High-intensity LEDs are embedded in the flash stimulation pad The small disc shape and silicone properties of the pad make it both flexible and lightweight Illuminance can be set up to 20,000 lux, and different light emission times and cycles can be chosen. A common system for placing electrodes is the “10-20 International System” which is based on measurements of head size (Jasper, 1958).  The mid-occipital electrode location (OZ) is on the midline. The distance above the inion calculated as 10 % of the distance between the inion and nasion, which is 3-4 cm in most adults Lateral occipital electrodes are a similar distance off the midline. To have reliable VEPs, Intraoperatively, the following factors are important Maintaining normal intraoperative physiological/hemodynamic parameters Use of TIVA instead of inhalational anesthesia Better stimulus delivery methods Recording intraoperative ERG to ensure good retinal stimulation and Employing optimal recording parameters

1. Intraoperative
Flash Visual Evoked Potentials
The New Frontier
Anurag Tewari MD
Medical Director

2. New ideas pass through three periods
It CAN NOT be done
It probably can be done, but it is NOT WORTH doing
I KNEW it was GOOD IDEA all along!
Arthur C. Clarke

3. Flash Visual Evoked Potentials
aka
Flash Visual Evoked Cortical Potentials

4. Electrical potentials initiated by brief visual stimuli
recorded from the scalp overlying the visual cortex

5. Sources of VEP
f-MRI (Occipital Areas) during VEP Stimulation
Red: Max blood flow Blue-Purple: Minimum Blood Flow

6. Intraoperative flash VEP measures the
FUNCTIONAL INTEGRITY
DETECTS DYSFUNCTION
anywhere in the optic pathway

8. HISTORY OF VEP
History: (from Greek ἱστορία, historia),
means
"Inquiry, Knowledge Acquired By Investigation"

9. HISTORY OF VEP
• 1930s: VEPs initiated by strobe flash were noticed during clinical EEG
• Recorded rhythmic potentials as high as 25/second using scalp
electrodes, while exposing the eyes to flickering light.
EEG following flash presentation (blue and yellow areas); typically the waveforms of the VEP.

10. Put some money on the VEPs

11. HISTORY OF VEP
• 1961, Ciganek L. studied the VEP in human beings and found that it
typically comprised of a triphasic waveform of waves I, II, and III.

12. HISTORY OF VEP
Neurosurgeons began using flash VEP intraoperatively:
• (1976): Wilson et al.
• 4 pts
• manipulation of the optic chiasm lead to
• Erratic VEP response
• Decrease in amplitude when compared to baselines
• (1985): Silva et al. Brain Research Group Netherlands
• Optic nerve manipulations caused
• Disturbance in EVP waveforms and
• Increase in latency of P100 with inhalational anesthesia

13. HISTORY OF VEP
• 1973: Wright et al
• Reported the first case where flash VEP monitoring was used during
Intra-Orbital tumor removal under GA

14. HISTORY OF VEP
• The clinical use of flash VEP monitoring under GA for preservation of
visual function was subsequently investigated
• but no clear utility was observed
• Potential obtained under GA, at that time were
• UNSTABLE
• POOR REPRODUCIBILITY

15. HISTORY OF VEP
• Intraoperative flash VEP monitoring eventually stopped being used in
clinical settings around 1990 as an intraoperative tool

16. HISTORY OF VEP
• Recent breakthroughs have rekindled interest in VEP
• Use of TIVA (Propofol + Remi)
• Reduced VEP suppression by anesthetics
• Light-emitting diodes (LEDs)
• with strong illuminance
• Photo-stimulation devices using LEDs
• allowed the retina to be more strongly photo-stimulated

17. HISTORY OF VEP 2010
• .

18. HISTORY OF VEP
The authors reported
• Reproducible flash VEP recording possible in
• 93.5% (187/200 eyes) Sasaki et al
• 97.2% (103/106 eyes) Kodama et al
• A strong relationship between
• Intra-Op potential changes and Post-Op visual function

19. Previous Work on VEPs
Author Device N Recording
electrode
Anesthesia Flash Frequency (Hz) Bandpass
Filter (Hz)
ERG Result
Cedzich et al. -
1988
Red LED 45 Oz-Fz Inhalational 1.9 5-100 No
VEP recordable, but no
correlation between
intraoperative findings and post-
operative visual function
Chacko et al. -
1996
Red LED 36 Oz-Fz Inhalational 1.9 5-100 No
30.8% improvement in the visual
field defect in the monitored group
compared to 18.4% in the controls.
No correlation between
intraoperative findings and
postoperative visual function
Harding et al. -1990
Stroboscopic flash
light over dilated
pupils
57 Oz-Fz Inhalational 1.6 30-Jan No
Loss of VEP >4 min cause post-
operative decrease in visual
function (Visual Acuity)

20. Previous Work on VEPs
Author Device N Recording electrode Anesthesia Flash
Frequency
(Hz)
Bandpass
Filter (Hz)
ERG Result
Wiedemayer et al.
(2003)
Red LED 32 Oz-Fz, Oz- A1/A2 TIVA 1.5 Feb-30 No
Pre-operative high inter-individual variability. No
stable recording.
Chung et al. -2012
Red LED avg
Luminosity 2000
Lx, for max 4 sec.
65
Oz, LO, RO, Cz ref to
A1,A2
TIVA 1 for 20msec - No
VEP recordable, but no correlation between
intraoperative findings and post operative visual
function
Kodama et
al.
2010
Thin curved
goggles with 15 red
light emitting
diodes
53
ERG, A1,A2, Lt,LO, Rt,
RO, Oz
TIVA 1 for 40msec 10-1000 Yes
103/106 (97%) VEP recorded.
INTRAOPERATIVE VEP
CORRELATE WITH POST OP
VISUAL FUNCTION.

21. WERE WE LOOKING AT A NEW BEGINNING?

22. Previous Work on VEPs
Author Device N Recording
electrode
Anesthesia Flash Frequency
(Hz)
Bandpass
Filter (Hz)
ERG Result
Sasaki et al.
(2010)
16 red high
luminosity LEDs
(100 mCd)
embedded in a soft
round silicone disc
100
ERG, 4cm above and
lateral to inion
TIVA 1 for 20msec 20-500 Yes
• 93.5% reproducibility of the
waveforms.
• 2 false negatives (no intraoperative
VEP change with impaired
postoperative visual function).
• All other intraoperative findings
correlate with post op visual function.
Kamio et al.
(2014)
16 red high
luminosity LEDs
(100 mCd)
embedded in a soft
round silicone disc
33
ERG, 4cm above and
lateral to inion
TIVA 1 for 20msec 20-500 Yes
• 28/33 (84.8%) had stable
intraoperative VEP recording.
• In 4/28 cases, VEP amplitude
decreased transiently,
• In 1/28 cases VEP amplitude did not
recover.
Luo et al.
(2015)
19 Red light
emitting diodes with
illuminance set to
20000 Lux
46
A1, A2, O1, O2, Oz,
Cz, Fz. A1 and A2
were linked and
served as recording
reference.
TIVA
For 1st 36 patients 1.1
Hz for 40 ms, and next
10, 1.1 Hz for 10 ms
5-100 No
• VEP was recorded in 62 eyes with
normal pre-operative vision.
• 3 false negatives and
• 2 false positives.

23. ANATOMY

24. The flash stimulus input to the
retina is transmitted to the
Optic nerve
Optic chiasm
Optic tract
Lateral Geniculate Body
Optic Radiation
Visual Cortical Area
VEP waveform is recorded from
the occipital region.

25. RETINA
• The retina is formed by seven layers from the interior to the exterior
• Ganglion cell layer
• Inner plexiform layer
• Inner nuclear layer
• Outer plexiform layer
• Outer nuclear layer
• Photoreceptor layer, and
• Pigmented epithelial layer
• PLEXI=plexus: (Latin for "braid“) an intricate network or nerves or vessels

26. • PHOTO-STIMULUS
reaches the RETINAL SURFACE
stimulates
• PRIMARY NEURONS (photoreceptor cells)
stimulation is transmitted
• SECONDARY NEURONS (bipolar and
horizontal cells)
• TERTIARY NEURONS (ganglion and
amacrine cells),
transmit the information
• OPTIC NERVE
Via the axons of the GANGLION CELLS
to the CENTRAL NERVOUS SYSTEM.

27. OPTIC CHIASM
• .

28. LATERAL GENICULATE
BODY.

29. PRIMARY VISUAL CORTEX
• .

30. VISUAL PATHWAY
• Light information input into
• NASAL retina propagates to the
contralateral lateral geniculate
body by intersecting at the optic
chiasm.
• TEMPORAL retina propagates to
the ipsilateral lateral geniculate
body.
The lateral geniculate body contains almost no neurons that receive information from both eyes.

31. VISUAL PATHWAY
• Synthesis of information from the left and right eyes occurs in
the cerebral cortex
• The VISUAL CORTEX receives information from the lateral geniculate
body corresponds to BRODMANN AREAS 17, 18, and 19

32. VISUAL PATHWAY
• A large part of the PRIMARY VISUAL CORTEX (Brodmann area 17) is embedded in
the medial aspect of the occipital lobe.

33. How do we
get to our
destination?

34. Measurement Principles: Types of VEP
• VEP obtained from the visual cortex by applying photo-
stimulus to the retina exposed to
• FLASH STIMULATION or
• PATTERN REVERSAL STIMULATION

35. Measurement Principles: Types of VEP
• Neurons of the visual cortex are highly sensitive to visual stimuli by
graphics containing CONTOURS AND CONTRAST
• Pattern reversal stimulation involving black and white lattices exchanging
position at regular intervals was developed using this principle
• Excellent for effectively stimulating neurons of the visual cortex

36. Measurement Principles: Types of VEP
• However, pattern reversal stimulation cannot be performed under GA
• As patient co-operation and intact cognition is required

37. Measurement Principles: Types of VEP
• FLASH STIMULATION is performed, whereby a strong light is
delivered to the retina
• LEDs photo-stimulate the retina (20,000 lux of illuminance)

38. Methods of recording
Intraoperative Flash VEP
•Stimulation methods
•Recording methods
•Methods for recording ERG
•Intraoperative assessment of VEP

39. STIMULATION METHODS
• High-intensity LEDs are embedded in the flash stimulation pad
• The small disc shape and silicone properties of the pad make it both
flexible and lightweight
• Illuminance can be set up to 20,000 lux, and different light emission times
and cycles can be chosen.

40. STIMULATION METHODS
• The flash stimulation pad is placed over both eyelids
• The eyes are closed and fixed with cornea protecting tape or another
fixative to prevent it from dislodging.
• Covering the pad with a light-shielding sheet is an effective means of
preventing light, such as surgical lighting, from entering when performing
flash stimulation.

41. RECORDING METHODS
•Clinical VEPs are usually recorded from occipital scalp
overlying the calcarine fissure.

42. RECORDING METHODS
• A common system for placing electrodes is the “10-20 International
System” which is based on measurements of head size (Jasper, 1958).
• The mid-occipital electrode location (OZ) is on the midline.
• The distance above the inion calculated as 10 % of the distance between the inion
and nasion, which is 3-4 cm in most adults
• Lateral occipital electrodes are a similar distance off the midline.

