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Visual Evoked Potentials

Visual evoked potentials (VEPs) record electrical signals from the scalp in response to visual stimuli. VEPs are useful for objectively assessing visual function, especially of the retina and optic nerve. The VEP involves presenting a visual stimulus such as a flashing light or alternating checkerboard pattern. Electrodes placed on the scalp record the P100 waveform generated in the striate and peristriate cortex in response to the stimulus. Analysis of the P100 latency, amplitude, and interocular latency difference can help detect and localize abnormalities in the retina, optic nerve, optic tract, and visual cortex.

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Visual Evoked Potential Presenter - Dr. Rahul Jain Moderator - Dr. B. K. Bajaj
• Evoked Potentials (EP) - electrical signals generated by the nervous system in response to sensory stimuli. • VEP - Electrical potential differences recorded from the Scalp in response to visual stimuli.
• Only means available for Objective assessment of visual function, especially of Retina (ERG) and Optic Nerve (VEP). • Requires less active participation of the patient - than the subjective evaluations of perimetry and visual acuity testing. • Discriminating disease of retina vs optic nerve • Distinguishing types of injuries of ON, glaucoma, inflammatory, metabolic • Useful in monitoring conditions like MS
CLINICAL POINT Ganglion cell layer Nerve fiber layer Inner plexiform layer Inner nuclear layer Outer plexiform layer Outer nuclear layer Photoreceptor layer Pigment epithelium Cells Inner limiting membrane Axons at surface of retina pass optic nerve, chiasm, and tract geniculate body Ganglion cell Müller cell (supporting glial ce Amacrine cell Bipolar cell Horizontal cell Rod Cone Pigment cells of choroid Section through retina Retinal Layers 14.25 THE RETINA: RETINAL LAYERS The retina is a tissue-paper-thin piece of CNS tissue that con- tains the photoreceptors; it is attached to the vascular tunic at the ora serrata. The layers of the retina in the interior of the eyeball are oriented from outer to inner. The pigment epithe- lium is at the outer margin, followed by the outer nuclear layer (photoreceptors), the inner nuclear layer (bipolar neurons, amacrine and horizontal cells), and the ganglion cell layer. The outer and inner plexiform layers are the zones of synaptic connectivity. The ganglion cell axons form an inner nerve metric midpoint. The fovea consists purel vision (photopic); these cone projection involve very little convergence. In the fove one-to-one-to-one relationship among neurons, and ganglion cells. The periphe ceptors are mainly rods, for night vision huge convergence of rods onto bipolar neu converge onto single ganglion cells. T achieved in the fovea, the region for color Outer to Inner X type Retinal GC / Parvocellular Y Type Retinal GC/ Magnocellular Small axonal diameter Large axonal diameter Cone Vision Rod Vision Concentrated in central visual field Wide receptive field, peripheral retina Low sensitivity in motion High Sensitivity in motion Central 1 degree of retina - Cones (peak density) From the centre of retina 6-8 degrees - Rods (peak density)
• Because signals from the temporal field of each eye cross at the optic chiasma • Patterns appearing in only one hemifield go entirely to the opposite occipital lobe. • With Large field stimulus (full field stimulation), patterns from both the hemifields, produce vectors which ADD up to produce VEP Maximum at occipital midline
Left eye Right eye Chiasm Chiasm Prechiasmatic Postchiasmatic Optic tract Optic nerve (Optic nerve) Optic tract Crossed (nasal) fibers Uncrossed (temporal) fibers Key Optic radiations Occipital cortex Superior nasal fibers Superior Temporal Nasal Retinal fibers Nasal Inferior Temporal Inferior nasal fibers Inferior nasal fibers decussate in anterior chiasm and then project into optic tract as anterior fibers Superior view Optic pathway (superior view) with E.Hatton 2:1
Limbic cingulate cortex Thalamus Pituitary gland bes and functional areas Pons mental motor cortex Medulla oblongata Frontal Limbic Primary motor cortex Parietal Occipital Precentral sulcus Paracentral lobule Somatosensory association cortex Corpus callosum Visual association cortex Calcarine fissure Primary visual cortex Cerebellum UB LB
A. Lobes and functional areas Pons Medulla oblongata Cereb 8 9 6 4 7 7 23 19 18 17 17 18 31 24 32 32 10 12 25 33 3 1 2 RAIN: unique architec- 6 6 6 4 3,1,2 3,1,2 3,1,2 22 22 22 41 42 40 40 40 39 39 37 37 37 37 21 20 20 21 5 5 7 7 7 19 19 19 19 19 18 18 18 17 4 4 1909. His numbering of cortical areas is still used as a short- hand for describing the functional regions of the cortex, par- ticularly those related to sensory functions. Some overlap 17 - Primary Visual Cortex V1 18 - Secondary Visual Cortex V2 19 - Association Visual Cortex V3, V4, V5 UB LB Foveal projection more than peripheral retina - Foveal Magnification GENERATOR OF VEP - STRIATE AND PERI STRIATE CORTEX
VISUALLY EVOKED POTENTIALS BY DONNELL J. CREEL, https://webvision.med.utah.edu/book/electrophysiology/visually-evoked-potentials/
Retina Alternating checkerboard pattern displayed Optic nerve Optic chiasm Optic tract Lateral geniculate nucleus Cochlear Series of Inferior colliculus Latency Amplitude Latency Amplitude Lateral lemniscus Medial geniculate body Acoustic area of temporal lobe cortex Primary visual cortex P1 N1 I VI VII II III IV V VI VII N2 I. Visual Evoked Potential II. Brainstem Auditory Evoked Potential Retino - Geniculo - Calcarine pathway X type Retinal GC Y Type Retinal GC Small Large Cone Vision Rod Vision Concentrated in central visual field Wide receptive field, peripheral retina Low sensitivity in motion High Sensitivity in motion PATTERN SHIFT VEP FLASH VEP RETINO- GENICULO - CALCARINE PATHWAY EXTRA GENICULATE PATHWAY
X type Retinal GC Y Type Retinal GC Small Large Cone Vision Rod Vision Concentrated in central visual field Wide receptive field, peripheral retina Low sensitivity in motion High Sensitivity in motion PATTERN SHIFT VEP (Patterned Visual Stimuli) FLASH VEP (Non patterned) RETINO- GENICULO - CALCARINE PATHWAY EXTRA GENICULATE PATHWAY Less - Inter & Intra - individual variability More variability Detects Minor abnormality, More Sensitive & Accurate Cannot detect Minor abnormality, Less Sensitive & Accurate For those who can’t fixate, Steady State VEP
