Chapter IV

Performing the pf-NCS




PART I - PF-NCS Basic Steps

Step #1 Breaking Electrical Impedance: By far, the most important thing for the examiner to understand is that the voltage intensity required to break the skin's resistance is usually higher than the intensity causing the A-delta fibers to fire a threshold spike (action potential). This means that until the skin's resistance is broken, the voltage cannot reach the nerve. This also means that the sensation the patient feels when the resistance is broken is stronger than what s/he will feel on the second test, when the minimum voltage causes firing. This difference in sensation should be explained to the patient so s/he understands that the second test will probably feel much weaker.


The skin's electrical resistance is overcome in the same way that the air's electrical resistance is overcome before lightning strikes. As energy builds in a storm cloud, the air becomes charged (ionized), which forms a pathway along which the electricity can flow. When the air becomes sufficiently ionized the lightning strikes, traveling between the cloud and the earth (ground).

The pf-NCS is the same as lightning. As the dial is turned up and the energy increases, an ion charged pathway is formed between the sponge ground (earth) and the saline-soaked cotton tip of the test electrode (cloud). The difference is that lightning releases all its energy when it strikes, and the ion pathway collapses. In the pf-NCS, when the skin's resistance is overcome the ion pathway persists so long as the dial is not turned too close to zero. This is why the testing protocol is to break the skin's resistance, and then immediately turn the dial down to a little below 10, but not to zero. At a setting of 10, approximately 1 mA at 2.5 volts of electricity is passing through the body to maintain the ion pathway. This allows an approximate measurement of the threshold to be made the second time the dial is turned up.


In Figure A, the dial is being turned up but the ion pathway has not as yet formed and the current is not passing through the body. In Figure B, the resistance is broken at a dial setting of 32 and electricity passes between the test electrode and the ground. Notice that the voltage has triggered the nerve membrane to fire, and a second or two later the brain feels the sensation. In Figure C, the dial is back at 10, and without firing the nerve, the trickle of electricity is maintaining the ion pathway.

Step #2 Estimating A-delta Fiber Threshold: The second time the dial is turned up, a close estimate of the voltage intensity causing an action potential can be made. He may be expecting the sensation to be as strong as the first. Therefore, It is important to say to the patient: "This time it will feel much weaker, so pay close attention and say ‘NOW' the INSTANT you feel the slightest tickle." Wait 3 seconds to allow the patient time to focus his attention back on the test site. In Figure D, the patient feels the second stimulation at 26. Again, the dial is turned back immediately to 10, which maintains the ion pathway.

Step #3 - Close In On The Threshold: Step #3 to repeat Step #2 until you are satisfied you have found the threshold. On these test is sufficient to say; "Again" and WAIT 3 SECONDS before starting the dial up, since the patient is now expecting the sensation to feel weaker.

Step #4 Exact - Objective - Measurement: Same as Step #2-3, tell the patient to say NOW the INSTANT he feels the sensation, wait 3 seconds before starting the dial up. When the dial is 2 points below the setting where he has been saying NOW, step on the foot peddle while you continue to turn the dial up. When he says NOW step on the foot peddle again. The peddle starts and stops the recording of the real-time waveform.

Sensory injury causes a reduction in glutamine neurotransmitters, so a stronger than normal stimulus is needed to initiate firing of a threshold A-delta action potential (nerve impulse).

REVIEW - THRESHOLD ACTION POTENTIAL: At threshold a subject feels a slight sensation when a stimulus is strong enough to cause a sudden increase in the number of fibers that are firing. For example, near threshold the number of firing fibers ramps up, say from 200 to 400 to 1000, then, within a few milliseconds (ms) the number firing jumps to >3 million. This spontaneous jump is termed Action Potential Summation. Potential = Voltage. Action Potential = Moving Voltage (i.e., voltage moving along the nerve fiber as each voltage-gate opens producing a burst of voltage that opens the next and the next.) Motor fibers are 50 times larger than A-delta fibers and require a loss of at least 50% of the myelin covering before EMG/NCV can begin to detect changes in velocity, latency or configuration. Therefore, these responses are of no known diagnostic value in A-delta fibers, however, combined they verify the strength of stimulus required to cause a threshold firing.

