Electrodiagnostics (EDX) testing is used to evaluate the integrity and function of the peripheral nervous system (most cranial nerves, spinal roots, plexi, and nerves), NMJ, muscles, and the central nervous system (brain and spinal cord). EDX testing is performed as part of an EDX consultation for diagnosis or as follow-up of an existing condition. EDX studies can provide information to:
In some medical conditions the electrical activity of the muscles or nerves is not normal. Finding and describing these electrical properties in the muscle or nerve may help diagnose your condition. EDX may aid with the diagnosis of nerve compression or injury (such as carpal tunnel syndrome), nerve root injury (such as sciatica), and with other problems of the muscles or nerves. Less common medical conditions include amyotrophic lateral sclerosis, myasthenia gravis, and muscular dystrophy.
EMG is most often used when people have symptoms of weakness, and examination shows impaired muscle strength. It can help to tell the difference between muscle weakness caused by injury of a nerve attached to a muscle and weakness due to neurologic disorders.
Dorsal (back) of spinal cord transmits sensory information from sensory units to brain, Ventral (front) of cord transmits motor information from brain to muscle
People usually have a small amount of discomfort during EMG testing because of pin insertion. Disposable needles are used so there is no risk of infection.
During nerve conduction studies, small electrodes are taped to the skin or placed around fingers. You typically experience a brief and mild shock, which may be a bit unpleasant. Most people find it only slightly annoying.
It is very important to note that most EMG/NCV tests are not 100% accurate. Most physicians will admit that the tests have at least a 10% margin of error. Very often individuals with nerve damage will have normal EMG/NCV tests even though they are experiencing nerve damage.
During EMG, small pins or needles are inserted into muscles to measure electrical activity. The needles are different than needles used for injection of medications. They are small and solid, not hollow like hypodermic needles. Because no medication is injected, discomfort is much less than with shots.
You will be asked to contract your muscles by moving a small amount during the testing.
With nerve conduction studies, small electrodes will be taped to your skin or placed around your fingers. You typically will experience a mild and brief tingling or shock, which may be a bit unpleasant.
The person who administers the test will explain the procedure. Often muscle activity is monitored through a speaker during the test, which may make a popping or soft roaring noise. The EMG technician will be looking at an oscilloscope, which looks like a small TV set during the procedure.
A minimal evaluation for radiculopathy includes 1 motor and 1 sensory NCS and a needle EMG examination of the involved limb. However, the EDX testing can include up to 3 motor NCSs (in cases of an abnormal motor NCS, the same nerve in the contralateral limb and another motor nerve in the ipsilateral limb can be studied) and 2 sensory NCSs. Bilateral studies are often necessary to exclude a central disc herniation with bilateral radiculopathies or spinal stenosis or to differentiate between radiculopathy and plexopathy, polyneuropathy, or mononeuropathy.
H reflexes and F waves can provide useful complementary information that is helpful in the evaluation of suspected radiculopathy and can add to the certainty of electrodiagnostic information supporting a diagnosis of root dysfunction.
Additional testing may be indicated in patients with a differential diagnosis which includes peripheral neuropathy, cervical radiculopathy, brachial plexopathy, or more proximal median neuropathy.
To determine the level of the lesion in a focal mononeuropathy, and in order to exclude radiculopathy, plexopathy, or polyneuropathy, it may be necessary to study 3 motor and 3 sensory nerves including the clinically affected nerve, the same nerve on the contralateral side, and an unaffected ipsilateral nerve. F-wave studies provide additional diagnostic information. A needle EMG examination in the affected limb is indicated.
In order to characterize the nature of the polyneuropathy (axonal or demyelinating, diffuse or multifocal) and in order to exclude polyradiculopathy, plexopathy, neuronopathy, or multiple mononeuropathies, it may be necessary to study 4 motor and 4 sensory nerves, consisting of 2 motor and 2 sensory NCSs in 1 leg, 1 motor and 1 sensory NCS in the opposite leg, and 1 motor and 1 sensory NCS in 1 arm. H-reflex studies and F-wave studies from 2 nerves may provide additional diagnostic information. At least 2 limbs should be studied by a needle EMG examination. Studies of related paraspinal muscles are indicated to exclude some conditions such as polyradiculopathy.
To diagnose a myopathy, a needle EMG examination of 2 limbs is indicated. To help exclude other disorders such as polyneuropathy or neuronopathy, 2 motor and 2 sensory NCSs are indicated. Two repetitive motor nerve stimulation studies may be performed to exclude a disorder of neuromuscular transmission.
There are many clinical situations where good medical management requires repeat testing, such as in the following examples:
Repeat EDX consultation is therefore sometimes necessary and, when justifiable, should be reimbursed. Reasonable limits can be set concerning the frequency of repeat EDX testing per year in a given patient by a given EDX consultant for a given diagnosis. The following numbers of tests per 12-month period per diagnosis per physician are acceptable:
Spinal nerves have motor fibers and sensory fibers. The motor fibers innervate certain muscles, while the sensory fibers innervate certain areas of skin. A skin area innervated by the sensory fibers of a single nerve root is known as a dermatome. A group of muscles primarily innervated by the motor fibers of a single nerve root is known as a myotome. Although slight variations do exist, dermatome and myotome patterns of distribution are relatively consistent from person to person.
The ventral (anterior) gray matter of the spinal cord contains nerve cells that send axon fibers out, through the nerves, to their end points on the muscles that they activate. Sensory information from the body and arriving instructions from the brain all cause movement by giving instructions to these “motor neurons” in the spinal cord gray matter.
Myotomes – Relationship between the spinal nerve & muscle and are best evaluated with EMG
Dermatomes – Relationship between the spinal nerve & skin and a combination of EMG and NCS is used to define pathology.
Each muscle in the body is supplied by a particular level or segment of the spinal cord and by its corresponding spinal nerve. The muscle, and its nerve make up a myotome. This is approximately the same for every person and are as follows: