This document provides an overview of electrophysiological studies in ophthalmology, with a focus on electroretinography (ERG), electrooculography (EOG), and visual evoked potentials (VEP). It describes the visual pathways and electrical activity in the retina. The main tests - ERG, EOG, VEP - are explained in detail, including recording procedures, stimulus parameters, waveform analysis, and clinical applications. ERG assesses retinal function, EOG assesses retinal pigment epithelium and photoreceptor interaction, and VEP evaluates visual pathways up to the occipital cortex. Standardization of these tests allows for meaningful clinical use and research communication.

1. Electrophysiological studies in
ophthalmology
Dr.Vaibhav.k
P.G.Ophthalmology
Navodaya medical college

2. • The electrical activity of the visual system is
what converts the image of a beautiful picture
into a meaningful signal for the brain to
understand and for us to get the wonderful
perception of ‘seeing’.

3. • The visual pathways start from the photoreceptor and
retinal pigment epithelial (RPE) layers in the retina,
proceeding through the inner retinal layers and the
retinal ganglion cells.
• The two optic nerves meet at the optic chiasm where,
in a normal, approximately 50% of the fibres project to
the ipsilateral hemisphere of the brain and
approximately 50% decussate to the contralateral
hemisphere.
• From the optic chiasm the two optic tracts project to
the lateral geniculate bodies of the thalami, and thence
to the occipital cortex via the optic radiations.

4. • Electrical activity in the retina & visual pathway is the
inherent property of the nervous tissues which remain
electrically active at all times & the degree of activity alters
with stimulation.
• Visual electrophysiology is an extremely powerful tool to
assess the functional integrity of various levels of this visual
system.

5. History
• First described by Prof. E. D. Reymond who showed
that cornea is electrically positive with respect to
posterior pole of eye.
• In 1908 Einthoven & Jolly showed that a triphasic
response could be produced by simple flash of light
on retina.

6. Electrophysiological tests
Electroretinography (ERG).
Electrooculography (EOG).
Visually evoked response (VER).

7. The main tests available are the
• Electrooculogram (EOG) - which examines the
function of the RPE and the interaction between
the RPE and the photoreceptors
• Electroretinogram or ERG - the responses of the
retina to full-field luminance stimulation that,
through alterations in stimulus parameters and
the adaptive state of the eye, enable the
separation of the function of different retinal cell
types and layers

8. • Pattern ERG (PERG) - which objectively assesses
the macula and the central retinal ganglion cells
• Photopic negative response (PhNR) - which
allows assessment of global ganglion cell function
• Multi-focal ERG (mfERG) - to demonstrate the
spatial distribution of central macular cone
function
• Visual evoked potential (VEP) - which provides
information about functional integrity of visual
pathways up to the occipital cortex.

9. • Electrophysiological responses are strongly
related to stimulus and recording parameters,
and the adaptive state of the eye, and
standardization is therefore mandatory for
meaningful scientific and clinical communication
between laboratories.
• The International Society for Clinical
Electrophysiology of Vision (ISCEV) has published
Standards and Guidelines for the main tests.

10. Electrooculogram (EOG)

11. DEFINITION
 Electrooculography is a technique for measuring the corneo-
retinal standing potential that exists between the front and
the back of the human eye. The resulting signal is called the
ELECTROOCULOGRAM.
 Measurement of eye movements is done by placing pairs of
electrodes either above and below the eye or to the left and
right of the eye.
 If the eye moves from center position toward one of the
two electrodes, this electrode sees the positive side of the
Retina and the opposite electrode sees the negative side of
the retina. potential difference occurs between the
electrodes.

12. PRINCIPLE
The eye acts as a dipole in which the anterior pole is positive
and the
posterior pole is negative.
1. Left gaze: The Cornea approaches the electrode near the outer
Canthus of the left eye, resulting in a negative-trending change
in the recorded potential difference.
2. Right gaze: the Cornea approaches the electrode near the
inner Canthus of the left eye, resulting in a positive - trending
change in the recorded potential difference.

