MEP monitoring in Neurosurgery ByungGun Lim M D

MEP monitoring in Neurosurgery Byung-Gun Lim, M. D. , Clinical Assistant Professor Department of Anesthesiology and Pain Medicine, Guro Hospital, College of Medicine, Korea University JUN 14, 2011

Contents Ø Structures of MEP Ø Real Interpretation of MEP in Neurosurgery: Usefulness of MEP Ø Factors affecting MEP: – Variable Artifacts Ø Risk of MEP

Intraoperative monitoring (IOM) machine

IOM statistics (1994~2009; SMC) 월 평균 70건; MVD 34%, Spine 22%, 뇌종양 20%, Aneurysm 16%

IOM statistics (2010. 8. ~; KUMC)

Structures of MEP N: 음극파 P: 양극파

Structures of MEP

Interpretation of MEP in Neurosurgery

Spinal stenosis (Achondroplasia): M/19

Neuromuscular scoliosis (cerebral palsy, MR): M/27 153 cm, 35 kg, m. relaxant (-)

STA-MCA (superficial temporal artery to middle cerebral artery) Bypass, Rt (Moyamoya dz): F/50

Aneurysm clipping, MCA bifur An Lt: F/48

Factors affecting MEP

Physiologic Factors affecting MEP Ø Systemic blood pressure, temperature (local and systemic), and blood gas tensions • Mean Arterial Pressure < 50 mm. Hg: decrease in amplitude. • Hypothermia: increases in latency and decreases in amplitude. – Below 28 ℃ : marked decrease in amplitude – Hyperthermia: decreases in amplitude • Hypoxia: decrease in amplitude of SSEP • Anemia during isovolemic hemodilution: – progressive increases in latency of SSEPs – Changes in amplitude were variable until very low hematocrits (approximately 7%: decrease in amplitude)

Surgical Factors affecting MEP • Patient positioning: – axillary compression (brachial plexus pressure↑ due to shoulder taping in cervical spine op. ), – proper padding of extremities, – overextension/flexion of cervical spine (Mayfield frame) • Ischemic changes: vasospasm, excessive blood loss, spinal correction • Mechanical changes: hardware placement, surgical instrument manipulation, resection (tumor, cord, etc. )

Factors affecting MEP

Anesthetic factors affecting MEP

Anesthetic factors affecting MEP

Influence of neuromuscular blockade on MEP • Avoid muscle relaxant Pajewski TN, et al, Eur Spine J 2007; 16: 115 -29. • TOF 1 -2 Kalkman CJ, et al, Anesth Analg 1992; 75: 584 -9. • TOF not less than 2 Chen N, et al, Zhonghua Yi Xue Za Zhi 2009; 89(7): 445 -8. • maintaining T 1 twitch height at around 30% of control values to prevent excessive movement during the operation Edmonds HL, et al, Spine 1989; 14: 683.

Syndromic Scoliosis (TIVA with propofol, remifentanil): 14/F, 130 cm, 39 kg 3 hrs after induction without additional m/relaxant, TOF 95%

Syndromic Scoliosis (TIVA with propofol, remifentanil): 14/F, 130 cm, 39 kg 1 hr after cisatracurium infusion, 1 mcg/kg/min, TOF 41%

Syndromic Scoliosis (TIVA with propofol, remifentanil): 14/F 2 hr after cisatracurium infusion, 1 mcg/kg/min, TOF 1~5%

Typical effects of anesthetics: general loss

Sevoflurane 1 MAC + N 2 O 50%

TIVA (Propofol + Remifentanil)

Influence of BIS monitoring was used to assess the depth of anesthesia on MEP • When BIS indices were 40 or below, detection was difficult and higher intensities of electrical stimulation were necessary to evoke m-MEPs. • Risk of seizure was relatively high due to stronger electrical stimulation. • A BIS index above 50 and below 70 was optimal range. Yamaguchi F, et al, No Shinkei Geka. 2002; 30(11): 1181 -8

Factors affecting MEP in children • Age, anesthetic agent • Inhalational agents – Greater stimulation needed (higher intensities : trains and voltages) – Less steady MEP amplitudes were produced – The number of alerts of MEP loss or reduction was higher • These effects were more prominent in younger age patients.

Factors affecting MEP in children

Technical Factors MEP: affecting Variable Artifacts

Technical factors affecting MEP • Stimulating and recording electrode placement and improper connections, • Stimulation intensity (train/volts) issues, • Equipment malfunction (hardware and software)

Variable artifacts • Retractor or drill • Electrical artifacts (60 Hz artifacts): blood warmer, microscope, warm air blanket, rapid infusor, C-arm, surgical table, electrocautery etc.

Artifact Example of MEP Free run RADM : 60 Hz Time Base : 200 us Time Base : 2 sec

Artifact of MEP Free run Left: surface electrode, Right: needle electrode

Risk of MEP

Safety Issues • Thermal injury (burn) of the scalp or brain and electrochemical or excitotoxic injury to the brain • Seizures or cardiovascular alterations. • Movement-induced injury • Epidural electrode complications • Contraindications.

Summary Ø Ø Ø Mean Arterial Pressure > 50 mm. Hg Body temperature: baseline ± 2 -2. 5℃ Maintain Normal Hb/Hct, p. H, PO 2, PCO 2 Patient positioning: avoid compression/traction Anesthetic technique for Mlutipulse TCe. MEP: – TIVA (propofol + remifentanil, propofol + N 2 O* + remifentanil, propofol + dexmedetomidine or ketamine ± remifentanil, etomidate or ketamine + dexmedetomidine ± remifentanil, etc. ) • *: less recommended – Muscle relaxant: avoid (or TOF 4), but SEP comonitoring, TOF count 2 -4

Summary • Maintain BIS 50 -60 • Consider underlying dz (DM, HTN, etc. ) • Children: use TIVA; longer trains and higher voltages are required for patients aged 7 and under • Understand variable artifacts including electrical artifacts: use only inevitable equipment, keep proper use and distance • Understand risk and complication: a bilateral bite block between molars is required; use only allowed range of intensity