needle electromyography (EMG)

If nerve conduction studies are like testing the cables from outside the wall, needle electromyography is like opening up a single junction and listening to it directly. A very thin needle, which doubles as a recording electrode, is placed into a muscle, and the examiner listens to and watches the electrical chatter of the muscle fibres around the needle tip, both at rest and while the patient gently contracts. It is the second half of a standard electrodiagnostic study and the part that lets the examiner read a muscle from the inside.

The needle is solid, not hollow, and nothing is injected; the discomfort is real but brief, more a series of small pinpricks than the deep ache of a blood draw. The examiner samples several muscles, listening for three things: insertional activity, the brief burst when the needle first moves; spontaneous activity, whether the muscle crackles abnormally on its own while at rest; and, on voluntary contraction, the shape and recruitment of the motor unit action potentials, the building blocks of muscle firing. Each muscle is chosen deliberately, because its particular nerve supply and root level let the pattern of findings localize a lesion.

Needle EMG is what gives the electrodiagnostic study much of its power and its ability to localize. Because a muscle's electrical behaviour changes in characteristic ways after its nerve is injured, after a few weeks of axon loss, or in muscle disease, the needle exam can distinguish a nerve problem from a muscle problem, time roughly when an injury happened, and map which roots or nerves are involved, answering questions imaging often cannot. It is also the part patients fear most, so good examiners explain it plainly and work efficiently.