Pre-Test Evaluation • Explain the procedure, co-operation. • Avoid Hair Spray or Oil after last hair wash • Usual glasses, to be Worn during the test • Ophthalmological Exam results to be reviewed. - Visual acuity, pupillary diameter, visual field • Avoid any miotic/ mydriatic drug 12hrs before the test
Procedure • Standard disc EEG electrodes used. • Skin should be prepared by abrading and degreasing. • Two Channel vs Four Channel VEP (Electrode Placement)
Electrode Placement 2 channel VEP Channel 1 - Oz - Fz Channel 2 - Oz - linked ear 4 channel VEP Channel 1 - Oz - Fz Channel 2 - Pz - Fz Channel 3 - L5 - Fz Channel 4 - R5 - Fz Impedance < 5 kilo ohms Oz - actually located at the middle point of Variation Range of Calcarine fissure 12 cm above the nasion Reference Electrode Recording Electrode 5 cm above the Inion
International 10 - 20 System • Oz - 3-4cm above inion (10% of distance between inion and nation, which is about 3-4 cm in normal adults) • Fz - 11cm above nasion • O1, O2 - 2.5 cm lateral to Oz • A1 A2 ear • Cz - ground Queens Montage • MO (Mid Occipital) - 5cm above inion, in midline • MF (mid frontal) - 12cm above nasion, in midline • LO, RO (left right occiptial) - 5cm left and right of MO • A1/A2 - ear • Ground - Vertex Source - Guidelines on Visual Evoked Potentials, American Clinical Neurophysiology Society, 2008
Transient Pattern Reversal VEP • Peak response occurs approximately 50-250 ms after stimulus. By convention in neurophysiology - Upward - Negative, Downwards - Positive. • First negative - N75 (suggested to be input from Dorsal LGN to the striate cortex) Lack of consistency in latency, and are sometimes present / absent • Second positive - P100 (suggested to be excitatory outflow of 17 to 18,19/ secondary inhibitory response at 17) • Third Negative - N145 • Inter ocular latency difference - significant - 6-10 ms, as they are less variable, hence more important than the absolute latency measurements (more variable) Source - Aminoff’s Electrodiagnosis in Clinical Neurology 6th ed.
FIG. 2. PVEP to full-field stimulation shows the rostral—caudal extent of the occipital components N75, P100, and N 145. These may be maximal above or below the midoccipital site in normal individuals with a minimal amplitude response as the midoccipital lead. The frontal N100 component may be recorded widely over the anterior ad . C c , 30 ; a , .97/ ; ; 256 . Source - Guidelines on Visual Evoked Potentials, American Clinical Neurophysiology Society, 2008
P100 is the most positive peak • Prolonged P100 - demyelination (in most typical abnormality, all peaks prolonged)
• Two primary features - time elapsed since stimulus (latency), magnitude of deflection from baseline (amplitude) • Amplitude of P100 is highly variable, therefore it is difficult to establish normal values. Some labs measure N70 to P100, P100 to N145, • Depends on state of arousal, and other patient and condition specific factors. Therefore before labelling abnormal, caution should be exercised • Constant monitoring is required. Patient should maintain visual fixation, throughout the study. (vs Malingering pt) Source - Aminoff’s Electrodiagnosis in Clinical Neurology 6th ed. , Clinical Neurophysiology by UK Misra & J Kalita, 3rd ed
• Average of 100 - 200 stimuli - one response. With low amplitudes upto 400 stimuli. • The values can be compared to the standard normative data. Peak latencies are more consistent, than amplitudes. • At least 2 responses to be recorded. The P100 latency measured in these should be within 2.5 ms difference, (>6-10ms abnormality), in between - technical difference. • Peak to peak amplitude of N75 - P100
Kothari R, Bokariya P, Singh S, Singh R. A Comprehensive Review on Methodologies Employed for Visual Evoked Potentials. Scientifica (Cairo). 2016;2016:9852194. doi:10.1155/2016/9852194 Time period analysed after each stimulus 1/5th to 1/2 a second Luminance - intensity of light from visible spectrum per unit area traveling in a given direction, P100 latency increases with decrease of luminance VEPs are generally recorded under ambient photopic conditions in a standard, normally illuminated room. Contrast - luminance difference between two adjacent elements in the visual scene, Low contrast responses have smaller amplitudes, broader peaks, prolonged latency
Produces reversal pattern every 500ms/0.5 s
• Routine Traditional VEP, done in Pattern Shift, checkerboard pattern is FULL FIELD STIMULATION. • That means, the Stimulus varies consistently ACROSS A LARGE PORTION OF THE VISUAL FIELD.
Abberant waveform • A VEP waveform is considered aberrant if it can be recorded Reproducibly but a P100 peak can not be identified. • Many of aberrant responses, contain too many peaks, rather than too few. • W shaped P100 - bifid P100 - when frontal N100 and occipital P100 are asynchronous. This is seen only in frontal or central reference. i.e. Oz- Fpz / Oz - Pz • Can be seen normally in Oz - A1 leads. (occipital leads) • Can be abolished by asking the patient to look at the upper edge of the pattern, rather than central. Normal Normal Supernumerary P175 etc. - Transient oscillations of occipital cortex
Walsh P, Kane N, Butler S The clinical role of evoked potentials Journal of Neurology, Neurosurgery & Psychiatry 2005;76:ii16-ii22.
Multifocal Transient VEP • In contrast to full field stimulation, Multifocal technique divides the Visual Field into a fixed number of sectors, each of which follows its own sequence of stimulus changes. • Hence, each sector can have on of the two states, inverted checkerboards. • Smaller check sizes in the centre for stimulating the macular vision, larger check sizes in periphery, to stimulate the peripheral retinal vision. • The multifocal VEP demonstrates equivalent, or superior test repeatability when compared to standard automated perimetry. Source - Aminoff’s Electrodiagnosis in Clinical Neurology 6th ed.
Source - Aminoff’s Electrodiagnosis in Clinical Neurology 6th ed. • The waveforms are usually the same as standard VEP, but phase reversal takes place at the mid horizontal meridian, caused by the calcarine fissure
Source - Aminoff’s Electrodiagnosis in Clinical Neurology 6th ed.
Source - Aminoff’s Electrodiagnosis in Clinical Neurology 6th ed.
Source - Aminoff’s Electrodiagnosis in Clinical Neurology 6th ed.
a. Full field VEP recording from Oz - Fpz, 53 yr old woman, leading to temporal dispersion which is difficult to interpret b. Multifocal VEP confirms abnormality by demonstrating central amplitude decrement in left eye (blue) Source - Aminoff’s Electrodiagnosis in Clinical Neurology 6th ed.