44. Flash VEP electrode setup
QUEEN SQUARE SYSTEM
• STANDARD SURFACE ELECTRODES : Lateral canthus of each eye
• Records early potentials from the retina (electroretinogram; ERG)
• Evoked potentials from the visual cortex recorded with electrodes on the
scalp overlying the occipital lobe
Label Name Placement
MO MID-OCCIPITAL Positioned 5 cm above the external occipital protuberance (inion)
LO LEFT OCCIPITAL Placed 5 cm lateral to MO at left occipital positions.
RO RIGHT OCCIPITAL Placed 5 cm lateral to MO at right occipital positions.
MF REFERENCE
ELECTRODES
Placed in the mastoid process bilaterally as well as a mid-frontal location 12 cm
above the nasion.

45. FLASH VEP
RECORDING PARAMETERS
Recording Channel Filter Bandpass Time-base
Left – Right lateral canthus 10Hz – 750 Hz 30 mSec/div
LO – mastoid 20Hz – 500 Hz 30 mSec/div
MO – mastoid 20Hz – 500 Hz 30 mSec/div
RO – mastoid 20Hz – 500 Hz 30 mSec/div
MO – MF 20Hz – 500 Hz 30 mSec/div
Six High Intensity Diodes

46. RECORDING METHODS
• The mechanical settings for flash VEP recording under GA are as follows:
SETTINGS RECOMMENDED PARAMETERS
Light stimulus illuminance 10000–20000 Lx
Duration 10mSec
Frequency 1.1–3.0 Hz
Average 50–200 responses
Analysis time 200 mSec
Band-pass filter 20 Hz (low), 500 Hz (high)

47. RECORDING METHODS
• At least 2 waveforms recorded to establish reproducibility
• ERG recording used to confirm stimulation of the eyes
• The best occipital recording channel from the baseline used for
monitoring

48. RECORDING METHODS
• The baseline VEPs is assessed according to the following criteria:
• At least 2 VEP waveform sets acquired approximately every 30
minutes during surgery
• VEP recordings repeated more often as necessary if a significant VEP
change is identified

49. VEP ALERT
• Significant VEP waveform changes triggering an alert based on:
• Assuming a stable monitorable baseline:
Loss of VEP – absence of a repeatable waveform
Decrease of VEP – reproducible ≥50% decrease in amplitude
Latency increase – increase of VEP latency by ≥10%
• Assuming the baseline is considered marginal:
Only a loss of VEP considered significant

50. VEP ALERT INTERVENTION
• Following steps are taken if a VEP alert occurs (significant waveform changes):
Check for possible TECHNICAL issues
Position of the stimulator,
Connections,
Hardware malfunction, etc.
Check for ANESTHETIC causes (e.g. I/V bolus)
Evaluate an estimated BLOOD LOSS
Check BLOOD PRESSURE
Evaluate SURGICAL factors (for intracranial cases)

51. Methods for recording ERG
• ERG must be recorded while monitoring flash VEPs
• Confirms that the flash stimulus has reached the retina
• Sasaki T, et al. Intraoperative monitoring of visual evoked potential: introduction of a clinically useful method. J Neurosurg 2010; 112: 273-84.
• Kodama K, et al. Standard and limitation of intraoperative monitoring of the visual evoked potential. Acta Neurochir (Wien) 2010; 152: 643-8.
• Kamio Y, et al. Usefulness of intraoperative monitoring of visual evoked potentials in transsphenoidal surgery. Neurol Med Chir (Tokyo) 2014; 54: 606-11.

52. Methods for recording ERG
• Flash stimulation-induced ERG can be recorded using
electrodes placed anywhere around the eyes
• ERG is processed by the same averaging as the VEPs.
ERG monitoring is particularly important during frontal craniotomy

53. WHAT WAVEFORM WOULD WE BE LOOKING AT?

54. Normal VEP waveforms
PHYSIOLOGICAL IMPLICATIONS
• A typical VEP waveform has an amplitude of approximately 5–20 μV
The I to III waveforms with maximum amplitude and shortest latency at the occipital region are recorded

55. Normal VEP waveforms
PHYSIOLOGICAL IMPLICATIONS
• At least SEVEN components seen after flash stimulation
• Early Components (I-III)
• Late components (IV-VII)
A proper evoked potential induced by flash stimulation should be able to exhibit both early and late components.

