THE ROLE OF INTRAOPERATIVE

NEUROMONITORING IN SPINE SURGERY

Shane Burch MD

ASSISTANT PROFESSOR SURGERY

DEPT OF ORTHOPEDICS UCSF

NEURAL INJURY DURING SPINE SURGERY

• Spinal Cord Injury

• Nerve root injury more frequent than spinal cord injury

• 2.9% adult spinal deformity surgery (Pateder, Spine 2005)

• Buchowski (Spine 2007)

• n=108, 11.1% root injury

• SSEPs, NMEPs, EMG

• None detected

Hamilton, D. K., J. S. Smith, et al. (2011). "Rates of New Neurological Deficit Associated with Spine Surgery Based on 108,419 Procedures: A Report of the Scoliosis Research Society Morbidity and Mortality Committee." Spine (Phila Pa 1976).

Hamilton, D. K., J. S. Smith, et al. (2011). "Rates of New Neurological Deficit Associated with Spine Surgery Based on 108,419 Procedures: A Report of the Scoliosis Research Society Morbidity and Mortality Committee." Spine (Phila Pa 1976).

MECHANISMS OF INJURY• Degenerative (without

manipulation)

• Direct

• Indirect

• Deformity (with manipulation)

• Direct

• Instrumentation

• Decompression / dural tear

• cautery

• Indirect

• Spinal manipulation

• hypotension

Buchowski, J. M., K. H. Bridwell, et al. (2007). "Neurologic complications of lumbar pedicle subtraction osteotomy: a 10-year assessment." Spine (Phila Pa 1976) 32(20): 2245-2252.

UTILITY OF NEUROMONITORING

• Diagnosis

• Prediction / Prevention

• Intra-operative Surgical Response

MODALITIES

• Wake-up test• SSEPs

• Posterior tib. nerve to scalp• Examine the continuity of the dorsal columns

• EMGs• Free run• Direct - Stimulus applied to the nerve root• CMAP

• TcMEPs• Stimulus applied to the scalp• CMAP in different muscle groups

• Direct Spinal Cord Stimulation

ALARM CRITERIA

• Threshold

• CMAP

• Measured Voltage increase to obtain baseline amplitude

• Amplitude

• CMAP

• Measured drop in amplitude from baseline

• >50%

• Latency

UNKNOWNS

• CMAP

• Amplitude?

• Latency vs AUC?

• Change during injury

• Stepwise vs linear?

• Root Dominance

• Hemodynamics / Ca2+

• Anesthetics

BACKGROUND

• Lieberman, J. A., R. Lyon, et al. (2008). "The efficacy of motor evoked potentials in fixed sagittal imbalance deformity correction surgery." Spine 33(13): E414-24.

• Hsu, B., A. K. Cree, et al. (2008). "Transcranial motor-evoked potentials combined with response recording through compound muscle action potential as the sole modality of spinal cord monitoring in spinal deformity surgery." Spine 33(10): 1100-6.

• Kelleher, M. O., G. Tan, et al. (2008). "Predictive value of intraoperative neurophysiological monitoring during cervical spine surgery: a prospective analysis of 1055 consecutive patients." J Neurosurg Spine 8(3): 215-21.

CASE EXAMPLE ALIF

• 62 Y FEMALE

• SCOLIOSIS

• ALIF

MECHANISM OF INJURY

CASE EXAMPLE

• 81 Y FEMALE

• 45 DEGREE SCOLIOSIS

• BACK AND LEG PAIN

• 2 STAGE APPROACH

• TRANSPSOAS / PSF

• Determine sensitivity and specificity of TcMEPs to detect and predict isolated nerve root injury.

• Transforaminal lumber interbody fusion (TLIF) at L4/5, L5/S1 level.

• Retrospective chart review of 79 patients undergoing TLIF

• NASS 2009

CLINICAL TRANSLATION: TLIF

• Primary TLIF at levels L4-5, L5-S1: 42 L4/5 and 37 L5/S1

• Average age 59.2 yrs- 29 Male, 50 Female

• Multi-myotomal MEP

METHOD

• Warning criteria >80% drop in MEP amplitude In at least one myotome during sustained retraction of L4/L5 nerve root During diskectomy/ insertion of cage.

• Nerve root damage = sustained changes on examination at hospital departure

• EMG warning criteria= >5s tonic EMG activity

METHOD RESULTS

CLINICAL TLIF

Minutes of Retraction

% d

rop fro

m b

asel

ine

% d

rop fro

m b

asel

ine 0%

20%

40%

60%

80%

100%

120%

1.00 2.00 3.00 4.00 5.00 6.00

Nerve Root Retraction

X

X

X

X

X

• 7 patients had threshold EMG activity

• 2 of 5 deficits accurately predicted on EMG

• 3 false negative results produced

• Multimyotomal MEP 100% sensitive and 83% specific to isolated nerve root injury

RESULTS

HOW DID WE MISS NERVE INJURIES?

