Electrophysiological tests for vareious occular disorder and interpretation
ELECTROPHYSIOLOGICAL TESTS FOR
VARIOUS OCCULAR DISORDERS
AND THEIR INTERPRETATION
Dr. Pragya rai
Rohilkhand medical college bareilly
• 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.
• These tests give a recording similar to ECG ,
that helps to know the functional integrity of
various layer of retina and ant. visual pathway.
• Sometimes clinical examination of eye cannot
explain the exact cause of decrease in vision.
• This test categorize the site of lesion in visual
pathways ,from the various layers of retina to
optic nerve and brain cortex.
• Help know the extent , seriousness and stage
• 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.
The electro- oculogram
• EOG examines the function of the RPE and the
interaction between the RPE and the
• Based on the measurement of resting
potential of the eye which exists b/w the
cornea (+ve) & back of the retina (-ve) during
fully dark-adapted & fully light-adapted
• Retinitis pigmentosa
• Bests’ vitelliform macular
• Stargardt’s disease
• Choroidal melanomas
• Drug toxicity against RPE
• Gyrate atrophy
• Leber congenital amaurosis
• Metallosis bulbi
• Vit A deficiency
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.
• 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
Arden ratio=LP/DT * 100
• Normal light rise values are
185 or above
• Abnormal values are below
Limitations of EOG
• Pt with poor central fixation .
• Uncooperative adults
• Media opacities
• Illumination levels
• There exists a potential difference of 1 mV
b/w cornea and posterior pole of the eye.
• Known as the corneoretinal potential .
• Modification in the corneoretinal potential in
response to brief flash light known as
• Various techniques are in clinical use to asses
the electrical response of retinal cell to light.
• Common of these is the full-field flash ERG.
• Others- pattern ,focal , and multifocal ERG.
• It is the composite of electrical activity from
the photoreceptors, Muller cells & RPE.
FULL-FIELD FLASH ERG
• It is the mass response of retina to full-field
• Reflects the function of photoreceptor and
inner nuclear layers of the retina in response
to light stimulation.
Technique of ERG recording
• Recording of ERG requires:-
1. Recording, reference & ground electrodes.
2. Ganzfeld bowl stimulator.
3. Signal averager & amplifier.
4. Display monitor & printer.
• Dilate the pupils.
• Contact lens electrodes or thin nylon fibre.
• Darkroom .
• 30 min for dark adapted and 10 min for light
• No FFA before test, if done dark adaptation for
• Main electrodes-
• Placed on cornea
embedded in contact lens
after topical anaesthesia.
• Bipolar and non bipolar
• Non bipolar reference
electrode should be
positioned at the ipsilateral
• Electrodes that contact the
cornea recommended for
full field recording.
• Reference electrode
• silver chloride electrode
• Placed on forehead or near each orbital rim
• Ground electrode
• Forehead or ear
• Stimulus is an illuminated bowl which projects
diffuse light into all part of fundus.
• intensity and frequency of flashes should be
RECORDING AND AMPLIFICATION OF
• The elicited response is then recorded from
the anterior corneal surface by the contact
• The signal is then channeled through
consecutive devices for pre-amplification,
amplification & finally display.
Full pupillary dilatation
30 minutes of dark adaptation
Maximum combined response
10 minutes of light adaptation
single flash cone response
30 Hz flicker
• A WAVE- Negative wave .
• Arise from photoreceptors
• B WAVE- Positive wave .
• Arise from Muller cells.
• Representing activity of bipolar cell layer
• Disturbed by a ripple of three or four small
wavelets known as oscillatory potential.
• C WAVE- Prolonged positive wave with lower
• Generated from the RPE in response to rod signals
• 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:-
• Photopic ERG refers to testing pt. in bright
• Typical cone response elicited by photopic
ERG shows a lower amplitude and shorter
• 5 to 8 million cones are contributing.
• Scotopic ERG recorded 20 minutes after dark
• Large amplitude and longer implicit time.
• 6 to 8 million cones and 125 million rods contribute.
• Cones are capable of responding to flickering stimuli of
up to 50 Hz, after which point individual responses are
no longer recordable(critical flicker fusion).
• 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.
• Dark adapted
• Flash of very dim light or blue light.
• Reduced amplitude and longer implicit time.
• However, if a bright light stimulus is used in
the dark adapted state both the rods & cones
will respond called mesopic ERG.
• 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
• Considering only a-wave and b-wave response
the duration of ERG is less than 1/4th sec.
• International society for clinical
electrophysiology of vision
• Standard protocol for ERG,EOG,VEP
• 5 basic responses recommended by ISCEV.
Principle Responses demanded by
• Scotopic rod response
• Scotopic maximal combined response
• Photopic single flash cone response
• Photopic 30 Hz flicker response
• Oscillatory potentials
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-dilated pupils &
nystagmus can cause an abnormal ERG
• ERG reaches its adult value after the age of 2
• ERG size is slightly larger in women than men
Abnormal ERG response
• Full field Ganezfeld graded as---
• Supernormal response-
• Amplitude is > two standard deviation above the
mean both a and b wave
• Such response is seen in-
• Subtotal circulatory disturbances of retina.
• Early siderosis bulbi
• Subnormal response- potential < 2 standard
deviations beneath the mean normal
• Indicates that a large area of retina is not functioning .
