Anesthetic considerations in IOM
- Inhalational Anaesthetics (IHA)
- SSEPs: dose related
- Inc. latency &
- Dec. amplitude
- MEPs: easily abolished
- Not recommended but if used: no bolus or acute changes
- Total Intravenous Anaesthesia (TIVA) i.e. propofol
- Dose dependent: dec. amplitude MEPs
- Choice of anaesthesia for IOM
- Combine the use with Remifentanyl: can dec dose of propofol and inc. dose of remifentanyl to reduce the dec. in amplitude MEP.
Clinical decision
Intraoperative electrophysiologic monitoring changes
- EP change criteria to trigger notification of surgeon
- SSEP:
- 50% decrease in peak signal amplitude from baseline
- Increase in peak latency >10%
- Complete loss of a waveform
- TCMEP: sustained 50% decrease in signal amplitude
- DEP: decrease in signal of >60%
- Interventions for SSEP/TCMEP changes during spine surgery (Vitale checklist)
- TIPP
- Time
- Irrigation
- Papaverine
- Pressure (blood pressure)
- When loss or degradation of SSEP/TCMEP during spine surgery is due to compression, the prognosis may be good. Vascular injuries generally do not fare as well.
- Options/suggestions include (adapted/excerpted from the “Vitale checklist”) (this is a list, not a step-by-step protocol - items are not necessarily performed in order listed):
- Verify that the change is real.
- Rule out 60Hz interference from other equipment
- OR table,
- C-arm,
- Microscope…
- Anything with a plug
- Check connections
- Verify that stimulating electrodes & recording leads are making good contact
- Place OR on alert status
- Announce intraoperative pause and stop the case
- Eliminate possible distractions (music, unnecessary conversations…)
- “Muster the troops”:
- Attending anaesthesiologist,
- Senior neurologist or neurophysiologist
- Experienced nurse are called to the room.
- Consult a surgical colleague if necessary (even if by phone)
- Anaesthetic/metabolic considerations
- Optimize mean arterial pressure (MAP usually> 85mm Hg preferred)
- Check hematocrit for anemia (could contribute to cord ischemia)
- Optimize blood pH (rule out acidemia) and pCO2
- Normalize patient body temperature
- Check anaesthetic technical factors: assess extent of paralytics, inhalational agents…
- Discuss possibility of “Stagnara wake up test” with attending anesthesiologist and scrub nurse
- Surgical considerations (maneuvers are restricted to those that do not destabilize the spine):
- Visually check patient position on table: arms, legs, shift in torso, malfunction of positioning frame; reverse step that immediately preceded change in potentials (if feasible)
- Remove traction if used
- Decrease distraction or other corrective forces
- Remove rods
- Remove screw that could correlate with the change, and re-probe for breach
- Perform intra-op X-ray to rule out shift in patient position
- Check for spinal cord compression
- Check for nerve root compression at osteotomy sites
- Obtain intraoperative imaging (CT or O-arm if available)
- Consider staging operation if practical
- Perform Stagnara wake-up test if feasible
- An intraoperative test of voluntary motor function.
- The patient is allowed to wake up enough from anesthesia to be able to wiggle the toes on command (the patient is kept intubated and the wound is not closed).
- Less commonly used in the current era of electrophysiologic monitoring (EPM), but may be employed in cases where EPM is felt to be possibly unreliable.
- Since no specialized equipment is required, it can be used in times or places where access to EPM is limited.
- Is more efficacious if planned in advance, in which case short-acting anesthetic agents are employed and the patient can be briefed ahead of time.
- Cons:
- Can only be performed sparingly during surgery (time consuming and often challenging), so it may delay detection of a change that might have been identified earlier by EPM.
- Patients often will try to get up or move.
- Tests only motor function, and may miss subtle deficits.
- Consider IV steroids
Uses of neurophysiology
- A diagnostic tool that can quantify type and severity of damage to the central and peripheral nervous system,
- A means of monitoring the safety of structures within and around the surgical site,
- A method to map structures.
- Antidromic conduction refers to the travelling of an action potential from the periphery towards the cell body.
- Orthodromic conduction refers to an action potential travelling away from the cell body.
Action potential/ membrane potentials
- An action potential occurs when the cell membrane is depolarized to a threshold value.
- Resting membrane potential
- Around 60– 70 mV.
- This potential is caused by differences in ion concentration between intra- and extracellular fluid.
