2. Clinical electrophysiological tests are objective tests which allow
assessment of nearly the entire length of visual pathway.
Electrophysiological tests:
Electroretinogram (ERG)
Electrooculogram (EOG)
Visually Evoked Potentials (VEP)
3. Uses
To locate the site of pathology in case of unexplained visual
loss
To document the extent of the pathology
To detect drug toxicity
To document the amount of ischaemic damage in case of
vascular events
5. ELECRORETINOGRAM (ERG)
ERG is an electric potential generated by retina in response to brief
stimulus of light.
The ‘amplitude of ERG’ (amount of electric potential generated) is
directly proportional to area of functioning retina stimulated.
6. 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.
7. ERG WAVEFORMS
‘a wave’ : It’s a ‘negative’
(downward) wave & reflects
photoreceptor function.
‘b wave’ : It is a ‘positive’ (upward)
wave & reflects bipolar cell
activity.
‘Oscillatory potentials ‘: Small
rippling currents produced by inner
plexiform layer.
8. ERG RESPONSES
ERG has 4 distinct responses depending in stimulus strength:
Rod response(scotopic)
stimulus strength less than standard flash stimulus
Maximal combined response
bright standard flash stimulus
Single flash cone response(photopic)
Standard flash stimulus repeated at intervals of >0.5 sec
30 Hz flicker response
Standard flash stimulus repeated at intervals of <0.5 sec
Dark adapated
Light adapted
9. ROD RESPONSE (Photopic ERG) :
Produced by dark adapting patient for 20 min.
& then stimulating retina with dim light flash
which is below cone threshold.
The resultant waveform has ‘prominent b
(positive) wave ‘& no detectable ‘a (negative)
wave’.
10. MAXIMAL COMBINED RESPONSE :
It is a larger waveform generated by using
bright flash in dark adapted state which
maximally stimulates both rods & cones.
It results in prominent ‘a (negative) wave &
‘b (positive) wave’ with ‘oscillatory potentials’
which are superimposed on ‘b wave’.
11. CONE RESPONSES(scotopic) :
‘Single flash response’ is obtained by
maintaining the patient in light adapted state &
stimulating the retina with bright white flash.
The rods are suppressed by light adaptation &
do not contribute to the waveform.
With patient in light adapted state, a
flickering stimulus at 30 Hz can also be used to
filter rod response & measure cone response
(30 Hz flicker response)
12. RECORDING OF ERG:
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)
Filament type electrode placed on lower lid include Gold foil
electrode, DTL Fiber electrode and HK-Loop electrode
13. Reference electrode
The silver chloride electrode.
Placed on the patient’s forehead, it serves as the negative pole as it
is placed closer to the electrically negative posterior pole of the eye.
Ground electrode
It’s placed on the earlobe.
14. ELECTRODES USED IN ERG
Jet Electrode Gold Plated Electrode Skin
Electrode
DTL Electrode HK Loops Burian
Allen Electrode
15. 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.
Recording & amplification
The elicited response is then recorded from the anterior corneal surface by
the contact lens electrode
The signal is then channelled through consecutive devices for pre-
amplification, amplification & finally display.
16. SPECIALISED FORMS OF ERG
BRIGHT FLASH ERG:
Used for assessment of retinal function in ‘severely traumatized
eye‘ or ‘eye with dense media opacity’ like dense VH, corneal opacity
or advanced cataract.
The flash used is about 10.000 times brighter than that used in
standard ERG.
In this procedure successive responses are obtained with flashes of
increasing intensity, allowing the time for re-adaptation in between
flashes.
A non recordable flash ERG is an ominous sign for visual prognosis.
17. FOCAL ERG (fERG):
Used for detecting small focal lesions or pathologies which are
missed by standard full field ERG.
A small stimulus of 4o
size is projected on area of retina to be
tested.
Due to light scattering & poor signal to noise ratio, this technique
is mostly used in research setting than in clinical setting.
18. Clinical uses of fERG :
Early detection of cone dystrophy or macular disease before the
fundus changes are evident.
Can differentiate between early macular & optic nerve pathology.
Can be used for evaluation of any type focal macular pathology.
