Electrodiagnostics

Testing may take 30-60 minutes. The nerve conduction part usually takes longer than the needle exam. That is because we take measurements and do math during it. On average, for one arm or leg, the nerve conduction part takes 15 to 30 minutes. The needle exam for one arm or leg usually takes 15 to 20 minutes. Plan on about one hour in the exam room if only one limb is tested. It takes longer if more limbs need testing.

The least testing for radiculopathy (a pinched nerve root) is 1 motor and 1 sensory NCS plus a needle EMG of the problem limb. But EDX testing can include up to 3 motor NCSs and 2 sensory NCSs. If a motor NCS is abnormal, we can test the same nerve on the other side and another motor nerve on the same side. Testing both sides is often needed. It rules out a central disc herniation pinching roots on both sides, or spinal stenosis. It also tells radiculopathy apart from plexopathy, polyneuropathy, or mononeuropathy.

H reflexes and F waves can add useful information when we suspect radiculopathy. They can make us more certain the nerve root is the problem.

More testing may be needed when the list of possible causes includes peripheral neuropathy, cervical radiculopathy, brachial plexopathy, or median nerve damage higher up the arm.

When one nerve is damaged (focal mononeuropathy), we need to find where along it the damage sits. We also need to rule out radiculopathy, plexopathy, or polyneuropathy. That may take testing 3 motor and 3 sensory nerves. They include the problem nerve, the same nerve on the other side, and a healthy nerve on the same side. F-wave studies add more information. A needle EMG of the problem limb is called for.

Polyneuropathy is damage to many nerves. To pin down its type, we ask two things. Is the nerve core (axonal) or the nerve coating (demyelinating) damaged? Is it spread out (diffuse) or in patches (multifocal)? We also need to rule out polyradiculopathy, plexopathy, neuronopathy, or multiple mononeuropathies. That may take testing 4 motor and 4 sensory nerves. That means 2 motor and 2 sensory NCSs in 1 leg, 1 motor and 1 sensory NCS in the other leg, and 1 motor and 1 sensory NCS in 1 arm. H-reflex and F-wave studies from 2 nerves may add more information. At least 2 limbs should get a needle EMG. Testing the muscles next to the spine helps rule out some conditions such as polyradiculopathy.

To diagnose a myopathy (a muscle disease), we do a needle EMG of 2 limbs. To help rule out other problems such as polyneuropathy or neuronopathy, we do 2 motor and 2 sensory NCSs. Two repeated motor nerve stimulation studies may be done. They rule out a problem where nerve signals fail to cross to the muscle.

Motor neuronopathy is disease of the nerve cells that run the muscles (for example, amyotrophic lateral sclerosis [ALS or Lou Gehrig’s disease]). To diagnose it, we must rule out look-alikes such as multifocal motor neuropathy or polyneuropathy. We may test up to 4 motor nerves and 2 sensory nerves. Needle EMG of up to 4 limbs (or 3 limbs and face or tongue muscles) is often needed. It shows whether muscles have lost their nerve supply in many places. It also rules out a myopathy. One repeated motor nerve stimulation study may be needed to rule out a problem where nerve signals fail to cross to the muscle.

A brachial plexopathy is damage to the nerve network in the shoulder. To describe it and tell it apart from cervical radiculopathy and mononeuropathies, we often test every major nerve that is easy to test in both arms. That means the sensory nerves (radial, median, ulnar, and medial and lateral antebrachial cutaneous sensory). It also means the motor nerves (radial, median, ulnar, and possibly axillary and musculocutaneous motor). We also do a needle EMG in both arms. A lumbosacral plexopathy is damage to the nerve network in the low back and pelvis. To describe it and tell it apart from lumbar radiculopathy and mononeuropathies, we often test every major nerve that is easy to test in both legs (superficial peroneal and sural sensory; peroneal and posterior tibial motor). We also do a needle EMG in both legs. F-wave studies in the motor nerves and soleus H reflexes add useful information too.

