Electroretinogram

Electrooculogram

Anuraag

Singh

ERG History

Holmgren in 1865 first demonstrated that

an alteration in electrical potential

occurred when light fell on retina.

In1877. Dewar recorded light evoked

electrical response, ERG, from humans

for the first time.

In1941, Riggs introduced the contact lens

electrode in humans

ERG

Full-field Electroretinogram (ERG) is a

mass electrical response of the retina to

photic stimulation.

Basic method of recording is by

stimulating the eye with a bright light

source such as a flash produced by LEDs

or a strobe lamp.

The flash of light elicits a biphasic

waveform recordable at the cornea.

Basic wave forms The two components that are most often

measured are the a- and b-waves.

a-wave is the first large negative

component, followed by the b-wave which

is corneal positive and usually larger in

amplitude

Physiology of ERG “a” wave aka late receptor potential

When light falls on photo receptors

Hyperpolarisation

Outer portion of photoreceptor becomes

positive

Inner part becomes negative

A wave shows downward deflection

Reflects the potential of photoreceptors in outer

retina

b wave b wave - Reflects the function of the inner

layers of the retina, including the ON

bipolar cells and the Muller cells.

Muller cell is a glial cell ( has no synaptic

connection )

It respond to potassium concentration in

extracellular space

Light strikes a photoreceptor

Potassium released from photorecptors

( amount dependent on Light Intensity )

Muller cell respond by changing its

membrane potential

b wave is dependent on the electrical

activity within photoreceptors

Muller cells can provide a b wave from

eithyer cone or rod receptors

c Wave Positive wave

Reflects function of pigment epithelium in

response to rod signals only

d wave

Reflects Off bipolar cells

Ops - some wavelets that occur on the rising

phase of the b-wave known as oscillitatory

potentials (OPs).

OPs are thought to reflect activity in amacrine

cells

Two principal measures of the ERG waveform are taken:

1) The amplitude 2) The time

Amplitude:- a wave- from the baseline to the negative

trough of the a-wave b-wave measured from the trough of the a-

wave to the following peak of the b-wave

Time:- (t)a from flash onset to the trough of the a-

wave (t)b from flash onset to the peak of the b-wave

These times, reflecting peak latency, are referred to as “implicit times”

ERG recording electrodes Pupils are dilated

Different types of electrodes used

Burian Speculum that hold the eye open

and have a contact lens with a wire ring

that “floats” on the cornea supported by a

small spring.

Cotton wick electrodes

Gold Mylar tape that can be inserted

between the lower lid and sclera/cornea.

Light stimulation for ERGs. Strobe lamp and LEDs - mobile and can be

easily placed in front of a person whether sitting

or reclining.

Mobility of a strobe lamp or an array of LEDs is

a necessity in some situations such as at the

hospital bedside or in the operating room

Ganzfeld stimulation globe The Ganzfeld allows the best control of

background illumination and stimulus

flash intensity.

Rod and cones erg Implicit times and amplitudes vary

depending upon whether the eye is dark

adapted or not, and brightness and color

of the light stimulus.

These parameters allow separation of rod

and cone activity in retina.

Normally there are120 million rods in each

retina and about 6-7 million cones.

The ERG following a white flash is

dominated by the mass response of the

rods( due to large number )

Rod and cone activity can be isolated

Adaptation level

Background illumination

Rate of stimulation

Color of the flash

Flash intensity

Color stimulus Peak wavelength sensitivity for rods is around

510 nm and the peak sensitivity of cones as a

group is about 560 nm

By using color filters such as the Kodak Blue

and Red Wratten series rods and cones can be

differentiated

Rod and cone ERGs can also be isolated using

dim flash stimuli into photopic (cone)and

scotopic (rod) signals

Dim red flashes stimulate both rod and cone

function producing a small photopic component

bx and larger rod b-wave.

Rods are about three log units more sensitive

than cones.

Cones recover faster than rods.

Rate of stimulus

Rates (flicker) of stimulus presentation

also allows rod and cone contributions to

the ERG to be separated.

Even under ideal conditions rods cannot

follow a flickering light up to 20 per

second whereas cones can easily follow

a 30 Hz flicker.

This is the rate routinely used to test if a

retina has good cone physiology.

Types of ERG

Standard Full Field ERG ISCEV Standard ERG Protocol

In 1989, the International Society for

Clinical Electrophysiology of Vision

(ISCEV) developed a protocol to

standardize ERG testing so test results

could be compared worldwide.

The protocol consists of five separate

tests, each designed to evaluate different

areas or functions of the eye.

