complex repetitive discharges.

Increased insertional activity:
Complex Repetitive Discharges

When there is nerve fiber injury, be it from entrapment neuropathy such as Carpal Tunnel Syndrome or root compression such as from disc herniation or bony Stenosis with axonal/nerve fiber injury, denervation potentials develop in the muscle known as positive sharp waves (PSW) and fibrillation potentials (FP) in the nerve distribution distally. The sound of positive sharp waves have been described as, like ‘claps of distant thunder’, and fibrillation potentials a higher pitched and shorter duration sound, like ‘rain on a tin roof’. The degree of denervation, i.e. PSW & FPs, is often graded on a scale from 1 – 4, grade 1- occasional; to grade 4- ‘filling the screen’, reflecting a much more severe degree of nerve injury. Below, grade 1-2+ is demonstrated.

Spontaneous Activity/Denervation Potentials:
Positive Sharp Waves & Fibrillation Potentials-
Grade 1-2+

Muscle Recruitment Pattern

The firing pattern of muscles during a light contraction provides invaluable information about the health and abundance of motor units, and thus the number of functioning nerve fibers supplying the muscle being tested.  Normal motor units are between 200 mV to 5 mV and usually have a crisp bi- or tri-phasic appearance.  Upon a full muscle contraction, the motor units fill the screen.

Normal Recruitment Pattern

After nerve injury, there can be recovery from several mechanisms.  Nerve fibers can re-grow down the old nerve sheath, or remaining nerve fibers can sprout (like shooters that develop after trimming a tree limb).  However, the new sprouts are de-synchronous and thus polyphasic in appearance ( > 5 phases), and as the reconstituted motor units mature, they become much larger in amplitude than normal (up to 15-20 mV) or 4-5 X the normal size.  Given nerve fiber thinning, the recruitment pattern can be described as neurogenic or ‘picket fence’, i.e. only a few large complex motor units firing with hollow voids.

Abnormal Recruitment Pattern:
Polyphasic-hypertrophic motor units
Neurogenic/’Picket-fence’ recruitment pattern

During EMG testing, the earliest consequence of nerve injury is abnormal muscle membrane instability or increased insertional activity. However, this can also be seen as a consequence of direct muscle trauma, in muscle diseases particularly those with inflammatory infiltrates such as polymyositis, as well as metabolic and electrolyte disturbances. However, denervation potentials, i.e. positive sharp waves and fibrillation potentials, are only seen in consequence to nerve injury. The higher the grade of denervation potentials, the more severe is the nerve injury.

Denervation Potential
Click here to view a larger image

As but one example, if a disc ruptures causing pressure upon a nerve root, or triggers an inflammatory reaction resulting in axonal/nerve fiber injury, denervation potentials will develop in the distribution of muscles supplied by that nerve root.  The grade of denervation is a function of the degree of axonal/nerve fiber injury and loss.

EMG Abnormal Muscle
Click here to view a larger image

Subsequent to acute axon/nerve fiber injury, there is a time delay for Wallerian degeneration downstream.  It takes almost 2 days to see early consequences just distal to a sectioned nerve, and 7-10 up to 14 days to see characteristic changes in the innervated muscle, i.e. denervation potentials in response to injury. 

Wallerian Degeneration
Click here to view a larger image

Subsequent to acute consequences of nerve injury, through a process of: (1) Axonal/nerve regrowth; and (2) axonal collateral sprouting, motor units that are usually triphasic and not larger than 5 mV become complex and polyphasic on EMG.  As these motor units mature, “giant” complex motor unit reorganization develops up to 15-20 mV+, and thus 4-5 X the usual maximal size.  The recruitment pattern becomes “picket fence” reflecting the degree of axonal thinning with rapid firing (increasing the frequency of muscle fiber firing as one mechanism to compensate by attempting to generate a stronger muscle contraction.

By combining the findings on EMG - Assessing both the: (1) Resting muscle membrane, and (2) the recruitment pattern, the physician can develop a clear picture of acute vs. subacute vs. chronic injury, and the severity ranging from minimal to very severe.

Acute and Chronic Response to Injury
Click here to view a larger image

This slide highlights the different appearance of a normal motor unit in a muscle absent prior nerve injury, with an abnormal muscle demonstrating chronic motor unit change: polyphasic, hypertrophic reorganization (up to 5 fold magnitude difference ); while manifesting a “picket fence” recruitment pattern with rapid firing.  (see A-V clip above on Abnormal Recruitment pattern).

Normal & Abnormal Nerve Conduction Study (NCS) Findings

 Below are a few examples of common EMG/NCV abnormalities in several peripheral nerve disorders.

Carpal Tunnel Syndrome (CTS) is the most common peripheral entrapment neuropathy.  Only the median nerve travels through the tunnel, and thus only the median motor & sensory latencies are delayed across the wrist.  Below, NCS reveals not only significant delay of all median latencies relative to predicted, but also by differential median compared to ulnar and radial latency comparisons, particularly so for the median midpalm sensory latency (a short distance latency across the carpal tunnel), compared to the ulnar midpalm sensory latency. There is also median vs. ulnar motor latency delay.  The more the delay, particularly when lost amplitude and waveform dispersion, the more significant is the compression.  Additionally, EMG details mild active denervation in the median innervated muscle, as well as chronic motor unit reorganization, and thus a total picture for a moderately-severe form of CTS. 

Electrodiagnostic Pattern of CTS
Click here to view a larger image

Ulnar neuropathy at the elbow is probably the second most common entrapment neuropathy.  Below, NCS reveals absence of the ulnar sensory responses above the elbow, and significant ulnar slowing with a near complete neurapraxic conduction block at the level of the elbow defined by inching technique to be located precisely behind the tip of the medial epicondyle.  EMG shows denervation in the ulnar distribution as well as motor unit reorganization with rapid firing reflecting axonal/nerve fiber injury and loss. 

Ulnar Neuropathy of the Elbow
Click here to view a larger image

True Tarsal Tunnel Syndrome (TTS) which is an unusual distal posterior tibial entrapment neuropathy of the ankle and foot, the analog to CTS of the wrist and hand, should not be overlooked as it is treatable.  This must be differentiated from musculoligamentous problems such as plantar fasciitis vs. isolated branch plantar neuropathy vs. generalized distal lower extremity peripheral neuropathy (“sick” nerve neuropathy). 

Below left, left sided medial and lateral plantar sensory responses are easily defined, but right sided responses are absent.  Below right, left sided medial and lateral plantar motor latencies across the ankle/tarsal tunnel are normal, but right sided plantar latencies across the ankle are prolonged with the amplitude of responses diminished (reflecting axonal/nerve fiber loss). The normal left sided motor and sensory responses and bilateral motor conductions rules out a generalized peripheral neuropathy.

True R TTS Prolonged
Click here to view a larger image Click here to view a larger image

Generalized lower extremity peripheral neuropathy in distinction to Tarsal Tunnel Syndrome.  There is diffuse and not focal motor and sensory abnormalities. 

Below left, the bilateral medial and lateral plantar sensory responses are symmetrically prolonged and of tiny amplitude nearing unobtainable.  Below right, the bilateral medial and lateral plantar motor latencies are prolonged and of tiny amplitude with significant but symmetric motor conduction slowing defined ankle to knee.

GPN not TTS Bilaterally
Click here to view a larger image Click here to view a larger image


Back to Services | Back to top