- Membrane potential fluctuates in response to external stimuli which alter permeability to specific ions. Such stimuli can consist of
- Electrical current across the membrane, or
- The action of neurotransmitters in the extracellular fluid.
- These stimuli cause the opening of specific ion channels → increasing the permeability of the membrane to that ion → driving the membrane potential towards the equilibrium potential for that ion.
- Neurotransmitters that
- Increase membrane permeability to Na or Ca cause → depolarization (membrane potential moves towards 0 mV),
- Increased permeability to K or Cl causes hyperpolarization (membrane potential moves away from 0 mV).
- Threshold value for an action potential
- Around – 60 to – 50 mV.
- At this threshold, a cascade of voltage gated sodium channel activation leads to a sudden increase in sodium permeability causing a rapid change in membrane potential which propagates along the membrane at speeds of up to 100 m/ s in myelinated axons.
- An action potential can also be generated if a current is applied externally; spontaneous and externally generated volleys of nerve impulses are both useful in clinical testing.
Synaptic function
- The synaptic cleft measures around 20 nm.
- Action potential arrives at the presynaptic membrane → neurotransmitters are released into the synaptic cleft → neurotransmitters bind to receptors on the postsynaptic membrane → opening ion channels → triggering a change in voltage across the post synaptic membrane.
- Gamma aminobutyric acid (GABA)
- Most common inhibitory neurotransmitter.
- Binds to
- GABA A receptors → open chloride channels → hyperpolarizing the postsynaptic membrane
- GABA B receptors which are linked through G- proteins to potassium channels → hyperpolarize the postsynaptic membrane.
- Glutamate
- An excitatory neurotransmitter
- Binds to
- N- methyl- D- aspartate (NMDA) receptor,
- AMPA receptor
- Kainite receptors.
- These receptors open cation channels that allow sodium, potassium, or calcium ions in to the postsynaptic neuron → membrane depolarization.
- The neurones can be activated or inhibited by input from dentrites of other neurons.
Classification of types of nerve fibres
A fibres
- Larger diameter
- Myelinated
- Have high conduction velocity (10– 100 m/ s).
- Subdivided into:
- A alpha fibres
- In muscle spindles and Golgi tendon organs.
- A beta fibres
- In cutaneous mechanoreceptors, and as secondary muscle spindle receptors.
- A gamma fibres
- Are motor neurons controlling muscle spindle activation.
- A delta fibres
- Conduct fast painful stimuli as free nerve endings.
B fibres
- Smaller diameter
- Myelinated.
- Autonomic nerves
- Conduction velocity of up to 15 m/s
C fibres
- Small and unmyelinated.
- Conduction velocity is hence lower, up to 2 m/ s.
- They transmit slow pain signals and temperature.
Summary of different types of monitoring
MODALITIES | SSEPs | MEPs/CMAPs | EMG | EEG |
Stimulation site | Peripheral sensory nerves | Transcranial scalp electrodes | Free-running: none Triggered: bipolar stimulation of a specific structure | (None) |
Recording site | Cortical and cervicomedullary junction | Extremity muscles (e.g., thenar muscles, tibialis anterior) | Myotome specific | Scalp |
Alert threshold | 50% reduction in amplitude 10% increase in latency | Disappearance of signal (all-or-none phenomenon) | Sustained activity (>2 sec) | |
Advantages | Sensory specificity Continuous signal capture | Motor specificity; large amplitude signal | Continuous monitor Allows for surgical correlation with pedicle screw stimulation | Monitors cerebral integrity and anaesthetic depth |
Limitations | Low amplitude; requires averaging (possible introduction of delays) | TIVA preferable; intermittent signal; variable stimulation thresholds with age | No neuromuscular block; Difficulty distinguishing innocuous from selious injuy; Insensitive to complete newe injury |
- Strengths and weaknesses Charalampidis 2020
- SSEP: somatosensory sensory evoked potential; tcMEP: transcranial motor-evoked potential; EMG: electromyography.
