Overview
Neuroselective sensory Nerve Conduction Threshold (sNCT) studies are
performed by applying a mild electrical stimulus through a pair of small
disposable surface electrodes placed on the skin or mucosa. Three painless,
neuro-selective electrical stimuli are applied to a peripheral/visceral
nerve or dermatome innervation field in order to determine, confirm and
record sensory threshold responses. Each stimulus evaluates the functioning
of one of the three major subpopulations of sensory nerve fibers which
together comprise more than 90% of the typical sensory nerve. sNCT studies
are used to localize or map a sensory nerve dysfunction. The painless
Current Perception Threshold (CPT) measures are used to detect and evaluate
hyperesthetic conditions that precede progressive nerve impairment and
hypoesthetic conditions that reflect a loss of functioning. The studies are
appropriate for any patient capable of indicating when they detect a change
of sensation at the site being stimulated.
Neurometer devices have various
Modes of Operation to
provide different types of measures.
sNCT studies follow a
standardized,
automated procedure to generate objective, sensitive and reliable
measures of sensory nerve function. Measures are obtained using
microprocessor controlled constant alternating current (AC) sinusoid
waveform stimuli presented at intensities ranging from 0.01 mAmperes to 9.99
mAmperes and at frequencies of 5 Hz, 250 Hz and 2000 Hz. The measures are in
the form of Current Perception Threshold (CPT) values, each of which
represents the minimum intensity of a neuroselective, transcutaneous
constant electrical current required to reproducibly evoke a sensation.
The electrical stimulus produced by the sNCT equipment is self-calibrating
and able to maintain a constant current output regardless of normal
variations in skin thickness and impedance. The system monitors the
impedance at the skin electrode interface and instantly warns operators when
conditions cause excessive impedance that could distort the accuracy of the
measures. The system also monitors the consistency of a patient’s responses
to guard against false readings due to improper procedures or
non-compliance.
The equipment used for sNCT studies is battery powered and portable and
doesn’t require any special electrical shielding for safe and reliable
operation. Patients can be evaluated almost anywhere they can be made
comfortable and in an environment free from interruptions. Studies have
demonstrated the reliability of sNCT evaluations conducted under a wide
range of conditions, both in and out of clinical settings.
How to Apply Electrodes for
Testing
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Snap in Electrodes |
Apply Tape to Electrode Holder |
Apply Electrode Gel |
Apply One Electrode |
Wrap Tape Around Finger |
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Apply Second
Electrode |
Wrap Tape
(Electrodes Ready!) |
Wrap Around
Electrode Placement |
Trigeminal Nerve
Test Site |
Fourth Lumbar
Test Site |
Summary of Services
Pre-Service Work: The examiner determines which nerves are to be
studied based upon the physician’s prescription and the available clinical
information.
Intra-service Work: The physician or technician supervises and/or performs patient
preparation including electrode placement, explains the procedure and begins
the automated threshold determination study. Each nerve test site is
evaluated with the three sNCT neuroselective electrical stimuli to assess
both large and small fiber functioning. Additional testing may be prescribed
during the course of the study in response to the information obtained.
Post-service Work: Examination data are entered into software that
evaluates and grades the sNCT measures based upon a comparison with
standardized, clinically established normative values. These data are
subjected to range analyses, intra-site and inter-site comparisons. The
physician integrates the graded sNCT findings with clinical and other
laboratory findings into a report and defines or generates the diagnosis. On
occasion, suggestions for additional work-up will be included.
Conducting
a sNCT™
CPT®
Study
The patient is placed in a comfortable position - typically sitting - and in
a location with minimal interruptions. The examiner connects the electrode
cable to the sNCT device, attaches a new set of 1 cm. diameter, disposable
gold plated electrodes to the cable, and then powers up the equipment. The
examiner then performs a Pre-Exam Cable Test by following the directions
displayed on the device's LCD screen. Successful completion of this test
confirms the proper functioning of the equipment, electrodes and cables
prior to each examination.
The examiner explains the general nature of the test to the patient, tells
them what they can expect and answers any questions they may have. The
examiner then examines the prescribed skin test sites to confirm that they
are free of any signs of recent trauma which could effect the sNCT measures.
Finally, the examiner prepares each test site using a mildly abrasive skin
prep paste that cleanses and hydrates the skin and facilitates the testing.
The electrodes are coated with a thin layer of electro-conductive gel and
then taped to the test site on the patient. Next, the patient is presented
with an automated or manual Intensity Alignment procedure that quickly
narrows down the threshold level to a range of +/- 50 μAmperes out of a
total range of 0 to 9.99 mAmperes. The Auto Test Mode then begins, which is
a fully automatic, double-blind, forced choice procedure that determines the
actual CPT measures. Patient are presented with randomly generated sets of a
real and a placebo stimulus and must indicate which of the two - if either -
felt stronger. Responses can be made verbally, by pushing a button or any
other means through which the patient’s intent can be communicated.
Depending upon the patient’s response, the sNCT device automatically
re-adjusts the output intensity of the stimulus and randomly generates a new
testing order for the next pair of tests in the series. Because the sNCT
testing methodology is completely automated, neither the subject nor the
operator can influence the testing sequence. The Auto Test Mode follows a
testing methodology similar to that used in standard objective auditory
tests and determines the patient’s CPT measures with a resolution of +/- 20
µAmperes to a p<0.006. When a sufficient number of correct consecutive
responses have been obtained, the sNCT device calculates and displays the
CPT value for the test series and optionally prints out the results. The
device also monitors patient responses for consistency and accuracy, and in
the event that there are inconsistent responses, the operator is alerted so
that the test may be repeated or discontinued. This testing sequence is
repeated for each of the three stimulus frequencies at each site being
studied.
The average time required to complete a three-frequency single site
electrodiagnostic sNCT evaluation is approximately 10 minutes. A typical
evaluation could involve testing 2, 4 or 6 sites (bilateral evaluation of 1
to 3 peripheral nerves or dermatomes) and take approximately 20-60 minutes
to complete plus an additional 5-10 minutes for the initial equipment set-up
and patient orientation. Electrodiagnostic testing is considered dynamic,
however, in that one test may indicate that an additional test or tests are
indicated or not necessary.
Statistical Evaluation of
sNCT™
Data
sNCT/CPT measures are evaluated by software
(link for .pdf file) that compares them to clinically determined ranges
of healthy measures that have been established for dozens of different test
sites. Both the CPT values and their ratios are considered when determining
the degree of sensory nerve impairment and both contribute to the overall
neurological diagnosis. The data analysis may include a determination of
hyperesthetic and/or hypoesthetic conditions, and a detailed laboratory
report and narrative are generated. As with any neurodiagnostic test, a
clinician’s interpretation including a clinical correlation is essential and
necessary for diagnostic purposes.
Interpreting the Clinical Significance of sNCT™
Findings
A licensed physician with appropriate training interprets the results of an
sNCT study by using the graded data generated by the computer analysis in
conjunction with other laboratory data and clinical impressions of the
patient. The location, distribution, neurospecificity and severity of the
CPT abnormalities (if present) help the physician develop and confirm a
diagnosis. Clinical certification courses are available for physicians which
include a review of the neurophysiological basis and clinical use of the
sNCT electrodiagnostic evaluation and the interpretation of the test
results. Technical certification courses are also offered for technicians
operating the sNCT equipment that emphasize examination techniques and use
of the evaluation software.
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