Electromyography & Nerve Conduction Velocity
Electromyography (EMG) and Nerve Conduction Velocity (NCV) are used to assess the integrity of the peripheral nervous system.
EMG
Needle EMG is used to detect electrical abnormalities within an individual muscle which may indicate nerve damage or other pathological process. The needle is the "active elctrode" which allows for observation and assessment of motor units and detects electrical abnormalities.
NCV
NCV tests the speed of conduction of action potentials along a nerve. Sensory or motor large diameter, myelinated fibers are tested. This can be used to evaluate disease or damage to the peripheral nerves. This allows for an assessment and diagnosis of the severity and location of any peripheral nerve entrapment.
Testing
NCV testing looks primarily at three things:
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Latency - the time it takes for sensory or motor units to respond after stimulation. This measurement is only assessed for the most distal segment being tested for a motor nerve. This is measured in milliseconds. An increase in time here would indicate a problem with demyelination of the nerve.
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Velocity - this is found by dividing distance by latency. This measurement is assessed in place of latency for the more proximal segments being tested. This is measured in meters per second. A decreased velocity here would indicate a problem with demyelination of the nerve or a conduction block.
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Amplitude - the power needed to reach maximal intensity of a motor or sensory nerve. This is measured in mV for motor nerves and uV for sensory nerves. A decrease here would indicate damage to or a loss of axons. Reduction would be considered pathologic at 50%.
When testing sensory nerves, the stimulation is applied at a distance of 14cm proximal to the cathode.
When testing motor nerves, the stimulation is applied at a distance of 8cm proximal to the cathode. The additional motor sites that are tested are found and marked, then measured. This cannot be standardized as everyone has different arm lengths.
With EMG, a needle is inserted into a muscle belly and the insertional activity of that muscle is assessed. Seeing an increase or decrease in insertional activity are both bad. Spontaneous activity is also bad and could signify axon loss. Motor unit potentials are analyzed by looking at the size, shape, recruitment and interference of those muscle fibers. Motor units typically consist of 12-15 muscle fibers. The patient will be asked to contract lightly at first and then increase to a full contraction.
Abnormalities are
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increased speed = loss of units
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increased size = loss of units, but now there is some recovery
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decreased size = myelopathy
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Polyphasic MUP = previous damage with axon regrowth