Step#1: Break the skin impedance.
1. Tell the patient; "You will feel a very slight tickling sensation. Say NOW the INSTANT you feel it here (wiggle the Q-tip) at or near this electrode." WAIT 3 SECONDS BEFORE STARTING THE DIAL UP.
2. When he says NOW, quickly turn the dial back to 10 - DO NOT LIFT THE Q-TIP - DO NOT TURN TO ZERO.

A patient's mind is wondering, so always give a warning that you are going to test again. Without a warning threshold measurements will be inaccurate.

Step# 2: Find the dial setting where he says NOW:
1. Tell the patient; "This time it will be MUCH WEAKER, Say NOW the INSTANT you feel anything." WAIT 3 SECONDS BEFORE STARTING THE DIAL UP, so he has time to refocus back on the Q-tip.
2. When he says NOW, turn back to 10 and say; "Again" WAIT 3 SECONDS before starting the dial up.
3. Repeat, and each time turn a little slower to find the dial setting where he says NOW. ALWAYS TURN BACK TO 10 IMMEDIATELY, and NEVER LIFT THE ELECTRODE or TURN THE DIAL TO ZERO.

Step# 3: Record Waveform:
The brain takes a second or two to process the sensation and say now, so firing actually occurs at a setting just below where he says NOW. For our example let's say he is saying NOW at 30.
1. After finding the setting where is says now, test one last time and press the foot-peddle at 28 (2 points below where he says NOW). Turn up slow and smooth and press the peddle as you hit the number. The first foot-peddle press starts the waveform recording.
2. When he says NOW, press the foot-peddle again to stop the recording.


PART II - Recruitment - More Objective Proof

Recruitment is a process wherein, as nerve fibers fatigue, adjacent fibers are induced (recruited) to fire. Recruitment increases total voltage output detected by the potentiometer. It is more important to prove that a nerve is abnormal than to verify that a nerve is normal. Recruitment gives the strongest evidence that abnormal A-delta fiber hypo-functioning exists.

After completing the test, return to the nerve with the highest amplitude measurement, which is likely the main pathological nerve. Break the skin's impedance as explained in Step #1 and then proceed through to Step #3. Do not expect the measurement to be exactly the same as before because testing often shifts the nerve's sensitivity. This will be explained later in this chapter. Once the threshold is detected, turn the dial down 2 points and leave the dial at this lower setting for 60 seconds. If the nerve is not firing the potentiometer number will fluctuate within a small range but will not increase. Next, turn the dial up the 2 points to where the sensation is felt and leave the dial at this setting for 60 seconds. If the nerve is firing the potentiometer number will increase. Recruitment at the higher amplitude, and not the lower, verifies that the higher dial amplitude causes conduction of an action potential.


PART III - Detect Malingering
& Technical Misconceptions

Malingering: If recruitment is noted within the normal stimulus intensity and the patient reports he feels nothing, then he is trying to defeat the test. Given enough time recruitment will eventually increase to the point that the patient will experience an involuntary withdrawal reflex, which proves he is a malingerer.

There are a number of reasons why an experienced examiner can easily detect an attempt to defeat the test based on the patient's perception alone. A) Weber & Fechner Principle: Based on the sensory discrimination principle of Weber and Fechner, sensations innate to humans, such as temperature, pressure and etc., cannot be judged within the same narrow range that we can detect a voltage threshold sensation. Therefore, if a patient repeatedly reports that he feels the sensation within 1 dial point he can only be reporting the threshold sensation. B) Brain Interpretation: The cuneate nucleus is a major switching relay for somatosensory signals, and it is felt that sensory experience is influenced by the crossovers of transmissions in this region. C) Multimodality of A-delta Fibers: A-delta fibers are multimodal in that they transmit several types of sensations, from temperature (hot to cold) to pricking pain, tickling, vibration, pressure and tickling sensations. D) Neuron Patterns/Ensembles: Firing is felt to switch between ensembles of neurons that transmit different sensations. The concept is that the first firing may involve an ensemble that sends, for example, pressure signals. Once fired this ensemble is not as easily potentiated to fire as easily as a second ensemble that may transmit vibration, and so forth. Whatever the mechanism, or mechanisms involved, the fact is that the sensation changes with each threshold serial test. An attempt to defeat the pf-NCS involves the patient not reporting when he feels the first sensation, but waiting until it feels stronger of different and then repeatedly reporting that different or stronger sensation to be the threshold. The problem for the malingerer is that the sensation changes. The first threshold sensation may feel like a vibration, while the second is heat, the third pressure, etc. As a result, the malinger cannot give consistent responses as can a patient who is honestly reporting when he feels the threshold sensation.