P100 on standard pattern reversal VEP - 178, 179 normal brain mri Normal P100 latency on multifocal VEP Source - Aminoff’s Electrodiagnosis in Clinical Neurology 6th ed.
Stimulation Patterns • Pattern Shift VEP - Checker Board pattern. • The size of the stimulus, at the retina will depend on its size on the monitor, and the distance of the subject from the screen. • The total stimulus size will determine the area of the visual field subject to stimulation, and check size will impact the neuronal cell population which is generating the response. • Hence, larger check sizes, will stimulate more of peripherally located RGCs • Smaller check sizes, will stimulate more of centrally located RGCs • But smaller check sizes - have risk of error due to refractive errors. Larger check sizes, will not produce adequate central field stimulation. (pattern shift VEP) • IFCN recommends, check size that subtend 24 - 32 minutes of visual arc.
W = 380 mm / 8 large squares = 47 mm B = 57.3 X 47 mm divided by 90 mm = 30 minutes of visual arc subtended by both the vertical and horizontal dimensions. The visual angle subtended by an indi id al elemen f he a e n a he bjec e e i e e ed in ei he min e degrees of arc. The tangent of the visual angle is equal to the check width divided by the distance from the eye to the screen. The angle can therefore be calculated as: B = arctan W/D This can be approximated for small angles by the formula: B= A* W/D where W is the width of the unit in millimeters, D is the distance from the screen to the eye in millimeters, and B is the visual angle in minutes of arc when A = 3438 and in degrees of arc when A = 57.3. Size of stimulus field. The size of the total stimulus field is specified by the visual angle i b end a he bjec e e, mea ed a de c ibed ab e f individual pattern units. Location and designation of stimulus field types. A fixation point must be provided for the subject that is distinct from the reversing pattern itself. The location of the fixation in i h ega d he im l field de e mine he egi n f he bjec i al field to be stimulated. A pattern that extends equally to both sides of the fixation point is referred to as a full-field stimulus. A pattern restricted to a small region of central vision, such as 2-4 , with central fixation is designated a central-field stimulus. A pattern presented to one side of the fixation point in one-half of the visual field, such as right half or left half, is designated a half-field or hemi-field stimulus. A pattern presented to a small sector of the visual field is designated a partial-field stimulus, with the location described relative to the fixation point. Half-field stimuli presented in an alternating fashion with reversal of left and right half fields sequentially, with a central fixation spot, are designated alternating half field stimuli. Whenever half-field or partial-field stimuli are used, the fixation point should be displaced to the nonstimulated visual field by a small amount, such as 1 check width of 10. This helps prevent stimulation of both retinal hemifields or regions outside the
Checkerboard Pattern with Red Fixation Point Vertical Grating Pattern Shift Vep Used when checkerboard pattern misses trivial defects. Used. Most Commonly Simplicity Reliability Because the primary visual system is arranged to emphasise detection of edges and movement, shifting patterns with multiple edges and contrasts are the most appropriate way to assess visual function, rather than bright flashes of unpatterned light.
• Stimulus Field Types - Fixation point is provided for the subject that is distinct from the reversing pattern itself. 1. Full field stimulation 2. Central field stimulation 3. Partial field stimulation (not yet useful in clinical practice) 4. Alternating half field stimulation (not yet useful in clinical practice) Anterior to chiasma Posterior to chiasma
Full Field Stimulation Left eye Right eye Chiasm Chiasm Prechiasmatic Postchiasmatic Optic tract Optic nerve (Optic nerve) Optic tract Crossed (nasal) fibers Uncrossed (temporal) fibers Key Optic radiations Occipital cortex Superior nasal fibers Superior Temporal Nasal Retinal fibers Nasal Inferior Temporal Inferior nasal fibers Inferior nasal fibers decussate in anterior chiasm and then project into optic tract as anterior fibers Superior view Optic pathway (superior view) with E.Hatton • Most sensitive in detecting visual system Anterior to Optic Chiasma • Majority of P100 responses arise in neural elements of the eye subserving the central 8-10 degrees of the visual field. • Lesions that produce half or partial visual field deficits but that spare much of central vision will usually not produce significant changes in P100 response latency or amplitude. Such partial lesions in prechiasmal, postchiasmal, or chiasmal locations may produce changes in response topography, but are best tested for using partial visual field stimulation Performed Mono-ocularly Visual Fixation should be centre of the screen (red dot)
Flash VEP • More variable than pattern VEPs across subjects, but are usually quite similar between eyes of an individual subject. • Multiple positive and negative peaks. • Source - extra geniculate pathway • Great variability, limits their utility, as it is difficult to differentiate between normal and abnormal • Hence, American Clinical Neurophysiology society has advised that ONLY COMPLETE ABSENCE OF A FLASH RESPONSE CAN BE CONSIDERED DEFINITELY ABNORMAL.
•They are useful for patients who are unable or unwilling to cooperate for pattern VEPs, and when optical factors such as media opacities prevent the valid use of pattern stimuli, INFANTS •Provide rudimentary information, only that the visual information is reaching the brain. •Can be done with both eyes closed •Can be done with opacity of the media •Not affected by refractive errors •Preservation of response of FVEP suggests that visual pathways are at least partially intact, absence indicates no useful visual function. •Asymmetry of FVEP may occur at site of structural lesions.
• The VEP to flash stimulation consists of a series of negative and positive waves. The earliest detectable component has a peak time of approximately 30 ms poststimulus and components are recordable with peak latencies of up to 300 ms. Peaks are designated as negative and positive in a numerical sequence. • This nomenclature is recommended to differentiate the flash VEP from the pattern-reversal VEP. The most robust components of the flash VEP are the N2 and P2 peaks. Measurements of P2 amplitude should be made from the positive P2 peak at around 120 ms to the preceding N2 negative peak at around 90 ms. • Flash rate - 1 HZ Note that with a e is recorded Fig. 4 A normal flash VEP 123
Note that with a e is recorded Fig. 4 A normal flash VEP 123 124 124 130 130 90 90
cs often make it difficult to compare specific response components betwe ubjects. ash VEP to LED goggle stimulation. Response waveforms may be quite tency and amplitude abnormalities must be interpreted with caution (see
Stimulation of half of the field gives rise to stimulation of contralateral occipital cortex P100 to be expected to be prolonged on the opposite to side of hemi field stimulation Walsh P, Kane N, Butler S The clinical role of evoked potentials Journal of Neurology, Neurosurgery & Psychiatry 2005;76:ii16-ii22.
