In electromyography chapter an important section is neurography, the most important, and essential
aspect is the studying of nerve function with sensory conduction velocity ( SCV ).
To calculate SCV you need to be able to record electric potentials generated by signal transit through
the nerve .
Sensory NCV tests are performed by electrical stimulation of a peripheral nerve and recording from a
purely-sensory portion of the nerve, such as on a finger. The nerve is constituted by a lot of filaments
(axons), along each filament an electric impulse (Action Potential) can propagate. The action potential
has a low voltage and can be recorded only with electrodes directly positioned on the nerve. In order not
to damage the nerve using needle electrodes, to record the sensory action potential signal, cup
electrodes are used. Stimulating more axons rise up sensory signal and further synchronised average
helps obtaining a cleaner response.
Sensory latencies are measured in ms. Sensory amplitudes are measures in microvolts (µV). The
sensory NCV is calculated based upon the latency and the distance between the stimulating and
recording electrode. This can’t be changed. Note: while natural delay between stimulation and start of
nerve response does not normally affect velocity calculation, it must be considered in case stimulation
and recording electrode distance is very low.
Usually SCV test is performed on short nervous paths (e.g. stimulating on a finger and recording on the
wrist) with a distance of about 9-12 cm, and the two stimulation and the two recording electrodes have
an inter distance of 2-3 cm. Therefore, it is important to have a standard way to place negative and
positive stimulation and recording electrodes. Usually the stimulation electrode is placed closer to the
recording cathode (negative) electrode and the first peak on the response, typically biphasic, is referred
to the electrode closest to the stimulation area. The distance has to be calculated between cathode and
the derivation electrode closest to this one, and the propagation time is measured between the
beginning of the trace and the first peak of the sensitive response.
Electrode positioning
The S response (sensitive) can be recorded in several nerves. The recording electrodes have to be
positioned over the nerve to be analysed.
This response must be studied in an orthodromic sense depending if the stimulation is distal or proximal
to the derivation point (see the above pictures). On the orthodromic response you stimulate in a nerve
point and you record in a proximal position along the usual direction of sensitive fibres.
It’s preferable to place the ground close to the stimulating point, between the stimulating and the
recording area, but there can’t be a direct connection between this one and the other electrodes.
Because of the small distance between the electrodes and the ground, there can be a short circuit
caused by the presence of water or conductive gel, generating artefacts.
To have an high signal quality the electrode impedances should be lower than 5 Kohm and balanced
between the different electrodes. In order to have a small stimulus artefact it is also important to have a
low contact impedance of stimulation electrodes too (using a gel on stimulating electrodes greatly
enhance signal quality).
Stimulation
The stimulation intensity is regulated in a way to be felt by the patient without pain, but it cannot be too
much low, especially if you have to evaluate the sensory response. So you have to stimulate with an
intensity sufficient to have a maximal response without activating other nervous structures different to
the nerve under examination.
The stimulus duration can be short (100 µs), but not shortest if you don’t want to increase the stimulation
intensity. Average intensity of the fingers stimulation are 8-12 mA, for the wrist nerves (median and
ulnar) 6-10 mA, for the sural nerve 30-40 mA.
Acquisition parameters
Number channels: 1
High Pass filter: 20 Hz
Low Pass filter: 2000 Hz
Base Time: 10 – 50 msec
Max Signal 400 µV
Gain/Div 20-100 µV/div
Averaging 20-40 means
After electrode application, and after inviting the patient to stay relaxed in order to avoid the generation
of electromyographic activity, you have to start stimulation using a low intensity (4-5 mA) increasing the
intensity until the impulse is felt by the patient and the sensitive response is clear on the online EMG
trace. Then begin the average calculation, until you get a clean signal. Usually 5 to 20 averages are
sufficient to obtain a good signal, but more averages may be required for difficult studies. Correct
electrode positioning is essential for obtaining a good response.