EMG, NCV, and Ultrasound Basics

NCV studies

Nerves are like electric wires; they carry tiny bioelectric signals from different parts of body to the brain and from brain to muscle groups to make your hands and legs move. These signals travel at a high speed much like a highway and the nerves are in essence the information highways in the body. You can get an idea about the speed by simply asking someone to touch you at the wrist and figuring out how long it took to feel the touch (only about 20 milliseconds!!). When there is injury or prolonged pressure on a nerve, as in carpal tunnel syndrome, the speed of conduction slows down or stops in that area, due to loss of insulation (myelin) and eventually the contents (axons) of the nerve (figure 1).  The purpose of the nerve conduction velocity study (NCV) is to detect areas where there is conduction delay or block.

It is necessary to initiate an electrical signal in the nerve so that we can measure the speed at which it travels through the length of the nerve. In our body bioelectric signals are generated by selective transit of ions (sodium, potassium and chloride) through selective channels in the covering membrane of the axons. By applying a small electric current on the skin close to the nerve (figure 2) we can alter the ionic equilibrium in the tissue around the nerve causing the generation of a signal (action potential) in the nerve, which will travel along the nerve.

NCV studies may show slowing or block of conduction over a specific anatomic area like the cubital tunnel (figure 3) or may show diffuse slowing in disorders like diabetic neuropathy. The NCV studies are of immense value in the diagnosis of nerve injuries, entrapments and polyneuropathies.

NCV studies have been done for more than 75 years and are considered quite safe. However, if you have implanted devices such as pacemakers, some precautions are necessary, and you should alert the office in advance.

Needle EMG

One of the major function of nerves is to cause muscle contraction. The nerve communicates with the muscle by releasing a chemical transmitter at the neuromuscular junction. Muscle contraction is accompanied by electrical activity which provides information regarding abnormal nerve and muscle function. It is necessary to record from specific muscles, which can be done only by inserting an electrode into that muscle. The electrode is a thin wire, which is shaped like a needle. You will be asked to contract the muscle once the electrode is in position. Depending upon the suspected disorder, one or many muscles may have to be studied. The discomfort is like that of a finger stick for blood sample. There is usually only minimal bleeding. However, if you have bleeding tendency or you are on blood thinners, alert the physician.

What other studies are done at the Neurodiagnostic Center?

In some cases we provide ultrasonic evaluation of nerves and muscles (fig. 4). Unlike NCV/EMG which evaluates the function/malfunction of nerves and muscles, ultrasonography looks at the anatomy/pathology of nerves and muscles. It is particularly useful in detecting nerve tumors and injuries and muscle disorders like muscular dystrophy and myositis.  It complements the information derived from NCV/EMG.

We can stimulate point A and record the signal over point B which gives the travel time; knowing the distance between A and B the conduction velocity can be calculated. The recording electrodes are taped to the skin (they won’t stay on the skin if there is cream or oil on the skin). While the electric shocks can be unpleasant, they are not necessarily painful and last only a fraction of a millisecond each time.