Technical Misconceptions: There are both anatomical and physiological factors which make testing large fibers different from testing small pain fibers. The difference between large fibers and small pain fibers is not only size, but also the fact that pain fibers have little or no myelin (see Figure #2 on page 22). Conventional EMG/NCV and NCS are based on gross myelin loss or severe nerve damage, which changes conduction velocity, latency and configuration. Pain fibers are too small for such tests and, since they have little or no myelin, the velocity, latency and configuration would not be diagnostic even if they could be tested using conventional large fiber methods. The diagnostic measurement for pain fiber function is the intensity/amplitude of voltage causing the conduction of a nerve impulse (action potential). Capturing the real- time amplitude, velocity/latency and configuration of the action potential summation verifies that conduction occurs at a specific intensity of voltage stimulus. It is not known if these measures are of any diagnostic significance in and of themselves, but they do act to verify that the threshold firing takes place at a specific stimulation intensity.

The color coded dial markings of most pf-NCS devices are separated by 2 point increments between the large numbers. These color marks allow the examiner to make a quick mental note of the dial position at a glance. This allows the examiner to immediately turn the dial back to 10 and then take his time looking at the dial to determine the exact measurement.

To allow the examiner to focus on the dial without distraction, it is important for the hand holding the test electrode to be anchored on the patient or table. Anchoring also prevents any sight motion, which the patient could mistake for the conduction sensation.

When the patient reports the sensation, the examiner immediately turns the dial down, NOT TO ZERO. Turning too close to zero will stop the electrical flow, in which case, breaking the skin's impedance would be again necessary. Hence, the examiner must turn the dial down to a little below 10. Meanwhile DO NOT LET THE TEST ELECTRODE BREAK SKIN CONTACT. Breaking contact stops the electrical flow in the same way that turning the dial too close to zero can stop the electrical flow.

Exception to turning back to 10: There are exceptions when the dial should not be turned back to 10. One exception is in cases where the skin's impedance is extremely high. This is most often seen when testing sites such as the plantar branches on the bottom of the foot. Skin thickness can cause the ion pathway to collapse at rather high settings. This can be controlled by placing the ground closer to the site being tested. For example, in testing the foot the ground can be placed under the calf of the side being tested. The closer proximity of the ground aids in maintaining the ion pathway. However, even with the ground close to the test site, the ionization pathway may collapse at a high dial setting. In such cases what indicates collapse is that subsequent measurements are high and unstable because each test is breaking the constantly shifting impedance (this is called shifting). If sifting is noticed, turn the dial back about halfway and ask the patient if he feels any sensation. You might even try turning the stimulus off to let him feel no sensation (just the cotton tip) then, once again, turn it up until he feels it and immediately back to the halfway point to let him compare this to no sensation. In this way, the test can continue with assurance that collapse of the ion pathway is not a factor.

Inter and Intra-Operator Repeatability: The pf-NCS examiner must be efficient and fast to avoid shifting the threshold. The good news is that there is an ample window of time within which an accurate and repeatable measurement can be made - usually a minute or two. However, regardless of how fast and efficient the examiner may be, the electrical stimulation initiates either a cascading release of histamine or desensitization by accommodation. For this reason, comparing inter and intra-operator repeatability is not practicable. Conventional electrodiagnostic tests use inter and intra-operator repeatability to demonstrate efficacy, but these tests are based on fixed anatomical changes such as gross myelin loss or severe nerve fiber damage. By contrast, the pf-NCS is based on functional sensitivity which the test itself may alter. The pf-NCS should be looked upon as a snapshot of the state of the A-delta fibers. Testing a few minutes later or hours later, in most cases, cannot be expected to result in nearly identical measures. The general pattern may be similar a day or two later, but injury and entrapment change over time, making sensitivity transient and unstable.

Patient complains of pain, but pf-NCS is normal: Findings can be completely normal in spite of the fact that the patient may complain of severe pain. In such cases, the problem is not due to nerve damage; pain is due to damage of other tissues and the A-delta fibers are simply telling the patient that he has damaged tissues. Another reason for a negative test is that at that time the patient had little or no symptoms. This points out that it is important to determine how severe the patient's symptoms are when the examination is performed, so always have him grade his symptoms using the visual analog scale (VAS).


