Neuro Protection

Neuroprotection for Scoliosis Surgery

Mary Ellen McCann, MD, MPH

Associate Professor of Anaesthesia Harvard Medical School

Children’s Hospital Boston [email protected]

Disclosure

• I have no financial relationships to disclose • I will not be discussing off label or

investigational use in this talk

Overview

• Scoliosis seen in up to 4 % of population • 70-80% idiopathic • Non idiopathic causes include neuromuscular

disorders, congenital scoliosis, prior thoracic surgery in infancy, infections and trauma.

• Risks of surgery include major blood loss, venous air embolism and spinal cord injury

• Spinal cord injury- ischemia during derotation and distraction of the spine.

• Spinal cord deformity measured by Cobb angle • Surgery is indicated when angle exceeds 50

degrees

IOM definition

• Intraoperative Monitoring (IOM) • Can be unimodal or multimodal

- The use of modalities (test(s)) to help detect surgical or peri-surgically induced insults to neural structures, in order to prevent permanent neurologic deficits

The Evidence for Intraoperative Neurophysiological Monitoring in Spine

Surgery: Does It Make a Difference? Fehlings, Spine 2010

• 1990-2009 • 32 articles • HLE –MIOM sensitive and specific for detecting intraoperative

neurologic injury • HLE-MEPs are better than SSEPs • LLE that IOM reduces the rate of new or worsened periperative

neuro deficits • VLLE that intraop responses to NM alert reduces rate of periop

neuro deterioration.

IOM

• Spinal sensory evoked potentials (SSEP) • Transcranial Motor evoked potentials (TcMEP) • Neurogenic Motor Evoked Potentials • Electromyography • Pedicle screw stimulation • Stagnara wake-up test

Spinal Sensory Evoked Potential

• Stimulation site: - peripheral nerve (post tib) - Constant current stimulation 25-50mA - 300-500 trials averaged

• Recording from scalp (sensory cortex) • Meaningful Change

- Measure: conduction latency>10% and amplitude<50%

SSEP’s

• Somato-Sensory Evoked Potentials: - Peripheral stimulus

• Peripheral nerve (median/ulnar, posterior tibial)

• Record the response - Peripheral) - Cervical response - Cortical response

• Follow the dorsal column pathway - Definite

Lower Extremity SSEP

Tibial nerve Stimulation

Cortical response

Cervical response

Peripheral response

Cortical response

SSEP

• Limitations, Disadvantages: - Motor pathways more sensitive than sensory to

ischemia - Affected by inhalational anesthetic agents,

intravenous agents, hypotension and hypothermia - Changes must be averaged over 5 minutes

• Advantages: - Can use NMBs - Successful in infants of >3 mo

Transcranial Stimulation for Motor Evoked Potentials – TCMEP’s

• Stimulation: - Cortical structures across intact cranium

• Response: - Evoked potentials recorded in peripheral

structure • True motor pathway • Direct information about cortical function

Transcranial Stimulation

• Transcranial Electrical Stimulation (TcMEP) • Transcranial Magnetic Stimulation (TcMS)

TCMEPs

• TransCranial Electrical Motor Evoked Potentials

• Stimulate motor cortex • Record in muscle (hand,

foot) • Follow the lateral

corticospinal pathway

TcMEP’s • TcMEP’s-activation of motor neurons of the

corticospinal tract via application of strong electrical impulse across scalp into the cranium

• 2 waves generated which can be recorded by epidural or direct spinal cord probe: - D wave (direct): unaffected by anesthesia

• depolarization of axon directly correlates to post-op motor function

- I wave (indirect): affected by anesthetic agents • unstable waveform, difficult to reliably reproduce

TcMEP’s

• Monitors motor spinal cord motor pathway integrity during surgery - Single stimulus technique - Multiple stimulus technique

• 3-5 pulses at 500 hz

TcMEP

• Multiple stimulus technique: - Short train of repetitive electrical stimuli to cortex - EMG needles in extremities

• compound muscle action potential is produced • Can be combined with D-wave recording to have

combined D-wave + EMG • Latter is sensitive to pathology in spinal cord with

little affect from anesthesia

TcMEP’s

• In young children: - Sutures still open - Electrodes away form open sutures - Motor pathways not mature until age 18 months

• Response may be incomplete, unobtainable <6 years

TcMEP’s

• Limitations: - Poor cortical tissue:

• Severe CP • Severe hydrocephalus

- Suppressed cortex • Severe seizure disorder on lots of seizure meds

- Poor peripheral nerves: • Peripheral neuropathy

- No motors • Severe myopathy

TcMEP’s

• Some safety concerns - Bite blocks to avoid tongue lacerations, jaw injury - Seizures can rarely be triggered (5 in 15,000 cases.

