Electromyography
EMG

Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph, to produce a record called an electromyogram. An electromyograph detects the electrical potential generated by muscle cells when these cells are electrically or neurologically activated. The signals can be analyzed to detect medical abnormalities, activation level, recruitment order or to analyze the biomechanics of human or animal movement.

Quick Link
Dictionary

A - B          C - D

E - F          G - H

I - K            L - M

O - P           R - S

T - U           V - W

Signs &
Symptoms

A - B           C - D

E - F           G - H

I - K             L - M

O - P            R - S

T - U            V - W
Signs & Symptoms Of
Autoimmune Disease

Inflammation
&
Autoimmune Disease

Body Parts Affected
By
Autoimmune Diseases

Medical Terminology

Muscle Biopsy

Skeletal System

Muscular System

Nervous System

Endocrine System

Electromyography
(EMG)

     Electromyography (EMG) is a diagnostic procedure to assess the health of muscles and the nerve cells that control them (motor neurons). Motor neurons transmit electrical signals that cause muscles to contract. An EMG translates these signals into graphs, sounds or numerical values that a specialist interprets. An EMG uses tiny devices called electrodes to transmit or detect electrical signals. During a needle EMG, a needle electrode inserted directly into a muscle records the electrical activity in that muscle. A nerve conduction study, another part of an EMG, uses electrodes taped to the skin (surface electrodes) to measure the speed and strength of signals traveling between two or more points. EMG results can reveal nerve dysfunction, muscle dysfunction or problems with nerve-to-muscle signal transmission.
Picture
Surface Electrodes
Your doctor may order an EMG if you have signs or symptoms that may indicate a nerve or muscle disorder. Such symptoms may include:

  • Tingling
  • Numbness
  • Muscle weakness
  • Muscle pain or cramping
  • Certain types of limb pain

EMG results are often necessary to help diagnose or rule out a number of conditions such as:

  • Muscle disorders, such as muscular dystrophy or Polymyositis
  • Diseases affecting the connection between the nerve and the muscle, such as myasthenia gravis
  • Disorders of nerves outside the spinal cord (peripheral nerves), such as carpal tunnel syndrome or peripheral neuropathies
  • Disorders that affect the motor neurons in the brain or spinal cord, such as amyotrophic lateral sclerosis or polio
  • Disorders that affect the nerve root, such as a herniated disk in the spine
An EMG is done to:

  • Find problems that damage muscle tissue, nerves, or the spots where nerves and muscles join. These problems may include a herniated disc, amyotrophic lateral sclerosis (ALS), or myasthenia gravis (MG).
  • Find the cause of weakness, paralysis, or muscle twitching. Problems in a muscle or the nerves going to a muscle can cause these symptoms. So can problems in the spinal cord or the area of the brain that controls a muscle. The EMG does not show brain or spinal cord diseases.

A nerve conduction study is done to:

  • Find damage to the peripheral nervous system. This includes all the nerves that lead away from the brain and spinal cord. It also includes the smaller nerves that branch out from those nerves. This test is often used to help find nerve problems such as carpal tunnel syndrome or Guillain-Barré syndrome.
How To Prepare

Be sure to tell your doctor about all the medicines you take, even over-the-counter ones. Many medicines can change the results of this test.

  • You may need to stop taking some medicines before you have this test.
  • If you take blood thinners, your doctor will tell you if you should stop taking them before the test. These medicines include warfarin (Coumadin), clopidogrel (Plavix), and aspirin. Make sure that you understand exactly what your doctor wants you to do. NSAID's

Wear loose clothing to make it easier to do the test. You may get a hospital gown to wear.

The electrodes are attached to your skin. So make sure your skin is clean. Don't use any sprays, oils, creams, or lotions.

You may be asked to sign a consent form that says you understand the risks of the test and agree to have it done.

Talk to your doctor if you have any concerns about the need for the test, its risks, etc.
The nervous system specialist (neurologist) conducting the EMG will need to know 
if you have certain medical conditions. Tell the neurologist and other EMG lab personnel if you:

  • Have a pacemaker or any other electrical medical device
  • Take blood-thinning medications
  • Have hemophilia, a blood-clotting disorder that causes prolonged bleeding

Questions to ask

When you schedule your EMG, you may want to ask the following questions:

  • What time do I need to arrive?
  • Where is the EMG lab, and what's the best way to find it in the hospital or clinic?
  • Can a friend or relative be with me during the exam?
Bathing

Take a shower or bath shortly before your exam in order to remove oils from your skin. Don't apply lotions or creams before the exam.
     There are two kinds of EMG in widespread use: surface EMG and intramuscular (needle and fine-wire) EMG. To perform intramuscular EMG, a needle electrode or a needle containing two fine-wire electrodes is inserted through the skin into the muscle tissue. A trained professional (such as a neurologist or EMG technician) observes the electrical activity while inserting the electrode. Certain places limit the performance of needle EMG by non-physicians. A recent case ruling in the state of New Jersey declared that it cannot be delegated to a physician's assistant. The insertional activity provides valuable information about the state of the muscle and its innervating nerve. Normal muscles at rest make certain, normal electrical signals when the needle is inserted into them. Then the electrical activity when the muscle is at rest is studied. Abnormal spontaneous activity might indicate some nerve and/or muscle damage. Then the patient is asked to contract the muscle smoothly. The shape, size, and frequency of the resulting electrical signals are judged. Then the electrode is retracted a few millimetres, and again the activity is analyzed until at least 10–20 motor units have been collected. Each electrode track gives only a very local picture of the activity of the whole muscle. Because skeletal muscles differ in the inner structure, the electrode has to be placed at various locations to obtain an accurate study.
You'll likely be asked to change into a hospital gown for the procedure and lie down on an examination table. The following explanations can help you understand what will happen during the exam:

  • Electrodes. The neurologist or a technician places surface electrodes at various locations on your skin depending on where you're experiencing symptoms. Or the neurologist may insert needle electrodes at different sites depending on your symptoms.
  • Sensations. The electrodes will at times transmit a tiny electrical current that you may feel as a twinge or spasm. The needle electrode may cause discomfort or pain that usually ends shortly after the needle is removed.