Lidocaine Paradox: Though no formal study has been conducted, it has been reported that A-delta measurements return toward normal function following epidural injection of Lidocaine. This constitutes an apparent paradoxical reaction in which the A-delta fibers become more, rather than less, sensitive in the presence of Lidocaine. This paradoxical shift has also been reported with other types of analgesics. This possible paradox is an area ripe for study.

PART IV - Patient Setup

Before discussing patient preparation for the pf-NCS examination, this is an appropriate point to address exactly what the test is not and, thereby, avoid possible confusion.

Pf-NCS vs. Dermatome Tests: There are three things that make a dermatome test different from the pf-NCS: 1) Dermatome tests evaluate sensation mediated through the receptors: 2) Dermatome tests cover an area supplied by the cutaneous branches coming from a specific nerve-root: 3) The pf-NCS does not test a large derma (skin) tome (layer), but assesses the function of A-delta fibers in a major nerve that originates from a specific nerve-root. The pf-NCS tests a single site along the nerve and evaluates that nerve tract.

Pf-NCS vs. Quantitative Sensory Testing (QST): Many aspects make the pf-NCS different from a QST. First, QST employs naturally occurring stimuli that are innate to humans, such as hot, cold, pressure, vibration, etc. This is different from pf-NCS which uses electrical stimulation, which by definition makes it an electrodiagnostic examination (EDX). QST requires the patient to make a judgment as to a change in the strength of a stimulus, such as hotter, colder, more or less pressure, etc. The pf-NCS does not require a judgment of change, but instead a recognition of any sensation and this measurement is verified by a potentiometer, which detects the amplitude of millivoltage produced by the action potential. This objectively verifies that the nerve fires at a certain voltage stimulus amplitude, which makes the pf-NCS unlike a dermatome test or QST.

Patient Preparation

History: A comprehensive history requires listening. Since the A-delta fibers down-regulate, the source of pain is incorrectly localized by over 50% of patients, so it is critical to ask the patient where he felt pain at the onset of symptoms. If the patient can accurately recall the anatomical location of his first symptoms, it is highly probable this will correlate with the pf-NCS findings. The problem is that patients cannot be trusted to remember where they first felt pain. Therefore, if you are sure the pf-NCS was performed correctly, trust it even if the findings show the problem to be on the opposite side from where the patient reports symptoms. The pf-NCS has sensitivity approaching 100%, which is not true of any patient's memory.

Patient cooperation: We all know how a subject can be hypnotized and given the suggestion that an ink pen is a hot poker. A similar thing can happen with patients. Most physicians are unaware that establishing rapport is identical to the first stage of hypnosis.xliii Another word for rapport is trust. From this state of trust the patient is transferred to a nurse who proceeds to the next level of deepening suggestibility. The nurse asks the patient to do something he normally does not do for others - he removes his clothes. This may heighten the hypnotic state and in this heightened state the patient is told what to expect during the test.

Patient Instructions: The worst statement an examiner can make is to tell the patient, "This won't hurt". From childhood we have learned that when we hear this, it often means exactly the opposite, it will hurt. Additionally, most people are afraid of electricity, so avoid using the term "electrical sensations" and, simply, use the term "sensation". "Electrical" will only conjure up the idea of electrocution and pain. The examiner may want to be honest and tell the patient of all the possible sensations that s/he might feel, but this would be a mistake in most cases because the majority of patients will only remember the things that make a strong impression, such as pricking, burning or electric shock. Many examiners instruct patients by saying; "Tell me when you feel anything". However, this does not convey the need to say, "NOW" as soon as the sensation is noticed, and the patient is left to decide for himself how the sensation is going to feel. Furthermore, the patient may have already heard about other nerve tests and how painful they are, so we need to put the patient at ease.

The best instruction: Let the patient form an idea in his mind as to what to expect. Hence, the best thing to say is, "You're lucky because this test doesn't use needles and electric shocks. We call it the tickle test. We can test 5-year old children and they just laugh. I'll touch you with this cotton tip and all you do is say ‘NOW' the INSTANT you feel the slightest tickle. And best, the test won't take long. The doctor does most of the work analyzing the results. OK, let's start. Say ‘NOW' the INSTANT you feel the slightest tickle." The patient has now internalized that the test is painless, it just tickles, and s/he needs to say "NOW" the "INSTANT" that s/he feels the tickle.