Pattern Onset/Offset VEP Blocks of black and white appear and disappear, followed by plain gray background. Without decrease in luminance • greater inter-subject variability than pattern-reversal VEPs. • Effective for detection or confirmation of malingering • evaluation of patients with nystagmus • as the technique is less sensitive to confounding factors such as poor fixation, eye movements or deliberate defocus. • Standard VEPs to pattern onset/offset stimulation typically consists of three main peaks in adults; C1 (positive, approximately 75 ms), C2 (negative, approximately 125 ms), and C3 (positive, approximately 150 ms). Amplitudes are measured from the preceding peak. mately 150 ms) (see Fig. 3). Amplitudes are mea- sured from the preceding peak. comparison of amplitud the sensitivity of the V Fig. 3 A normal pattern onset/offset VEP. Note that with a 300 ms sweep only the pattern onset response is recorded Fig. 4 A normal flash VEP
• Stimulation rate - 4-8 Hz • The responses overlap each other, and appear as sinusoidal wave form, which persist during the period of stimulation. • Also known as Repetitive Evoked Potentials. • Repetitive evoked potentials whose constituent discrete frequency components remain constant in amplitude and phase over an infinitely long time period. • Children Source - Clinical Neurophysiology by UK Misra & J Kalita, 3rd ed Steady State VEP’s
Factors affecting VEP • Not affected by direction of change of checks in checkerboard pattern. Horizontal or Vertical • Size of checks - Smaller check size - largest peak amplitude, smallest peak latency. • A person with good visual acuity - produces shortest latency and largest amp. With Small check size, person with poor visual acuity - with Large check size.
Patient Factors • Sex - P100 latencies, shorter for women than men • Age - multifocal VEP latency, increases with age, particularly in men, (@ 2.5 ms/ decade after 5th decade) • Pupil size - pupil dilatation, will reduce P100 latency, mainly in Full Field Pattern stimulation and vice versa, not in Full field flash VEP. Standard - recording with pupils normal. (dilatation - more light, constriction - less light) Average pupillary constriction of 1.75mm increases average latency by 4.6ms. • Eye Dominance - Dominant eye - shorter latency, higher amplitude • Eye Movement - reduced P100 amplitude, Latency not affected. eg. nystagmus patients • Body Temp - only in demyelinating lesions, like MS. • Some studies have shown, P100 latency to decrease when concentrating more
Source - Clinical Neurophysiology by UK Misra & J Kalita, 3rd ed
Usefulness • Disease of anterior optic pathway may produce prolonged P100 latencies without detectable alteration in Visual Acuity, Colour Vision, Pupillary Reactivity, Fundoscopy, or Perimetry. • If Visual Acuity is decreased, and PVEP is normal, it is very unlikely that optic nerve or chiasma is the lesion.
Diseases 1. Multiple Sclerosis • Screening for Asymptomatic Lesions in MS is the most common use. (eg. dissemination in space) • It has been calculated that a plaque of 1 cm in size , can lead to P100 delay of about 25 ms. • Classical pattern - Assymetrical prolongation of P100 latency, with relative preservation of amplitudes. • No relationship between VEP and Visual Clinical complaints was found.
2. NMO • Unrecordable P100 waveform, and reduced amplitude of P100 have reported to be characteristic unlike prolonged P100 latency and normal P100 amplitude in MS.
3. Optic Neuritis • If in Uni Ocular ON, VEP is prolonged only on the affected side, not on opposite side, lesser chances of it to be MS. • Optic neuropathy is found in peripheral neuropathies like - CMT, giant axonal neuropathy, neuropathy associated with macroglobulinemia
4. Ischemic Optic Neuropathy • Reduction of P100 amplitude, before latency prolonged.
Other Demyelinating diseases like • Adreno-Leuko dystrophy • Meta-chromatic Leuko dystrophy • Hereditary spastic paraparesis Vitamin B12 deficiency - asymmetric, bilateral prolongation of P100 latency. Vitamin E deficiency in patients with Abetalipoproteinemia
Optic Neuropathy in HIV patients. Alcohol, Tobacco CO poisoning, reduced amplitude and prolonged latency Ethambutol, Desferoxamine, Quinine Vigabatrin, Valproate Amiodarone
• Not been found to be useful in Cortical Lesions. • Abnormal in Optic Nerve Glioma, NF1
Compressive lesions affecting VEP • Papilledema - later stages, when it becomes severe enough to compress the optic nerve • Raised ICT, hydrocephalus, pseudotumor cerebri • Extrinsic compression of the Anterior Chiasmal pathway results in Loss of amplitude, destruction of wave form, prolonged P100 latency. (latency prolongation less as compared to demyelinating disorders).
• Most imp. Differential for optic neuritis is retinal disorder - central serous retinopathy. • Many patients have been labelled as MS in the past falsely, due to prolonged VEP, having CSR • CSR resolves spontaneously. • PVEP delays can be seen with a variety of other acute and chronic retinal disorders. Hence, complete ophthalmological examination is mandatory.
Electro Retinogram • ERG can be recorded from corneal surface by a contact lens embedded with a corneal ring electrode, and a conjunctival reference from periorbital skin , by small electrodes attached at the lateral canthi. • Stimulus - Strobe light, (ganz field - whole field) • Cones - shorter refractory period - therefore a high frequency stimulus can be used to assess. (30hz) • Rods - (20hz) • Typically recorded with patient’s pupils Dilated after mydriatics • ERG responses classified negative when dips below baseline, positive when goes above. A - stimulation of photoreceptors (rods and cones) 15ms B - activation of retinal interneurons from inner nuclear layer 30-35ms C wave - retinal pigment epithelium D wave - OFF bipolar cells
• Pattern Reversal ERG can be recorded with alternating checkerboard stimulus. • PERG - characterised by negative wave around 50ms N50 and positive deflection at around 95ms P95 • P95 is thought to reflect the b wave - activation of RGC. • It was found that in individuals with abnormal VEP, P95 was prolonged, but P50 was almost always preserved. But in those with normal VEP, both could be prolonged. • Hence also distinguishes abnormality between Photoreceptors and RGCs.
Thank You

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Visual Evoked Potentials

  • 1.
    Visual Evoked Potential Presenter -Dr. Rahul Jain Moderator - Dr. B. K. Bajaj
  • 2.
    • Evoked Potentials(EP) - electrical signals generated by the nervous system in response to sensory stimuli. • VEP - Electrical potential differences recorded from the Scalp in response to visual stimuli.