13. EYE MODEL BASED IN EOG (BIDIM-EOG)
Bi Dimensional Bi Polar Model (BiDiMEOG)

14. Technique of recording
• Electrodes are placed over the orbital margin near the medial
& lateral canthi.
• A forehead electrode serves as a ground electrode.
• Pt. sits in a room in erect position.

15. • Head position is controlled at a certain fixed distance from 3
fixation lights (dimly lit, usually red), which are placed in pts.
line of vision.
• The central light serves for central fixation & the 2 side lights
which can be fixed after an excursion of anywhere from 30-60
degrees serves as the right or left fixation lights.

16. • an arrangement is made to make the eyes light adapted with
the help of a bright, long duration stimulus.
• Pupil size is controlled by instillation of mydriatics.
• Ordinarily, a pupil size > 3 mm allows a little variation of EOG.

17. Recording
• The patient is asked to move the eye sideways (medially &
laterally) by fixating the right & left fixation lights alternately
& keep there for few seconds, during which the recording is
done.
• In this procedure the electrode near the cornea becomes
positive.
• The recording is done every 1 min.
• To begin with, the recording is started with the stimulus lights
on.

18. EOG recording

19. • After a standardized period of light adaptation, all lights are
extinguished (except for fixation lights) & responses recorded
for 15 min under dark adapted conditions
• The stimulus lights are then turned on again & responses
recorded for 15 min under light adapted conditions

20. Measurement & interpretation
• Normally the resting potential of the eye progressively
decreases during dark adaptation reaching a dark trough in
approx. 8 - 12 min.
• With subsequent light adaptation the amplitude starts rising
& reaches to light peak in approx. 6-9 min.

21. Results of EOG are interpreted by finding the Arden ratio as follows:-
• The largest peak to
trough amplitude in the
light is divided by the
smallest peak to trough
amplitude in the dark.
• Ratio of light peak to
dark adapted baseline is
also acceptable EOG
measure.

22.  ARDEN’S RATIO :
 It is the ratio of ‘largest EOG amplitude during light
adaptation’ (light peak) to ‘least amplitude during dark
adaptation’ (dark trough).

23. • >180 Normal
• 165—180
Borderline
• <165
Subnormal
• Difference of >10% in
BE is significant
• Good pt cooperation is
required

24. • As a general rule those conditions which cause a reduction in
size of b-wave in ERG also produce reduction in value of
Arden ratio.
• EOG serves as a test that is supplementary & complementary
to ERG.
• In certain conditions it is more sensitive than ERG e.g.,
patients with vitelliform macular degeneration, fundus
flavimaculatus, & generalized drusen often show a striking
EOG reduction in presence of normal ERG.

25.  Like ERG, EOG reflects activity of entire retina & used to
evaluate combined photoreceptor-RPE activity.
 As validity of results depends upon consistent tracking of
fixation target over 30 min., this test is not suitable in unco-
operative patients & children.
 Also EOG depends upon a minimum degree of light
adaptation so it is not reliable in patients with dense
cataracts.

26.  CORNEOFUNDAL POTENTIAL :
 It is the source of voltage obtained in EOG & it renders the
cornea positive by 0.006 to 0.010 V as compared with the
back of the eye.
 The corneofundal potential results from metabolic activity
of RPE (mainly) as well as corneal & lens epithelium.
 Contributions of corneal & lens epithelium are not
photosensitive but that of RPE is, which is substantially
increased during light adaptation & decreased during dark
adaptation.

27.  For EOG to be normal, it requires as little as 20-25 % of
normal functioning retina.
 Thus abnormal EOG indicates a dense pathology involving
entire retina.

28.  BEST’S DISEASE :
 Abnormal EOG with normal ERG is a
hallmark.
 Other examples of ERG to EOG
dissociation are :
 Diffuse fundus flavimaculatous
 Pattern dystrophy of RPE
eg. Butterfly Macular Dystrophy.
 Chloroquine retinopathy
 Metallosis bulbi
EOG IN CLINICAL CASES

29. • Asymptomatic genetic carriers of the mutation show EOG
abnormality, and if Best’s disease is suspected in a young child
who is not capable of doing the EOG, testing of both parents
should be performed.
• As Best’s disease is dominantly inherited, one of the parents
must carry the mutation if the child is affected and will
manifest the characteristic EOG abnormality.

