General information
- Spontaneous potentials
- Recorded during investigations such as EEG and electromyography (EMG).
- Resultant waveforms contain peaks that are named
- N (negative—upward deflection) or
- P (positive—downward deflection)
- Latency: time till peak from stimulation
- GA
- Most Evoked potentials are attenuated or blocked completely by anaesthetic agents
- With the best results in theatre being obtained by total intravenous anaesthetic with propofol and remi- fentanyl.
- Muscle paralysis blocks MEPs and EMG,
- Although if paralysis is administered during intubation it has usually worn off by the time monitoring is required.
Definition
- Electrical signal generated in response to an external stimulus
- Evoked by external stimulus
- Evoked potentials are averaged EEG waveforms recorded following repetitive stimulation
- The process of averaging nulls-out EEG activity that is not time-locked to the stimulus.
Types of Evoked potentials
Somatosensory (SSEP)/Sensory (SEP) evoked potentials
- Peripheral stimulation evokes a response that can be recorded proximally within the nervous system
- Mainly used by neurosurgeons for intraoperative monitoring purposes.
- May use
- Electrical stimulation of peripheral nerves (somatosensory evoked potentials (SSEP))
- Produced by short (0.1– 1.0 msec) electrical stimulation (1– 5 Hz) of a peripheral nerve and the generated responses are recorded over a proximal part of the nerve or sensory pathway
- Auditory clicks through earphones (auditory or AEP, AKA BAER (brainstem auditory evoked response))
- Flashing lights through goggles (visual EP or VEP).
- VEPs can be evoked by patterned or un-patterned stimuli and may be used during surgery around the optic chiasm or optic tract
- Results under anaesthesia are not reliable.
- Typical waveforms
- BAER= brainstem auditory evoked response;
- UE/LE SSEP = upper/lower extremity somatosensory evoked potential;
- PR VER= pattern reversal visual evoked response which requires patient cooperation and visual attention as opposed to flash VER which may even be done through closed eyelids
- Evoked potential waveforms (values may differ from lab to lab)
- Typical stimulus values for intra-op evoked potentials
- Input characteristics for acquiring evoked potentials
- ᵃM = mastoid (“i” is ipsilateral to stimulus, and “c” is contralateral)
Test | Freq (Hz) | Duration (mcS) | Magnitude | Comment |
BAER | 23.5 | 150 | 85–100 dB | Rarefaction usually better than compression |
UE SSEP (median nerve at wrist) | 4.7 | 300–700 | up to 50 mA | Supramaximal stimulus (sensory threshold + motor threshold) |
LE SSEP (posterior tibial at ankle) | 4.7 | 300–700 | up to 100 mA | Supramaximal stimulus |
PR VER | 1.97 | — | — | 16 × 16 checks, 1.6 cm each, at 1 meter (subtends 55" arc visual angle) |
Test | Input filter (Hz) | Sensitive (mcV) | Duration (mS) | Reps | Electrode derivations |
BAER | 150–3000 | 25 | 15 | 1500 | M₁ᵃ–CzZ, M₂*–Cz, ground = FZ |
UE SSEP | 30–3000 | 50 | 55–60 | 600 | Fz–Erb’s point, Cv7–FPZ, C3–FPZ, C3’–NC (non-cephalic, e.g. shoulder) |
LE SSEP | 30–3000 | 50 | 60 | 600 | Popliteal fossa (front to back), Cz–FPZ, back (L5–T12) (difficult in obese or elderly), Cᵢ–Cc (optional: somatosensory ipsilateral to contralateral) |
PR VER | 5–100 | 50 | 500 | 100 | O₁–A₁, OZ–A₁, O₂–A₁, OZ–Cz |
- Intraoperative evoked potentials
- General information
- The need to average a number of waveforms results in a short delay which limits their usefulness in avoiding acute intraoperative injury.
- Somatosensory evoked potentials (SEPs)
- Intraoperative SSEPs may also be used to localize primary sensory cortex in anesthetized patients (as opposed to using brain mapping techniques in awake patients) by looking for phase reversal potentials across the central sulcus.
- Median somatosensory response (SEP)
- Elicited by electrical stimuli— about 200 stimuli applied at the wrist— giving responses normally detectable by averaging at
- Erb’s point
- Over the cervical spine
- Cortex
- Response
- At sensory cortex
- A negative waveform at 20 msec (N20),
- An indicator of cortical function.