  • 3.
    • Only meansavailable for Objective assessment of visual function, especially of Retina (ERG) and Optic Nerve (VEP). • Requires less active participation of the patient - than the subjective evaluations of perimetry and visual acuity testing. • Discriminating disease of retina vs optic nerve • Distinguishing types of injuries of ON, glaucoma, inflammatory, metabolic • Useful in monitoring conditions like MS
  • 4.
    CLINICAL POINT Ganglion celllayer Nerve fiber layer Inner plexiform layer Inner nuclear layer Outer plexiform layer Outer nuclear layer Photoreceptor layer Pigment epithelium Cells Inner limiting membrane Axons at surface of retina pass optic nerve, chiasm, and tract geniculate body Ganglion cell Müller cell (supporting glial ce Amacrine cell Bipolar cell Horizontal cell Rod Cone Pigment cells of choroid Section through retina Retinal Layers 14.25 THE RETINA: RETINAL LAYERS The retina is a tissue-paper-thin piece of CNS tissue that con- tains the photoreceptors; it is attached to the vascular tunic at the ora serrata. The layers of the retina in the interior of the eyeball are oriented from outer to inner. The pigment epithe- lium is at the outer margin, followed by the outer nuclear layer (photoreceptors), the inner nuclear layer (bipolar neurons, amacrine and horizontal cells), and the ganglion cell layer. The outer and inner plexiform layers are the zones of synaptic connectivity. The ganglion cell axons form an inner nerve metric midpoint. The fovea consists purel vision (photopic); these cone projection involve very little convergence. In the fove one-to-one-to-one relationship among neurons, and ganglion cells. The periphe ceptors are mainly rods, for night vision huge convergence of rods onto bipolar neu converge onto single ganglion cells. T achieved in the fovea, the region for color Outer to Inner X type Retinal GC / Parvocellular Y Type Retinal GC/ Magnocellular Small axonal diameter Large axonal diameter Cone Vision Rod Vision Concentrated in central visual field Wide receptive field, peripheral retina Low sensitivity in motion High Sensitivity in motion Central 1 degree of retina - Cones (peak density) From the centre of retina 6-8 degrees - Rods (peak density)
  • 5.
    • Because signalsfrom the temporal field of each eye cross at the optic chiasma • Patterns appearing in only one hemifield go entirely to the opposite occipital lobe. • With Large field stimulus (full field stimulation), patterns from both the hemifields, produce vectors which ADD up to produce VEP Maximum at occipital midline
  • 6.
    Left eye Righteye Chiasm Chiasm Prechiasmatic Postchiasmatic Optic tract Optic nerve (Optic nerve) Optic tract Crossed (nasal) fibers Uncrossed (temporal) fibers Key Optic radiations Occipital cortex Superior nasal fibers Superior Temporal Nasal Retinal fibers Nasal Inferior Temporal Inferior nasal fibers Inferior nasal fibers decussate in anterior chiasm and then project into optic tract as anterior fibers Superior view Optic pathway (superior view) with E.Hatton 2:1
  • 7.
    Limbic cingulate cortex Thalamus Pituitarygland bes and functional areas Pons mental motor cortex Medulla oblongata Frontal Limbic Primary motor cortex Parietal Occipital Precentral sulcus Paracentral lobule Somatosensory association cortex Corpus callosum Visual association cortex Calcarine fissure Primary visual cortex Cerebellum UB LB
  • 8.
    A. Lobes andfunctional areas Pons Medulla oblongata Cereb 8 9 6 4 7 7 23 19 18 17 17 18 31 24 32 32 10 12 25 33 3 1 2 RAIN: unique architec- 6 6 6 4 3,1,2 3,1,2 3,1,2 22 22 22 41 42 40 40 40 39 39 37 37 37 37 21 20 20 21 5 5 7 7 7 19 19 19 19 19 18 18 18 17 4 4 1909. His numbering of cortical areas is still used as a short- hand for describing the functional regions of the cortex, par- ticularly those related to sensory functions. Some overlap 17 - Primary Visual Cortex V1 18 - Secondary Visual Cortex V2 19 - Association Visual Cortex V3, V4, V5 UB LB Foveal projection more than peripheral retina - Foveal Magnification GENERATOR OF VEP - STRIATE AND PERI STRIATE CORTEX
  • 9.
    VISUALLY EVOKED POTENTIALSBY DONNELL J. CREEL, https://webvision.med.utah.edu/book/electrophysiology/visually-evoked-potentials/
  • 10.
    Retina Alternating checkerboard pattern displayed Optic nerve Optic chiasm Optic tract Lateral geniculatenucleus Cochlear Series of Inferior colliculus Latency Amplitude Latency Amplitude Lateral lemniscus Medial geniculate body Acoustic area of temporal lobe cortex Primary visual cortex P1 N1 I VI VII II III IV V VI VII N2 I. Visual Evoked Potential II. Brainstem Auditory Evoked Potential Retino - Geniculo - Calcarine pathway X type Retinal GC Y Type Retinal GC Small Large Cone Vision Rod Vision Concentrated in central visual field Wide receptive field, peripheral retina Low sensitivity in motion High Sensitivity in motion PATTERN SHIFT VEP FLASH VEP RETINO- GENICULO - CALCARINE PATHWAY EXTRA GENICULATE PATHWAY
  • 11.
    X type RetinalGC Y Type Retinal GC Small Large Cone Vision Rod Vision Concentrated in central visual field Wide receptive field, peripheral retina Low sensitivity in motion High Sensitivity in motion PATTERN SHIFT VEP (Patterned Visual Stimuli) FLASH VEP (Non patterned) RETINO- GENICULO - CALCARINE PATHWAY EXTRA GENICULATE PATHWAY Less - Inter & Intra - individual variability More variability Detects Minor abnormality, More Sensitive & Accurate Cannot detect Minor abnormality, Less Sensitive & Accurate For those who can’t fixate, Steady State VEP
  • 13.
    Pre-Test Evaluation • Explainthe procedure, co-operation. • Avoid Hair Spray or Oil after last hair wash • Usual glasses, to be Worn during the test • Ophthalmological Exam results to be reviewed. - Visual acuity, pupillary diameter, visual field • Avoid any miotic/ mydriatic drug 12hrs before the test
  • 14.
    Procedure • Standard discEEG electrodes used. • Skin should be prepared by abrading and degreasing. • Two Channel vs Four Channel VEP (Electrode Placement)
  • 15.
