Electrophysiological studies in ophthalmology

Dr.Vaibhav.k

P.G.Ophthalmology

Navodaya medical college

• 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’.

• 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 contralateralhemisphere.

• 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.

• 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.

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.

Electrophysiological tests

Electroretinography (ERG).

Electrooculography (EOG).

Visually evoked response (VER).

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

• 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.

• 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.

Electrooculogram (EOG)

DEFINITION

Electrooculography is a technique for measuring the corneo-retinal standing potential that exists between the front andthe back of the human eye. The resulting signal is called theELECTROOCULOGRAM.

Measurement of eye movements is done by placing pairs ofelectrodes either above and below the eye or to the left andright of the eye.

If the eye moves from center position toward one of thetwo electrodes, this electrode sees the positive side of theRetina and the opposite electrode sees the negative side ofthe retina. potential difference occurs between theelectrodes.

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.

EYE MODEL BASED IN EOG (BIDIM-EOG)

Bi Dimensional Bi Polar Model (BiDiMEOG)

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.

• 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.

• 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.

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.

EOG recording

• 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

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.

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.

ARDEN’S RATIO :

It is the ratio of ‘largest EOG amplitude during light adaptation’ (light peak) to ‘least amplitude during dark adaptation’ (dark trough).

• >180 Normal

• 165—180 Borderline

• <165 Subnormal

• Difference of >10% in BE is significant

• Good pt cooperation is required

• 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, fundusflavimaculatus, & generalized drusen often show a striking EOG reduction in presence of normal ERG.

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.

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.

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.

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

• 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.

Visual evoked potential

• 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.

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

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.

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.

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.

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.

• 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.

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

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.

Recording parameters

• Amplification and filtering:

Amplification of the input signal by 20,000–50,000 times is usually appropriate for recording the VEP.

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

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

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.

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.

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.

• Standard VEPs to pattern onset/offset stimulation typically consists of three main peaks in adults;

• C1(positive, approximately 75 ms)

• C2 (negative,approximately125 ms)

• C3 (positive, approximately 150 ms)

• Amplitudes are measured from the preceding peak

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.

• 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.

Multi-channel recording for assessmentof 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.

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

• 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

• 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.

• unilateral disease is relatively easy to detect

• Useful in:

• Optic neuritis

• Multiple sclerosis

• glaucoma, with local visual field effects

• Ischemic optic neuropathy

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.

• 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

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.

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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.

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.

• 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.

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.

• However, the differentiation of tract lesion from that of optic radiation lesion is difficult.

• Decreased amplitude of VER recorded over the contralateralhemisphere, when each eye is stimulated separately indicates a bitemporal visual deficiency & may localize the site of chiasmal pathology

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.

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

Electroretinogram

• 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.

TYPES OF ELECTRORETINOGRAM

ERG

Full field ERG

Focal ERG

Multifocal ERG

Pattern ERG

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).

Additional protocols of ERG

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.

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.

ELECTRODES USED IN ERG

Jet Electrode Gold Plated Electrode Skin Electrode

DTL Electrode HK Loops Burian Allen Electrode

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)

• Lens lubricant and corneal anaesthesia used

• Filament type electrode placed on lower lid include Gold foil electrode ,DTL Fiber electrode and HK-Loop electrode

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.

• 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

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

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 cdm-2 with a scotopic strength of 75 scotopic cd m-2.

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.

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.

• 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

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 systemresponses)

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

Normal Waveforms

a-wave

• Initial negative wave.

• In dark adapted condition primarily from photoreceptors

B-wave

• Large positive wave.

• Arises from the Muller cells, representing the activity of bipolar cells.

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

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

• 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:-

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.

• 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.

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

Measurement of ERG Components

Amplitude

a wave measured from the baseline to the trough of a-wave.

b wave

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

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

ths

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

Scotopic maximal combined response

• Standard flash(0dB)

• Rod and cone response

• Large a & b waves

• Example-normal, early cone dystrophy,RP.

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,Progressivecone dystrophy

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

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

• 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.

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

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,steroidadministration,optic atropy,retinal vascular disrders

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

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

Negative response

• Characterized by large a-wave.

• Indicates gross disturbances of retinal circulation as seen in arteriosclerosis, giant cell arteritis, CRAO & CRVO.

• 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.

• 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.

• RETINOSCHISIS :

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

• 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.

• CRAO :

• In vascular occlusions like CRAO, ERG typically shows showsabsent b wave.

• Ophthalmic artery occlusions usually results in unrecordableERG.

• 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.

• 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 moreas compared with normal eye.

• No intervention finally results into an unrecordable ERG (Zero ERG)

• 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

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.

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

• Thank you.