30. Visual evoked potential

31. • The VEP is an evoked electrophysiological potential that can
be extracted, using signal averaging from the ongoing
EEG(electroencephalographic activity) activity recorded at the
scalp.
• The VEP is assumed largely to arise in the occipital cortices
and allow assessment of the functional integrity of the visual
pathways.
• It is the only objective technique to assess
clinical and functional state of visual syst.beyond
retinal ganglion cells.

32. Protocols of VEP
• Pattern VEP
• Flash VEP
Specialized and extended VEP protocols not covered by the ISCEV Standard
• Steady state VEP
• Sweep VEP
• Motion VEP
• Chromatic (color) VEP
• Binocular (dichoptic) VEP
• Stereo-elicited VEP
• Multi-channel VEP
• Hemi-field VEP
• Multifocal VEP
• Multi-frequency VEP
• LED Goggle VEP

33. Electrodes
• Skin electrodes such as sintered silver–silver chloride,
standard silver–silver chloride, or gold disc electrodes are
recommended for recording VEPs.
• The skin should be prepared by cleaning and a suitable paste
or gel used to ensure good, stable electrical connection.
• The electrode impedances should be below 5 kilo ohms
measured between 10 and 100 Hz and, to reduce electrical
interference, they should not differ by more than 20%
between electrode sites.

34. Placement of electrodes
• The scalp electrodes should be placed relative to bony
landmarks, in proportion to the size of the head, according to
the International 10/20 system .
• The anterior/posterior midline measurements are based on
the distance between the nasion and the inion over the
vertex.
• The active electrode is placed on the scalp over the visual
cortex at Oz with the reference electrode at Fz.
• A separate electrode should be attached to a relatively
indifferent point and connected to the ground; commonly
used ground electrode positions include the forehead, vertex
(Cz), mastoid, earlobe (A1 or A2), or linked earlobes.

35. Stimulus
• Pattern stimuli
• The standard pattern stimulus is a high contrast black and
white checkerboard.
• The viewing distance, typically between 50 and 150 cm, can
be adjusted to obtain a suitable field size and the required
checksizes for any physical size of display screen.

37. Luminance and contrast
• The mean luminance of the checkerboard should be 50 cd/
m2 (40–60 cd/ m2)
• Contrast between black and white squares should be high .
• The luminance and contrast of the stimulus should be uniform
between the center and the periphery of the field.

38. • Pattern-reversal stimuli:
• For the pattern-reversal protocol, the black and white checks
change phase abruptly (i.e., black to white and white to black)
and repeatedly at a specified number of reversals per second.
• There must be no overall change in the luminance of the
screen, which requires equal numbers of light and dark
elements in the display, and no transient luminance change
during pattern reversal.
• A reversal rate of two reversals per second (±10%) should be
used to elicit the standard pattern-reversal VEP.

39. Pattern onset/offset stimuli
• For pattern onset/offset, the checkerboard pattern is abruptly
exchanged with a diffuse gray background.
• The mean luminance of the diffuse background and the
checkerboard must be identical with no change of luminance
during the transition from pattern to diffuse blank screen.
• Pattern onset duration should be 200 ms separated by 400 ms
of diffuse background.
• The ISCEV standard onset/offset response is the onset
response

40. Flash stimulus
• The flash VEP should be elicited by a brief flash that subtends
a visual field of at least 20, presented in a dimly illuminated
room.
• The strength (time-integrated luminance) of the flash
stimulus should be 3 (2.7–3.3) photopic candelas seconds per
meter squared (cd s m-2).
• This can be achieved using a flashing screen, a hand held
stroboscopic light or by positioning an integrating bowl
(ganzfeld) such as that used for ERG tests in front of the
patient.
• The flash rate should be 1 per second.