    Electrode Placement 2 channelVEP Channel 1 - Oz - Fz Channel 2 - Oz - linked ear 4 channel VEP Channel 1 - Oz - Fz Channel 2 - Pz - Fz Channel 3 - L5 - Fz Channel 4 - R5 - Fz Impedance < 5 kilo ohms Oz - actually located at the middle point of Variation Range of Calcarine fissure 12 cm above the nasion Reference Electrode Recording Electrode 5 cm above the Inion
  • 16.
    International 10 -20 System • Oz - 3-4cm above inion (10% of distance between inion and nation, which is about 3-4 cm in normal adults) • Fz - 11cm above nasion • O1, O2 - 2.5 cm lateral to Oz • A1 A2 ear • Cz - ground Queens Montage • MO (Mid Occipital) - 5cm above inion, in midline • MF (mid frontal) - 12cm above nasion, in midline • LO, RO (left right occiptial) - 5cm left and right of MO • A1/A2 - ear • Ground - Vertex Source - Guidelines on Visual Evoked Potentials, American Clinical Neurophysiology Society, 2008
  • 17.
    Transient Pattern Reversal VEP •Peak response occurs approximately 50-250 ms after stimulus. By convention in neurophysiology - Upward - Negative, Downwards - Positive. • First negative - N75 (suggested to be input from Dorsal LGN to the striate cortex) Lack of consistency in latency, and are sometimes present / absent • Second positive - P100 (suggested to be excitatory outflow of 17 to 18,19/ secondary inhibitory response at 17) • Third Negative - N145 • Inter ocular latency difference - significant - 6-10 ms, as they are less variable, hence more important than the absolute latency measurements (more variable) Source - Aminoff’s Electrodiagnosis in Clinical Neurology 6th ed.
  • 18.
    FIG. 2. PVEPto full-field stimulation shows the rostral—caudal extent of the occipital components N75, P100, and N 145. These may be maximal above or below the midoccipital site in normal individuals with a minimal amplitude response as the midoccipital lead. The frontal N100 component may be recorded widely over the anterior ad . C c , 30 ; a , .97/ ; ; 256 . Source - Guidelines on Visual Evoked Potentials, American Clinical Neurophysiology Society, 2008
  • 19.
    P100 is themost positive peak • Prolonged P100 - demyelination (in most typical abnormality, all peaks prolonged)
  • 20.
    • Two primaryfeatures - time elapsed since stimulus (latency), magnitude of deflection from baseline (amplitude) • Amplitude of P100 is highly variable, therefore it is difficult to establish normal values. Some labs measure N70 to P100, P100 to N145, • Depends on state of arousal, and other patient and condition specific factors. Therefore before labelling abnormal, caution should be exercised • Constant monitoring is required. Patient should maintain visual fixation, throughout the study. (vs Malingering pt) Source - Aminoff’s Electrodiagnosis in Clinical Neurology 6th ed. , Clinical Neurophysiology by UK Misra & J Kalita, 3rd ed
  • 21.
    • Average of100 - 200 stimuli - one response. With low amplitudes upto 400 stimuli. • The values can be compared to the standard normative data. Peak latencies are more consistent, than amplitudes. • At least 2 responses to be recorded. The P100 latency measured in these should be within 2.5 ms difference, (>6-10ms abnormality), in between - technical difference. • Peak to peak amplitude of N75 - P100
  • 22.
    Kothari R, BokariyaP, Singh S, Singh R. A Comprehensive Review on Methodologies Employed for Visual Evoked Potentials. Scientifica (Cairo). 2016;2016:9852194. doi:10.1155/2016/9852194 Time period analysed after each stimulus 1/5th to 1/2 a second Luminance - intensity of light from visible spectrum per unit area traveling in a given direction, P100 latency increases with decrease of luminance VEPs are generally recorded under ambient photopic conditions in a standard, normally illuminated room. Contrast - luminance difference between two adjacent elements in the visual scene, Low contrast responses have smaller amplitudes, broader peaks, prolonged latency
  • 24.
    Produces reversal patternevery 500ms/0.5 s
  • 32.
    • Routine TraditionalVEP, done in Pattern Shift, checkerboard pattern is FULL FIELD STIMULATION. • That means, the Stimulus varies consistently ACROSS A LARGE PORTION OF THE VISUAL FIELD.
  • 33.
    Abberant waveform • A VEPwaveform is considered aberrant if it can be recorded Reproducibly but a P100 peak can not be identified. • Many of aberrant responses, contain too many peaks, rather than too few. • W shaped P100 - bifid P100 - when frontal N100 and occipital P100 are asynchronous. This is seen only in frontal or central reference. i.e. Oz- Fpz / Oz - Pz • Can be seen normally in Oz - A1 leads. (occipital leads) • Can be abolished by asking the patient to look at the upper edge of the pattern, rather than central. Normal Normal Supernumerary P175 etc. - Transient oscillations of occipital cortex
  • 34.
    Walsh P, KaneN, Butler S The clinical role of evoked potentials Journal of Neurology, Neurosurgery & Psychiatry 2005;76:ii16-ii22.
  • 35.
    Multifocal Transient VEP •In contrast to full field stimulation, Multifocal technique divides the Visual Field into a fixed number of sectors, each of which follows its own sequence of stimulus changes. • Hence, each sector can have on of the two states, inverted checkerboards. • Smaller check sizes in the centre for stimulating the macular vision, larger check sizes in periphery, to stimulate the peripheral retinal vision. • The multifocal VEP demonstrates equivalent, or superior test repeatability when compared to standard automated perimetry. Source - Aminoff’s Electrodiagnosis in Clinical Neurology 6th ed.
  • 36.
    Source - Aminoff’sElectrodiagnosis in Clinical Neurology 6th ed. • The waveforms are usually the same as standard VEP, but phase reversal takes place at the mid horizontal meridian, caused by the calcarine fissure
  • 37.
    Source - Aminoff’sElectrodiagnosis in Clinical Neurology 6th ed.
  • 38.
    Source - Aminoff’sElectrodiagnosis in Clinical Neurology 6th ed.
  • 39.
    Source - Aminoff’sElectrodiagnosis in Clinical Neurology 6th ed.
  • 40.
    a. Full fieldVEP recording from Oz - Fpz, 53 yr old woman, leading to temporal dispersion which is difficult to interpret b. Multifocal VEP confirms abnormality by demonstrating central amplitude decrement in left eye (blue) Source - Aminoff’s Electrodiagnosis in Clinical Neurology 6th ed.
  • 41.
