Electroretinogram

Scotopic Balancing

If a long-wavelength (red) light produces an ERG with the same amplitude as does a short-wavelength (blue) light in a dark-adapted animal, then the stimuli are said to be scotopically balance - can be used to characterize diseases that have a predilection for either rods or cones - procedure: dark adapt animal -> record ERG to dim blue and red flashed (dim white background) - if the ERG's of the patient don't have equal amplitudes for the red and blue stimuli, retinal function is NOT normal (so stimuli can reveal underlying causes of abnormal ERGs)

"Standard" ERG

- dark adapted subject - single, bright, white flash (5ms) - mixture of photopic (cone) and scotopic (rod) response components *as recording techniques have gotten more advanced, more parts of the ERG graph have been identified

Scotopically balanced ERG

- patient with a normal retina will give an equal amplitude response to the blue and red stimuli under dark- or light-adaptation - a patient with a rod dystrophy would demonstrate a lower-amplitude response to blue than to red (not shown)

Photopic ERG

- predominantly cone function (but rods too) - single, red flash (b/c less likely to stimulate rods) - high frequency flicker (>20Hz) -> rods recover more slowly than cones so they cannot follow a high frequency flicker - high intensity (bright) diffuse flash also minimized rod participation

Scotopic ERG

- predominantly rod function (but cones too) - dark-adapted subject - single, dim, blue flash (b/c less likely to stimulate cones and disrupt dark-adaptation; rods are more sensitive to blue light than are cones) **No a-wave and b-wave has smaller amplitude

Scotopic Threshold Response (STR)

- recording: fully dark-adapted eye, very dim (near-threshold), full-field flash - Origin: Proximal inner retina (muller cells and Aii Amacrine cells) - clinical value: Juvenile X-linked retinoschisis and Early Diabetic Retinopathy

ERG: The big picture

- ERG recording, the fast oscillations (FO) are the primary focus - the fast oscillation is generated by a light-evoked decreased in chloride ion concentration within the RPE cells and the accompanying hyperpolarization of the basal membrane of the RPE - when clinicians and researchers talk about the ERG, they're generally referring to those changes in the retinal potential that occur within the first few seconds of a flash of light - in clinical practice the most important components of the ERG are the a and b waves (circled in red) *FO in normal subjects is very sensitive to abrupt changes in blood glucose -> there is an increase in the amplitude of the fast oscillations

ERG components, Granit

- Granit studied the ERG by slowly increasing the amount of anesthesia given to an animal while recording ERG P I: a slow, "cornea-positive" response was the first to disappear P II: an earlier "cornea-positive" response, disappeared second P III: the "cornea-negative" response that remains when P I and P II are extinguished -> this also disappears with further anesthesia

Oscillatory Potentials and Pathology

- OPs are very sensitive to ischemia, so their attenuation in an ERG in which the a and b waves are normal can be used as an indication of mild ischemia in the inner retina - OPs are severely attenuated in a number of retinal degenerations, including diabetic retinopathy, hypertensive retinopathy, central retinal artery and vein occlusions, etc. - each individual OP can be attenuated or abolished by specific experimental conditions or pathologies, suggesting that each could represent a separate electrical event, structure, or pathway **however, the precise origin of each OP is still unknown -> when the origin is discovered the OPs will likely become more clinically useful

ERG measurements of interest

- amplitude - implicit time - latency

Focal ERG and Epimacular Membrane

- before surgery, VA in affected eye = 20/125 - 6 months after surgery, VA = 20/32 - post-surgically, focal ERG has greater amplitude and faster implicit time than pre-surgically

Diffuse flash ERG

- can be applied to a dark or light-adapted eye - "Standard" (white) stimulus - scotopic - photopic = use flicker - scotopic balance

Focal ERG

- small, flickering spot (no rod input once flicker is fast enough) - signal averaging is required - stray light can be a problem when it lights up other areas - most often used for macular disorders but can be applied to any small area of the retina (~10deg diameter)

Pattern ERG (PERG)

- use JET, gold foil, or DTL fiber electrode b/c need clear optical path! - pattern (contrast) reversal stimulus - response origin: inner retina (mainly ganglion cells) - clinical value: glaucoma, CRA occlusion, optic nerve trauma

ERG types

1. Diffuse Flash 2. Focal (fERG) 3. Multifocal (mfERG) 4. Pattern (PERG)

Standard flash ERG components

1. Early Receptor Potential (ERP) 2. a-wave 3. Oscillatory Potentials (OPs) 4. b-wave 5. Afterpotential 6. c-wave

When we are evaluating and ERG for changes from normal, what do we look for?