41. Recording parameters
• Amplification and filtering:
Amplification of the input signal by 20,000–50,000 times is
usually appropriate for recording the VEP.

42. Analysis time
• The minimum analysis time (sweep duration) for all adult
transient flash and pattern-reversal VEPs is 250 ms
poststimulus.
• To analyze both the pattern onset and offset responses
elicited by onset/offset stimuli, the analysis time (sweep
duration) must be extended to 500 ms.
• The VEP in infants has longer peak latencies and a longer
sweep time will be required to adequately visualize the
response

43. Preparation of the patient
• Pattern stimuli for VEPs should be presented when the pupils
of the eyes are unaltered by mydriatic or miotic drugs.
• Pupils need not be dilated for the flash VEP.
• Extreme pupil sizes and any anisocoria should be noted for all
tests.
• For pattern stimulation, the visual acuity of the patient
should be recorded and the patient must be optimally
refracted for the viewing distance of the screen.
• With standard electrodes and any additional electrode
channels attached, the patient should view the center of the
pattern field from the calibrated viewing distance.
• Monocular stimulation is standard.
• Care must be taken to have the patient in a comfortable, well-
supported position to minimize muscle and other artifacts

44. The ISCEV standard VEP waveforms
• VEP waveforms are age dependent.
• Standard responses reflects the typical waveforms of adults
18–60 years of age.
• Peak time : The time from stimulus onset to the maximum
positive or negative deflection or excursion of the VEP.

45. Pattern-reversal VEPs
• The pattern-reversal VEP
waveform consists of N75, P100,
and N135 peaks.
• These peaks are designated as
negative and positive followed by
the typical mean peak time.
• It is recommended to measure
the amplitude of P100 from the
preceding N75 peak.
• The P100 is usually a prominent
peak that shows relatively little
variation between subjects,
minimal within-subject
interocular difference, and
minimal variation with repeated
measurements over time.

46. Pattern onset/offset VEPs
• Pattern onset/offset VEPs show greater inter-
subject variability than pattern-reversal VEPs.
• Pattern onset/ offset stimulation is effective
for detection or confirmation of malingering
and for evaluation of patients with nystagmus,
as the technique is less sensitive to
confounding factors such as poor fixation, eye
movements or deliberate defocus.

47. • 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)
• C3 (positive, approximately
150 ms)
• Amplitudes are measured
from the preceding peak

48. Flash VEPs
• Flash VEPs are more variable than pattern VEPs across
subjects, but are usually quite similar between eyes of an
individual subject.
• 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.

49. • 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
sequences.
• The most robust components of the
flash VEP are theN2 (75)and P2
(125)peaks.

50. Multi-channel recording for assessment
of the posterior visual pathways
• With dysfunction at, or posterior to, the optic chiasm,
or in the presence of chiasmal misrouting (as seen in
ocular albinism), there is an asymmetrical distribution
of the VEP over the posterior scalp.
• Chiasmal dysfunction gives a ‘‘crossed’’ asymmetry
whereby the lateral asymmetry obtained on
stimulation of one eye is reversed when the other eye
is stimulated.
• Retrochiasmal dysfunction gives an ‘‘uncrossed’’
asymmetry such that the VEPs obtained on stimulation
of each eye show a similar asymmetrical distribution
across the hemispheres.

51. Multifocal visually evoked potential
• provides local topographic information
• mfVEP recording technique is similar to
that for a standard VEP, but the stimulus
and analysis techniques are different

52. • Typical stimulus array
for the mfVEP is a
dartboard display
composed of a number
of sectors, each with a
checkerboard pattern.
• The sectors vary in size
with retinal eccentricity

53. • Each sector is an
independent stimulus
that reverses in contrast
in a pseudo-random
fashion (m-sequence).
• mathematical algorithm
is used to extract
separate responses for
each of the sectors from
a single continuous EEG
signal.