    P100 on standardpattern reversal VEP - 178, 179 normal brain mri Normal P100 latency on multifocal VEP Source - Aminoff’s Electrodiagnosis in Clinical Neurology 6th ed.
  • 42.
    Stimulation Patterns • PatternShift VEP - Checker Board pattern. • The size of the stimulus, at the retina will depend on its size on the monitor, and the distance of the subject from the screen. • The total stimulus size will determine the area of the visual field subject to stimulation, and check size will impact the neuronal cell population which is generating the response. • Hence, larger check sizes, will stimulate more of peripherally located RGCs • Smaller check sizes, will stimulate more of centrally located RGCs • But smaller check sizes - have risk of error due to refractive errors. Larger check sizes, will not produce adequate central field stimulation. (pattern shift VEP) • IFCN recommends, check size that subtend 24 - 32 minutes of visual arc.
  • 43.
    W = 380mm / 8 large squares = 47 mm B = 57.3 X 47 mm divided by 90 mm = 30 minutes of visual arc subtended by both the vertical and horizontal dimensions. The visual angle subtended by an indi id al elemen f he a e n a he bjec e e i e e ed in ei he min e degrees of arc. The tangent of the visual angle is equal to the check width divided by the distance from the eye to the screen. The angle can therefore be calculated as: B = arctan W/D This can be approximated for small angles by the formula: B= A* W/D where W is the width of the unit in millimeters, D is the distance from the screen to the eye in millimeters, and B is the visual angle in minutes of arc when A = 3438 and in degrees of arc when A = 57.3. Size of stimulus field. The size of the total stimulus field is specified by the visual angle i b end a he bjec e e, mea ed a de c ibed ab e f individual pattern units. Location and designation of stimulus field types. A fixation point must be provided for the subject that is distinct from the reversing pattern itself. The location of the fixation in i h ega d he im l field de e mine he egi n f he bjec i al field to be stimulated. A pattern that extends equally to both sides of the fixation point is referred to as a full-field stimulus. A pattern restricted to a small region of central vision, such as 2-4 , with central fixation is designated a central-field stimulus. A pattern presented to one side of the fixation point in one-half of the visual field, such as right half or left half, is designated a half-field or hemi-field stimulus. A pattern presented to a small sector of the visual field is designated a partial-field stimulus, with the location described relative to the fixation point. Half-field stimuli presented in an alternating fashion with reversal of left and right half fields sequentially, with a central fixation spot, are designated alternating half field stimuli. Whenever half-field or partial-field stimuli are used, the fixation point should be displaced to the nonstimulated visual field by a small amount, such as 1 check width of 10. This helps prevent stimulation of both retinal hemifields or regions outside the
  • 44.
    Checkerboard Pattern with RedFixation Point Vertical Grating Pattern Shift Vep Used when checkerboard pattern misses trivial defects. Used. Most Commonly Simplicity Reliability Because the primary visual system is arranged to emphasise detection of edges and movement, shifting patterns with multiple edges and contrasts are the most appropriate way to assess visual function, rather than bright flashes of unpatterned light.
  • 45.
    • Stimulus FieldTypes - Fixation point is provided for the subject that is distinct from the reversing pattern itself. 1. Full field stimulation 2. Central field stimulation 3. Partial field stimulation (not yet useful in clinical practice) 4. Alternating half field stimulation (not yet useful in clinical practice) Anterior to chiasma Posterior to chiasma
  • 46.
    Full Field Stimulation Lefteye Right eye Chiasm Chiasm Prechiasmatic Postchiasmatic Optic tract Optic nerve (Optic nerve) Optic tract Crossed (nasal) fibers Uncrossed (temporal) fibers Key Optic radiations Occipital cortex Superior nasal fibers Superior Temporal Nasal Retinal fibers Nasal Inferior Temporal Inferior nasal fibers Inferior nasal fibers decussate in anterior chiasm and then project into optic tract as anterior fibers Superior view Optic pathway (superior view) with E.Hatton • Most sensitive in detecting visual system Anterior to Optic Chiasma • Majority of P100 responses arise in neural elements of the eye subserving the central 8-10 degrees of the visual field. • Lesions that produce half or partial visual field deficits but that spare much of central vision will usually not produce significant changes in P100 response latency or amplitude. Such partial lesions in prechiasmal, postchiasmal, or chiasmal locations may produce changes in response topography, but are best tested for using partial visual field stimulation Performed Mono-ocularly Visual Fixation should be centre of the screen (red dot)
  • 47.
    Flash VEP • Morevariable than pattern VEPs across subjects, but are usually quite similar between eyes of an individual subject. • Multiple positive and negative peaks. • Source - extra geniculate pathway • Great variability, limits their utility, as it is difficult to differentiate between normal and abnormal • Hence, American Clinical Neurophysiology society has advised that ONLY COMPLETE ABSENCE OF A FLASH RESPONSE CAN BE CONSIDERED DEFINITELY ABNORMAL.
  • 48.
    •They are usefulfor patients who are unable or unwilling to cooperate for pattern VEPs, and when optical factors such as media opacities prevent the valid use of pattern stimuli, INFANTS •Provide rudimentary information, only that the visual information is reaching the brain. •Can be done with both eyes closed •Can be done with opacity of the media •Not affected by refractive errors •Preservation of response of FVEP suggests that visual pathways are at least partially intact, absence indicates no useful visual function. •Asymmetry of FVEP may occur at site of structural lesions.
  • 49.
    • The VEPto flash stimulation consists of a series of negative and positive waves. The earliest detectable component has a peak time of approximately 30 ms poststimulus and components are recordable with peak latencies of up to 300 ms. Peaks are designated as negative and positive in a numerical sequence. • This nomenclature is recommended to differentiate the flash VEP from the pattern-reversal VEP. The most robust components of the flash VEP are the N2 and P2 peaks. Measurements of P2 amplitude should be made from the positive P2 peak at around 120 ms to the preceding N2 negative peak at around 90 ms. • Flash rate - 1 HZ Note that with a e is recorded Fig. 4 A normal flash VEP 123
  • 50.
    Note that witha e is recorded Fig. 4 A normal flash VEP 123 124 124 130 130 90 90
  • 51.
    cs often makeit difficult to compare specific response components betwe ubjects. ash VEP to LED goggle stimulation. Response waveforms may be quite tency and amplitude abnormalities must be interpreted with caution (see
  • 54.
    Stimulation of halfof the field gives rise to stimulation of contralateral occipital cortex P100 to be expected to be prolonged on the opposite to side of hemi field stimulation Walsh P, Kane N, Butler S The clinical role of evoked potentials Journal of Neurology, Neurosurgery & Psychiatry 2005;76:ii16-ii22.
