Neuromonitoring in the Operating Room

1. Neuromonitoring in the Operating Room Dr. Gary Simon LHSC Anesthesiology

2. Introduction • Define neuromonitoring • Risk of CNS damage • Spinal cord anatomy • Mechanisms of injury LHSC Anesthesiology

3. Introduction • Stagnara and Clonus testing • Somatosensory evoked potentials • Motor evoked potentials • Electromyography LHSC Anesthesiology

4. Define neuromonitoring • Ensure that functional integrity is maintained • Identify anatomical structures • Detect injury patterns LHSC Anesthesiology

5. Risk of CNS damage • Elective AAA (0.16%-0.25%) • Scoliosis instrumentation (0.4%-1.6%) • Coarctation of the aorta (0.4%-1.5%) • Thoraco-abdominal aneurysm (16%-25%) • Risk factors: dissection, hypotension, long period of aortic crossclamping, increased ICP, sacrifice of critical intercostal or lumbar arteries, and extent of aortic disease LHSC Anesthesiology

6. High risk scoliosis procedures • Combined anterior and posterior repair • Hyperkyphosis • Significant rigid curves • Neuromuscular vs idiopathic LHSC Anesthesiology

7. Spinalcord injury • Ischemia, disruption, compression, concussion, or distraction. • Type, intensity, location and duration of injury determines the extent of damage. • Experienced neuromonitoring teams can decrease deficits by 50% for scoliosis surgery (ie 0.46% vs 1.04 %) LHSC Anesthesiology

8. Spinal cord and brain anatomy • Gray and white matter • Motor and sensory • Anterior and posterior spinal arteries • Low thoracic/lumbar blood supply • Anterior horn cell LHSC Anesthesiology

9. Brain sensory & motor areas LHSC Anesthesiology

10. Brain sensory & motor areas LHSC Anesthesiology

11. Brain anatomy LHSC Anesthesiology

12. Spine Anatomy LHSC Anesthesiology

13. Spine Anatomy LHSC Anesthesiology

14. Sensory and motor pathways to and from the brain Three sensory modalities: • Discriminative touch • Touch, pressure and vibration • Pain and temperature • Free nerve endings are responsive to nerve damage, prostaglandins, histamine and substance P • Proprioception LHSC Anesthesiology

15. Discriminative touch • Enter the cord and ascend on the ipsilateral side • Synapse in the medulla and cross to the contralateral side • Secondary afferents project to the ventroposterior lateral nucleus where they synapse and ascend to the cortex LHSC Anesthesiology

16. Pain and temperature • Afferents enter the dorsal horn and synapse within one or two segments • Cross to the opposite side and ascend in the spinothalamic tract • Synapse again in the ventro- posterior lateral nucleus and project to the cortex LHSC Anesthesiology

17. Motor pathways • Large pyramidal neurons send axons into the brain that finally end up in the medulla • They form pyramids which cross at the caudal part of the medulla • 15-20% of fibres do not cross descend • in the ant. corticospinal tract • Majority of fibres decussate and • descend in the lat corticospinal tract • Fibres enter the grey matter at the • appropriate level and synapse • Alpha motor neuron sends axon to • muscle LHSC Anesthesiology

18. Spinal cord vascular supply • Arteries of the brain and spinal branches of the subclavian, aorta and iliac arteries • Anterior spinal artery supplies the ventral 2/3 • Posterior spinal arteries supplies the dorsal sensory part of the spine • No communicating branches between the anterior and posterior spinal arteries LHSC Anesthesiology

19. Spinal cord vascular supply LHSC Anesthesiology

20. Spinal cord vascular supply LHSC Anesthesiology

21. Anterior horn cell is the most sensitive structure in the cord to ischemia Axons (white matter) are relatively resistant to ischemia Anterior horn cell LHSC Anesthesiology

22. Scoliosis etiology • Idiopathic • Neuromuscular (flaccidity, spasticity, dyskinesis) • Neuropathic • Upper motor neuron • C.P., spinocerebellar deg., springomyelia, tumour, trauma • Lower motor neuron • Polio, trauma, spinal muscle atrophy, dysautonomia • Myopathic • Arthrogryphosis, muscle and myotonic dystrophy, cong. hypotonia, fibre type disproportion LHSC Anesthesiology

23. Scoliosis etiology • Congenital • 20% have congenital GU malformations;10-15% have congenital heart disease; high association with spinal dysraphism • Syndromes • Neurofibromatosis • Marfan syndrome • Compensatory • Leg-length discrepancy LHSC Anesthesiology

24. Mechanisms of injury • Ischemia, disruption, compression, concussion or distraction • Distraction or compression affects region • Extent of damage determined by- type, intensity, location and duration of insult • Recovery depends on interval of time and reversal of precipitating events. LHSC Anesthesiology

25. Stagnara wakeup test • Stagnara wakeup – Clin Orthop 1973 • Gold standard • Difficulties • Unsuitable patients • Not able to provide ongoing monitoring • Limited repetition capabilities • Air embolism • Extubation • Recall LHSC Anesthesiology