Legatt 2004) • Exclusion criteria:

- Metal plates in skull - Cochlear implants,cardiac pacemakers,dorsal column

stimulators or other implanted device that might be impaired by high intensity electrical stimulation

• Young patients with immature brains-less myelination can have suboptimal TcES

Anesthetics Effect MEPs Effect SSEPs Isoflurane +++ +++ Sevoflurane +++ +++ Nitrous Oxide ++ ++++ Barbiturates +++ +++ Benzodiazepines ++ ++ Propofol ++ ++ Ketamine -+ -+ Fentanyl - _

Volatile Agents

• Most centers use .5 MAC or less • 0.6% with 50% nitrous compatible with

reproducible signals in 9/10 pts Ubags 1998

Isoflurane + 50%N20

Amplitude Latency

0% 761 uV 29.9 msec .2% 560 uV 30.7 msec .4% 337 uV 32.5 msec .6% 184 uV 34.1 msec

Nitrous Oxide

• 60% N2O compatible with multipulse stimulation Van Dongen 1999

• Hi dose propofol not compatible Sakamoto 2001

• Suppression augmented by hypothermia in rabbits Kakimoto 2002

Propofol

• Superior MEPs compared to isoflurane + N20 • Keep dose <200 ug/kg/min

Dexmedetomidine

• Similar to propofol Mahmoud 2010

• May allow lower doses of propofol and diminish risk of PRIS

Ketamine

• No effect on MEPs until high doses • 1 mg/kg no effect on human volunteers Kalkman

1994

• 41% patients will have dysphoria at 1-2 mg/kg/hr

• 14% at 1 mg/kg/hr when paired with low dose propofol Kawaguchi 2000

Midazolam

• Similar effects to other types of TIVA

Evoked Potential Fade

• Gradual MEP amplitude fading and threshold increase normal with either TIVA or inhalational agents

• Estimated that increase 11 V/hr intact pts, 23 V/hr myelopathic pts Lyon 2005

Optimal Physiologic Parameters

• Blood Pressure MAP>65-70 mm Hg • Normal temperature • Normovolemia • Hematocrit>21% • Normal Cardiac Output

Effect of Hemorrhage and Hypotension on Transcranial Motor-evoked Potentials in Swine

• 12 swine-prop/ket/fent-hemorrhaged to TcMEPs 40% baseline

• Treatment with colloid or phenylephrine did not improve TcMEPs

• Treatment with epinephrine did improve TcMEPs • Decrease in TcMEPs associated with decrease in CO

and DO2 but not MAP

Neurogenic Motor Evoked Potentials – NMEP’s

• Stimulation site: - Spinal cord

• Percutaneous needle or open spinous process • Epidural electrode

• Parameters: - Stimulation current less than 300mA - Average of 100 trials

• Recording site: - Neurogenic MEP

• Popliteal fossa/post tib nerve for mixed motor sensory response

- Myogenic MEP - use the muscle of choice for electromyography

NMEP

• Disadvantages: - affected by inhalational anesthetic agents,

intravenous agents, hypotension, hypothermia (but less than SSEP’s)

- NMEP-probably not a true indicator of motor pathway function

- Myogenic MEP - cannot paralyze patient pharmacologically

Electromyography

• Continuous free-running electro-myographic monitoring

• Stimulus-triggered electromyography

Continuous Free-Running Electromyography

• Paired intramuscular needle or wire electrodes - Monitor muscles innervated by nerves or

nerve roots considered to be at risk during surgery

- High frequency EMG activity • Neurotonic discharges

• Trauma to roots or peripheral nerve • Previously irritated root

Stimulus-Triggered Electromyography

• Electrical stimulation of motor nerves: - Compound muscle action potentials (CMAPs) in

innervated muscles • Intraoperative stimulus-triggered EMG:

- Integrity of instrumented pedicles • Cortical bone has high impedence to passage of

current • Perforation of pedicle:

• Lowers the impedence • Activates the local nerve root at a lower

stimulus intensity

Testing Pedicular Instrumentation

• Each hole or screw can be tested individually • A misplaced hole can be redirected and retested • Stimulus thresholds of less than 4-6-8 mA are

suggestive of cortical bony perforation - Triggered EMG study had 99.6% screw “left in

place” with threshold 8.0mA or greater (Raynor et al SRS 2004) in lumbar spine

• Thoracic triggered EMG’s not as reliable

Pedicle Screw Stimulation

• C3-4 Trapezius • C5-6 Biceps • C6-7 Triceps • C7-8 Ex Dig communis • T1 Abductor pollicis brevis • T7-12 Ext Oblique and Rec Ab

Pedicle Screw Stimulation

• L1-2 Iliacus • L2-4 Vastus Medialis • L4-5 Tibialis anterior • S1-2 Medial Gastrocnemius • S3-4 Anal and Urethral Sphincter

What is a significant change?

• SSEPs - 50% in amplitude - 10% in latency

• TCeMEPs - 80% in amplitude

• EMG - Sustained activity

• Triggered EMG - < 8 mA stim

Stagnara Wake-up Test

• Gold Standard? • Limitations:

- Not continuous ( although can be)

- Patient must follow commands: “squeeze hands, wiggle toes, move feet up and down and in and out”

- Someone at head and feet - Inhalational WU faster than

TIVA

Stagnara Wake-up Test

• Risks: - Extubation: - Movement off table

• Dislodge lines • Dislodge, plough implants

- Air embolism - flood wound with saline sponges

Strategies for changed IOM

• Three basic causes: - Technical

• Equipment • Anesthetic • Positioning?

- Mechanical change to neural structure

- Vascular compromise of neural structure


• Maximize perfusion: - BP - Hct - Temp

• What did you last do? (or do before that?) - May be significant lag - Reverse last (or last several manuevers) - If no improvement, consider

• Remove all correction • Retain stabilization if spine unstable


• Wake-up test • Significance of resolved IOM change?

- Early warning - Indicator of cord status

Inadvertent benefits of IOM:

• Communication with surgeon, neuromonitoring team

• Brain status • Early pick-up of upper extremity issues

Documents

Neuro Protection