If you're concerned about discomfort or pain, you may want to talk to the neurologist about taking a short break during the exam.

  • Instructions. During the needle EMG, the neurologist will assess whether there is any spontaneous electrical activity when the muscle is at rest — activity that isn't present in healthy muscle tissue — and the degree of activity when you slightly contract the muscle.

He or she will give you instructions on resting and contracting a muscle at appropriate times. Depending on what muscles and nerves the neurologist is examining, he or she may ask you to change positions during the exam.

      Intramuscular EMG may be considered too invasive or unnecessary in some cases. Instead, a surface electrode may be used to monitor the general picture of muscle activation, as opposed to the activity of only a few fibres as observed using an intramuscular EMG. This technique is used in a number of settings; for example, in the physiotherapy clinic, muscle activation is monitored using surface EMG and patients have an auditory or visual stimulus to help them know when they are activating the muscle (biofeedback).

    Nerve conduction testing is also often done at the same time as an EMG to diagnose neurological diseases. Some patients can find the procedure somewhat painful, whereas others experience only a small amount of discomfort when the needle is inserted. The muscle or muscles being tested may be slightly sore for a day or two after the procedure.
Normal Results

     Muscle tissue at rest is normally electrically inactive. After the electrical activity caused by the irritation of needle insertion subsides, the electromyograph should detect no abnormal spontaneous activity (i.e., a muscle at rest should be electrically silent, with the exception of the area of the neuromuscular junction, which is, under normal circumstances, very spontaneously active). When the muscle is voluntarily contracted, action potentials begin to appear. As the strength of the muscle contraction is increased, more and more muscle fibers produce action potentials. When the muscle is fully contracted, there should appear a disorderly group of action potentials of varying rates and amplitudes (a complete recruitment and interference pattern).  
Abnormal Results

EMG is used to diagnose diseases that generally may be classified into one of the following categories: neuropathies, neuromuscular junction diseases and myopathies.

Neuropathic disease has the following defining 
EMG characteristics:

  • An action potential amplitude that is twice normal due to the increased number of fibres per motor unit because of reinnervation of denervated fibres
  • An increase in duration of the action potential
  • A decrease in the number of motor units in the muscle (as found using motor unit number estimation techniques)
Myopathic disease has these defining EMG
characteristics:

  • A decrease in duration of the action potential
  • A reduction in the area to amplitude ratio of the action potential
  • A decrease in the number of motor units in the muscle in extremely severe cases only
Because of the individuality of each patient and disease, some of these characteristics may not appear in every case.

Abnormal results may be caused by the following medical conditions (please note this is nowhere
near an exhaustive list of conditions that can result in abnormal EMG studies):

  • Friedreich ataxia (inherited disease that affects areas in the brain and spinal cord that control coordination, muscle movement, and other functions)
  • Guillain-Barré syndrome (autoimmune disorder of the nerves that leads to muscle weakness or paralysis)
  • Lambert-Eaton syndrome (autoimmune disorder of the nerves that causes muscle weakness)
  • Multiple mononeuropathy (a nervous system disorder that involves damage to at least 2 separate nerve areas)
  • Mononeuropathy (damage to a single nerve that results in loss of movement, sensation, or other function of that nerve)
  • Myopathy (muscle degeneration caused by a number of disorders, including muscular dystrophy)
  • Myasthenia gravis (autoimmune disorder of the nerves that causes weakness of the voluntary muscles)
  • Peripheral neuropathy (damage of nerves away from the brain and spinal cord)
  • Polymyositis (muscle weakness, swelling, tenderness, and tissue damage of the skeletal muscles)
  • Radial nerve dysfunction (damage of the radial nerve causing loss of movement or sensation in the back of the arm or hand)
  • Sciatic nerve dysfunction (injury to or pressure on the sciatic nerve that causes weakness, numbness, or tingling in the leg)
  • Sensorimotor polyneuropathy (condition that causes a decreased ability to move or feel because of nerve damage)
  • Shy-Drager syndrome (nervous system disease that causes body wide symptoms)
  • Thyrotoxic periodic paralysis (muscle weakness from high levels of thyroid hormone)
  • Tibial nerve dysfunction (damage of the tibial nerve causing loss of movement or sensation in the foot)
What is the Risk?

     An EMG is very safe. You may get some small bruises or swelling at some of the needle sites. The needles are sterile, though. EMG is a low-risk procedure, and complications are rare. There's a small risk of bleeding, infection and nerve injury where a needle electrode is inserted. When muscles along the chest wall are examined with a needle electrode, there's a very small risk that it could cause air to leak into the area between the lungs and chest wall, causing a lung to collapse (pneumothorax). There is no chance of problems with nerve conduction studies. Nothing is put into your skin, so there is no chance of infection. The voltage of electrical pulses is too low to cause an injury.
Click To Download
FREE Information On

Autoimmune
Diseases

Signs &
Symptoms PDF

Diseases List PDF

FAQ's PDF
Medical
Terminology

Muscle
Biopsy

MRI Magnetic
Resonance Imaging
Body Parts Affected
By Autoimmune Disease

Inflammation &
Autoimmune Diseases

Risks &
Symptoms Checklist

All information on this page
was last checked and
verified on April 12th, 2018