56. Normal VEP waveforms
PHYSIOLOGICAL IMPLICATIONS
• Early Components (I-III) are
• Relatively STABLE in the same individual
• Left and Right waveforms are almost SIMILAR

57. Normal VEP waveforms
PHYSIOLOGICAL IMPLICATIONS
• Early Components (I-III) represent the action potentials of the
primary visual cortex and relay zone

58. Normal VEP waveforms
PHYSIOLOGICAL IMPLICATIONS
• Late components (IV-VII): Result due to activity in cortical
areas other than the optic pathway of the visual cortex
traveling from the lateral geniculate body

59. Normal VEP waveforms
PHYSIOLOGICAL IMPLICATIONS
• The first three components amplify gradually
• Wave I and II have small amplitudes and are often indistinguishable (buried
in background noise)
• The third component is the waveform that can be confirmed in all cases.

60. Normal VEP waveforms
PHYSIOLOGICAL IMPLICATIONS
• The fifth waveform reaches its maximum amplitude and
shortest latency in the parietal region

61. Normal VEP waveforms
PHYSIOLOGICAL IMPLICATIONS
• Flash VEP under GA are evaluated using peak-to-peak amplitude
between the III and IV waveforms
• i.e., between N75 and P100

62. Methods of recording intraoperative flash VEP
Intraoperative assessment of VEP
• When the peak-to-peak distance between N75 and P100 decreases by
at least 50% from the reference amplitude
• Report to the surgeon as a significant change in the VEP

63. Methods of recording intraoperative flash VEP
Intraoperative assessment of VEP
• However, as the VEP amplitude is low, at least two or more
recordings of the same waveform must be confirmed to verify
reproducibility.
• The continuous disappearance of VEP waveforms can be
interpreted as the onset of severe postoperative visual
impairment.

64. Clinical Intra-Op Flash VEP Monitoring
•Surgeries that pose a risk of visual impairment
•Neurosurgical procedures
•Spinal surgery performed in the prone position
•Cardiovascular surgery
•Robot-Assisted Prostate surgery
• with the head tilted downward

65. Neurosurgical Procedures
• Tumor Resection near the optic chiasm
• Pituitary adenomas
• Craniopharyngiomas
• Tuberculum sellae meningiomas, etc.
• Resection of brain tumors in the vicinity of the optic pathway
• Optic nerve
• Optic radiation
• Occipital lobe
• Internal Carotid Artery aneurysm clipping (risk of impeding blood flow to the
ophthalmic artery)

66. Factors that Affect Flash VEP
•Preoperative visual function
•Body temperature
•Partial pressure of carbon dioxide in the blood
•Hypoxemia and hypotension
•Hemodilution

67. Know your
opposition

68. Factors that Affect Flash VEP
Preoperative visual function
• Severe visual impairment prior to surgery causes
• low reproducibility and
• are difficult to record.
• As the optic nerve cannot be sufficiently stimulated by flash
stimulation.
• Suitability of intraoperative VEP monitoring must be
determined in accordance with the preoperative visual function

69. Factors that Affect Flash VEP
Body temperature
• A drop in temperature by 1°C reduces
• peripheral conduction by 5% and
• central conduction by 15%
• The optic pathway, is a polysynaptic
pathway, hence considered sensitive
to hypothermia.

70. Factors that Affect Flash VEP
Body temperature
• Synaptic transmission is more susceptible to the effects of hypothermia
than axial propagation.
• Due to the effects of hypothermia on VEPs, caution is required during
GA when changes in body temperature are prone to occur.