  • 55.
    Pattern Onset/Offset VEP Blocksof black and white appear and disappear, followed by plain gray background. Without decrease in luminance • greater inter-subject variability than pattern-reversal VEPs. • Effective for detection or confirmation of malingering • evaluation of patients with nystagmus • as the technique is less sensitive to confounding factors such as poor fixation, eye movements or deliberate defocus. • Standard VEPs to pattern onset/offset stimulation typically consists of three main peaks in adults; C1 (positive, approximately 75 ms), C2 (negative, approximately 125 ms), and C3 (positive, approximately 150 ms). Amplitudes are measured from the preceding peak. mately 150 ms) (see Fig. 3). Amplitudes are mea- sured from the preceding peak. comparison of amplitud the sensitivity of the V Fig. 3 A normal pattern onset/offset VEP. Note that with a 300 ms sweep only the pattern onset response is recorded Fig. 4 A normal flash VEP
  • 56.
    • Stimulation rate- 4-8 Hz • The responses overlap each other, and appear as sinusoidal wave form, which persist during the period of stimulation. • Also known as Repetitive Evoked Potentials. • Repetitive evoked potentials whose constituent discrete frequency components remain constant in amplitude and phase over an infinitely long time period. • Children Source - Clinical Neurophysiology by UK Misra & J Kalita, 3rd ed Steady State VEP’s
  • 57.
    Factors affecting VEP •Not affected by direction of change of checks in checkerboard pattern. Horizontal or Vertical • Size of checks - Smaller check size - largest peak amplitude, smallest peak latency. • A person with good visual acuity - produces shortest latency and largest amp. With Small check size, person with poor visual acuity - with Large check size.
  • 58.
    Patient Factors • Sex- P100 latencies, shorter for women than men • Age - multifocal VEP latency, increases with age, particularly in men, (@ 2.5 ms/ decade after 5th decade) • Pupil size - pupil dilatation, will reduce P100 latency, mainly in Full Field Pattern stimulation and vice versa, not in Full field flash VEP. Standard - recording with pupils normal. (dilatation - more light, constriction - less light) Average pupillary constriction of 1.75mm increases average latency by 4.6ms. • Eye Dominance - Dominant eye - shorter latency, higher amplitude • Eye Movement - reduced P100 amplitude, Latency not affected. eg. nystagmus patients • Body Temp - only in demyelinating lesions, like MS. • Some studies have shown, P100 latency to decrease when concentrating more
  • 59.
    Source - ClinicalNeurophysiology by UK Misra & J Kalita, 3rd ed
  • 60.
    Usefulness • Disease ofanterior optic pathway may produce prolonged P100 latencies without detectable alteration in Visual Acuity, Colour Vision, Pupillary Reactivity, Fundoscopy, or Perimetry. • If Visual Acuity is decreased, and PVEP is normal, it is very unlikely that optic nerve or chiasma is the lesion.
  • 61.
    Diseases 1. Multiple Sclerosis •Screening for Asymptomatic Lesions in MS is the most common use. (eg. dissemination in space) • It has been calculated that a plaque of 1 cm in size , can lead to P100 delay of about 25 ms. • Classical pattern - Assymetrical prolongation of P100 latency, with relative preservation of amplitudes. • No relationship between VEP and Visual Clinical complaints was found.
  • 62.
    2. NMO • UnrecordableP100 waveform, and reduced amplitude of P100 have reported to be characteristic unlike prolonged P100 latency and normal P100 amplitude in MS.
  • 63.
    3. Optic Neuritis •If in Uni Ocular ON, VEP is prolonged only on the affected side, not on opposite side, lesser chances of it to be MS. • Optic neuropathy is found in peripheral neuropathies like - CMT, giant axonal neuropathy, neuropathy associated with macroglobulinemia
  • 64.
    4. Ischemic Optic Neuropathy •Reduction of P100 amplitude, before latency prolonged.
  • 65.
    Other Demyelinating diseases like • Adreno-Leuko dystrophy • Meta-chromatic Leuko dystrophy • Hereditary spastic paraparesis Vitamin B12 deficiency - asymmetric, bilateral prolongation of P100 latency. Vitamin E deficiency in patients with Abetalipoproteinemia
  • 66.
    Optic Neuropathy inHIV patients. Alcohol, Tobacco CO poisoning, reduced amplitude and prolonged latency Ethambutol, Desferoxamine, Quinine Vigabatrin, Valproate Amiodarone
  • 67.
    • Not beenfound to be useful in Cortical Lesions. • Abnormal in Optic Nerve Glioma, NF1
  • 68.
    Compressive lesions affectingVEP • Papilledema - later stages, when it becomes severe enough to compress the optic nerve • Raised ICT, hydrocephalus, pseudotumor cerebri • Extrinsic compression of the Anterior Chiasmal pathway results in Loss of amplitude, destruction of wave form, prolonged P100 latency. (latency prolongation less as compared to demyelinating disorders).
  • 69.
    • Most imp.Differential for optic neuritis is retinal disorder - central serous retinopathy. • Many patients have been labelled as MS in the past falsely, due to prolonged VEP, having CSR • CSR resolves spontaneously. • PVEP delays can be seen with a variety of other acute and chronic retinal disorders. Hence, complete ophthalmological examination is mandatory.
  • 72.
    Electro Retinogram • ERGcan be recorded from corneal surface by a contact lens embedded with a corneal ring electrode, and a conjunctival reference from periorbital skin , by small electrodes attached at the lateral canthi. • Stimulus - Strobe light, (ganz field - whole field) • Cones - shorter refractory period - therefore a high frequency stimulus can be used to assess. (30hz) • Rods - (20hz) • Typically recorded with patient’s pupils Dilated after mydriatics • ERG responses classified negative when dips below baseline, positive when goes above. A - stimulation of photoreceptors (rods and cones) 15ms B - activation of retinal interneurons from inner nuclear layer 30-35ms C wave - retinal pigment epithelium D wave - OFF bipolar cells
  • 73.
    • Pattern ReversalERG can be recorded with alternating checkerboard stimulus. • PERG - characterised by negative wave around 50ms N50 and positive deflection at around 95ms P95 • P95 is thought to reflect the b wave - activation of RGC. • It was found that in individuals with abnormal VEP, P95 was prolonged, but P50 was almost always preserved. But in those with normal VEP, both could be prolonged. • Hence also distinguishes abnormality between Photoreceptors and RGCs.
  • 74.