54. • unilateral disease is
relatively easy to detect
• Useful in:
• Optic neuritis
• Multiple sclerosis
• glaucoma, with local
visual field effects
• Ischemic optic
neuropathy

55. Clinical applications
Optic nerve disease
1. Optic neuritis:-
• Involved eye shows a reduced amplitude & delay in
transmission i.e. increased latency as compared to normal eye
• These changes occur even when there is no defect in the VA,
color vision or field of vision.

56. • Following resolution, the amplitude of VER waveform may
become normal, but the latency is almost always prolonged &
is a permanent change.
2. Compressive optic nerve lesions:-
• Usually associated with a reduction in the amplitude of the
VER without much changes in the latency

57. 3.During orbital or neurosurgical procedures:-
• A continuous record of the optic nerve function in
the form of VER is helpful in preventing inadvertent
damage to the nerve during surgical manipulation.

58. .
Measurement of VA in infants, mentally retarded & aphasic pts
• VER is useful in assessing the integrity of macula & visual
pathway.
• Pattern VER gives a rough estimate of VA objectively.
• Peak VER amplitude in adults occurs for checks b/w 10 & 20°
of arc & this corresponds to a VA of 6/5.

59. Malingering & hysterical blindness
• pattern evoked VER amplitude & latency can be altered by
voluntary changes in the fixation pattern or accommodation.
• However, the presence of a repeatable response from an eye
in which only light perception is claimed indicates that pattern
information is reaching the visual cortex & thus strongly
suggests a functional component to the visual loss.

60. • A characteristic of hysterical response seems to be large
variations in the response from the moment to moment.
• The first half of the test may produce an absent VER & 2nd half
a normal VER.

61. Lateralizations of defects in the visual pathway
• VER provides a useful information for localizing the defects in
visual pathway in difficult cases e.g. children & non-
cooperative elderly pts.
• Asymmetry of the amplitudes of VER recorded over each
hemisphere implicit a hemianopic visual pattern.

62. • However, the differentiation of tract lesion from that of optic
radiation lesion is difficult.
• Decreased amplitude of VER recorded over the contralateral
hemisphere, when each eye is stimulated separately indicates
a bitemporal visual deficiency & may localize the site of
chiasmal pathology

63. Unexplained visual loss
• useful in general & in pts. with orbital/head injury
Assessment of visual potential in pts. with opaque media
• like corneal opacities, dense cataract & vitreous hemorrhage.

64. Amblyopia
• flash VER is normal but pattern VER shows
decrease in amplitude with relative sparing of
latency .
So pattern VER is used in the detection of
amblyopia & in monitoring the effect of occlusion
on the normal as well as the amblyopic eye, esp.
in small children.
Glaucoma
• helps in detecting central fields

65. Electroretinogram

66. • ERG is the corneal measure of an action
potential produced by the retina when it is
stimulated by light of adequate intensity.
• It is the composite of electrical activity from
the photoreceptors, Muller cells & RPE.

67. TYPES OF ELECTRORETINOGRAM
ERG
Full field
ERG
Focal ERG
Multifocal
ERG
Pattern
ERG

68. Six standard protocols of ERG
• These are named according to the stimulus (flash strength in cdsm-2) and the state
of adaptation.
• 1. Dark-adapted 0.01 ERG (a rod-driven response of on bipolar cells).
• 2. Dark-adapted 3 ERG (combined responses arising from photoreceptors and
bipolar cells of both the rod and cone systems; rod dominated).
• 3. Dark-adapted 10 ERG (combined response with enhanced a-waves reflecting
photoreceptor function).
• 4. Dark-adapted oscillatory potentials (responses primarily from amacrine cells).
• 5. Light-adapted 3 ERG (responses of the cone system; a-waves arise from cone
photoreceptors and cone Off- bipolar cells; the b-wave comes from On- and Off-
cone bipolar cells).
• 6. Light-adapted 30 Hz flicker ERG (a sensitive cone-pathway-driven response).

