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70
R. Schneider and C.-W. Wu
not only larger but also has more nodules. The start of resection on the dominant side is directed toward the leading thyroid pathology responsible for most symptoms.
Informed Consent
With regard to the treatment of their disease, patients are
very interested in shared decision-making. Patient autonomy
requires that the informed consent form species the use of
CIONM and claries the need for a staged procedure in case
of loss of EMG signal on the rst side of resection.
CIONM Equipment
It is performed with the following commercially available
devices:
• hand-held monopolar stimulator probe (4 pulses/s, 100
μs, 1 mA; Medtronic) for intermittent IONM,
• the automatic periodic stimulation (APS) electrode (2.0
or 3.0 mm; Medtronic) for CIONM,
• noninvasive surface electrodes embedded or inked on the
endotracheal tube (NIM EMG Endotracheal Tube;
Medtronic) to contact with vocal cord for evoked EMG
signal recording, and
• nerve monitoring systems (NIM Vital; Medtronic) with a
pulse generator for continuous stimulation (1 Hz, 100 μs,
1 mA) and an EMG amplier.
Anesthesia andEndotracheal Tube Positioning
Monitored thyroid surgery is performed under general anesthesia, using the short-acting muscle relaxants to facilitate
tracheal intubation and avoid prolonged neuromuscular
block. The endotracheal tube is inserted under visual control
or video laryngoscope to ensure correct placement of the
recording surface electrode at the level of the VC [9].
Maintenance of an electrode impedance of <5 kΩ with an
imbalance of <1 kΩ indicates optimal electrode-mucosal
contact according to recently published guidelines for neural
monitoring standards [7, 10].
Procedure
To obtain meaningful EMG signals, the following points
must be observed:
1. Collar incision, ranging from small incisions to exten-
sive Kocher incisions.
2. Opening the carotid sheath at the level of the thyroid cartilage through a short incision, the length of which
depends on the dimensions of the continuous stimulating electrode, and identifying the VN before exposing
the thyroid gland through an anterior (midline) or lateral
(between the sternohyoid and sternocleidomastoid)
approach (Figs.14.1 and 14.2). The VN most commonly
courses medially and posteriorly (73%) and less commonly medially (15%), laterally (8%), or anteriorly
(4%) to the internal jugular vein and common carotid
artery [11]. The central approach is suitable for small,
benign goiters, whereas the lateral approach, which
allows greater exposure, is more appropriate for large
goiters and reoperations. If the electrical wires can be
routed outside the surgical eld, the risk of accidental
dislocation of the vagus electrode is much lower.
3. Morphological and functional examination of the VN
with the hand-held IONM probe (4.0 Hz, 100 μs, 1 mA)
(Figs.14.3 and 14.4) before attaching the CIONM electrode to the nerve. If stimulation remains unsuccessful,
the troubleshooting algorithm for IONM [2] must be followed. The right VN should be stimulated further down
the neck to rule out the presence of a nonrecurrent inferior laryngeal nerve (1.4%) [12].
4. Circumferential dissection of a short segment (<1 cm) of
the VN while preserving its blood supply (Figs.14.5 and
14.6). The VN is enormously robust to the usual careful
and meticulous surgical technique in the neck.
5. Placement of the APS electrode on the VN with initial
stimulation at 1 mA and 1Hz in a position where it cannot interfere with subsequent resection. The electrode
has to be carefully positioned on the nerve at a 45-degree
angle with the tabs of the housing held open with forceps
and resting on the nerve (Fig.14.7).
6. Calibration of the IONM system. After connecting the
APS electrode to the NIM Vital nerve monitoring system, the baselines for latency and amplitude will be
automatically calibrated (Fig.14.8). For stable and reliable EMG signals, the baseline amplitude should reach
≧500 μV [17]. When the baseline amplitude is less than
500 μV, the anesthesiologist should position the endotracheal tube to achieve a waveform >500 μV.To optimize
sensitivity to impending nerve injury, the baseline amplitude should be maximized.
