Neurodiagnostic Evaluation of the Pain Patient
Herbert N. Chado, M.D., Senior Medical Consultant
Neurotron, Inc.,
410-664-0800;fax 410-664-0831,
E-mail: servcust@neurotron.com
Abstract
Subjective sensory abnormalities, such as radiating pain,
dysesthesia and llodynia, are a common feature of patients presenting to a pain
clinic. Objectively quantifying those
abnormalities using subjective tests (eg tuning fork, safety pin) may be
difficult. There are a number of non-invasive neurodiagnostic technologies
available to supplement the history and physical exam including MRI, thermal
perception threshold and vibratory perception threshold test. The sensory Nerve Conduction Threshold (sNCT)
electrodiagnostic evaluation generates Current Perception Threshold (CPT) and
Pain Tolerance Threshold
(PTT) measures provide objective measures of sensory nerve
functional integrity from the periphery to the central nervous system. Neuroselective sinusoidal electrical
stimuli permits the sNCT evaluation to quickly, painlessly and
objectively quantify the conduction and functional integrity of the large and
small myelinated and unmyelinated sensory nerve fibers. Areas of abnormal function are localized to
aid in diagnosis, determine clinical severity and evaluate the therapeutic
outcome.
Introduction
Subjective sensory abnormalities are a common feature of
patients presenting to a pain clinic.
Reliable documentation of the degree and distribution of a patient's
sensory abnormalities is essential to arriving at a diagnosis. Accurate timely diagnosis of pathology at the
earliest stages is preferable, improving the prognosis and reducing the cost of treating advanced
stages of pathology. A thorough history can be very effective at elucidating a
description of the symptoms, however, objectively
quantifying these abnormalities can be difficult.
The Visual Analogue Scale (VAS) is the most frequent
diagnostic pain measure used. The VAS represents the patients' subjective
rating of their pain intensity. Other
commonly used measures include clinical examinations with safety pins, tuning
forks or pinwheels. These measures are subjective,
highly variable and prone to examiner bias making their use for serial
assessment difficult. There are a number
of non-invasive neurodiagnostic technologies available to supplement the
history and physical exam. Imaging
studies provide a structural view, however, a bulging disc on Magnetic
Resonance Imaging (MRI) does not necessarily indicate pathology[1].
Sensory Nerve Conduction Velocity (sNCV) studies provide a physiological
measure that is limited to the sub-population of large myelinated nerve fibers
confined within a segment of a peripheral nerve, however, a nerve conduction
block does not necessarily indicate a complete loss of nerve function[2]. Lesions occurring proximal to the dorsal root ganglion, such as a
radiculitis secondary to nerve root
inflammation are generally not detectable by either sNCV or MRI studies. Sensory electrodiagnostic procedures supply
an objective measure that can be compared to established normative data
providing a gauge of the severity of the neuropathology.
The electrodiagnostic sensory Nerve Conduction Threshold
evaluation generates
neuroselective Current
Perception Threshold (CPT) and Pain Tolerance Threshold
(PTT) measure which provide an objective measure of
sensory nerve functional
integrity from the periphery to
the central nervous system. The
quantitative vibratory detection threshold test provides a measure of
mechanoreceptive sensory end-organ transduction to the large diameter sensory
fibers. Variations in skin thickness,
temperature and bone conduction are confounding variables of this measure.
Quantitative thermal discrimination tests evaluate thermal receptor
transduction to the smaller diameter sensory fibers. This test is also affected by variations in
skin thickness and temperature. The sNCT quantitative evaluation uses a
neuroselective electrical stimulus to directly excite both the large and
small diameter sensory nerve fibers in a manner not affected by skin thickness
or temperature.
Definition of CPT and PTT
The CPT measure represents the minimal amount of a
painless, neuroselective,
transcutaneous electrical
stimulus required to reproducibly evoke a sensation. This test provides an
objective, quantitative gauge of sensory nerve integrity and is obtainable from
any cutaneous site[3].
The CPT evaluation achieves neuroselectivity by using different
frequencies of an electrical sine wave stimulus. The 2000 Hz stimulus measure
reflect large myelinated fiber function, the 250 Hz stimulus measure reflect
small myelinated fiber function and the 5 Hz stimulus measure reflects small
unmyelinated fiber function[4]. Electrodes can be applied to any cutaneous site and an
automated double blind forced choice testing procedure determines CPT measures
to within ±20 microAmperes (p<0.006).
CPT measures are compared to established normative values to determine
if the patient's values are normal or not.
Abnormally low CPT measures indicate hyper-sensitive nerve function
(commonly associated with inflammation or irritation) reflecting a
hyperesthetic condition. Abnormally
elevated CPT measures indicate a loss of nerve function reflecting a
hypoesthetic condition. Pain Tolerance
Threshold (PTT) is the maximum amount of electrical stimulus which evokes pain
that can be tolerated. The PTT test uses
the same neuroselective stimulus (double blind automated methodology) to evoke
pain from the large myelinated
fibers (when present), small myelinated and
unmyelinated fibers. The PTT evaluation is atraumatic in that it
does not cause tissue damage. The PTT measure permits the evaluation of
allodynia. The sNCT evaluation CPT and PTT measures are obtained using
Neurometer(R) CPT/C
neuroselective sNCT devices
(Neurotron, Inc, Baltimore, MD USA). These diagnostic devices are constant
current simulators which are microprocessor controlled.