- The N20 can be lost with ischaemia from vascular spasm (e.g. in aneurysm or insular surgery)
- Latency increases and reduced amplitude of responses indicate pathology at all levels of wrist- cortex.
- SSEP waveform is often quite variable and should therefore be monitored before surgery begins and throughout the procedure.
- To separate the SSEP signal from background noise a technique called signal averaging is used to combine a large number of SSEP signals.
- The time required to perform signal averaging means that SSEP changes may not be seen until a few minutes after the neurological injury has occurred.
- Posterior tibial somatosensory response
- Posterior tibial stimuli at the ankle gives a cortical P40 response.
- Posterior tibial responses travel in the gracile fasciculus;
- Uses
- To identify midline
- Left and right ankle stimuli gives a guide to the midline of the cord and can be useful to find safe entry if anatomy is distorted over intramedullary tumours.
- Monitor posterior column function in the cord in spinal procedures rather than motor function
- SSEP monitoring during spine surgery
- Paralytics actually improve SSEP recordings by reducing muscle artefact
- But will abolish the visible twitch that confirms that the stimulus is being received.
- Typical stimulus sites:
- Median nerve
- Ulnar nerve
- Tibial nerves
- Impulses ascend in the ipsilateral posterior column.
- UE SSEPs travel primarily in the dorsal columns, but LE SSEPs are carried mostly in the dorsolateral fasciculus (Lissauer’s Tract) which is supplied by the anterior spinal artery ????.
- UE and LE SSEPs are more sensitive to direct mechanical effects primarily on the posterior spinal cord (sensory)
- May remain unchanged with some injuries to the anterior cord (motor);
- LE SSEPs can detect ischemic effects on the anterior cord by virtue of involvement of the anterior spinal artery ???.
- Brainstem auditory evoked responses (BAER)
Motor evoked potentials (MEP)
- General
- Can be recorded wherever distal muscle is accessible,
- Down to the level of S3
Indicated
- Cranial surgery
- Facial nerve monitoring during
- Vestibular Schwannoma
- Is the commonest type of intraoperative EMG used in cranial neurosurgery
- Recording electrodes are placed in the
- Orbicularis oris
- Orbicularis oculis muscles.
- Muscle activity is recorded if
- A surgeon’s intentional stimulation of the nerve
- A surgeon inadvertent nerve stimulation during manipulation, mechanical, or thermal injury.
- Optimal facial nerve EMG recordings are obtained when neuromuscular blockade is avoided
- But facial muscles appear relatively resistant to the effects of muscle relaxants.
- Hartman’s solution irrigation should be used instead of saline irrigation to avoid stimulation of the facial nerve.
- 4th ventricle floor Or Brainstem entry
- Stimulation of the facial colliculi with a probe
- Recordable responses in the face and tongue
- Motor cortex identification
- Awake surgery
- A phase reversal of the N20/ P20 peak indicates that those electrodes straddle the central sulcus.
- Direct cortical stimulation may be used to map the motor cortex
- Asleep surgery
- A phase reversal of the N20/ P20 peak indicates that those electrodes straddle the central sulcus.
- Spinal surgery
- Selective dorsal rhizotomy
- For treatment of spasticity
- Monitoring of pudendal MEPs and sacral EMG recording from the anal sphincter
- Helps identify nerve rootlets that can be selectively sectioned.
- Spinal instrumentation: spinal root monitoring
Generated by stimulating at (Via an electrical or magnetic stimulus)
- Motor cortex: Transcranial motor evoked potentials (TCMEPs)
- Transcranial magnetic stimulation OR
- Magnetic stimuli can be used preoperatively to map motor and speech areas.
- Transcranial electric stimulation using corkscrew or needle electrodes
- During anaesthesia trains of electrical stimuli are normally employed
- A ‘train of 5’ is commonly used, consisting of five pulses with an interpulse interval of 2– 4 ms.
- Taniguchi:
- Stimulation of primary motor cortex with high frequency (300– 500 Hz) of train of 5 pulses to map motor cortex with a monopolar probe, and monitor the corticospinal tract under anaesthesia
- Usually from a subdural strip electrode contact on the cortex
- In awake procedures the cathode is better located in an anaesthetized area of scalp (e.g. the vertex).
