Intraoperative neuromonitoring in intracranial surgery

Question 1

Why use neuromonitoring for intracranial surgery

Answer 1

• It provides a crucial opportunity to salvage at-risk neural tissue before there is irreversible damage.
• It can be used pre-op to “map” critical anatomical structures

Question 2

Motor evoked potentials

Answer 2

stimulate a neural pathway with electricity and measure the response in volts at the effector muscles

Question 3

Evoked potential waveform

Answer 3

• Peak amplitude
• Peak latency time (time taken to arrive after stimulation)
• Pattern

Question 4

Somatosensory Evoked Potential Warning Signs

Answer 4

• Amplitude reduction of 50%
• Latency increase of 10%

Question 5

Mechanisms of CNS pathway interoperative injury

Answer 5

• Mechanical disruption from surgical manoeuvres
• Thermal injury from diathermy
• Pressure damage from patient positioning
• Ischaemia due to hypoperfusion

Question 6

Perioperative factors affecting intraoperative neuromonitoring

Answer 6

Physiological Factors
- Temperature- both high and low prolong latency
- Hypo and Hypercapnia- due to changes in CPP
- Hypoxia
- Haemodilution- maintain Hct >30%
- Hypotension
- Positioning- neural compression and vascular compromise

Pharmacological Factors
- Volatiles- dose-dependent decrease in amplitude and increase in latency
- IV anaesthetic agents- dose dependent but to a much lesser degree than volatiles
- Ketamine- increase SSEP and MEP at low doses but >1mg/kg will decrease
- NMBD- Obliterate MEPS
- Opioids- minimal but will affect SSEP latency at high doses
- BZDs- minimal effect
- a2 agonists- minimal at normal dose range

Question 7

Issues with using intraoperative neuromonitoring

Answer 7

• It will cause the patient to move
• Damage to oral cavity if bite block not used
• Needle use can damage IV devices or tracheal tubes depending on placement

Question 8

Response to a change in intraoperative neuromonitoring

Answer 8

Neurophysiologist
- Run again to check not false +ve
- Check wiring and equipment

Anaethetist
- Ensure not related to drug admin
- Check temp
- Check O2 + CO2
- Maintain MAP to at least 10-20% above baseline (or >70)
- Optimise pH, Hct, glucose

Surgeon
- Stop current manipulation
- Evaluate events
- Consider reversing actions if possible
- Irrigate field
- Consider suspending procedure

Question 9

Types of surgery Neuromonitoring can be used for

Answer 9

• Resection of intracranial tumours esp if near motor pathways or eloquent areas
• Repair of AV malformations
• Deep brain stimulation surgery
• Epilepsy surgery
• Microvascular decompression for trigem neuralgia
• Surgery at the brainstem