Intraoperative Monitoring

1. Intraoperative Monitoring BehrouzZamanifekri, MD Neurophysiology Fellow KUMC March 2013

2. Intraoperative monitoring The most primitive method of monitoring the patient 50 years ago were continuous palpation of the radial pulsations throughout the operation or wake up test!!

3. History • 1921, Dr Penfield, intraoperative neurophysiology research • 1950s, Dr Penfield, electrical stimulation to find epileptic foci • 1970s, Dr Brown used SSEP for scoliosis operation • 1974, among 7,800 operations conducted with Harrington instrumentation, 87 patients had subsequently developed significant spinal cord problems • Early 1980, IOM in operations for large skull base tumors • 1980, the American Society for Neurophysiological Monitoring [ASNM])

4. Introduction What is IOM ? • use of neurophysiological recordings for detectingchanges caused by surgically induced insults • assess the function of specific parts of the nervous system continuously during an operation • It is becoming part of standard medical practice

5. What is the purpose of IOM? 1. Reduce the risk of postoperative neurological deficits 2. Identify specific neuronal structures and landmarks that cannot be easily recognized 3. Research purposes in basic science, pathophysiology and therapeutic management

6. What are the most common types of recording? • Spontaneous activity EEG EMG • Evoked responses (through external stimulation of a neural pathway) Sensory: 1- visual 2-auditory 3-somatosensory Motor • The type of test to be used and the sites of recording and stimulation are chosen on a case by case

7. Complications during surgery • ischemia • mechanical insult

8. PRACTICAL ASPECTS OF MONITORING SPINAL CORD

9. Spinal Monitoring • Spinal cord, nerve roots, and blood vessels are frequently placed at risk for injury • Electrophysiological modalities for monitoring: SSEPs MEPs free run or spontaneous EMG (sEMG) triggered EMG (tEMG)

10. Spine Surgery: • surgical insults to the ventral parts of the cord, using motor evoked potentials (MEPs) • dorsal columns of the spinal cord , SEP • the purpose of IOM is to detect response changes due to surgery, not to make a clinical diagnosis

11. Monitoring of Somatosensory Evoked Potential • Earliest used method in IOM • 1970s in operations for scoliosis • Stimulation of peripheral nerve and recorded from scalp • Only monitor dorsal(sensory) spinal cord • patient sensory examination for position and vibration is recommended prior to surgery

12. SSEP • By electrical stimulation of peripheral nerves • Median nerve at wrist for injury above C8 • Posterior tibial nerve at ankle for injury below C8

13. spinal cord through the dorsal roots, ascending pathways, thalamus and, finally, to the primary sensory cortex

14. Recording • P9 from brachial plexus • P11 Dorsal horn • P14-16 Dorsal column nuclei • P20 Primary sensory cortex(contralat.) upper limb SSEP • N37 Primary sensory cortex(contralat.) lower limb SSEP

16. Location of the stimulating and recording posterior tibial nerve SEPs.

17. It is important to note - Earlier peaks tend to be less sensitive to anesthesia - used to differentiate SSEP monitoring changes resulting from anesthetic effects from surgical manipulation.

18. Alarm criteria -50% reduction in amplitude -10% increase in latency Factors that affect the SSEP amplitude include halogenated agents, nitrous oxide, hypothermia, hypotension, and electrical interference

19. Normal SSEPs from median nerves and posterior tibialnerves

20. SSEP in Peripheral nerves? • sciatic nerve injury during pelvic fracture • Injuries to brachial plexus in positioning of pt is common • Prolong latency of all peaks and decrease amplitude

21. Nerve root • SSEP: insensitive to changes in nerve root function • Why?

22. SSEP in nerve root injury? • SSEP used during placement of pedicle screws • Risk of spinal nerve root injury • If one root damaged, no change in SSEP Dermatomal stimulation is better

23. Inhalational anesthetics, cortical responses • Intravenous Agents - Propofolincreases the latency by approximately10% - Benzodiazepines reduce the amplitude of cortical SEP - Etomidate : cortical SEP amplitude augmented 200–600%, increases SEP latencies - Opiates, cause a slight increase in SEP latency - Muscle relaxants, notaffect SEPs

24. SEP changes due to surgical maneuvers (e.g., spinal distraction) or ischemia (e.g., after placement of an artery clamp) are abrupt and localizedand only one side of the body may be affected • whereas changes due to anesthesia or body temperature changes are relatively slower

25. Detection of cord injury due to misplaced instrumentation • just after placement of instrumentation, • both the cortical (peak N45) and • cervical (peak N30) responses disappear

26. SEPs obtained after cross-clamping of the internal carotid ,whichresulted in ischemia (time 9:45) that later deteriorated (9:55). After placement of a shunt,response amplitude is restored to within normal limits (time 10:01).