7. Start CIONM and stimulation of the VN with the handheld intermittent nerve stimulation (V1) (Fig.14.9). All
subsequent EMG changes are measured against the calibrated amplitude and latency baselines and displayed on
the screen as a function of time (Fig.14.10). Audible and
visual alarms can also be dened at thresholds that signal impending nerve injury and require release of the
affected nerve. An upper limit for latency and a lower
limit for amplitude were displayed as separate alarm
14 Continuous Intraoperative Neuromonitoring inThyroid Surgery
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71
Fig. 14.1 Medial approach after collar incision with dissection of the short straight neck muscle from the right thyroid capsule
Fig. 14.2 Complete mobilization of the short neck muscles laterally,
medialization of the right thyroid lobe, and exposition of the carotid
sheath
Fig. 14.3 Opening the carotid sheath at the level of the thyroid cartilage through a short incision, the length of which depends on the
dimensions of the stimulating electrode, and identifying the vagus
nerve. Morphological and functional examination of the vagus nerve
with the hand-held intermittent nerve stimulation probe before attaching the CIONM electrode to the nerve
72
R. Schneider and C.-W. Wu
Fig. 14.4 Monitor screen with normal EMG of the right vagus nerve as setup check
Fig. 14.5 Circumferential dissection of a short segment (<1 cm) of the
vagus nerve while preserving its blood supply. The vagus nerve is enormously robust to the usual careful and meticulous surgical technique in
the neck
Fig. 14.6 Careful elevation of the vagus nerve by driving under it with
the nerve hook
14 Continuous Intraoperative Neuromonitoring inThyroid Surgery
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Fig. 14.7 Placement of the APS electrode on the vagus nerve at a
45-degree angle with the tabs of the housing held open with forceps and
resting on the nerve
73
lines. After establishing the APS baseline for amplitude
and latency of the evoked response, continuous and
intermittent stimulated EMG signals will be recorded.
8. In addition to CIONM with the APS electrode, the handheld probe can be used for intermittent direct nerve stimulation by the surgeon to facilitate identication of the
RLN (R1) (Figs.14.11 and 14.12) and performance of
the procedure. Dissection of the thyroid lobes is usually
performed in a caudal to cranial direction with close
observation of the course of the RLN. For larger
retrosternal goiters, the upper pole can be mobilized rst
to expose the retrosternal portion of the goiter. After dissection of the thyroid lobe, the hand-held probe helps to
monitor and conrm the nal RLN function (R2)
(Figs.14.13 and 14.14).
9. At the end of surgery before removal of the APS electrode, the VN nerve is stimulated with the hand-held
Fig. 14.8 Calibration of the IONM system. After connecting the APS electrode to the NIM vital nerve monitoring system, the baselines for
latency and amplitude were automatically calibrated. For stable and reliable EMG signals, the baseline amplitude should reach ≧500 μV
74
Fig. 14.9 Start CIONM and stimulation of the vagus nerve with the
hand-held intermittent nerve stimulation (V1)
R. Schneider and C.-W. Wu
intermittent nerve stimulation probe proximal (V2)
(Figs.14.15 and 14.16) to the electrode contact site to
rule out electrode-induced segmental VN injury. EMG
documentation is required in the patient's medical
records and includes time-traceable measurements of
V1, R1, R2, and V2 for each side.
10. Postoperative laryngeal examination (L2) is important to
improve the prognostic correlation between CIONM
stimulation and postoperative VC function.
