Intraoperative Neuromonitoring In Intracranial Surgery

Question 1

What are common indications for IONM?

Answer 1

1. Resection of tumours
2. Repair of vascular abnormalities
3. Surgery within posterior fossa, cerebellopontine angle and brainstem

Question 2

Name some IONM modalities

Answer 2

1. MEPs - Motor Evoked Potentials
2. SSEPs - Somatosensory Evoked Potentials
3. BAEPs - Brainstem Auditory Evoked Potentials
4. VEPs - Visual Evoked Potentials
5. EMG - Electromyography

Question 3

What are mechanisms of intraoperative injury in neurosurgery?

Answer 3

1. Mechanical disruption from surgical manoeuvres
2. Thermal injury from coagulation and diathermy
3. Pressure damage from patient positioning
4. Ischaemia as a result of local or global hypoperfusion

Motor pathways are more vulnerable compared with sensory pathways partly because of decreased redundancy in perfusion.

MEPs detect ischaemia approx. 15 min earlier and with greater sensitivity than SSEPs

Question 4

What are perioperative physiological factors affecting IONM?

Answer 4

Physiological factors:

1. Temperature
2. Hypo and hypercapnia
3. Hypoxia
4. Haemodilution
5. Hypotension
6. Positioning

Question 5

Describe details of SSEP neuromonitoring technique

Answer 5

1. Recorded from scalp at 1st cervical point and Erb’s point, 2-3cm above he clavicle
2. Function of dorsal column and thalamocortical pathways to primary sensory cortex is assessed
3. Peripheral stimulation sites include mixed sensors and motor nerves:
   - Median (C6-T1)
   - Ulnar (C8-T1)
   - Posterior Tibial (L4-S2) nerves

Stimulation activates large diameter, fast conducting Ia muscle afferent and Group II cutaneous nerve fibres which produce the recorded response

Question 6

Describe details of MEPs neuromonitoring technique

Answer 6

Performed by stimulation of primary motor cortex directly or transcranially

MEPs are recorded from muscles using surface or needle electrodes

Recording of muscle action potentials allow for testing between critical surgical manoeuvres

Question 7

Describe details of BAEPs neuromonitoring technique

Answer 7

Reflect the function of the ascending auditory pathways of the brainstem after acoustic stimuli

They are performed by click stimulation of the ears and recorded over scalp

Question 8

Describe details of VEPs neuromonitoring technique

Answer 8

Reflect function of the visual pathways from retinal stimulation

During surgery, performed with flash stimulation, using either red or white light and recorded from the scalp

Question 9

Describe details of the blink reflex neuromonitoring technique

Answer 9

Blink reflex is a way to continuously assess the function of the trigemino-facial pathways of the Brainstem

Question 10

What are benefits of using IONM for resection of intracranial tumours?
What are common modalities used?

Answer 10

Benefits:
Useful for demarcation of eloquent areas and functional pathways (especially motor pathways)

Modalities:
SSEPs
MEPs
VEPs
EEG
Language mapping
Language monitoring

Question 11

What are benefits and modalities of IONM for intracranial vascular procedures?

Answer 11

Benefits:
- Determine adequacy of blood flow
- determine tolerance to vessel occlusion
- determine Systolic BP
- SSEPs can identify ischaemia from vasospasm, retractor pressure and hyperventilation

Modalities:.
SSEPs
MEPs

Question 12

What are benefits and modalities of IONM for functional neurosurgery? (Epilepsy or DBS)

Answer 12

Benefits:
- useful to demarcate seizure focus from surrounding functional or eloquent areas
- useful for assessment of disabling motor symptoms and adverse effects while optimising position for DBS electrodes

Modalities:
EEG
SSEPs
Language mapping
EMG

Question 13

What are benefits and modalities of IONM for posterior fossa surgery?

Answer 13

Benefits:
- evaluate Brainstem viability in posterior fossa surgery including SOLs and vascular malformations in the cerebellum
- allows continuous monitoring of motor cranial nerves where corticobulbar responses are at risk

Modalities:
BAEPs
EMG
SSEPs
MEPs
Brainstem reflexes

Question 14

What Anaesthetic factors might cause IONM signal suppression?

Answer 14

1. Anaesthetic agent or change in technique tends to result in global signal change whereas surgical injury may manifest as focal or asymmetrical change.

Recovery due to Anaesthetic agent may take 30 mins or more to correct - assist by increasing MAP and oxygen

Question 15

What are complications associated with IONM?

Answer 15

1. SSEPs and MEPs can result in patient moving
2. Excessive movement can cause bite injury, interference with Arterial Line or pulse oximeter and impede surgical team at critical point during surgery
3. Vigilance regarding disposal of sharps with electrode placement and removal
4. Vigilance when electrodes are places close to intravascular devices or ETT

Question 16

What are the perioperative pharmacological factors affecting IONM?

Answer 16

Pharmacological factors:

1. GA agents exhibit a dose dependent suppression of Evoked potential transmission, with MEPs more sensitive than SSEPs
2. Volatile agents have greater effect than IV agents
   - All volatiles except halothane cause an initial increase in frequency of frontal EEG readings, followed by decrease in frequency and amplitude at increasing doses, with burst suppression seen at -1.5 MAC
3. IV agents
   - TIVA is considered agent of choice
   - Large boluses of Ketamine can decrease MEPs
   - Induction doses of Thiopentone can decrease VEPs
   - NMB - modest dose given at induction should be allowed to wear off before baseline IONM signal acquisition or give reversal agent

• Opioids
  Remifentanil allows fo adequate IONM even at high doses
• Benzos
  Do not suppress SSEPs or MEPs at low doses

Alpha Adrenergic agonists
- dexmedetomidine has minimal effect at low doses
- favourable for its lack of respiratory depression