Gonzalez - SEP Monitoring (H)

1/22/2015
SEP Monitoring
Andres A Gonzalez, MD
Director, Surgical Neurophysiology
Keck Medical Center of USC
University of Southern California
Outline
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Development of SEPs
Stimulation and recording techniques
Predictive value of SEP
Uses of SEP monitoring
Outline
•
•
•
•
Development of SEPs
Stimulation and recording techniques
Predictive value of SEP
Uses of SEP monitoring
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ORIGINS OF EVOKED
POTENTIALS
• 1947 Dawson was the first to
described changes in the
electrical potentials in response to
stimulation of peripheral nerves
• 1969 Donaghy and Numoto was
probably first one to discuss the
prognosis significance of SSEPs in
animal following spinal cord
injury.
Dawson, G. D.: Cerebral Responses to Electrical Stimulation of Peripheral
Nerve in Man. J. Neurol., Neurosurg. and Psychiat., 10: 137-140, 1947.
Early Monitoring
• Prior to the use of EPs the technique used to to
functionally monitor the nervous system during
done spinal cord procedure was …
• Stagnara et al Wake up test in 124 patients
• Goal: discontinue general anesthesia after
placement of hardware and asses neurological
function
• Advantages: simple, easy to undo procedure if
problems detected
• Disadvantages: cooperative patient, one time
assessment
VAUZELLE, C.; STAGNARA, P.; and JOUVINROUX, P.: Functional
Monitoring of Spinal Cord Activity During Spinal Surgery. Clin.
Orthop., 93:173-178, 1973.
Somatosensor
y
1973 D`Angelo studies (in cats)
demonstrated that SSEPs
corresponded to ipsilateral
posterior column. Also, the
severity of SEP changes
correlated with the severity of
spinal cord damage
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SEP monitoring
• Introduced in the early 1980’s
• Early report successful monitoring in 5080% of cases
• However, with improved training,
anesthetic protocols and equipment
resulted in more reliable monitoring
Advantages of SSEP monitoring
• No need to wake up the patient
• Patient cooperation not required
• Provides continuous assessment of cord
function
Outline
•
•
•
•
Development of SEPs
Stimulation and recording techniques
Predictive value of SEP
Uses of SEP monitoring
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SEP
modalities
UPPER
Median Nerve (C6,7,8, T1) Surgeries above C6
Ulnar Nerve (C8, T1) Sx above C8
LOWER
Posterior tibial (L4,5,S1,2)
Peroneal (L4,5,S1) only when PT unavailable
Stimulation parameters
• Constant current stimulator
• Monophasic square wave current 100300μs
• Intensity 30-40 mA
• Cathode: proximal between the PL-FCR
• Anode: 2-3 cm distal
• Stimulation rate: 2-8/sec (avoid multiple of
60)
Recording potentials
An Evoked Potential is an electrical response to
sensory stimulation: vision (VEPs), sensation (SSEP),
hearing (BAERs).
Size of signals: amplitudes of EP are small
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Motor NC:> 4000 μV
Sensory NC: > 15 μV (or > 6 for sural)
EEG: PDR: 15-50 μV
VEPs: ~ 10 μV (photic driving response)
SSEPs: 0.8 μV
BAERs: 0.2 μV
40000 times small
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Recording Parameters
• Number of repetitions: 250-1000 (SNR)
• Analysis time: 50 ms (uppers), 100 ms
(lowers)
• Filters: 30-1 kHz
• Montages
Typical Upper Limb Montage
CPc-FPz
CPc-CPi
CPi-EPc
C5s-Epc
EPi-EPc
= scalp channel (N20, N18, P14)
= scalp channel (isolated N20)
= scalp-non-cephalic channel (N18, P14)
= the cervical channel (N13 )
= the erb point channel (N9)
Localization of SEP Waveforms
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Typical montage .
– CPi FPz = coronal scalp channel (P37) better. CPi’ (rather than CPc
dipole.
– CPz FPz = midline scalp channel (P37)
– FPz C5sp= cervical channel or scalp non cephalic channel (P31, N34)
– T12 IC = lumbar channel (N22) LP
– PF K = popliteal fossa (N8)
Location of Lower Extremity SEP
Comparison
Arm
Leg
Cortical
N20
P37
Thalamus
N18
N34
Posterior
column
Distal
N13
N22 (LP)
N9 (EP)
N8 (pop)
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SEP Montage
SEP Montage
Channel Considerations
Multiple channels
for lower extremity
P37-N45
recommended
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Channel Considerations
• If C5Sp-FPz channel is used, a collection of
N13, inverted P14 are consolidated into
one larger waveform
Channel Considerations
• Usefulness of two peripheral potentials:
– ERBs point (EP)
– Popliteal Fossa (PF) potential
Channel Considerations
• ERBs point intact, loss of subcortical and
cortical channels= spinal cord dysfunction
• Loss of ERBs, subcortical and cortical=
limb malposition
• Loss cortical, preserved subcortical=
cerebral ischemia
• Loss cortical bilateral, preserved
subcortical= anesthetic effect
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Outline
•
•
•
•
Development of SEPs
Stimulation and recording techniques
Predictive value of SEP
Uses of SEP monitoring
Intraoperative Neurophysiological
Monitoring
Electrophysiological testing during
operations in which portions of the
nervous system are specifically at risk in
order to minimize the probability of
neurological damage
Clinical Efficacy of SSEP
• Based on a study by Nuwer et al. in 1995
• Survey of spine surgeons and
neurophysiologists regarding 51,263 spine
surgeries performed
– Sensitivity of 92%
– Specificity of 98.9%
• High negative predictive value (99.93%), and
low positive predictive value (42%)
• Can reduce major neurologic deficit by 60%
M.R. Nuwer et al. / Electroencephalography and clinical
Neurophysiology 96 (1995) 6-11
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50/10 Criteria
• 50% drop in amplitude
• 10 % increase in latency
Caveats on reading the
literature
• False positive caveat:
– IOM changes wakes up no deficits.