Dim Scotopic Flash ERG This is the first step in the International

Society for Clinical Electrophysiology of

Vision (ISCEV) standard ERG protocol.

It is conducted with a -25 dB flash.

In a dark-adapted eye, a dim flash tests

a response arising from the rods primarily

and associated glial cells.

Maximum Scotopic Flash

ERG This is the second step in the

International Society for Clinical

Electrophysiology of Vision (ISCEV)

standard ERG protocol.

It is conducted with a 0 dB flash.

In a dark-adapted eye, a moderate flash

tests a response from both the rods and

cones.

Oscillatory Potentials (OPs)

This is the third step

The oscillatory potentials are high-frequency

oscillations or wavelets seen on the leading-

edge of the b-wave.

The oscillatory potentials measure of function of

the amacrine cells and become abnormal early

in retinal ischemia.

Photopic Flash ERG / Single

Flash

Cone Response This is the fourth step

It is conducted with a 0 dB flash. In a

light-adapted eye, a moderate flash tests

a response arising from the cones

30 Hz Flicker ERG This is the fifth step

In a light-adapted eye, a flicker ERG tests

a response arising from the cones.

The flicker ERG has also been shown to

be useful in patients with diabetic

retinopathy.

ERG in Retinitis pigmentosa

The first two responses are scotopically matched blue and red ERGs.

The blue flash was dim enough that no a-wave can be discerned in a normal patient leaving only the rod-dominated slower b-wave.

The red flash is bright enough that photopicoscillations and bx component can be observed just after the a-wave.

Bright white flash in the dark produces the largest amplitude ERG.

The 30 Hz flicker illustrates the response of the rapidly recovering cones.

Photopic response is representative of a normal response with the more sensitive rods bleached by background illumination.

Oscillatory potentials on the ascending b-wave are seen in responses to moderate-high intensity white flashes and in response to red, yellow, and green flashes

Stationary rod dystrophies Congenital stationary night blindness

(CSNB) is found in several forms.

Two types.

Type 1 have an abnormal dim scotopic

ERGs but the bright flash ERG maintains

oscillatory potentials on the ascending

limb of the b-wave.

Type 2 has a very abnormal dim scotopic

ERG and the bright flash scotopic ERG

has a large a-wave and no b-wave.

Oscillatory potentials are also missing

The bright flash ERG b-wave is selectively attenuated

in:

Juvenile retinoschisis

Coat’s disease

Central retinal vein occlusion and central retinal artery occlusion

Myotonic dystrophy

Congenital stationary night blindness Type 2

Oguchi’s disease

Lipopigment storage diseases (Batten’s disease)

Creutzfeldt-Jacob (CJD)

Disorders result in a completely extinguished

ERG

Leber’s congenital amaurosis

Severe retinitis pigmentosa

Retinal aplasia

Total detachment of retina

Ophthalmic artery occlusion

ERG in cone dystrophies

ERGs of a patient with a cone dystrophy

exhibit good rod b-waves that are just

slower.

The early “cone” portion (bx) 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 and photopic white

ERGs dependent upon cones are very

poor.

ERGs in retinal vascular disease Vascular occlusions –

avascular appearance to

select areas of the fundus

ERG with no b-wave

Ophthalmic artery

occlusions usually result

in unrecordable ERGs.

Foreign bodies and Trauma

A small piece of stainless steel or plastic

outside the macula may have a minor

affect on the retina.

A piece of copper or iron have deleterious

affects within a few weeks

In general if b-wave amplitudes are

reduced 50% or greater compared to the

fellow eye, it is unlikely that the retinal

physiology will recover unless the foreign

body is removed.

Drug toxicities. Several drugs taken in high doses or for

long periods of time can cause retinal degeneration with pigmentary changes.

Thioridazine

Chlorpromazine

Vigabatrin

Chloroquine

Hydroxychloroquine

The effects of toxic medications can be detected and quantified using ERGs.

The effects of toxic medications can be

detected and quantified using ERGs.

Chloroquine retinopathy appears as a

characteristic “bullseye” maculopathy

The better substitute for chloroquine,

Plaquenil, can also have macular effects

noticeable by multifocal

electroretinograms.

Hydroxychloroquine (Plaquenil) is usually

less disruptive to the retina than

chloroquine, but ERG changes can still

occur.

Vigabatrin, a pediatric seizure medication,

can be toxic to the retina.

Attenuation of full-field ERG b-wave

amplitudes can detect toxicity.

