ERG Dr. Ricky mittal

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ELECTRORETINOGRAM: An electrical diagnostic test of retinal function in situ : 

ELECTRORETINOGRAM: An electrical diagnostic test of retinal function in situ Dr Ricky Mittal Institute of ophthalmology, JNMC AMU Aligarh

INTRODUCTION : 

INTRODUCTION Electrodes placed on cornea Retina stimulated Resultant wave recorded and analyzed Helps study eye, systemic diseases

ELECTRORETINOGRAM(ERG) consists of : 

ELECTRORETINOGRAM(ERG) consists of Electro -part Currents, wires, voltage, resistance Retino - part Cell types, membrane potential, radial currents. Gramo - part Diagnostic test of patient retinal health Research test retinal circuitry, cell function, disease states, drug efficacy

Electro Part : 

Electro Part ERG is a recording of the temporal sequence of changes in potential in the retina when stimulated with a brief flash of light.

Types of Electrodes : 

Types of Electrodes Two types Corneal -Burian Allen (Bipolar) -Jet electrode (Unipolar) Non corneal electrode -DTL fiber electrode (Dawson-Trick-Litzkow) -Gold foil electrode -LVP Zari electrode

Electrode impededin a special contact lens. : 

Electrode impededin a special contact lens.

Burian-Allen Electrode for Human Use : 

Burian-Allen Electrode for Human Use

GANZFELD : 

GANZFELD ERG is recorded using Ganzfeld which is integrating sphere used to deliver stimuli Provides flash stimulation, diffuse background & fixation lights

Slide 9: 

The Grass xenon-arc photostimulator can also be used for delivering stimuli

Procedure : 

Procedure Dark adapt 30-45 min Anesthetize subjects cornea (paracaine) Dilate iris (tropicamide; phenylephrine) Attach electrodes: Forehead (Reference electrode- neg) Corneal (pos) (DTL microfiber, Burian Allen) -using non viscous coupler (CMC) Behind Ear (Ground electrode)

Recording methods : 

Recording methods The light stimulus consist of flashes of about 5 ms so that each flash is considerably shorter than integration time of any photo receptor A standard flash strength is defined as one that produces a stimulus strength of 1.5 – 4.5 cd.s.m-2

Recording methods : 

Recording methods Same flash attenuated by 2.5 log units of neutral density filter (the scotopic rod response) be recorded under full dark adaptation Now Ganzfeld produces steady & uniform background luminance of 17-34 Cd.m-2 for 10 min and photopic response is recorded Following which, light adaptation photopic transient and flicker ERGs are recorded. 30 Hz is usually used for the flicker ERG.

Slide 13: 

In addition to the presence of a rod-saturating background in the Ganzfeld, the rods have poor temporal resolution and cannot respond to a 30 Hz flicker.

Slide 14: 

Dual retina: Great amounts of time and energy have been devoted to separating rod- and cone-driven responses

Retino Part : 

Retino Part There are more photoreceptors than ganglion cells so there is a convergence pattern. Many photoreceptors terminate into one bipolar cell and many bipolar cells terminate into one ganglion cell. The convergence rate is greater at peripheral parts of the retina than at the fovea. Rods (10 million) are for vision in dim light and cones (3 million) are for color vision in brighter light.

Retino part cont…. : 

Retino part cont….

Retino part cont…. : 

Retino part cont…. The a-wave, sometimes called the "receptor potential“, reflects the general physiological health of the photoreceptors in the outer retina. The b-wave reflects the health of the inner layers of the retina, including the ON-bipolar cells and the Müller cells C-wave: pigment epithelial layer. D-wave: off-response retina.

Gramo Part : 

Gramo Part Basic Clinical ERG tests Dark adapted, bright (white) flash response Generates Max a-wave, b-wave, also generates OPs : Dark adapted, dim (blue) flash response Isolated rod-driven response Light adapted, bright flash Isolated cone-driven response 30 Hz Flicker Another method of isolating cone responses.

Dark adapted, bright (white) flash response (Scotopic) : 

Dark adapted, bright (white) flash response (Scotopic) It is also called as maximal combined response It consist of sharp negative a-wave & a much larger, rapidly rising b-wave which comes to base line very slowly A standard white flash is used. Diffrence between two flash is at least 10 sec (to remove effect of bleach)

Electroretinogram (ERG) : 

Electroretinogram (ERG)

Dark adapted, dim (blue) flash response (Scotopic) : 

Dark adapted, dim (blue) flash response (Scotopic) To isolate signal of Rod system, a dim white flash of (2.5 log unit below SF) A blue stimulus is equally effective It is also called as isolated Rod response, has almost no a-wave , slowly rising & broad b-wave

Slide 22: 

b-wave is a post receptor phenomenon i.e inner retinal cell response driven by only Rod photoreceptor With increasing stimulus intensity, amplitude of a wave start increasing