69. Additional protocols of ERG

70.  BASIC PRINCIPLE OF ERG :
 Sudden illumination of retina.
 Simultaneous activation of all the retinal cells to generate
the current.
 Currents generated by all the retinal cells mix, then pass
through vitreous & extra cellular spaces.
 High RPE resistance prevents summated current from
passing posteriorly.
 The small portion of the summated current which escapes
through the cornea is recorded as ERG.

71. Full field electroretinogram
• ERG is the record of an action potential produced by
the retina when it is stimulated by light of adequate
intensity.
• A small part of the current escapes from the cornea,
where it can be recorded as a voltage drop across the
extracellular resistance, the ERG.

72. ELECTRODES USED IN ERG
Jet Electrode Gold Plated Electrode Skin Electrode
DTL Electrode HK Loops Burian Allen Electrode

73. Application of electrodes
Active electrode
• It’s the main electrode.
• Recording electrodes are
of various types-
• Hard contact lenses that
covers sclera such as
Burian-Allen electrode,
Doran gold contact lens,
Jet electrode(disposable)

74. • Lens lubricant and
corneal anaesthesia
used
• Filament type electrode
placed on lower lid
include Gold foil
electrode ,DTL Fiber
electrode and HK-Loop
electrode

75. Reference electrodes
• Reference electrodes (those connected to the negative
input of the recording system) may be incorporated
into the contact lens-speculum assembly in contact
with the conjunctiva.
• These ‘‘bipolar electrodes’’ are the most electrically
stable configuration although ‘‘monopolar’’ contact
lens electrodes with a separate reference generally
produce larger amplitudes.
• Alternatively, skin electrodes placed near each orbital
rim, temporal to the eye are used as the reference
electrode for the corresponding eye.

76. • Common electrode
• A separate electrode should be attached to an
indifferent point and connected to the
common input of the recording system.
• Typical locations are on earlobe, mastoid or
the forehead

77. Stimulus
• The Ganzfeld bowl is large white bowl which is used to
stimulate the retina during the recording of the ERG.
• It diffuses the light & allows equal stimulation of all parts of
retina.
• This ISCEV Standard is based on flash stimuli with durations
that are shorter than the integration time of any
photoreceptor. The maximum acceptable duration of any
stimulus flash is 5 ms

78. Strength of stimulus(2015 revised)
• The flash stimuli and light-adapting background used for
the ISCEV Standard ERGs described below
• The weak flash stimulus strength is 0.010 photopic cdsm-2
with a scotopic strength of 0.025 scotopic cdsm-2.
• The standard flash stimulus is 3.0 photopic cdsm-2 with a
scotopic strength of 7.5 scotopic cdsm-2.
• The standard strong flash stimulus is 10 photopic cdsm-2
with a scotopic strength of 25 scotopic cdsm-2.
• Light-adapting and background luminance is 30 photopic cd
m-2 with a scotopic strength of 75 scotopic cd m-2.

79. Recording & amplification
• The elicited response is then recorded from the
anterior corneal surface by the contact lens
electrode
• The signal is then channeled through consecutive
devices for pre-amplification, amplification & finally
display.

80. Clinical protocol
• The pupils should be maximally dilated, and the pupil size
noted before and at the end of recording
• Pre-adaptation to light or dark
• The recording conditions outlined below specify 20 min of
dark adaptation before recording dark adapted ERGs, and
10 min of light adaptation before recording light-adapted
ERGs.
• The choice of whether to begin with dark-adapted or light-
adapted conditions is up to the user, provided these
adaptation requirements are met.
• A brief period of additional dark adaptation, approximately
5 min, is recommended for recovery after lens insertion.