• false negatives

• IOM

• surgeon

• tech

VARIABILITY IN TCMEP

Figure 2. Average frequency of TcMEP in non-critical

and critical junctures of surgery by diagnosis. Figure 1. Average total frequency of TcMEP by diagnosis.

FALSE POSITIVE / NEGATIVES

• tcMEPs

• Trial to trial variability ~ 5%-10%

• Hemodynamic fade – hypotension

• Frequency of testing

• SSEPs

• Injury to ventral cord

• EMG

• Lesion proximal to stimulus

• Missed tonic EMG

IMPORTANT FOR TRANSPSOAS APPROACHES

POINT OF INJURY

HEAD FOOT

UNKNOWNS

• CMAP

• Amplitude?

• Latency vs AUC?

• Change during injury

• Stepwise vs linear?

• Root Dominance

• Hemodynamics / Ca2+

• Anesthetics

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PURPOSE

• To develop an animal model

• To monitor TcMEPS changes during nerve root injury: compression vs retraction

• To compare tcMEPs changes to EMGs for nerve root injury: compression vs retraction

ANIMAL MODEL

• No reliable large animal model

• Anesthetic issue

• Team: anesthesiologist / neurophysiologist / surgeon

ANIMAL MODEL

• Porcine model

• Lumbar nerves similar to humans

• Overcome thickness of the calvarium

• Anesthetic regimen

• Venous plexus

• Mapping of the nerve roots

REDUCTION IN TCMEP AFTER NERVE ROOT LIGATION (% BASELINE TCMEP)

0

0.28

0.55

0.83

1.10

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

tcM

EP

AM

PL

ITU

DE

% B

AS

EL

INE

TIME (MIN)

2N

4N

1N

EFFECT OF RETRACTION FORCE OF TCMEP AMPLITUDE IN THE TIBIALIS

ANTERIOR

0

2.50

5.00

7.50

10.00

0

2.50

5.00

7.50

10.00

% B

AS

EL

INE

EM

G T

HR

ES

HO

LD

L5

NE

RV

E R

OO

T

2N 4N 1N 2N 4N 1N 2N 4N 1N

0 10 20

TIME (MIN)

2N

4N

1N

EFFECT OF RETRACTION FORCE ON EMG THRESHOLD OF L5 NERVE ROOT

WHAT ABOUT COMPRESSION INJURIES?

0.00%

20.00%

40.00%

60.00%

80.00%

100.00%

120.00%

0 1 2 3 4 5 6 7 8 9 10

% B

AS

EL

INE

AM

PL

ITU

DE

TcM

EP

TIME (MINS)

2N v 1N COMPRESSION (Ligation subtracted)

Mean 2N compression

Mean 1N Compression

Power (Mean 2N compression)

Power (Mean 1N Compression)

y = 0.8339x-0.498 R² = 0.9148

y = -0.0585x + 1.0226 R² = 0.9656

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 1 2 3 4 5 6 7 8 9 10

% B

AS

EL

INE

AM

PL

ITU

DE

Tc

ME

P

TIME (MIN)

1N, 2N Compression vs. 1N, 2N, 4N Retraction

CLINICAL TLIF

Minutes of Retraction

% d

rop fro

m b

asel

ine

% d

rop fro

m b

asel

ine 0%

20%

40%

60%

80%

100%

120%

1.00 2.00 3.00 4.00 5.00 6.00

Nerve Root Retraction

X

X

X

X

X

DISCUSSION

• Curvilinear change in TcMEPs following sustained retraction and compression – stretch injury has a much slower change

• The rate of change proportional to force and predictable

• The recovery of TcMEPs and NRT is inversely proportional to force

• NRT and TcMEPs are correlated and proportional in both comporession and retraction

MODEL

Zone of Recovery

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ALGORITHM

• Short acting relaxant for exposure

• TIVA

• Dual modality testing

• EMG / tcMEP

• SSEP / tcMEP

• Maintain MAP

• Frequent tcMEPs

• Bight block

• Education

CHANGE IN PRACTICE

• TcMEPS may predict nerve root injury – application to patient set-up – application in indirect (stretch injury)

• Frequent motors during retraction OR spinal manipulation

• Development of automated algorithms and retractors to detect early changes THANK YOU