Light adapted, bright flash (Photopic) : 

Light adapted, bright flash (Photopic) A small a wave & rapidly rising b wave that rapidly return to baseline Better localization of cone fucntions Produce due to hyperpolarisation of bipolar cells & cone photoreceptors

Different conditions yield different responses : 

Different conditions yield different responses Rod Rod & Cone Cone

Oscillatory Potentials (OPs) : 

Oscillatory Potentials (OPs) Oscillatory potentials are small but high frequency oscillations on ascending limb of b wave of maximal combined response Generated by amacrine cells in middle & inner retinal cell layer

Oscillatory Potentials (OPs) : 

Oscillatory Potentials (OPs)

Slide 28: 

Other wavelets are removed by resetting of filters Usual setting of filter is at between 0.3Hz to 75 Hz To refine OPs filter is set between 75 to 300 Hz to get these high frequency wavelets

Oscillatory Potentials (OPs) : 

Oscillatory Potentials (OPs)

Oscillatory Potentials are delayed in diabetes : 

Oscillatory Potentials are delayed in diabetes

Flicker ERGs : 

Flicker ERGs Using the difference in the speed of the rod (slow) and cone (fast) responses to isolate rod- and cone-driven function in the retina

Flicker cone Response : 

Flicker cone Response Under photopic condition repetitive stimuli (10 to 30 Hz) given Rods are suppressed & incapable of responding Amplitude is measured from trough to crest of each response

ERG to 5 sec of Flicker : 

ERG to 5 sec of Flicker Three separate stimuli, each with a different frequency.

Faster flicker smaller response : 

Faster flicker smaller response

Slide 35: 

A 30 Hz response is sensitive measure of cone dysfunction, but generated at inner retinal level Response is affected in inner retinal ischemic states

Multifocal ERG : 

Multifocal ERG This technique is developed by Bearse & Sutter The response is recorded from many regions of retina Response is recorded to a scaled hexagonal reversal stimulus in photopic condition It allows assessment of focal retinal function

The multifocal ERG (mERG) : 

The multifocal ERG (mERG)

Pattern electroretinogram (PERG) : 

Pattern electroretinogram (PERG) The pattern electroretinogram (PERG) assesses the retinal response to a structured non-luminance stimulus such as a reversing black and white checkerboard. It provides useful information in the distinction between optic nerve disease and macular disease in patients with poor central visual acuity

Slide 43: 

CLINICAL USES Assessing Visual loss of unknown etiology Diffrentiating visual loss due to macular photoreceptor & macular inner retinal cells Monitor drug toxicity Assessing glacumatous damage of retina

Limitations of PERG : 

Limitations of PERG Amplitude is very small & require highly sophisticated equipments Not reliable in hazy media More Pt. cooperation is required

Slide 45: 

This recording has a much lower amplitude than the full-field ERG, and signal extraction using computer averaging is necessary.

APPLICATIONS OF ERG : 

APPLICATIONS OF ERG

Slide 47: 

Diagnosis of Retinitis Pigmentosa

Cone Rod dystroppy : 

Cone Rod dystroppy Normal RP Cone-Rod Dystrophy

Diagnosing leber’s congenital Amaurosis : 

Diagnosing leber’s congenital Amaurosis ERG is Unrecordable even in early cases of LCA

Slide 50: 

Talc Retinopathy -occur in long term I.V drug users

Diagnosing & Differentiating fundus dystrophies : 

Diagnosing & Differentiating fundus dystrophies

Slide 54: 

There are many other Less common Retinal Dystrophy in which ERG is helpful in making Diagnosis Also very useful in estimation of retinal function in eyes with Opaque media Determining Prognosis Melanoma associated Retinopathy

Monitoring drug toxicity : 

Monitoring drug toxicity Gentamycinkills the retina:Brief exposures are reversible.Dose dependent loss of b-wave.

Slide 56: 

Also useful in monitoring Drug Toxicity of certain other drugs as -hydroxychloroquine -chloroquine -Thioridazone Detection of carrier stage (eg. X linked RP, blue cone monochoromatism) Retinal Detachment

Bionic eyes : 

Bionic eyes Understanding ERG may be helpful in future research & development of Bionic Eyes

Limitations : 

Limitations 1. Diurnal variation in rod ERG, so serial measurement is required 2. Number of artifact can be produced because of -muscular tension -blink reflex -improper electrode placement 3. Age & high refractive error can effect results 4. Flash ERG is affected in only widespread retinal dysfunction 5. Photophobia, Claustrophobia & Pt.s cooperation may vary results

Refrences : 

Refrences Neema – Diagnostic procedure in ophthalmology Yanoff & Ducker ophthalmology Kanski – Clinical methods in ophthalmology American academy of ophthalmology Wikepedia www.emedicine.com www.medscape.com Rubin & Kraft,Ophth. 2007 Images downloaded from various internet sites

Slide 60: 

Thanks