• Such response seen in-
• Chloroquine and quinine toxicity.
• Vit. A deficiency , hypothyroidism ,
mucopolysaccharidosis, and anaemia.
• Extinguished response- complete absence of
• Total failure of rod and cone function .
• Such response seen in-
• Advanced cases of RP
• Advanced siderosis bulbi.
• Leber’s congenital amaurosis.
• Luetic chorioretinitis.
• Negative response- characterized by a large
a- wave .
• Such resopnse seen in-
• Gross disturbances of the retinal
• Gaint cell arteritis
CLINICAL APPLICATION OF ERG
• 1-DIAGNOSIS AND PROGNOSIS OF RETINAL
• Retinitis pigmentosa and other inherited
• RP show a marked reduction in
amplitude of ERG.
There is relationship b/w area of
functioning retina and ERG.
• Leber’s amaurosis- important for primary
• Under anaesthesia.
• Other retinal degeneration-
• Rod –cone dystrophy
• Gyrate atrophy.
• Pathological myopia
• Other variants of RP.
• DIABETIC RETINOPATHY-
• Oscillatory potential of
the ERG is selectively
• RETINAL DETACHMENT-
• Immediate reduction in
size of b-wave with loss of
• Depends on the extent of
• ERG guides surgical
• VASCULAR OCCLUSIONS OF THE RETINA-
• In CRAO disappearance of oscillatory potential
with reduction of b-wave.
• Less marked changes in CRVO.
• Predicts development of neovascularization.
• TOXIC AND DEFICIENCY STATES-
• Drugs stored in pigment epithelium cause
alteration in response.
• Chloroquine toxicity may be detected in this
• Other drugs include quinine , lead ,
phenothiazines , indomethacin etc.
• 2) TO ASSESS RETINAL FUNCTION WHEN
FUNDUS EXAMINATION IS NOT POSSIBLE
• Recorded with dense opacity ,dense cataract
and vitreous haemorrhage.
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
• Record macular action potential
• Not in widespread use .
• Response is recorded to a scaled hexagonal
pattern in photopic conditions.
• Some laboratories attempt to record scotopic
• Distinguish diseases of outer retina from
ganglion cell or optic nerve.
• Also used in drug toxicity
• Interpretation are still not standardized.
• Patterned stimulus
• Contrast reversing
pattern with no overall
change of luminance
• Display vary in size but
not beyond 20 degree
• Allows both a measure of central retinal
function and retinal ganglion cell function.
• Useful in electrophysiological differentiation
b/w ON and macular dysfunction.
RECORDINGS AND MEASUREMENTS
• Recorded without mydriasis .
• With refractive correction
• With non contact lens electrodes.
• Reference electrodes placed at ipsilateral
• Not on forehead or ear.
• Binocular stimulation preferred ,except in
• The transient PERG has 2 main components
:P50 at 50msec
• And P95 at 95 msec
• P50 reflects macular function
• P95 reflect ganglion cell function
• Assess visual loss.(unknown etiology)
• Visual loss due to macular photoreceptors and
inner retinal cells from diseases of ganglion
cell and optic nerve
• Also helpful in drug toxicity.
VISUALLY EVOKED POTENTIAL
• VEP is an averaged and amplified record of
action potentials in the visual cortex.
• VEP is macula dominated response.
• Only objective technique to assess clinically
the functional state of visual system beyond
the retinal ganglion cell.
TYPES OF VEP RECORDING
• 1-FLASH VEP- Recorded by using a calibrated
intense diffuse light or stimulus.
• Used in- Corneal opacity
• Vitreous haemorrhage
• 2- PATTERN VEP- Recorded by some patterned
stimulus displayed on TV screen (chess board).
• pattern appearance – black and white
checker board presented in on –off sequence.
• Pattern reversal – in this pattern of stimulus
RECORDING PROTOCOL FOR VEP
• FOR PATTERN VER-
• Spectacle correction
• 1 meter from screen
• Other eye is patched
• Focus on red dot on screen
• It depends on form sense and thus give rough
estimate of visual acuity
• FOR FLASH VER-
• Ganzfeld hemisphere bowl.
• Repeated 75 times for two run before the eye
• FLASH VER- complex compared to pattern
• M – shaped curve 2 positive and 2 negative
FACTORS INFLUENCING VEP
• 1- Stimulus.
• 2-Position of electrodes.
• 3-Age and sex .
• 4- Attention of the pt to stimulus.
• 5- Effect of diseases on VEP.
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.
• Following resolution, the amplitude of VER
waveform may become normal, but the
latency is almost always prolonged & is a
2. Compressive optic nerve lesions:-
• Usually associated with a reduction in the
amplitude of the VER without much changes
in the latency
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.
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
Malingering & hysterical blindness
• Pattern evoked VER amplitude & latency can be
altered by voluntary changes in the fixation
pattern or accommodation.
• 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.
• 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.
Lateralization of defects in the visual
• VER provides a useful information for
localizing the defects in visual pathway in
difficult cases e.g. children & non-cooperative
• Asymmetry of the amplitudes of VER recorded
over each hemisphere imply a hemianopic
• 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
Unexplained visual loss
• useful in general & in pts. with orbital/head injury
Assessment of visual potential in pts. with opaque
• like corneal opacities, dense cataract & vitreous
• 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.
• helps in detecting central