26. Clonus test • Motor response to stretch reflex • Clonus to rapid dorsiflexion of ankle • Normal CNS prevents clonus by sending inhibitory potentials to that reflex area in the spinal cord • Recovering from GA depresses central inhibition and allows clonus • Problems – timing is everything! LHSC Anesthesiology

27. Clonus test • The ankle clonus test is not a clinically useful measure of spinal cord integrity in children. Ewen A - Can J Anaesth - 01-MAY-2005; 52(5): 524-9 • Conclusion: We conclude that the specificity of the ankle clonus test is too low to be clinically useful as a measure of spinal cord integrity in children, both when isoflurane and sevoflurane are used as the primary anesthetic agent. LHSC Anesthesiology

28. SomatoSensory Evoked Potentials • History • Time locked neural patterns vs EEG • Comparison of electrical potentials: • EKG: 1000µV • EEG: 10-100µV • SSEP: 0.1-10 µV LHSC Anesthesiology

29. Homunculus LHSC Anesthesiology

30. International 10-20 markings LHSC Anesthesiology

31. International 10-20 markings LHSC Anesthesiology

32. Somatosensory evoked potentials • Repetition and averaging • Stimulating and recording electrodes • Latency and amplitude LHSC Anesthesiology

33. SomatoSensory Evoked Potentials • Approximately 100 – 500 stimulus repetitions. • Nerve conduction velocity is fairly constant • Evoked potentials can be monitored from the periphery to the surface of brain. LHSC Anesthesiology

34. Somatosensory evoked potentials • Negative predictive value – 99.9% • Positive predictive value – 42% • Factors that can alter traces: • Technical • Physiologic: temperature, hemoglobin, BP • Anesthetic effect on traces: Pentothal, propofol, narcotics and benzodiazepines LHSC Anesthesiology

35. c) Isoflurane Isoflurane Somatosensory evoked potentials • Factors that can alter traces: • Anesthetic: inhalational agents and N2O • Ketamine and etomidate (familalmyoclonic epilepsy) LHSC Anesthesiology

36. LHSC Anesthesiology

37. LHSC Anesthesiology

38. LHSC Anesthesiology

39. Somatosensory evoked potentials • Warning – latency increase by 10% • Amplitude decrease by 50% • Interventions • Check equipment, physiology, anesthesia • Increase blood pressure, reverse manipulation • Limitations • Afferent impulses are conducted nonsynaptically. Ten plus minutes needed for SSEP’s to deteriorate. • Posterior columns only are monitored LHSC Anesthesiology

40. Motor evoked potentials • Electrical or magnetic energy • Stimulating sites: • Transcranial electrical stimulation (TCES) • Neurogenic “motor” evoke potentials • Cervical or thoracic spinal cord • Nerve root or fibre • Free running or triggered EMG LHSC Anesthesiology

41. Motor evoked potentials-TCES • Exclusively motor potential • Paralysis • Specific risk to motor cord • Intramedullary tumour • Vascular abnormalities • Thoraco-abdominal aneurysm • Certain spinal corrections LHSC Anesthesiology

42. Motor evoked potentials-TCES • Stimulation over pre-central motor strip • 3-5 short duration but high voltage pulses • Facilitation of cortical neurons • Spatial and temporal summation • Amplification of myogenic response • Not ECT – seizure should not occur LHSC Anesthesiology

43. Motor evoked potentials-TCES • Stimulus through the skull to motor cortex. • Response can be monitored in epidural space or at the level of muscle. LHSC Anesthesiology

44. Motor evoked potentials-TCES • Recording sites- • Epidural: resistant to anesthetic, more consistent • Muscle: anterior horn cell, lateralizing information • Anesthesia agents • Inhalational agents, N2O • TIVA drugs • Muscle Relaxants LHSC Anesthesiology

45. Motor evoked potentials-TCES • Warning signs – present/absent • Risks include: • Seizures, skin burns, electrode site infections, patient movement, tongue lacerations and jaw fracture • Contraindications: • Epilepsy, convexity skull defects, increased ICP, significant cardiac disease, intracranial electrodes, vascular clips, shunts, pacemakers or other biomedical devices LHSC Anesthesiology

46. LHSC Anesthesiology

47. LHSC Anesthesiology

48. LHSC Anesthesiology

49. Motor evoked potentials-TCES Charles Dong: Ann Thoracic Surgery 74:S1873-6,2002. Intraoperative Spinal Cord Monitoring During Descending Thoracic and Thoracoabdominal Aneurysm Surgery. • 16 of 56 showed MEP evidence of spinal cord ischemia while only four of them had delayed associated SSEP changes. • 13 had reversal of MEP changes with (a) implantation of more segmental arteries (b) increasing blood flow (c) increase BP or (d) DHCA. None were paraplegic post-op. • 3 had no recovery of MEP traces and all awoke paraplegic (SSEP’s relatively preserved) LHSC Anesthesiology

50. Motor evoked potentials- EMG • Monitor individual nerve roots • Electrodes are placed in muscle at risk • Quadriceps femoris: L2-L4 • Anterior tibialis: L4-L5 • Biceps femoris: L5-S1 • Gastrocnemius: S1-S2 • Patients are not (or partially) paralysed LHSC Anesthesiology