Peripheral Cutaneous Sensory Nerve Anatomy
The large myelinated (A beta )
fibers which conduct up to 60 m/s, convey touch
sensation and comprise only
5-10% of the total number of fibers comprising the typical sensory nerve. The small myelinated (A delta) fibers which
convey sharp pain are also relatively few in number (<10%). Approximately 80% of the sensory fibers in
the sensory nerve are the small unmyelinated (C) fibers which convey sensations
such as temperature and dull pain[5].
The pain management specialist does not have to limit
patient evaluation to tests solely of large fiber function (sNCV) or smaller
fiber function (thermal perception threshold).
The sNCT evaluation is multimodal and can assess both large and small
sensory fiber function in a single procedure.
Both of those classes of fibers may mediate pain and should be
evaluated. All modes of sensation may be
effected in a patient with neuropathy, however, there may be selective
impairment of large or small fiber functions[6]. The fiber neuroselectivity of the sNCT
evaluation enhances its sensitivity for detecting sensory dysfunction by
testing the three major sub-populations of nerve fibers within any cutaneous
test site. This additional information
permits more appropriate selection of therapeutic intervention.
Clinical Research Establishing Neuroselectivity
There have been over 200 publications clinical and basic
science studies utilizing, referencing and validating the sNCT evaluation. These studies include:
1. Comparison
studies of the sNCT evaluation CPT measures with other
neurodiagnostic techniques
performed at the Manchester Royal Infirmary (UK),
Tokoku Kosei-Nenkin Hospital in Sendai (Japan) and other
institutions have
demonstrated that sensory
abnormalities detected with the 2000 Hz CPT measures
correlate best with the sNCV
test which evaluates large myelinated fiber function. The 250 Hz CPT measures correlate best with
quantitative vibratory tests which evaluate primarily smaller myelinated fiber
function. The 5 Hz CPT measures were
reported to correlate best with the thermal quantitative sensory tests which
evaluate primarily small unmyelinated fiber function[7,8].
2. Histological
studies of sural nerve biopsies from diabetics and healthy controls using
electron microscopy reported that low frequency 5 Hz CPT measures correlated
with small fiber density and the 2000 Hz CPTs correlated with large fiber
diameter. This same study failed to find
any correlation between unmyelinated fiber pathology and myelinated fiber
density with sNCV, vibratory and thermal perception threshold tests[9].
3. Pharmacologic
studies have demonstrated that the 5 Hz measures are most
sensitive to the effects of lidocaine[10,11,12]. This is consistent with evidence suggesting
that small unmyelinated fibers are most sensitive to the effects of local anesthetics[13,14].
4. Regeneration
studies show recovery of small fiber function first, correlating with 5 Hz and
250 Hz CPTs whereas recovery of large fiber function, correlating with 2000 Hz
CPTs, occurs approximately 9 months later[15].
5. Imaging studies
from Harvard University and the Massachusetts General Hospital using functional
MRI have demonstrated that the 5 Hz pain stimulus elicits metabolic activity in
the same cortical regions as heat pain stimuli.
Interestingly, there was no habituation to the electrical stimuli, however, there was habituation to the heat pain
stimulus. This is presumably because
heat sensation is nerve end-organ mediated, whereas the electrical stimulus
directly excites the nerve fibers. The
250 Hz and 2000 Hz activated different regions of the brain[16].
6. Human, as well
as, animal studies have demonstrated neuroselective and
reproducible pain
thresholds[3,17].
Clinical and Research Applications of the CPT Evaluation
The sNCT evaluation CPT and PTT measures are capable of
objectively quantifying
sensory abnormalities consistent
with polyneuropathy[18,19] compressive
neuropathy/peripheral nerve
injuries (eg carpal tunnel syndrome)[20],
radiculopathy[21], Complex
Regional Pain Syndrome (CRPS)[22]. Areas
of abnormal
function are localized to aid in
diagnosis, determine clinical severity and evaluate the therapeutic
outcome. The painless nature of the test
ensures high patient compliance with the follow up examinations.
Conclusion
The ability to objectively quantify neuroselective pain
and non-pain sensations is a powerful new tool for the pain clinician. More clinical and basic research utilizing
the sNCT evaluation CPT and PTT measurements in characterizing sensory
abnormalities and pain tolerance thresholds in various chronic pain conditions
and assessing the efficacy of various interventional strategies is needed. The effects of spinal cord stimulation, using
implantable technologies and commonly used oral agents (eg Neurontin) are just
a few of the areas needing further research.
The sNCT evaluation will continue to provide a
valuable measures in the objective evaluation of the patient with pain.
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