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SEP Monitoring

Andres A Gonzalez, MDDirector, Surgical Neurophysiology

Keck Medical Center of USC University of Southern California

Outline

• 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.

Somatosensory1973 D`Angelo studies (in cats) demonstrated that SSEPs corresponded to ipsilateralposterior 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 50-80% 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 100-300μs

• 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• 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 = scalp channel (N20, N18, P14)CPc-CPi = scalp channel (isolated N20)CPi-EPc = scalp-non-cephalic channel (N18, P14)C5s-Epc = the cervical channel (N13 )EPi-EPc = 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 CPcdipole.

– 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

N13 N22 (LP)

Distal N9 (EP) N8 (pop)

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SEP Montage

SEP Montage

Channel ConsiderationsMultiple 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 ValueSSEP Sensitivity Specificity Reference

Scoliosis 1995 92% 98.90%Nuwer, M.R. et al., 1995. clinical neurophysiology, 96(1), pp.6–11.

Carotid Endarterectomy 52% 99%Florence, G., Clinical neurophysiology, 34(1), pp.17–32.

Intracranial and Spinal Tumors

79% 96%Wiedemayer, H. et al., 2004. Journal of neurology, neurosurgery, and psychiatry, 75(2), pp.280–286.

Skull Base 58% 100%Bejjani, G.K. et al., 1998.Neurosurgery, 43(3), pp.491–8–discussion 498–500.

Cervical Spine 52% 100%Kelleher, M.O. et al., 2008. Spine, 8(3), pp.215–221.

Lumbar Spine 28% 98%Gunnarsson, T. et al., 2004. Spine, 29(6), p.677.

Tethered Cord 50% 100%Paradiso, G. et al., 2006. Spine, 31(18), pp.2095–2102.

Gonzalez et al. SEP Monitoring. In Husain 2015andresgo@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

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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 neuromonitoringconfigurations– 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 Repairwith 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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