Nerve conduction studies

In the arm: median and ulnar nerves

In the leg: tibial and peroneal nerves

In the arm: median, ulnar and radial nerves

In the leg: sural and superficial peroneal sensory nerves

Recording surface disc electrode is placed over APB

Reference surface disc electrode is placed 3-4cm away on inactive (non-muscle) area like a tendon or bone

A ground electrode is placed between the recording electrode and stimulator

The motor nerve that innervates the APB muscle (median nerve) is stimulated at the wrist and in the antecubital fossa

Current is applied in milliamps and is increased steadily until all muscle fibers have been depolarized

The resulting waveform is the Compound Muscle Action Potential (CMAP); it is the summation of all the muscle fibers that were depolarized from median motor nerve stimulation

Sensory nerve testing is done in a similar manner; the resulting action potential is termed Sensory Nerve Action Potential (SNAP)

Latency is the time interval between the onset of a stimulus and the onset of the motor or sensory response (CMAP/SNAP); for motor nerve conductions the latency = time it takes the nerve to depolarize, time it takes the signals to traverse the neuromuscular junction, and time it takes the muscle to depolarize

Amplitude is the maximal height of the action potential (CMAP)

Conduction Velocity is how fast the fastest conducting fibers carry the impulse down the nerve; for motor nerve conduction studies, two points of stimulation (distal and proximal) are needed to calculate the nerve’s conduction velocity

Antidromic electrical impulse travels in opposite direction as normal physiologic conduction

Orthodromic electrical impulse travels in the same direction as normal physiologic conduction

F-Wave/response is a CMAP evoked by a small population of anterior horn cells; it is achieved by antidromically stimulating a motor nerve at its most distal point using maximal electrical stimulus; the latency of the F-response represents the time required for the stimulus to travel antidromically toward the spinal cord, stimulate a population of anterior horn cells, and then travel orthodromically down the same nerve along a very small percentage of nerve fibers; F-responses are prolonged in polyneuropathies and entrapment neuropathies; their greatest use is in identifying early polyradiculopathy such as occurs in AIDP, where demyelination commonly begins at the nerve roots

H-reflex is a CMAP evoked by orthodromically stimulating sensory fibers (afferent Ia muscle spindle), the impulse then synapses on an alpha motor neuron which then sends an efferent signal down their axons to cause muscle contraction; it is the electrical correlate of the S1 tendon ankle reflex; lesions that may prolong H reflex include: polyneuropathy, proximal tibial and sciatic neuropathy, lumbosacral plexopathy, lesions of the S1 nerve root