Types of monitoring | Strengths | Weaknesses |
SSEP | - Allows continuous monitoring throughout the surgery - Does not preclude the use of neuromuscular blockade | - Its interpretation requires temporal summation, which can delay the detection of a signal change by up to 16 mins - Unable to detect motor changes - Individual nerve root function is not effectively monitored by SSEPs |
tcMEP | - Allows monitoring of the entire motor pathway (cortex, corticospinal tract, nerve root, peripheral nerve) - Sensitive in the detection of postoperative motor deficits - Sensitive for detecting spinal cord ischemia | - Does not allow for continuous monitoring - Precludes use of neuromuscular blockade - Highly sensitive to inhalational anesthetics, demanding rigid anesthetic protocols |
Spontaneous EMG | - Sensitive for nerve root injury - Provides real-time information about nerve root function throughout surgery - May be combined with SSEPs to improve specificity | - Sensitive to temperature changes - High rate of false positive alarms - Precludes use of neuromuscular blockade |
Triggered EMG | - High sensitivity for medial pedicle wall breach - Useful in minimally invasive surgery where anatomical landmarks may be challenging to visualize - Relatively easy technique | - Accepted set thresholds not firmly established - Less sensitive for thoracic pedicle screws than for lumbar pedicle screws - High rate of false positive alarms |
Checklist for the response to Intraoperative Neuromonitoring changes in patients with a stable spine
Gain Control of Room
- Intraoperative pause: stop case and announce to the room
- Eliminate extraneous stimuli (e.g. music, conversations, etc.)
- Summon ATTENDING anesthesiologist, SENIOR neurologist or neurophysiologist, and EXPERIENCED nurse
- Anticipate need for intraoperative and/or perioperative imaging if not readily available
Anesthetic/Systemic
- Optimize mean arterial pressure (MAP)
- Optimize hematocrit
- Optimize blood pH and pCO₂
- Seek normothermia
- Discuss POTENTIAL need for wake-up test with ATTENDING anesthesiologist
Technical/Neurophysiologic
- Discuss status of anesthetic agents
- Check extent of neuromuscular blockade and degree of paralysis
- Check electrodes and connections
- Determine pattern and timing of signal changes
- Check neck and limb positioning; check limb on table especially if unilateral loss
Surgical
- Discuss events and actions just prior to signal loss and consider reversing actions
- Remove traction (if applicable)
- Decrease/remove distraction or other corrective forces
- Remove rods
- Remove screws and probe for breach
- Evaluate for spinal cord compression, examine osteotomy and laminotomy sites
- Intraoperative and/or perioperative imaging (e.g. O-arm, fluoroscopy, x-ray) to evaluate implant placement
Ongoing Considerations
- REVISIT anesthetic/systemic considerations and confirm that they are optimized
- Wake-up test
- Consultation with a colleague
- Continue surgical procedure versus staging procedure
- IV steroid protocol: Methylprednisolone 30 mg/kg in first hr, then 5.4 mg/kg/hr for next 23 hrs
Determinants of high risk
Patient factors
- Non-idiopathic etiology
- Neurologic comorbidity
- Tethered cord
- Split cord malformation
- Presence of syrinx ≥4 mm
- Myelopathy
- Skeletal dysplasia
- Cardiopulmonary comorbidity
Curve and cord factors
- Large coronal Cobb angle
- High coronal deformity angular ratio (C-DAR), sagittal DAR (S-DAR), total DAR (T-DAR)
- Scoliosis with hyperkyphosis
- Bayoneted spine
- High curve rigidity
- High rate of deformity progression
- Type 3 spinal cord shape (SCS)
- Spinal cord signal change or significant myelomalacia on preoperative MRI
Surgical factors
- Prior IONM event
- Prior spinal deformity surgery (current surgery is revision surgery)
- Prior ASF with vessel ligation
- Prior intradural surgery (eg, spinal cord tumor resection, tethered cord release, diastematomyelia release)
- Missing/poor quality baseline IONM data
- 3CO (PSO / VCR)
Preventative strategies
Preoperative
- Preoperative HGT in the appropriately selected patient
- If applicable, obtain IONM signals during halo placement for HGT
- Planned staging if >10 hr surgery anticipated
- Planned inclusion of a second experienced surgical colleague
Intraoperative
- Obtain pre-positioning IONM signals
- Elevate MAP during at-risk portions of case (may be entire procedure)
- Careful use of intraoperative traction (especially in angular/stiff patients)
- Place temporary instrumentation prior to PCO (especially in angular deformities/type 3 SCS)
- Place temporary instrumentation prior to 3CO
- Leave out apical/periapical concave screws in the setting of PCO (especially in angular deformities/type 3 SCS)
- Place short rod on convexity prior to correction to avoid apical stretch
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