- Motor tract monitoring under anaesthesia is also feasible during insular glioma resection— first locating motor cortex using SEP phase reversal with a subdural strip, then using contact over the motor cortex to stimulate (as anode— a scalp electrode as frontal cathode)
Cortical excitatory score
- iRMT stands for "individual Resting Motor Threshold,"
- A neurophysiological measure used in transcranial magnetic stimulation (TMS) or intraoperative neurophysiological monitoring to assess the excitability of the motor cortex.
- It reflects the minimum stimulus intensity (often delivered as a percentage of maximum stimulator output) required to elicit a motor evoked potential (MEP) of a defined amplitude in a target muscle at rest.
- Pathological iRMT means that this threshold is abnormally high, indicating impaired cortical excitability, which may be present in various neurological conditions and is relevant for scoring cortical excitatory status.
- Cortical excitatory score
- 0: no pathological iRMT
- 1: 1 pathological iRMT present (either upper or lower limb)
- 2: 2 pathological iRMT present (both upper and lower limb)
- Higher grade gliomas located in motor eloquent areas are related with decreased RMT, increased latency and decrease amplitude of motor responses, for the lower limb.
- The cortical excitability score provides a biological correlate between the WHO grading of gliomas and pre-operative nTMS motor mapping.
- Recording
- Target
- Mentalis
- Deltoid or biceps
- Extensor digitorum communis
- Abductor pollicis brevis
- Abductor digiti minimi
- Vastus medialis
- Tibialis anterior
- Abductor hallucis
- Method
- Commonly from paired needles in the muscles
- Motor pathway distal to the motor stimulus
- D waves
- Anaesthetic requirements:
- Electrophysiology anaesthetic requirements
- Volatile anaesthetic agents produce an increase in latency and a decrease in amplitude of cortical sensory responses
- Volatile agents have drug effects at multiple synaptic receptor types
- Inhalation anaesthetics have a significant effect on MEPs, in particular I wave responses.
- D waves are not affected by inhalation anaesthetics, but the α- motor neurone is blocked at higher doses, even when stimulated by transcranial electric stimulation.
- Intravenous anaesthetics only have a mild effect on MEPs and total IV anaesthetic using propofol and etomidate is typically preferred when using MEPs as they are less suppressive than gases at similar depths of anaesthesia.
- However, excessively high doses of propofol may be similarly suppressive to inhalation agents.
- Benzodiazepines are usually avoided because they eliminate MEP responses.
- Neuromuscular blockade must be minimized to permit ≥ 2 out of 4 twitches.
- Neuromuscular blocking agents do not affect neurophysiological monitoring and can actually enhance I wave recordings but MUAPs cannot be recorded when the neuromuscular transmission of acetylcholine is completely blocked.
- The anaesthetist observes the M- responses to evaluate the degree of neuromuscular blockade
- Using direct electrical stimulation is limited in awake patients by local pain.
- Due to the large potentials, the acquisition time is shorter and feedback to the surgeon is almost immediate.
- However, due to patient movement from the muscle contractions, continuous recording is usually not possible
- Except with monitoring the response over the spinal cord
- Useful for surgery involving the spinal cord (cervical or thoracic)
- No utility for lumbar spine surgery.
- Seizures occur rarely, usually in patients with increased seizure risk and with high-rate stimulation frequency.
- Contraindications to MEP:
- History of epilepsy/seizures
- Past surgical skull defects
- Metal in head or neck
- Use special care with implanted electronic devices
Descending evoked potentials (DEP)
- Formerly referred to by the misleading term “neurogenic motor evoked potentials”.
- Rostral stimulation of the spinal cord with recording of a caudal neurogenic response from the spinal cord or peripheral nerve, or a myogenic response from a distal muscle.
- DEPs can be mediated primarily by sensory nerves and therefore do not represent true motor potentials.
- However, shown to be sensitive to spinal cord changes and may be useful when TCMEPs cannot be obtained.
- How DNEPs are stimulated and recorded
- The DNEP signal travels down the spinal cord to the lower extremities
- Stimulation at proximal spine
- Signal arrives and is recorded over the sciatic nerves at the popliteal fossa bilaterally
- DNEP epidural placement
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