81. • Fluorescein angiography, fundus photography and
other imaging techniques using strong illumination
systems should be avoided directly before ERG
testing.
• If these examinations have been performed, ISCEV
recommend least 30-min recovery time in ordinary
room illumination before beginning ERG testing

82. Recording protocol
Full pupillary dilatation
30 minutes of dark adaptation
Rod response(Dark-adapted 0.01 ERG)
Dark-adapted 3 ERG (combined rod and cone system
responses)
Dark-adapted 10 ERG (combined responses to stronger flash)
Oscillatory potentials
10 minutes of light adaptation
Light-adapted 3.0 ERG (single-flash cone response)
30 Hz flicker

83. Normal Waveforms
a-wave
• Initial negative wave.
• In dark adapted
condition primarily
from photoreceptors

84. B-wave
• Large positive wave.
• Arises from the Muller
cells, representing the
activity of bipolar cells.

85. • Distributed by a ripple
of 3 or 4 wavelets at the
ascending limb known
as oscillatory potential.

86. c-wave
• Prolonged positive wave
with a lower amplitude.
• Considerably slower so
not used clinically.
• Represents the
metabolic activity of
RPE in response to rod
signals only

87. • Thus the normal response is actually a summation of
individual rod & cone response.
• In order to derive clinical information from ERG
recording it is essential to separate out the cone
response from the rod response.
• This can be easily achieved by following techniques:-

88. Cone ERG
• The cone function in the ERG can be easily be
separated out by either light adapting the patient or
by using a flickering stimulus.
• In a light adapted condition (photopic) only the
cones respond as the rods get saturated.

89. • Cones are capable of responding to flickering stimuli
of up to 50 Hz, after which point individual responses
are no longer recordable
• Rods do not respond to flickering stimulus of more
than 10 to 15 Hz.
• Thus by using 30 Hz flicker stimulus, only the cone
function can be recorded.

90. Rod ERG
• In a dark adapted state (scotopic), only the rods are
sensitive enough to respond to dim light stimulus.
• Thus stimulating the dark adapted retina with a dim
white or blue light will elicit only rod response.
• However, if a bright light stimulus is used in the dark
adapted state both the rods & cones will respond
called mesopic ERG

91. Measurement of ERG Components
Amplitude
a wave measured from
the baseline to the
trough of a-wave.

92. b wave
• measured from the
trough of a-wave to
the peak of b-wave.

93. Time sequences
• Latency:- it is the time interval b/w onset of stimulus
& the beginning of the a-wave response. Normally
it’s 2 ms.
• Implicit time:- time from the onset of light stimulus
until the maximum a-wave or b-wave response.
• Considering only a-wave and b-wave response the
duration of ERG is less than 1/4
th
s

94. Scotopic rod response
• Measure of the rod
system of retina
• Low intensity flash
• Flash white or blue
• Slow positive going
response with only b
wave visible
Example –
Normal, cone dystrophy,
RP

95. Scotopic maximal combined response
• Standard flash(0dB)
• Rod and cone response
• Large a & b waves
• Example-normal, early
cone dystrophy,RP.

96. Photopic single flash
• Measure of the cone system
• 10 min adaptation to background
light suppresses rod activity
• a & b waves smaller
• Lesser implicit time
• Abnormal in congenital
achromatopsia,acquired cone
degeneration,RP
• Example-normal,RP,Progressive
cone dystrophy

97. Photopic 30-Hz flicker response
• Repetitive stimuli
flickered at a rate 30 Hz
• Cone activity
• Flicker implicit time
measure is sensitive
before amplitude
• Abnormal in RP,cone
dystrophy

98. Oscillatory potentials
• high-frequency
wavelets that are said
to be riding on the b-
wave
• To record OPs, the
band- pass filters on the
amplifiers are changed
to eliminate the lower
frequencies while
allowing the higher
frequencies to pass

99. • believed to represent a
complex feedback circuit with
bipolar cells, amacrine cells,
and interplexiform cells
• sensitive to the effects of
ischemia
• Central serous retinopathy,
CSNB Type 2,Birdshot
choroidopathy, Retinoschisis
Carriers of X-linked CSNB
• Example-normal ,early
DR,PDR.

