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Contamination trigeminal evoked potentials by muscular artifacts.

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Lontammation of
Trigeminal Evoked
Potentials by Muscular
Massimo Leandri, MD, Carlo Italo Parodi, MD, and
Emilio Favale, MD
The paper by Hashimoto [ 11reports scalp responses elicited
by air puffs or electrical pulses applied to the lower lip.
Electrical stimulation of the facial skin, though used by many
authors, leads to large electrical artifacts and two types of
muscular artifact (MA1 and MA2) easily picked up by scalp
leads. MA1 is due to direct activation of muscular or motor
nerve fibers lying underneath the stimulating electrodes; it
begins immediately after the stimulus and lasts about 10 to
12 ms [2). MA2 is due to muscular reflexes [3), which never
occur before 5 to 6 ms after the stimulus. To overcome
MA1 it is necessary to stimulate only sensory fibers; this can
be performed by inserting two needle-electrodes into the
infraorbital foramen (to stimulate the infraorbital nerve) [2)
or by applying electrical pulses to the gum. Stimulating the
infraorbital nerve gives a massive input with very low electrical artifact, so that the trace can be read from 0.5 ms after the
stimulus. With this technique, we have been able to record
from the scalp a series of waves of definite neural origin (W1,
W2, W3, P4, N5, P6, N7) 12, 31; of these, W1, W2, and W3
are particularly reliable and their origin has been demonstrated by means of intraoperative recordings {4].Gum
stimulation activates a smaller number of afferents and gives
rise to a high-amplitude, long-duration electrical artifact; it is
therefore not suitable to study the beginning of the trace.
MA1 can be ruled out when using air-puff stimulation, but
no early (< 10 ms) waves are seen when using this technique
[l}. Occurrence of MA2 cannot be avoided with any electrical stimulation (infraorbital nerve, lip, or gum) if applied in
awake subjects [3]. Experiments in anesthetized and
curarized subjects 13) showed that late (> 10 ms) cortical
responses could be recorded only after infraorbital nerve
stimulation during curarization. Lip or gum stimulation
elicited no scalp activity during curarization (possibly because
of the low number of afferents activated), so the responses
seen in awake subjects must be considered entirely artifactual. The muscle monitoring performed by Hashimoto did
not necessarily exclude artifacts. We are therefore led to
believe that the responses he obtained with electrical stimulation of the lip are not of neural origin, and we would
suggest that responses obtained by air-puff stimulation
should be tested in curarized subjects before their reliability
can be accepted, though it seems unlikely that a weak
stimulus like air puff could be more effective than electrical
stimulation of lip or gum in evoking cortical responses.
Department of Neurology
University of Genoa
Genoa, Italy
1. Hashimoto I. Trigeminal evoked potentials following brief air
pufE enhanced signal-to-noise ratio. Ann Neurol 1988;23:332338
2. Leandri M, Parodi CI, Favale E. Early evoked potentials detected
from the scalp of man following infraorbital nerve stimulation.
Electroencephalogr Clin Neurophysiol 1985;62:99-107
3. Leandri M, Parodi CI, Zattoni J, Favale E. Subcortical and cortical
responses following infraorhital nerve stimulation in man. Electroencephalogr Clin Neurophysiol 1987;66:253-262
4. Leandri M, Campbell JA. Origin of early waves evoked by infraorbital nerve stimulation in man. Electroencephalogr Clin
Neurophysiol 1986;65:13-19
Isao Hashimoto. MD
Our aim was to record cortical potentials El). Frontal reference and narrow band-pass filtering in our study preclude
registration of far-field subcortical potentials. Thus, the question Leandri and colleagues raised as to the absence of shortlatency far-field potentials in our records is irrelevant to the
air-puff stimulation technique per se. Robust and reliable
sensory evoked potentials (SEPs) can be elicited by air-puff
stimulation of any desired skin regions including the hand,
face, and sole of the foot [ 1, 2). Even “a weak stimulus” just
above sensory threshold produces clear-cut SEPs [3]. This is
due to the fast rise-time of the stimulus, synchronously activating skin mechanoreceptors, even though their number is
small [4). We have also demonstrated a surprisingly high
input-output function in the somatosensory system to natural
skin stimulation 11, 2). Conversely, “a massive input” is not
necessarily a prerequisite for obtaining cortical responses
[I, 21.
N o electromyographic responses are recorded from the
orbicularis oris or oculi muscle with air-puff stimulation of
the second and third trigeminal branches 11, 51. Normal
trigeminal SEPs (TSEPs) in complete facial nerve paralysis
following either air-puff or electrical stimulation provide further evidence for neural origin of the recorded responses.
General anesthesia abolishes muscle activity but at the same
time greatly diminishes cortical activity. Thus no scalp responses during anesthesia does not necessarily mean that
responses obtained in the awake state are due to muscle
artifacts. Otherwise, as Leandri and colleagues rightfully
speculated, this can be attributable only to their weak electrical stimulation of the lip or gum. Presumably, N 1 with a
mean peak latency of 15 ms in TSEPs corresponds to N20 of
SEPs following the median nerve stimulation, which represents the primary response of the somatosensory cortex [b}.
Blind insertion of needle-electrodes into the infraorbital
foramen gives subjects sharp and often unbearable pain and
may damage the infraorbital nerve. Thus the procedure is
too invasive to be acceptable in routine clinical testing [7-91.
Since sensory threshold increases exponentially with decreasing stimulus duration, a much higher stimulus current is required at the short stimulus duration (0.05 ms) that Leandri
and colleagues employed [7-91. This in turn can produce a
long-lasting damping oscillation, resulting in a longer duration of stimulus artifact.
Copyright 0 1989 by the American Neurological Association
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potential, trigeminal, artifact, muscular, evoked, contamination
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