1. amplitude (uV) 2. implicit time (msec)

ERG standard procedure (dark adapted)

1. dark adapt 35-40 mins 2. anesthetize subjects cornea (proparacaine) 3. dilate pupil (tropicamide & phenylephrine) 4. attach electrode: either corneal contact lens OR forehead (neg), corneal (pos), behind eye (reference/ground) 5. the retina is then illuminated with different wavelengths, intensities, and rates of flashed light stimuli 6. the electric responses are obtained by the electrode and then analyzed by computer **if light-adapted responses are to be obtained, these are done after the dark-adapted responses

Pattern ERGs wave components

N35, P50, N95 N: negative going P: positive going - these components are ONLY labeled this way for PREG **note: once the pattern flickers too quickly, the parts of the wave become unclear

ERG electrodes types

Non-disposable: - burian-allen electrode - JET electrode - Gold Foil Electrode Disposable: - DTL Fiber Electrode

Which retinal layers contribute to ERG?

Photoreceptors! (rods and cones only) - bipolars inwards, cannot contribute to ERG because their responses are blocked by interneurons

difference in response to a bright white diffuse flash between dark-adapted and light-adapted retina

Same intensity flash produced different ERG under dark- vs light-adapted conditions - remember that in both cases, both rods and cones contribute, but in light-adapted eyes, the rods are limited in how much they can contribute

Electroretinography

The recording of the temporal sequence of changes in the potential of the eye when the retina is stimulated with a brief flash of light - a standard clinical protocol for recording ERG's was developed in 1989 to make ERG's taken at different clinics and labs comparable - today the ERG is used primarily to diagnose retinal degenerations

Rods and Cones distribution throughout retina

They are distributed differently throughout the retina - even through cone density is greatest at the fovea, 90% of cones are located outside the fovea - 120 million rods, 9 million cones **remember that this means that a bright white flash on a dark-adapted retina, the response generated is by both rods and cones, but mainly rods

Anatomy of Dark-adapted ERG (brief, high energy diffuse white flash; "standard ERG")

a-wave: - negative-going (cornea-negative) - 'receptor potential' -rods and cones - light induced photoreceptor activity (i.e. change in dark current) b-wave: - positive-going (cornea-positive) - largest component of diffuse flash ERG - ON bipolar cells, w/some Muller cell contribution c-wave: - positive-going (cornea-positive), but slower than b-wave - 2 sub-components: smaller negative, neural retinal positive RPE - caused by light-evoked decrease in [K+] in subretinal space

what causes the a, b, and c-waves?

a-wave: photoreceptor b-wave: bipolar and Muller cells c-wave: RPE

Oscillatory Potentials (OPs)

ascending limb of the b-waves when generated with a bright flash - caused by amacrine cells (IPL)

Flicker ERG

faster flickering lights will only allow for cones to respond, so we can separate them from rods

ERG amplitude

high of wave reflects magnitude of the voltage change

Afterpotential

located on the descending limb of b-wave, where ERG goes below baseline voltage

Early Receptor Potential (ERP)

occurs before the a-wave - caused by photoreceptors (outer segments)

Fast Oscillation (FO)

rapid decrease in the standing potential of the eye occurs about 45 seconds to 1 minutes after he onset of light - important measure in ERG recording

Slow oscillation (SO)

slow increase in the standing potential of the eye that peaks about 12 min after the onset of light - important measure in EOG recording (shown in blue)

ERG electrode placement

the ERG can be measured with skin electrodes placed around the eye, but a more accurate recording can be made by using a corneal contact lens electrode centered on the cornea

Significance of Oscillatory Potentials (OPs)

the ERG is recorded over time; if we filter out the low temporal frequencies (<100Hz), then we filter out the large a and b waves and we reveal the oscillatory potential (high frequency) in the signal - recording: dark adapted subject, bright white flash (the brighter the flash, the more OPs that are generated, up to 3 major and many smaller ones) - origin: Amacrine cells? (IPL) - clinical value: diabetic retinopathy

ERG: dual retina

the ERG wave is a combination of the rods and cones responses - great amounts of time and energy have been devoted to separating rod- and cone-driven responses

ERG latency

the time between stimulus onset and beginning of a-wave

ERG implicit time

time between onset of the flash and the peak of the wave