Sometimes nerve signals fail to cross from nerve to muscle. To show and describe that, we do repeated nerve stimulation studies in up to 2 nerves and SFEMG (single-fiber EMG) in up to 2 muscles. If any of these are abnormal, we may do up to 2 motor and 2 sensory NCSs. Those rule out nerve diseases that can come with this signal problem. At least 1 motor and 1 sensory NCS should be done in a limb with symptoms. Ideally it is in the area of a nerve tested with repeated stimulation or SFEMG. At least 1 muscle far from the trunk and 1 muscle close to it should get a needle EMG. That rules out a nerve or muscle disease that can come with abnormal repeated stimulation studies or SFEMG. At least 1 of the muscles should have symptoms, and both muscles should be in limbs with symptoms.

Together, NCSs and a needle EMG may help most when done several weeks after the injury. But NCSs are often useful right after a nerve injury. One example is when a nerve may have been cut. In fact, if testing waits, the chance to find the exact spot of injury, or to act on it, may be lost. In some cases, even a needle EMG done right after a nerve injury can show abnormal motor unit action potential (MUAP) recruitment. It can also give a starting point. That helps record problems you already had, date the injury, or compare with later tests.

Nerve injuries vary a lot. So there is no simple rule on when to do EDX testing, and the AAEM has no specific advice on it. In every case, the AAEM urges physicians and payors to talk. It also urges physicians to use their own clinical judgment on when to test and which tests to do.

Good medical care often calls for repeat testing. Here are some examples:

• Second diagnosis. One diagnosis is made on the first visit. Later, the patient gets a new set of symptoms. More testing is needed for a second diagnosis.
• Unclear diagnosis. A serious disease (e.g., ALS) is suspected, but the needle EMG/NCS results are not enough to be sure. Then 1, or even more, follow-up studies are needed to prove or rule out the diagnosis.
• Fast-moving disease. In some diseases, the first EDX test may look normal (e.g., Guillain-Barré syndrome) in the first 1 to 2 weeks. An early diagnosis, proven by a repeat test, must come fast so treatment can start. Follow-up testing can be very useful to predict the course and watch how the patient is doing.
• Course of the disease. Some treatable diseases, such as polymyositis and myasthenia gravis, go up and down. Their response to treatment varies. The physician treating these patients needs to track the disease and the response to treatment. Follow-up test results may be needed to guide treatment choices.
• Unexpected course or change in course of the disease. Sometimes treating a known condition does not give the results we expect. Or new problems show up that may or may not be related (e.g., no improvement after surgery for radiculopathy). In these cases, testing again makes sense.
• Recovery from injury. Repeat tests may be needed to track healing, to help predict the outcome, and/or to decide whether and when surgery is needed (e.g., traumatic nerve injury).

So a repeat EDX visit is sometimes needed. When it is justified, insurance should pay for it. Fair limits can be set on how often one EDX doctor repeats testing in one patient for one diagnosis each year. These numbers of tests per 12-month period per diagnosis per physician are acceptable:

• Two tests for carpal tunnel-unilateral, carpal tunnel-bilateral, radiculopathy, mononeuropathy, polyneuropathy, myopathy, and NMJ disorders.
• Three tests for motor neuronopathy and plexopathy.

Spinal nerves have motor fibers and sensory fibers. The motor fibers run certain muscles. The sensory fibers serve certain areas of skin. A patch of skin served by the sensory fibers of one nerve root is called a dermatome. A group of muscles run mainly by the motor fibers of one nerve root is called a myotome. There are small differences, but dermatome and myotome maps are fairly steady from person to person.

The front (ventral, or anterior) gray matter of the spinal cord holds nerve cells. They send long fibers out, through the nerves, to the muscles they fire. Feeling from the body and orders from the brain both cause movement. They do it by giving orders to these “motor neurons” in the spinal cord gray matter.