71. Factors that Affect Flash VEP
Body temperature
• Decreases in body temperature gradually cause
• VEP amplitude to ATTENUATE
• latency to EXTEND
• waveforms completely DISAPPEAR at 25–27°C
•VEP latency at 33°C is extended by 10–20%

72. Factors that Affect Flash VEP
Partial pressure of Carbon Dioxide in blood
• Hypocapnia alters blood pH that promote the neuronal stimulation
• Acceleration of the conduction velocity
During VEP monitoring, GA should be managed to avoid any
major changes in partial pressure of carbon dioxide in the blood

73. Factors that Affect Flash VEP
Hypoxemia and hypotension
• Compared to the spinal cord and subcortex, the cerebral cortex
has a high metabolic rate; hence a low tolerance to hypoxia
• Decreases in the MAP that is beyond autoregulation affect the
evoked potential because the transport of oxygen to neurons is
reduced.
• VEP amplitude is decreased and latency is
extended under conditions of extreme hypoxia
and hypotension.

74. Factors that Affect Flash VEP
Hemodilution
• Use of Crystalloid and/or colloid replacement for intraoperative
hemorrhage causes hemodilution.
• Excessive hemodilution can change VEPs.
• Hematocrit below 15% extends VEP latency and reduces VEP amplitude
• VEPs have been reported to recover by increasing hematocrit ≥ 22%

75. Flash VEP and Various Anesthetics
• Anesthetics suppress synaptic transmission, and polysynaptic pathways
are easily suppressed.
• Visual pathway is strongly influenced by anesthetics because it passes
through three synapses

76. Flash VEP and Various Anesthetics
• Effects of Various Anesthetics on Flash VEP
Anesthetic Agent Effects
Inhaled Anesthetic Gases Isoflurane ↓↓
Sevoflurane ↓↓
Desflurane ↓↓
Nitrous oxide ↓↓
Intravenous anesthetics Thiopental ↓↓
Propofol ↓
Fentanyl — or ↓
Remifentanil — or ↓
Ketamine ↓↓
Muscle relaxants Vecuronium —
Rocuronium —
↓↓: strong suppressive effect, ↓: small suppressive effect, —: no suppressive effect

77. Flash VEP and Volatile Anesthetics
• All inhaled anesthetic gases suppress flash VEPs by extending latency
and reducing amplitude in a concentration-dependent manner, even at
low concentrations
• Nitrous oxide causes marked attenuation of the amplitude and the
disappearance of waveforms when combined with an inhaled anesthetic

78. Flash VEP and Various Anesthetics
• Among intravenous anesthetics, only propofol has a small suppressive
effect on flash VEPs
• While other intravenous anesthetics are not suitable as they markedly
suppress flash VEPs even at low concentrations.
• Thiopental extends the latency and attenuates the amplitude in a dose-
dependent manner, and causes waveform to disappear at a dose of 6
mg/kg
• Ketamine causes a slight extension in latency but markedly attenuates
amplitude

79. Flash VEP and Various Anesthetics
• At normal clinical doses opioids fentanyl and remifentanil have
no effects on flash VEPs.
• Use caution when administering fentanyl in a single large dose
(10–60 μg/kg)
• Muscle relaxants can be used because they have no effects on
flash VEP.

80. Flash VEP and Various Anesthetics
• The anesthetic method suitable for flash VEP monitoring under
GA is TIVA
{Propofol + narcotic (fentanyl or remifentanil), ± muscle relaxant}
• Though, even propofol suppresses the VEP when administered
in large doses
• Hence the depth of anesthesia must be regulated.

82. Conclusions
• To have reliable VEPs, Intraoperatively, the following factors are
important
• Maintaining normal intraoperative physiological/hemodynamic parameters
• Use of TIVA instead of inhalational anesthesia
• Better stimulus delivery methods
• Recording intraoperative ERG to ensure good retinal stimulation and
• Employing optimal recording parameters

83. Thank you