Test Sites: Since Benjamin Franklin's experiments and his invention of the lightning rod it has been understood that electricity follows the path of least resistance. Instead of the lightning passing through the wood house and setting it ablaze, Franklin's lightning rod conducts the electricity from a ground spike in the basement and through a wire cable to the rod in the roof. Likewise, since the test sites (Asia points) are nerve centers, they conduct electricity better than the surrounding tissues. This means that if the cotton tip of the test electrode is close to a site the electricity will travel to the site. For most sites the tip only needs to be within 2cm to 3cm of a test site.

Blemishes: It is important to avoid scars, moles, skin abrasions and other cutaneous defects that may conduct abnormally. Generally it is best to test proximal to such defects. Remember to always test the same location on the opposite side for comparison.

Anatomical Position: The description of the test site locations are based on the anatomical position. This is especially important in the upper extremity where lateral is the thumb side of the hand and medial is the little finger side.

body planes

Part V - Examination Protocols and Test Site

Review: Typical pf-NCS Exam: In the cervical and lumbosacral tests the patient is face up on the examination table. In the cervical study the ground sponge (DRIPPING WET WITH TAP WATER) is placed under T6. In a lumbosacral study the ground sponge is placed under L4. Place a towel under the ground to capture the excess water. It is important to anchor the hand holding the test electrode so it cannot move during the test and the examiner can focus on the dial. Notice in the picture how the examiner is resting his hand

on the pillow

on the pillow. Press enough to dimple the skin, but not so hard that it leaves an impression in the skin. The main thing is DO NOT ALLOW THE PROBE (TEST ELECTRODE) TO MOVE. Hold the test probe steady. If the probe moves the patient may mistake this movement as that of the stimulus sensation.

Cervical Plexus
Ground and potentiometer electrode placement
The examination forms in the Delta NCS System manual show the exact placement for the potentiometer electrodes. Note that some nerves are considered to be controls, however, all nerves and branches must be tested to allow for accuracy detecting pathology.

Waveform Potentiometer Electrode Placement

The Delta NCS System manual shows the exact placement for the potentiometer electrodes on the examination charts.

Alternative Ground Sponge Placement: When testing the lower extremities, especially the bottom of the foot, place the ground sponge under the calf and test that side. After testing one side, rewet the sponge (Dripping Wet), place it under the other calf and test that side. The potentiometer electrode placement: The potentiometer electrode can be moved as required to obtain clear measurements. If you cannot obtain what seems a reliable reading, the results may be recorded as EQ (equivocal). This is well within the parameters of EDX studies. In fact many times an entire EMG-type study is equivocal.

Upper Extremity Study (Graph Analysis): Place the ground sponge (Dripping Wet) under T6 (same as the cervical study). If the patient has a pacemaker, place the ground under one scapula and test that side. Then rewet the sponge (Dripping Wet) and place it under the opposite side and test that side.

on the lateral thumb

To test the palm and lateral thumb, have the patient open the hand at his side, with the palm up. After testing the palm sites and lateral thumb site, place a towel on the patient's abdomen and put his hand palm side down on the towel, then test the remaining digital sites. Dip the tip in the saline solution between each test site and briefly touch the tip to a towel to catch any drip. If the drip is left it may run across the skin and the patient may mistake this as the sensation. Repeat the above procedure on the opposite side.

nerve roots

Anatomical Considerations The C6-7 nerve-roots are the origin of the median and radial nerves. They separate in the brachial plexus and travel to their respective areas in the upper extremity.

The motor branch of the median nerve passes through the carpal tunnel. However, the sensory palmar branch of the median nerve does not pass through the carpal tunnel, nor does the sensory palmar branch of the ulnar nerve pass through Guyon's canal. Therefore, sensory palmar branch dysfunction suggests possible proximal pathology.

If the digital branches and palmar branches both demonstrate dysfunction, the problem could be a double crush - the wrist and proximal. The radial branches, with sites on the back of hand, have the same origin as the median nerve (C6-7), so the radial nerve rules in or out cervical radiculopathy. Testing above and below the medial elbow detects cubital tunnel entrapment. The non-symptomatic side is used as a control.

