Introduction
CMAP evaluation is to be performed on every patient at each study visit. MUNE
evaluation is to be performed at sites where a neurophysiologist or trained evaluator is
available to perform the studies (separate protocol). Two baseline assessments will be
obtained at the S1 and S2 visits when possible to collect test-retest data. We will use data
only from those children for whom at least 3 separate G1 electrode placements with
adequate waveform collections have been performed for the maximum CMAP studies.
Five separate G1 electrode placements are the ideal minimum. If additional attempts are
made because waveform integrity is compromised by artifact, the best five waveforms
should be recorded. Please see additional bolded comments below re: appropriate scoring
of CMAP waveforms on the CRF document. All waveforms will be printed and included
with the CRF document. All waveforms will undergo central review at the Data
Coordinating Center by either Dr. Kathryn Swoboda or Mark Bromberg. This mandatory
review will include documentation of minimum number of waveforms collected,
limitation of artifact, accurate placement of markers, and accurate recording of values on
the CRF document, prior to entry into the database. All sites are required to send a video
documenting their technique on 3 subjects, attend a live training session or undergo site
visit for live audit at least once yearly, to ensure rigorous attention to the protocol below,
to ensure integrity of the collected data.
Since temperature can affect maximum CMAP amplitude, temperature greater than 33 degrees centigrade should be documented prior to preparation of skin for electrode placement. Temperature should be measured using a surface probe on the lateral aspect of the hand just proximal to the fifth digit. If temperature is less than 33 degrees centigrade, a chemical warming pack or other warming mechanism should be used prior to collecting data.
The skin over the ulnar aspect of the palm, the back of the hand, the volar aspect of the
wrist are cleaned with alcohol to remove any sweat, lotion or oil. This is very important
in SMA patients as they tend to sweat excessively, to limit stimulus artifact obscuring
take-off of CMAP waveforms, particularly those of diminished amplitude. Electrode
placement is similar to that for standard ulnar motor studies. The G1 primary recording
electrode is placed over the motor point of the abductor digiti minimi muscle in the hand,
with the first G1 electrode placement at the midpoint of the line drawn between the ulnar
aspect of the fifth metacarpophalangeal joint (MP-5) and the ulnar aspect of the pisiform
bone (P) in the hand. The G2 reference electrode is placed at the base of the fifth
proximal phalanx where it intersects with the MP-5 joint. An adhesive ground electrode
is placed on the back of the hand.
Panel 1: Modified figure from “Anatomic Guide for the Electromyographer” Charles C. Thomas, Publisher, 1980, p4.
Panel 2: Modified figure from “Clinical Electromyography Nerve Conduction Studies” Shin J. Oh, Williams and Wilkins, 1993, p195
For affected infants and children, it is important to place them in the position most
comfortable for them, probably on their back or side. Distraction of attention from the
procedure is extremely helpful to enable the most efficient collection of reliable data. Use
of a portable DVD player with battery backup is extremely helpful in this context, and
parents are encouraged to bring their child’s favorite DVDs along. Alternatively,
providing music, bubbles or other distractions are helpful. In general, it is helpful to
engage parents in distracting the infant or young child. The coordinator or nurse can help
assist, but should remain alert to the child’s overall state with regard to handling of
secretions and oxygenation. The evaluator will need assistance in distraction for the
infant so that he or she can remain intent on getting the necessary data as efficiently as
possible.
We recommend the use of disposable non-gelled recording electrodes with a silver-silver
chloride sensor, and a recording area of 7 x 4 mm. For the CARNIVAL and CARNIVAL
TYPE I studies, we used part number 9013L0203, from Alpine Biomedical
Neurodiagnostics Accessories; however, this same eletrode configuration is available
from several other companies. It is important to maintain consistency of the EMG
machine and recording electrodes between visits when comparing data over time in the
same patient. The relatively small recording area of this electrode configuration allows its use in a wide range of settings from small infants to adults, and the sides can be easily
trimmed on either side of the recording surface without affecting electrode recording
characteristics. Because of excessive sweating and the need to clean and reapply surface
electrodes, pre-gelled electrodes are much less desireable. Use of non-gelled electrodes
allows them to be gently cleaned with alcohol, after which a small amount of gel can be
reapplied and reused throughout the study as the G1 placement is varied (use in infant
demonstrated below). It is helpful in this setting to have adhesive tape to help secure the
electrodes given the excessive sweating; we have found that transpore 3M tape works
very well for this purpose, since electrodes quickly lose their self-adhesive properties
when they are moved from site to site.