Often the first indication of toxicity is

reduced amplitude to 30 Hz flicker

Cis-platinum used to treat brain tumors

sometimes reaches ophthalmic vascularization

and causes a reduction in ERG waveform in the

affected eye (OD in this case)

Steroid Retinopathy

The fundus photo shows a cherry red

spot in the macula. The ERG response

was diminished in size particularly

following dim scotopic flashes

Talc retinopathy

Seen in iv drug abusers

Global ERG is attenuated

Multifocal erg

Limitation of fferg - Unless 20% or more

of the retina is affected with a diseased

state the ERGs are usually normal.

Erich Sutter adapted the mathematical

sequences called binary m-sequences

creating a program that can extract

hundreds of focal ERGs from a single

electrical signal.

This system allows assessment of ERG

activity in small areas of retina.

mferg allows assessment of ERG activity

in small areas of retina.

With this method one can record mfERGs

from hundreds of retinal areas in a

several minutes

merg in macular degeneration

Small scotomas in retina

can be mapped and degree

of retinal dysfunction

quantified.

61 or 103 focal ERG

responses can be recorded

from the cone-driven retina.

The tested area typically

spans 20-30 degrees to

each side of the fovea

Pattern erg

The pattern ERG provides a useful

measure of macular function and

generalized bipolar cell function.

The most common stimulus is a

checkerboard stimulus composed of

white and black squares

PERG generation requires physiological

integrity of anatomically present RGCs

Reduction of PERG amplitude reflect the

reduced activity of dysfunctional RGCs

PERG reflects inner retina activity under

light-adaptation.

The PERG should be used in

combination with a traditional light-

adapted luminance ERG to have an index

of outer retina function

PERG represents an important tool to

monitor the onset and the progression of

RGC dysfunction in optic nerve disease.

Example:-

Glaucoma, optic neuritis, ischemic optic

neuropathy, and mitochondrial optic

neuropathy

The normal pattern electroretinogram :

N35- a small negative component with a

peak time occurring around 35 ms;

P50- a prominent positive wave emerging

around 50 ms

N95- a wide negative wave around 95 ms

perg in Macular diseases:-

The P50 component was shown to be

altered in all patients with retinal and

macular diseases.

perg in Optic nerve disease:-

N95 component was abnormal in 81% of

patients with diseases of the optic nerve.

The P50 component remain normal.

ELECTRO-OCULOGRAPHY

Electrophysiological test of function of the

outer retina and retinal pigment epithelium

in which the change in the electrical

potential between the cornea and the

fundus is recorded during successive

periods of dark and light adaptation.

The eye has a standing electrical potential

between front and back, sometimes called

the corneo-fundal potential

The potential is mainly derived from the

retinal pigment epithelium (RPE), and it

changes in response to retinal illumination

The potential decreases for 8–10 min in

darkness.

Subsequent retinal illumination causes an

initial fall in the standing potential,

followed by a slow rise for 7–14 min (the

light response).

These phenomena arise from ion

permeability changes across the basal

RPE membrane.

The clinical electro-oculogram (EOG)

makes an indirect measurement of the

minimum amplitude of the standing

potential in the dark and then again at its

peak after the light rise.

This is usually expressed as a ratio of

‘light peak to dark trough’ and referred to

as the Arden ratio.

The calibration of the signal may be

achieved by having the patient look

consecutively at two different fixation

points located at known angle apart

and recording the concomitant EOGs .

By attaching skin electrodes on both

sides of an eye the potential can be

measured by having the subject move

his or her eyes horizontally a set

distance .

Standard method

After training the patient in the eye

movements, the lights are turned off.

About every minute a sample of eye

movement is taken as the patient is

asked to look back and forth between the

two lights .

After 15 minutes the lights are turned on

and the patient is again asked about once

a minute to move his or her eyes back

and forth for about 10 seconds.

Typically the voltage becomes a little

smaller in the dark reaching its lowest

potential after about 8-12 minutes, the so-

called “dark trough”.

When the lights are turned on the

potential rises, the light rise, reaching its

peak in about 10 minutes.

When the size of the "light peak" is

compared to the "dark trough" the relative

size should be about 2:1 or greater .

A light/dark ratio of less than about 1.7 is

considered abnormal.

Clinical uses of EOG

Retinal diseases producing an abnormal

EOG will usually have an abnormal ERG

too which is the better test for analysis of

scotopic and photopic measures.

A particularly good use for the EOG is in

following the affects of high dosage

treatment with antimalarials such as

chloroquine and plaquenil over the course

of treatment and before the ERG is

affected

Most common use of the EOG nowadays

is to confirm Best’s vitelliform disease

Vitelliform lesions represent the

accumulation of lipofuscin in the macular

area. Further effects of retinal pigment

epithelium (RPE) dysfunction include

accumulation of degenerated

photoreceptor outer segments in the

subretinal space.