Neuroprotection for Scoliosis Surgery

Transcription

1 Neuroprotection for Scoliosis Surgery Mary Ellen McCann, MD, MPH Associate Professor of Anaesthesia Harvard Medical School Children s Hospital Boston mary.mccann@childrens.harvard.edu

2 Disclosure I have no financial relationships to disclose I will not be discussing off label or investigational use in this talk

3 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

4 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

5 The Evidence for Intraoperative Neurophysiological Monitoring in Spine Surgery: Does It Make a Difference? Fehlings, Spine 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.

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

7 Spinal Sensory Evoked Potential Stimulation site: - peripheral nerve (post tib) - Constant current stimulation 25-50mA trials averaged Recording from scalp (sensory cortex) Meaningful Change - Measure: conduction latency>10% and amplitude<50%

8 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 SSEP s

9 Lower Extremity SSEP Cortical response Cortical response Cervical response Peripheral response Tibial nerve Stimulation

10 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

11 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

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

13 TCMEPs TransCranial Electrical Motor Evoked Potentials Stimulate motor cortex Record in muscle (hand, foot) Follow the lateral corticospinal pathway

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15 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 postop motor function - I wave (indirect): affected by anesthetic agents unstable waveform, difficult to reliably reproduce

16 TcMEP s Monitors motor spinal cord motor pathway integrity during surgery - Single stimulus technique - Multiple stimulus technique 3-5 pulses at 500 hz

17 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

18 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

19 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

20 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

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

22 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

23 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

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

25 Dexmedetomidine Similar to propofol Mahmoud 2010 May allow lower doses of propofol and diminish risk of PRIS

26 Ketamine No effect on MEPs until high doses 1 mg/kg no effect on human volunteers Kalkman % patients will have dysphoria at 1-2 mg/kg/hr 14% at 1 mg/kg/hr when paired with low dose propofol Kawaguchi 2000

27 Midazolam Similar effects to other types of TIVA

28 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

29 Optimal Physiologic Parameters Blood Pressure MAP>65-70 mm Hg Normal temperature Normovolemia Hematocrit>21% Normal Cardiac Output

30 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

31 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

32 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

33 Electromyography Continuous free-running electro-myographic monitoring Stimulus-triggered electromyography

34 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

35 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

36 Testing Pedicular Instrumentation Each hole or screw can be tested individually A misplaced hole can be redirected and retested Stimulus thresholds of less than 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

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

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

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40 What is a significant change? SSEPs - 50% in amplitude - 10% in latency TCeMEPs - 80% in amplitude EMG - Sustained activity Triggered EMG - < 8 ma stim

41 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

42 Stagnara Wake-up Test Risks: - Extubation: - Movement off table Dislodge lines Dislodge, plough implants - Air embolism - flood wound with saline sponges

43 Strategies for changed IOM Three basic causes: - Technical Equipment Anesthetic Positioning? - Mechanical change to neural structure - Vascular compromise of neural structure

44 Strategies for changed IOM 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

45 Strategies for changed IOM Wake-up test Significance of resolved IOM change? - Early warning - Indicator of cord status

46 Inadvertent benefits of IOM: Communication with surgeon, neuromonitoring team Brain status Early pick-up of upper extremity issues