Tests Performed
(Nerve Conduction & Needle Electromyography)
Motor nerve conduction: Tests the peripheral nervous system to the arms, legs and face. This test checks the motor fibers. Tests are performed using surface electrodes to record the compound potentials following maximum stimulation by short duration pulse (0.1 ms.) of electric current at two or more points of the main trunk of the motor nerve and recording orthodromically the latencies at a distal muscle supplied by the nerve.
Sensory nerve conduction: Tests the peripheral nervous system to the arms, legs and face. This test checks the sensory fibers. The conduction of sensory nerves is determined using surface electrodes to record the nerve potentials after maximum stimulation by short duration pulse (0.1 ms.) of electric current at one or more points of the main nerve trunk or branches of the sensory nerve. The recording can be done either orthodromically or antidromically in another point of the nerve.
F-Waves: F-waves determine the proximal conduction of the motor nerve to the spinal cord. It is obtained by supramaximal stimulation of the motor nerve and recording the antidromic activation of the motor neurons in the spinal cord that discharged back to the same motor nerve. Surface electrodes applied to a distal muscle record this activity. F-wave latency is obtained from an average of multiple responses.
H-reflexes: H-reflexes also determine the proximal conduction to the spinal cord. It is obtained with submaximal activation of a compound nerve with stimulation of the large size sensory fibers that give rise to a miotactic reflex with activation of motor neuron with and subsequent contraction of a distal muscle and related by the activated neuron. This activity is recorded by surface electrodes applied to the distal muscle. The H-reflex latency is also obtained from an average of multiple responses. This test is identical in respect to a deep tendon reflex, although it is electrically elicited instead of mechanically. (i.e. Achilles reflex)
Somatosensory Evoked Potentials: Somatosensory evoked potentials are determine from transcutaneous stimulation of the peripheral nerve using electrodes placed on the skirt of the selected nerve. The potential generated in the cerebral cortex is recorded with surface electrodes placed on the scalp. The potential recorded is obtained by averaging several hundred responses. SSEP's help to determine if there is a demyelinating lesion in the peripheral nervous system.
Dermatomal evoked potentials: Dermatomal evoked potentials are determine from transcutaneous stimulation of the sensory dermatome. The potential generated in the cerebral cortex is recorded with surface electrodes placed on the scalp. The potential recorded is obtained by averaging several hundred responses. DEP's test specific nerve root levels. This is very beneficial in helping diagnosed radiculopathy. And most cases the sensory fibers of the nerve are affected prior to the motor fibers. The needle EMG may produce false-negatives early on and miss these early signs since it takes about 21 days for changes to occur in the muscle.
Electromyography (EMG):
For an EMG, a needle electrode is inserted through the skin into the muscle. The electrical activity detected by this electrode is displayed on an oscilloscope, and may be heard through a speaker.
After placement of the electrodes, you may be asked to contract the muscle (for example, by bending your arm). The presence, size, and shape of the wave form -- the action potential -- produced on the oscilloscope provide information about the ability of the muscle to respond when the nerves are stimulated.
A nerve conduction velocity test is usually performed in conjunction with an EMG.
EMG is most often used when people have symptoms of weakness and examination shows impaired muscle strength. It can help to differentiate primary muscle conditions from muscle weakness caused by neurologic disorders.