– In literature is considered a false positive (
instead, should be true positive?)
Caveats on reading the
literature
• False positive caveat:
– IOM changes wakes up no deficits.
– In literature is considered a false positive (
instead, should be true positive)
• False negative caveat:
– No SSEPs changes wake up paralyzed.
– Is really not a failure of the test.
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SSEPs alone
• Nevertheless, at a practical level SSEP
monitoring is an acceptable indicator of
overall spinal cord function
• Supported by sensitivity 80-100% to detect
postoperative neurological deficits
2004 Wiedemayer H. J Neurol Neurosurg Psych
SSEP only
Wiedemayer, H. et al, 2004
Overall Predictive Value
SSEP
Sensitivity Specificity
Scoliosis 1995
92%
Carotid Endarterectomy
52%
Intracranial and Spinal
Tumors
79%
Skull Base
58%
Cervical Spine
52%
Lumbar Spine
28%
Tethered Cord
50%
Reference
98.90%Nuwer, M.R. et al., 1995. clinical
neurophysiology, 96(1), pp.6–11.
99%Florence, G., Clinical neurophysiology,
34(1), pp.17–32.
96%Wiedemayer, H. et al., 2004. Journal of
neurology, neurosurgery, and
psychiatry, 75(2), pp.280–286.
100%Bejjani, G.K. et al., 1998.
Neurosurgery, 43(3), pp.491–8–
discussion 498–500.
100%Kelleher, M.O. et al., 2008. Spine, 8(3),
pp.215–221.
98%Gunnarsson, T. et al., 2004. Spine,
29(6), p.677.
100%Paradiso, G. et al., 2006. Spine,
31(18), pp.2095–2102.
Gonzalez et al. SEP Monitoring. In Husain 2015
andresgo@usc.edu
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SSEPs alone
• In addition, reports of false negative
outcomes when using SSEP alone
illustrated the need for multimodal
monitoring
Other Modalities
• Numerous monitoring methods are now
available including
– Motor Evoked Potentials (MEPs)
– Continuous free running EMG (fEMG)
– Evoked or triggered EMG (tEMG)
– Other (epidural, mapping, etc)
• None of this test individually provide
global function, but in combination
Combined Value of SEP
Gonzalez et al. Neurosurgical focus 2009
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SEP
• SEP has been the primary spinal cord
monitoring modality for decades
• Serves as a surrogate marker for “global”
spinal cord function
• It has been complemented by the
introduction of MEP, free run and trigger
EMG.
Advantages of SEP monitoring
• Has a definable amplitude or latency
criteria
• The signals are reasonably stable
• Can be continuous
• Multiple recording sites allow anatomical
precision of injury
• Is capable of recording from peripheral to
central somatosensory pathways
Outline
•
•
•
•
Development of SEPs
Stimulation and recording techniques
Predictive value of SEP
Uses of SEP monitoring
13
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Utility of SEP monitoring
• Given that SEPs affect all levels of the
neuroaxis it is no surprise that SEP are the
staple of most neuromonitoring
configurations
– Hemispheric
– Deep brain surgeries
– Posterior fossa surgery
– Cervical or thoracic spinal surgeries
– Spinal nerve root surgeries
– Peripheral nerve surgeries
Anterior Lumbar Interbody Fusion
(ALIF)
• Significant changes see in left lower extremity SSEP
ALIF
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ALIF
• According to Salvador et l, 57% of patients
undergoing ALIF at the L4–L5 level are
subject to compression of the iliac vessel and
oxygen desaturation
• Vascular Compromise correlated with
changes in the lower extremity SSEP
• These changes are usually transient and
resolve with removal or replacement of the
retractor
• If SSEP recovery is not seen, need to rule out
thrombosis
Salvador et al. 2003, The Spine
Journal
Risk of Neurological Deficit
• Scoliosis surgery 0.5-1.6%
• Surgical decompression for spinal tumors
20%
• Descending thoracic aorta ~40%
2009 ACNS Guidelines 11B
Abdominal Aortic Aneurysm Repair
with Loss of Lower Extremity SSEP
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Central Sulcus Localization
• SSEPs can be recorded directly from the
cortical surface
• A recorded N20 response can be seen in
the somatosensory cortex and a P20/P22
response recorded over the motor cortex
• The place in between where a change in
polarity is seen is considered the central
sulcus
Central Sulcus Localization
Positivity up. Loftus CM, 1994
Central Sulcus Localization
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Central sulcus localization
Gugino, Gonzalez et al, 2001
Conclusions
• Are vital to IOM
• SSEPs are a reliable way to monitor the
somatosensory pathway from the
peripheral nerve up to the cortex
• Use of SEPs has not diminished even with
the advent of other modalities (MEPs)
• Can be used in a wide variety of surgeries,
including central sulcus localization
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