NCS Electrode Placements

Nerve Conduction Velocity lab
Motor NCS

Objectives

Set correct parameters of sweep speed, gain, stimulus duration, pulse rate frequency. Define stimulation and recording sites for median, ulnar, and peroneal nerves.
Evoke and record M waves for median, ulnar, peroneal nerves.
Record latency values; calculate NCV for median, ulnar, and peroneal nerves.
Describe physiologic rationale for any changes in CMAP amplitude, latency, NCV.
Describe possible pathology that may affect data.
Describe and explain possible sources of error affecting NCV validity, reliability.
Median nerve.

Recording electrode: Cathode over motor point abductor pollicis brevis, 1/3 distance between origin, insertion. Palpate muscle. Anode over tendon at MP joint.
Stimulating electrode: Wrist – between tendons of flexor carpi radialis, palmaris longus-6 cm proximal to recording site, cathode distal. Elbow-palpate brachial pulse. Cathode stimulation site over or medial to pulse.

MotorSensoryRecord

Ulnar nerve. Position arm in 90 degrees elbow flexion, shoulder abduction, external rotation.

Recording electrode: Palpate abductor digiti minimi-cathode over motor point, 1/3 distance between origin, insertion. Anode over tendon at MP joint.
Stimulating electrode: Wrist—medial or lateral to flexor carpi ulnaris tendon, 6 cm. Proximal to recording site.
Proximal forearm stimulation is in the forearm medial intramusculature septum. Another stimulation site is in the ulnar groove (medial intermuscular septum) proximal to the elbow.
Peroneal nerve

Recording electrode: cathode over center of palpable portion extensor digitorum brevis; anode over tendon at base 5th metatarsal of 5th toe.
Stimulating electrode: Ankle-fossa lateral to tibialis anterior tendon at level of proximal malleolus. Proximal stimulation at fibular head, or popliteal fossa medial to biceps femoris tendon. Stimulation should ensure a foot dorsiflexion response.

Norms Distal motor latency Amplitude NCV
Median <4.0 ms  > 4 mV >49 m/s
Ulnar <3.1 ms > 5 mV >49 m/s
Peroneal <6.0 ms > 2 mV >40 m/s

 

 

 

Sources of error.

Recording cathode not over motor point will produce small response, or absence of clean initial deflection.
Stimulating cathode not over nerve will produce small response, absence of crisp initial deflection, or cause current spread to near by nerve.
Inadequate stimulation will produce small response, or significant amplitude difference between stimulation sites.
Stimulation much greater than supramaximal may cause stimulus to nearby nerves.
Stimulation with anode instead of cathode.
Incorrect measurement of stimulation sites, calculation of NCV.

 

 

 

NCS method/procedure
 establish proper instrumentation gain (vertical sensitivity) and sweep speed.
fix recording electrode cathode and anode appropriately for the nerve to be studied.
identify nerve stimulation sites for the nerve to be studied.
ensure that recording electrodes are connected to amplifier, which is turned to on position.
place stimulating cathode over distal stimulation site. Advance stimulus intensity. Record and observe CMAP on oscilloscope, ensuring supra-maximal stimulation. Adjust stimulus pulse duration, instrument gain, sweep speed if necessary.
record latency value and CMAP amplitude.
repeat stimulation procedure again over proximal site(s).
calculate motor NCV and compared CMAP amplitude measurements between sites. Compare data to norms.

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Ray Jurewicz
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E-mail: rj@NerveStudy.com Web Site design by Larry Berman and Chris Maher

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