19. MULTIFOCAL ERG
(mfERG):
The stimuli consists densely
arranged black or white hexagonal
elements displayed on CRT
monitor.
These hexagonal elements change
from light to dark independently &
this change results into recording
of mfERG.
20. Based on retinal activity, the recorded mfERG appears in
‘topographic map form’ & also in ‘small ERG waveforms’ from
various parts of retina.
21. PATTERN ERG (pERG) :
It mainly represents inner retinal activity (especially ganglion cell
activity)
Useful in differentiating optic nerve disorders from macular
disorders.
Unlike flash ERG, pattern ERG is a very small response.
Recorded with full correction of refractive errors as visualization of
stimulus for extended time is essential for recording.
23. 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.
24. 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.
25. RETINOSCHISIS :
ERG in retinoschisis is typically characterized by marked decrease
amplitude or absence of b wave.
26. 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.
27. CRAO :
In vascular occlusions like CRAO, ERG typically shows shows
absent b wave.
Ophthalmic artery occlusions usually results in unrecordable ERG.
28. 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.
29. 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)
31. It is recording of standing potential of the eye
The electrodes are placed at inner & outer canthus of the eye with
reference electrode placed on forehead.
The patient is asked to look back & forth between a pair of fixation
lights separated by 30o
of visual angles on Ganzfeld globe.
32. 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.
33. 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 substantialy increased during light adaptation
& decreased during dark adaptation.
34. 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.
35. ARDEN’S RATIO :
It is the ratio of ‘largest EOG amplitude during light adaptation’
(light peak) to ‘least amplitude during dark adaptation’ (dark
trough).
Clinically normal value of this ratio is 1.85 or higher.
Values below 1.85 are considered subnormal & those below 1.30
are considered severely subnormal or extinguished.
37. 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
39. VISUALLY EVOKED POTENTIALS (VEP)
Also called as ‘visually evoked response (VER)’ or ‘cortical
potentials’.
It is the electrical response of the brain to sudden appearance /
disappearance / change of visual stimulus.
Like EEG, VEP is detected by placing surface electrodes at scalp
which can be placed anywhere, but should always include posterior
occipital area.
40. VEP ELECTRODES
The occipital electrode (Inion) lies near visual area thus called as
reference electrode.
The vertex electrode is placed over non visual area which detects
minimum activity in response to visual stimulation is called as active
electrode.
The 3rd
electrode is placed over forehead is called ground electrode.
41. The stimulus shown is a flash of light (diffuse
light spot, annulus ) or patterned stimulus
(illuminated checkerboard)
The stimuli are repetitively presented at
random within a short period of time. Eg. 1
cycle/second for 100 seconds.
42. The standard flash VEP is characterized by positive wave (P1 or
P100) which most commonly studied clinically & 2 negative waves
(N1 or N75& N2 or N135).
43. VEP Terminologies
Amplitude of VEP : Height of the
potential of P100 wave. Predominantly
affected in ischemic disorders.
Latency of VEP : Time from stimulus
onset to peak of the response.
Predominantly affected in demyelinating
disorders.
44. APPLICATIONS OF VEP
Recording visual acuity in nonverbal patients.
Macular function test.
Screening and early diagnosis of Multiple Sclerosis.
To identify optic nerve diseases, visual pathway abnormalities.
Amblyopia : latency relatively spared, so VEP can be used to
monitor response to occlusion therapy.
Detection of a malingerer.
To detect color blindness : Using chromatic patterned light stimuli.
46. TOXIC & COMPRESSIVE OPTIC NEUROPATHY :
Following 2 changes are seen :
Decreased amplitude of P100 wave.
Increase in latency period.
Decreased amplitude of P100 is more predominant than
increased latency period.
47. MULTIPLE SCLEROSIS :
Abnormalities in VEP are bilateral & seen 90 % of cases irrespective
of visual symptoms.
In MS, increase in latency period is more predominant than decrease
in P100 amplitude..
48. OPTIC NEURITIS :
In optic neuritis, VEP shows increased latency
period &/or decreased amplitude as compared
to normal eye.
These findings develop even before occurrence of
visual symptoms & color defects.
In recovery stage, amplitude may return to
normal but latency period continues to be
decreased.