The ground can be placed on the dorsum of the hand, as in older subjects (above). In the
lower photos, note relatively large size of electrodes on smaller hand size of type 1 infant
– certain electrodes in which the recording surface area is clearly delineated can be
trimmed on either side of the actual recording interface. The type of round disposable
electrode used as a ground electrode in the photo above can also be used as a recording
electrode. However, use consistent electrode size and types for all evaluations in an
individual subject.
In an infant or smaller child, for proper placement of recording (G1) and reference (G2)
electrodes, first measure from the base of the fifth metacarpal to the wrist crease.
Make an ink mark at the base of the fifth metacarpal for placement of the G2 electrode. It will remain at this site during all movements of the G1 electrode to try to ascertain the maximum motor point. Make another mark at the wrist crease, then mark a point 2/3 of the way from the base of the fifth metacarpal towards the wrist crease. This middle mark indicates the appropriate place for your first G1 electrode placement
You can make additional marks to indicate placement points to help guide you in moving
the G1 electrode. Remember that often the bulk of the adductor digiti mimini muscle is
somewhat posterior to a line transecting the ventral/dorsal meridian. In addition to a
recorded response at the first mark, move the recording electrode at least four additional
times to collect five distinct maximum CMAP waveforms, as indicated by white arrows
above.
A maximum ulnar compound motor action potential (CMAP) should be obtained by
stimulation at the wrist starting using a stimulus duration of 50 milliseconds and an
intensity sufficient to elicit a maximum CMAP (a good starting point for stimulus
intensity is usually around 15-20 mA, and the intensity should be further increased in
increments of 10- 15% above that until the stimulus intensity which elicits the maximal
response is reached). In many infants less than 6 months of age, or in chubby older
infants, a stimulus duration of 100 milliseconds is often necessary. An ideal maximum
CMAP response should not have an initial positive deflection, although this may be
difficult in very weak type I children due to the extremely low amplitude of the response.
Recleansing the skin and reapplication of electrodes may be required several times to
ensure the best response. If the initial positive deflection exceeds 30% of the maximum CMAP amplitude, this likely represents a placement that is too far distant from the motor
point; thus, it should not be transferred to the CRF. The G1 electrode should be moved in
1-2 mm increments proximal to the initial placement for ideally, two additional
placements, then anterior and posterior to the midline in the plane of the maximum
response, for a minimum total of 5 measures to identify a point of maximum CMAP
amplitude and negative peak area. A minimum of at least 3 G1 electrode placements is
required to use the data for analysis, and 5 G1 electrode placements is optimal. If the
responses are technically unsatisfactory, due to a >30% initial positive deflection
compared to the maximum CMAP, or due to electrical impedence resulting in a
unstable baseline, the best of those responses from a technical standpoint should be
chosen to record on the CRF. If no adequate response could be obtained due to >
30% positive deflection for all waveforms, a 0 should be recorded on the CRF for
each insufficient response to total 5. If electrical impedence causing artifact
prevents a stable baseline during all attempts to gather data, a line should be drawn
through the CRF, and the evaluator should indicate in the comments section re:
technical difficulties. The same machine and settings should be used for each study
visit. In order to use the data, attempts need to be adequately documented in terms
of number in order to use the data and score it appropriately.
Common causes of an unstable baseline in SMA infants and children include excessive
sweating, increased subcutaneous tissue and short distances between the stimulating and
recording electrodes. Frequent cleaning of electrodes and recording areas, and
reapplication of a small amount of gel may be necessary to record optimal CMAP results
in these very weak infants. Each time stimulating electrode is moved, the wrist should
again be wiped clean with an alcohol swab and allowed to dry briefly, to prevent
bridging. Anything in the room that is plugged into an electrical outlet should be
unplugged (don’t forget if you use a DVD player to have battery backup). Same is true if
child is monitored using pulse oximetry during the study.
Once you have collected at least five maximum waveforms from distinct recording sites,
print out all waveforms collected, and review carefully to ensure markers for takeoff,
negative peak amplitude, and return to baseline are accurately marked. If necessary,
adjust markers as needed prior to recording data on the CRF. Then, you may either print
out final waveforms to include with your CRF as part of the datafile, or submit final set
electronically, ensuring that no more than one waveform for each site is included.
Negative peak amplitude (in mV) and negative peak area (in mVms) are recorded for
each site; only the five best responses are recorded on the CRF.
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