Fig. 14.10 Monitor screen with normal EMG of the right vagus nerve
(left side). All subsequent EMG changes are measured against the calibrated amplitude and latency baselines and displayed on the screen as a
function of time (right side). Audible and visual alarms can also be
dened at thresholds that signal impending nerve injury and require
release of the affected nerve. An upper limit for latency and a lower
limit for amplitude were displayed as separate alarm lines
14 Continuous Intraoperative Neuromonitoring inThyroid Surgery
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Fig. 14.11 Mapping and identication using the hand-held probe with
intermittent direct nerve stimulation (R1)
75
Fig. 14.12 Monitor screen with normal EMG of the right recurrent laryngeal nerve (white EMG) (R1) and the right vagus nerve (blue EMG)
76
Fig. 14.13 After dissection of the thyroid lobe, the hand-held probe
helps monitor nal RLN function (R2)
R. Schneider and C.-W. Wu
Fig. 14.14 Monitor screen with normal EMG of the right recurrent laryngeal nerve (white EMG) (R2) and the right vagus nerve (blue EMG) after
dissection
14 Continuous Intraoperative Neuromonitoring inThyroid Surgery
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Fig. 14.15 At the end of surgery before removal of the APS electrode,
the VN nerve is stimulated with the hand-held intermittent nerve stimulation probe proximal (V2) to the electrode contact site to rule out
electrode- induced segmental VN injury
77
Fig. 14.16 Monitor screen with normal EMG of the right vagus nerve after dissection (V2)
78
1 Vocalis 1
m
Percentage of baseline (869 mV, 5.75 ms)
CIONM session time (hh:mm:ss)
R. Schneider and C.-W. Wu
Evaluation ofEMG Data
Impending Neuronal Damage
Skillful application of CIONM requires experience and
observation of the EMG screen to facilitate interpretation of
clinically relevant quantitative EMG signals from “normal”
EMG (Fig. 14.17) leads. Unfavorable “combined” EMG
events with specic concurrent changes in both signal amplitude and latency, dened as a decrease in amplitude of more
than 50% and increase in latency of more than 10% com-
180
160
140
V, 6.00 ms)
120
100
80
pared with baseline values, indicate clinically important
EMG thresholds indicative of impending nerve damage
(Fig.14.18).
Artifacts
Periods of minor decrease or increase in amplitude or
latency may occur more than once during CIONM.Repeated
increases or decreases in both amplitude and latency may
be artifactual, likely due to rotation or upward/downward
6.60 ms
(+10%)
60
40
Percentage of baseline (975
20
0
00:00:00
2 Vocalis 2
200
180
160
140
120
100
80
60
40
Amplitude% Latency%
00:10:00
CIONM session time (hh:mm:ss)
00:20:00 00:30:00 00:40:00
488 µV
(-50%)
6.33 ms
(+10%)
435 µV
(-50%)
20
0
00:00:00 00:10:00 00:20:00 00:30:00 00:40:00
Amplitude% Latency%
Fig. 14.17 Uneventful EMG tracing during thyroid surgery on the left side
1 Vocalis 1
Percentage of baseline (2353 mV, 6.13 ms)
CIONM session time (hh:mm:ss)
14 Continuous Intraoperative Neuromonitoring inThyroid Surgery
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140
120
100
80
79
6.74 ms
(+10%)
60
40
20
Percentage of baseline (2507 mV, 6.13 ms)
0
00:00:00
2 Vocalis 2
200
180
160
140
120
100
80
60
40
Amplitude% Latency%
00:10:00
CIONM session time (hh:mm:ss)
1254 µV
(-50%)
00:20:00 00:30:00
6.74 ms
(+10%)
1177 µV
(-50%)
20
0
00:00:00 00:10:00 00:20:00 00:30:00
Fig. 14.18 “Combined EMG event” during surgery on the left side. It
is dened as a simultaneous decrease in amplitude by more than 50% of
baseline and increase in latency by more than 10% of baseline. It is to
Amplitude% Latency%
aging maneuver (mainly traction) on the recurrent laryngeal nerve is
stopped immediately, the combined event remains reversible and normal postoperative vocal cord function results
be interpreted as a prodrome of an impending loss of signal. If the dam-
displacement of the EMG tube or tracheal shift with
impaired contact between the recording electrodes and VC
as a result of thyroid manipulation. In addition to transient
loss of EMG recording due to coagulation with bipolar forceps, dislocation of the vagus electrode may occur after
inadvertently pulling the lead wire; or a mismatch between
the size of the VN and the electrode may result in poor
stimulation characteristics. An “EMG storm” with repeated
transient lapses in EMG recording followed by increased
amplitude values is caused by poor vagus contact of an
oversized vagus electrode. Typically, and in contrast to the
dangerous traction- induced EMG changes, the harmless
artifacts resolve after repositioning of the thyroid gland to
its original position (Figs.14.19, 14.20, and 14.21). Isolated
amplitude or latency changes are not associated with VC
palsy.
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