Lower Extremity Exam (No Graph - Numerical Analysis): The physician chooses which of the nerves to test; sites are tested above and below the location of a suspected entrapment. Sites on the non-symptomatic side are tested for controls. Significant findings are usually noted by a 30% or higher measurement distal to the suspected entrapment, and verified by comparing the measurement to the opposite side. If the proximal site also tests high then the problem is likely proximal, such as radiculopathy.

foot nerves
foot nerves

Neuroma (No Graph - Numerical Analysis): The lumbosacral or lower extremity test may be performed before to rule in or out proximal entrapment or radiculopathy.

nerves / toes

Only branches suspected of pathology need to be tested. Ground placement is under the calf of the side being tested. Remember, rewet the ground and move it to the opposite side before testing the opposite side. Neuromas are detectable by testing the digital branches associated with the suspected neuroma and comparing the measurements with the same digital branches on the opposite (i.e., normal) side. Note: A Neuroma may cause HYPER-function. As with all studies, the measurements supply a baseline for future comparison.

Trigeminal (No Graph - Numerical Analysis):

nerve sites

Consider first performing a cervical study to rule in or out pain referred from the cervical spine. Ground placement is the same as cervical study. Mark sites by drawing a circle around each. Be careful not to over stimulate the area. The sites do not need to be the same as shown on the chart. Pick the sites and mark them on the examination sheet. Test the exact same sites right and left for comparison. A 20% or more difference suggests pathology.

Thoracic Exam (Graphic Analysis):


With upper thoracic problems consider a cervical study first to rule in or out referred symptoms. If the symptoms are in the lower thoracic region consider performing a lumbosacral study. Ground placement is under either thigh with the patient sitting.

Have the patient forward flex and extend the neck while your fingers are on the tips of the lower spinous processes of the cervicothoracic junction. The last spinous process moving forward and backward is C6. Count down to T4 and place a circle 1 inch to each side of T4 and 1 inch to the side of the next 11 spinous processes. Be careful not to over stimulate the area while drawing circles around the sites. Due to anatomical variances of the thoracic cutaneous nerves measurements give a general idea of the level of pathology. (See thoracic radiographic correlation page 148).

Custom Exam (No Graph - Numerical Analysis):


Always consider a study of the region proximal to the area of interest. For example, if cubital tunnel entrapment is suspected first perform a cervical study. If the problem is suspected in the area near the knee, perhaps a lumbar study should be performed first. Mark the sites to be tested by drawing a circle around the sites, being careful not to over stimulate the area. The anatomical site is typed into the software in brief terms. For example, typing, "knee femoral nerve", results in the report reading, "Above the right knee femoral nerve 45, below the right knee femoral nerve 83."


Controls ensure accuracy, so test the exact same sites on the opposite side for comparison. Deviations of 20% or more are significant above the navel and 30% or more is significant below the navel. Always take the history into consideration because this rule of 20% and 30% is the average deviation. In some cases lesser deviations may be significant. In any case the measures become a baseline for future comparison to evaluate change. Find the raw data at the bottom of the report.

20% / 30%


The device manuals show the steps for calibration. It should be kept in mind that a lower or high calibration will not affect accuracy in detecting pathology. All of the measurements are on the same scale, making the measurements relative. In other words, the measurements are all plus or minus by the same percentage so they are relative and, therefore, comparable. Ground Sponge
Many problems can be traced back to the sponge being too dry. Be sure the sponge is DRIPPING WET with TAP WATER. If the sponge is too dry, the current may not flow effectively. For example, the dial could be turned to 100 and still the patient feels nothing. Also, a too dry sponge can cause the ion pathway to collapse at a fairly high setting as the dial is being turned down. If a reading is high and shifts then suspect the sponge is too dry. Another possible problem with a dry sponge is the patient may feel the sensation at the sponge.

Do not soak the sponge in saline.
Tap water works quite well; saline can cause itching and possibly cause the patient to develop a mild rash.

In a cervical study where the patient has a pacemaker or in any situation where the examiner is not sure if it is safe to place the sponge between the shoulders, place the ground (Dripping Wet) under the scapula and test that side. Then rewetted sponge and move it under the other scapula and test that side. DO NOT ALLOW THE CURRENT TO PASS THROUGH THE CHEST. Pacemakers are a concern only in upper body tests, i.e., cervical, trigeminal and upper extremities.

1) Do not remove the sticky gel over the three black contacts.
2) Wipe the skin with alcohol. LET IT DRY before placing the electrode.
3) If the potentiometer screen reads MEMORY FULL, don't turn it off. Leave it turned ON and in about 15 seconds it will return to normal operation. It does this because the potentiometer has a memory which allows it to interface with a computer to generate a graph; this is not needed in the pf-NCS.
4) If the potentiometer does not seem to be working it is often the cable. Look in the manual for the page regarding ordering accessories. This page includes information related to the cables and electrodes, which are not covered by the limited warranty.