100. Interpretation of ERG
• ERG is abnormal only if more than 30% to 40% of retina is
affected
• A clinical correlation is necessary
• Media opacities, non-dilating pupils & nystagmus can cause
an abnormal ERG
• ERG reaches its adult value after the age of 2 yrs
• ERG size is slightly larger in women than men

101. Abnormal ERG response
• The b-wave with a potential of <0.19 mV or > 0.54 mV is
considered abnormal.
• Abnormal ERG is graded as follows:-
Supernormal response-
• Characterized by a potential above normal upper limit.
• Such a response is seen in:-
1. Sub-total circulatory disturbances of retina.
2. Early siderosis bulbi.
Cone dystropy,albinism,hyperthyroidism,steroid
administration,optic atropy,retinal vascular disrders

102. Sub-normal response
• Potential < 0.08 mV.
• Indicates that a large area of retina is not functioning.
• Seen in:-
1. Early cases of RP.
2. Chloroquine & quinine toxicity.
3. Retinal detachment.
4. Systemic diseases like vit. A deficiency, hypothyroidism,
mucopolysaccharidosis & anemia

103. Extinguished response
• Complete absence of response.
• Seen in:-
1. Advanced cases of RP.
2. Complete RD.
3. Choroideremia.
4. Leber’s congenital amaurosis
5. Luetic chorioretinitis

104. Negative response
• Characterized by large a-wave.
• Indicates gross disturbances of retinal circulation as seen in
arteriosclerosis, giant cell arteritis, CRAO & CRVO.

105. • DIABETIC RETINOPATHY :
• In DR there is reduction in amplitude &
delay of peak implicit times.
• These changes are directly proportional
to severity of retinopathy.
• Amplitude of oscillatory potentials (OP)
is a good predictor of progression of
retinopathy from NPDR to PDR.
• Abnormal amplitude of OP indicate high
risk of developing PDR.

106. • RETINAL DETACHMENT (RD) &
CENTRAL SEROUS RETINOPATHY
(CSR) :
• In RD & CSR there is significant
reduction in ERG amplitude.
• However there is no significant
change seen in waveforms of ERG.

107. • RETINOSCHISIS :
• ERG in retinoschisis is typically characterized by marked
decrease amplitude or absence of b wave.

108. • RETINITIS PIGMENTOSA :
• A full field ERG in RP shows
marked reduction in both rod &
cone signals although loss of rod
signals is predominant.
• There is significant reduction in
amplitude of both a & b waves
of ERG.

109. • CRAO :
• In vascular occlusions like CRAO, ERG typically shows shows
absent b wave.
• Ophthalmic artery occlusions usually results in unrecordable
ERG.

110. • CONE DYSTROPHY :
• ERG in cone dystrophy shows good
rod b-waves that are just slower.
• The early cone response of the
scotopic red flash ERG is missing.
• The scotopic bright white ERG is
fairly normal in appearance but with
slow implicit times.
• The 30 Hz flicker & photopic white
ERGs which are dependent upon
cones are very poor.

111. • RETAINED IOFB :
• A retained metallic FB like iron &
copper shows changes in ERG early as
well as late stages.
• A characteristic change is b-wave
amplitude is reduced by 50% or more
as compared with normal eye.
• No intervention finally results into an
unrecordable ERG (Zero ERG)

112. • ERG plays a diagnostic role include choroideremia, gyrate
atrophy, pathological myopia & other variants of RP
• Important role in juvenile diabetics with a disease duration
longer than 5 years has been shown to be valuable for the
identification of those at risk for the development of
proliferative retinopathy

113. To assess retinal function when fundus
examination is not possible
• ERG can be recorded even in presence of dense opacities in
the media such as corneal opacity, dense cataract & vitreous
hemorrhage.
• In these cases the stimulus should be sufficiently bright, the
response should be normal in absence of disease.

114. Limitations of ERG
• Since the ERG measures only the mass response of the retina,
isolated lesions like a hole hemorrhage, a small patch of
chorioretinitis or localized area of retinal detachment can not
be detected by amplitude changes.
• Disorders involving ganglion cells (e.g. Tay sachs’ disease),
optic nerve or striate cortex do not produce any ERG
abnormality

115. • Thank you.