27. Procedures involves the ICA, MCA, PCA, P.Com, or BA? Median nerve SEPs • procedure involves the ACA or the A.Com artery? Posterior tibialnerve

28. MONITORING SPINAL MOTOR SYSTEM

29. Introduction • SSEP for sensory pathway • MEP for motor • SSEP + MEP: Small reversible changes in SSEP that occur when motor pathway are injured

30. MEP • 1990s, TC-MEP as a method to monitor the corticospinal tracts • Prior to MEP monitoring, the only way to assess corticospinal tract during surgery was wake-up test

31. TC-MEPs • stimulation through the skull with signal recording at the level of muscle (CMAP) nerve (neurogenic MEP) spinal cord ( D-wave ) -the newer technologies is continuous free-running EMG throughout the surgery

32. Recording of Muscle EvokedPotentials • Stimulation of cortex, activation of coticospinal, EMG of distal( Hand m., abdhallucis, tibialis anterior) • Muscle relaxant can not be used

33. MEP

34. Interpretation of MEP Recording • 4 methods : 1) all-or-nothingcriterion: the most used method, complete loss of the MEP signal from a baseline recording is indicative of a significant event 2) amplitude criterion: 80% amplitude decrement in at least 1 out of 6 recording sites 3) threshold criterion: increases in the threshold of 100 V or more required for eliciting CMAP responses that are persistent for 1 h or more 4) morphology criterion: changes in the pattern and duration of MEP waveform morphology

35. TcMEPmonitoring contraindicated in -deep brain stimulators or cochlear implants Tongue biting is the most common complication

36. Normal MEPs

37. Recording of the response from spinal cord(D, I wave) • Recording from epidural electrodes • D (direct activation of corticospinal) • I ( indirect, through transsynaptic) • Not affected by muscle relaxants, but latencies increase with cooling • Subdural electrodes can be substituted for epidural electrodes • Needle electrodes can be place in interspinous ligaments both sides of surgery area • major benefits reported during intramedullary spinal cord tumor resection • a complete loss of MEPs with at least 50% preservation of the D-wave amplitude generally results in a transient paraplegia

38. Spontaneous EMG • monitor nerve roots • recording electrodes placed in the muscles • no stimulation is performed • monitoring of 2 muscles is recommended • C5 nerve root injury, The deltoid and biceps brachii • MEPs be obtained intermittently

39. sEMG • no paralytic agents • train-of-4 testing should indicate that at least 3 out of 4 • Myasthenia gravis, Botox treatments, and muscular dystrophy are classic conditions thatinterfere with EMG

40. Abnormal sEMG • spikes • Bursts • trains • Trains are continuous, repetitive EMG firing caused by continuous force applied to the nerve root.

41. Example of EMG activityindicating irritation of the nerve Baseline recordings. Note the low amplitude background activity High amplitude spikes are present

42. Artifacts may be mistaken for spikes or trains • a neurostimulator • the surgical table • the surgeon’s head light • bipolar electrocautery device

44. Triggered EMG (Pedicle Screw Stimulation) • used to determine whether screws have breached the medial or inferior pedicle wall and thus pose a risk to the exiting nerve root at that level • When a pedicle screw is accurately placed, the surrounding bone acts as an insulator to electrical conduction, and a higher amount of electrical current is thus required to stimulate the surrounding nerve root. • When a medial pedicle wall breach occurs, the stimulation threshold is significantly reduced

45. False negative response • muscles relaxants • fluid, blood, or soft tissue around the head of the screw , shunt current away from the screw • it is important that the stimulation probe be placed directly on the top of the screw and not the tulip, as these 2 structures are not structurally fused • Presence of preexisting nerve root injury. Injured nerve roots will have higher triggering thresholds,

46. Due to the variation in thickness and shape between thoracic and lumbar pedicles, different stimulation thresholds exist for these regions • A threshold < 10 mA for screw stimulation, suggest a medial wall breach in the lumbarpedicles • A thresholds > 15 mA indicate a 98% likelihood of accurate screw positioning • For thoracic pedicle screw placement, stimulation threshold < 6 mA suggest a medial pedicle breach

47. In cervical and thoracic procedures, the spinal cord are of greater importance • Conversely, in lumbar or sacral procedures the nerve roots are at greater risk of injury

48. Overview of IOM classified by spinal region

49. Conclusions • Multimodality neurophysiological monitoring is extremely valuable in the prevention of neurological injury • Knowledge of the benefits and limitations of each modality helps maximize the diagnostic value of IOM during spinal procedures

50. Neurosurg Focus / Volume 27 / October 2009 • A concise guide to intraoperative monitoring / George Zouridakis, Andrew C. Papanicolaou.2001 • Intraoperative neurophysiological monitoring / Aage R. Moller. -- 2nd ed.