Test Check List:
Avoid Patient Confusion
It only confuses the patient to provide a list of what he may feel. Instead explain to the patient that s/he may expect to experience a slight tickle, and that it will feel much weaker after the first sensation at each test site. After testing about four sites patients usually remark that it feels different at each site and even different each time at the same site. Confirm this experience by saying, "Yes, it may feel different, but remember to say NOW the INSTANT you feel any sensation."
Do not talk while turning the dial; this is a distraction.
Do not ask questions, such as; "Do you feel it?" If you think the patient missed the threshold sensation because the measurement is too high, turn down the dial to 10 and say, "Tell me, again, the INSTANT you feel the tickle." Wait 3 seconds and start turning the dial up. The patient will often feel the sensation at a distant point along the nerve being tested. For example, testing T1 (at the point above the medial elbow) the patient often reports feeling a sensation in the little finger (ulnar nerve). Assure the patient that this is normal. The brain is feeling the sensation from the nerve that comes from the little finger.

PART VII - Sympathetically Mediated/Maintained Pain, Allodynia

General Considerations: Usually an injured nerve's threshold sensation is not different from that of a normal nerve - both are a tickling sensation. However, the A-delta fibers transmit hot, cold, vibration and pressure. Therefore, a patient may report the threshold sensation as any of these. In patients diagnosed with one of the several types of pain syndromes, referred to as allodynia, it has been observed that the threshold sensation is not a tickle but a painful sensation, which causes a withdrawal reflex away from the electrode.

Testing a patient suspected of having allodynia is a challenge. If the skin's impedance is much higher than the threshold the breakthrough sensation experienced when overcoming the impedance may be uncomfortable. For this reason it is necessary to tell the patient to pay attention before the second test because the sensation may feel much weaker than the sensation associated with breaking the impedance. Note that if a patient reports an uncomfortable sensation on the first breakthrough, this does not ultimately suggest allodynia. However, if electrode contact is not broken and the dial not turned to zero and then the next test causes pain, this may suggest some type of allodynia.

ll To prevent the patient from pulling away from the electrode, it may be necessary to both assure the patient that the stimulus will be turned down immediately when he feels anything and to hold the test electrode in such a way that the patient cannot pull away. Only then can the second test determine if the nerve's threshold sensation is truly painful.

Baseline Data: If allodynia is suspected, it is recommended that the nerve-root levels above and below the suspected nerve be tested for threshold and pain tolerance threshold using all three frequency settings: 2000 Hz (A-beta), 250 Hz (A-delta) and, 5 Hz (C-Type) fibers. Threshold can be quickly estimated without using the potentiometer. Pain tolerance is measured by asking the patient, "Tell me what this feels like as I turn it up." Then, turn up the dial at a steady rate and the instant the patient pulls away from the electrode immediately remove your fingers from the dial. The dial setting is his pain tolerance threshold. These tests are invaluable as baselines to monitor these disorders.

When testing all three fibers, it is recommended that the 2000 Hz sensitive (A-beta) fibers be tested first, since they transmit painless light touch sensations. Next the 250 Hz (A-delta) fibers are tested for pain tolerance since they have already been tested for threshold. The last should be the 5 Hz (C-Type) fibers. Before each test it might be effective to say, "Let's see if you even feel this." If you tell the patient to tell you when it hurts, he may, due to the power of suggestion, say the first sensation hurts.

Polyneuropathies: Diabetic polyneuropathies advance from the lower extremities upwards. The earliest functional change occurs in the lower extremities and is detectable by hypo-function in the A-delta fibers. Though a comparison can be made between the general difference of the feet, hands and neck, looking for what is termed "stair-stepping", wherein the feet are more hypoesthetic than the hands, while the hands are more hypoesthetic than the neck, it is reported that simply performing a lumbosacral study looking at the general change in the long nerve: Saphenous (L4), peroneal (L5) and sural (S!), allows detection of early onset polyneuropathies. Indeed, it has been reported by many endocrinologists that early detection of A-delta fiber hypo- function has greatly reduced the number of diabetic patients undergoing lower extremity amputations. In these types of cases, if may be desirable to test with all three frequencies to supply more data to follow the progress of the disease.

Chapter III   -   Table of Contents   -   Chapter V