An EMG (electromyogram) is a diagnostic study that helps to assess muscle function. A NCV (nerve conduction velocity) test usually accompanies the EMG in order to assess how well the nerves that control their activity are functioning. An EMG/NCV can be valuable tools in delineating a variety of muscle and/or nerve disorders as well as disruptions between the nerve innervation to muscles.
Nerve cells or motor neurons transmit electrical signals that cause a muscle to contract or relax. These studies translate that data into numbers that can track how well the nerves and muscles are functioning.
An EMG looks at the electrical signals generated by our muscles when they are at rest and then activated.
An NCV looks at the speed at which the electrical signal travels down the nerve.
Both create data that can be measured and then compared to normal levels to define if there’s an abnormality.
Damage at any point can help pinpoint the cause for a multitude of symptoms:
- persistent numbness or tingling
- persistent muscle weakness, pain, or cramping
- loss of function of a particular muscle or muscle group
- tics or involuntary muscle movements
- persistent symptoms in the arms or legs
Resulting in possible etiologies:
- Muscle disorders such as muscular dystrophy, a group of genetic neuromuscular disorders that cause progressive breakdown and weakness of skeletal muscles
- Motor neurons that are incapable of getting a muscle to respond as in myasthenia gravis. A weakness and fatigue of muscles under voluntary control due to a breakdown in communication between the nerve and muscle it supplies.
- Radicular complaints that are caused by impacts on a nerve from a herniated spinal disc. These cause pain in the distribution of the impacted nerve down an extremity.
- Issues that arise when peripheral nerves (nerves not associated with the spinal cord) are affected as with carpal tunnel syndrome. In this case, the median nerve in the forearm as it moves across the wrist is compressed.
- Nerve disorders like ALS- amyotrophic lateral dystrophy- or Lou Gehrigs disease, cause nerve cells to break down which causes the muscles they innervate to lose function.
- Chronic inflammatory conditions cause irritation and swelling on nerves, which can lead to neuropathies- a burning or tingling pain in one or more peripheral nerves. They are often due to auto immune disease such as lupus, rheumatoid arthritis.
- Damage due to trauma or injury.
- Alcohol use. It’s not exactly clear how alcohol damages nerves. It’s thought to be due to direct damage on the nerves as well as toxic effects from nutritional deficiencies such as thiamine.
- Endocrine disorders such as diabetes where high sugar levels can damage the small blood vessels that support nerves, essentially preventing nutrients from reaching them. This can damage fragile nerve fibers. Hypothyroid slows metabolism which leads to fluid retention and swelling that can exert pressure on nerves.
- Medicines such as chemotherapy can damage healthy cells, including nerves, as well as cancerous cells.
- Nutritional deficiencies such as lack of B12 can damage the protective myelin sheath that surrounds nerves. Without it they can’t function properly.
- Cancer can damage a nerve if the tumor is growing too close to it or one is damaged after it’s surgically excised.
- Liver disease. When the liver doesn’t work properly, toxins build up which can damage nerves. Liver damage also allows fluids to build up in the lower extremities leading to pressure, inflammation and damage to nerves.
- Kidney disease. Renal impairment causes electrolyte imbalances which can prevent nerves from functioning normally. It also prevents the removal of toxins through the urine so they can build up and lead to nerve damage.
- Vasculitis is a systemic disease that causes inflammation in blood vessels. When it affects those supplying nerves it can lead to a neuropathy.
A common misconception is that it takes months of impact before these test are accurate. Once a nerve is damaged Wallerian degeneration occurs. This is the process a nerve goes thru to recover. It can be picked up by an EMG/NCV test 21 days after the inciting event.
How are the tests done
An NCV (nerve conduction test) is usually performed first. A mild electrical current is sent through a needle electrode that is taped to a particular area in order to stimulate a nerve signal into the muscle being tested, often causing a mild tingling feeling.
It assesses the conduction or health of peripheral nerves- those that start where the nerve root takes off from the spinal cord. A record is then made of the time and speed it took for the muscle to respond to the nerve impulse.
This is an objective measurement of how well the nerve being tested functions. A normal nerve conduction velocity is between 50-60 meters per second, but results are impacted by age, body part examined, even where you live. It helps to differentiate between injury to a nerve fiber versus the myelin sheath- the protective covering surrounding the nerve- to help distinguish between a nerve disorder and a condition where a nerve injury has affected the muscles. Anything outside the normal range implies the nerve is damaged or diseased but it doesn’t define why.
This time a very tiny, slim needle electrode is inserted into different muscle groups. A mild electrical current is then sent into the muscle being tested while a machine records its reaction. It’ll be both at rest and when you’re asked to tighten or contract the muscle. The activity is seen as wavy and spiky lives on a computer. It measures the electrical activity of a muscle group at rest, with minimal contraction and then a forceful one which causes more and more fibers to be activated. Both procedures allow the provider to detect there’s any damage to the nerves or muscles as well as the extent and their location.
It sounds painful but in the hands of a provider who trained and experienced it’s painless. For years I was told stories of patients traumatized by novice testers who took forever to get results leading to bruises and discomfort. It wasn’t until I was checked by a particular gentleman years ago who lacked compassion and caring that I understood their fears. It usually takes 20-30 minutes to read one limb, so if all the extremes are being tested expect to be a couple of hours.
If you have an area of numbness, tingling, pain, weakness or loss of motion in any extremity that doesn’t resolve quickly an EMG and NCV maybe a great way to diagnose the reason.