12- ERGs and Center Surround

What are the different charges of the eye?

front of the eye is more positive

back of the eye is more negative

What is the process of an electrooculogram?

1. lights are turned off and difference with each eye movement gradually decreases. get a minimum value after 10-15 minutes

2. light are turned off and difference in change gradually increases

3. get maximum value after 10-15 min

How should the EOG amplitude in the light compare to the amplitude in the dark?

EOG amplitude @ light peak should double compared to dark

When is the difference in EOG amplitudes in light vs dark considered abnormal?

if the light is <1.7x that of dark

EOG are a maker of what retinal cell function?

RPE function

What does an electroretinogram (ERG) give us?

provides measure of summed electrical activity in the retina in response to light stimuli

What factors does an ERG give us?

provides us with a waveform with A-wave and B-wave as well as implicit times of a + b waves

What does an a-wave of an ERG represent?

function of PRs (reduction of dark current in response to light [hyperpolarization])

Is the a-wave of an ERG negative or positive?

negative

What does the b-wave of an ERG represent?

responses of bipolar cells in response to light

Is the b-wave of an ERG positive or negative?

positive

Why does the b-wave of an ERG follow ON-bipolar cell responses (show + change/depolarization)?

there are so many more ON bipolar cells that off bipolar cells as rods only talk to on bipolar cells are there are so many more rods in the retina

Changes in what part of the retina affect the ERG?

outer retina only (PRs, BPCs)

How do a + b waves of an ERG in light adapted (photopic only) conditions? Why?

both a and b wave amplitudes are decreased

because only cones are responding (less PR response --> decreased a-wave, and removing rod ON bipolar response --> b-wave decreased)

How do a + b waves of an ERG change in dark adapted (scotopic) conditions tested with light flash compared to an ERG in high adapted conditions (photopic) ? Why?

a-wave amplitude increases + b-wave amplitude increases

both rods + cones responding --> increased a-wave

both rods + cones responding, now 120 million rod ON bipolar responding --> increased b-wave

How do the a + b waves and implicit time of an ERG changed in dark adapted (scotopic) conditions with a very dim flash of light? (compared to dark adapted with bright flash)

decrease a-wave amplitude compared to dark adapted with bright flash (mixed response)

increased B-wave amplitude (ON bipolars only, no Off responding)

increased implicit time

How would the ERG change if you got cone response only? (via bright background light, stimulus flicker, or red colored stimulus)

decreased a-wave + decreased b-wave

What rate of flicker is used to get cone only response in an ERG?

30Hz (can use anything >10Hz)

What are the ERG test conditions standards? (steps)

1. dilate pupil

2. dark adapted for 20 mins + electrode put on under dim red light

3. use dim flash to get rod only response

4. then use bright flash to get mixed response

5. increase background light to bleach rods + use bright flash

6. then get flicker response

What are the types of ERGs?

full field ERG

modified focal ERG

multifocal ERG

What does full field ERGs give us?

uses flashes of lights the illuminate entire retina to give us overall retinal activity

What are modified focal ERGs?

small flash used to illuminate specific part of retina

What are multifocal ERGs?

array of hexagonal stimuli, light is flashed on and off in each hexagon @ random, keeping 1/2 hexagons illuminated at given time

What response (rods, cones, or both) does multifocal ERGs give us?

gives us rods and cones response

What is the advantage to multifocal ERGs?

can pick up on certain focal defects of outer retina

What is a downside to multifocal ERGs?

requires patient to have minimal eye movements during test

What are the main clinical uses of ERG + EOG?

assist the diagnosis of hereditary retinal + choroidal diseases

What is used more for retinal disease, ERG or EOG? what is the exception?

ERG used more

except in Best's disease (always use EOG)

Why do we use EOG in best's disease?

In best's, EOGs is abnormal before the clinical signs AND abnormal in carriers

ERG is often normal in patients with best's disease

What is Best's disease?

vitelliform dystrophy which causes lipofuscin deposits in RPE + sub RPE space, especially in fovea

What clinical appearance does Best's disease present?

egg yolk lesion in beginning, then overtime lesion can leak + bleed, get focal RPE degeneration + choroid atrophy, resulting in scrambled egg appearance

When do initial pigment disturbances occur in Best's disease?

5-10 years old

How can ERG aid in diagnosis of Retinitis Pigmentosa?

ERGs changes come before fundus changes

ERG will have abnormal scotopic amplitudes + implicit times

What types of ERGs would be affect in RP?

scotopic ERG affected the most (lower amplitudes)

in late stage RP, both rod + cone ERG are severely reduced

What is congenital stationary night blindness (CSNB)?

retinal disorders in which neurotransmission between PRs+ BPCs is altered

What is type 1 CSNB (complete form)?

defect is localized to ON bipolar signaling, therefore complete absence of rod pathway

doesn't cause degeneration of PRs

What causes type 1 CSNB?

X-linked genetic defect of nyctalopin

or

AR defect in mGluR-6 or TRPM1

What is nyctalopin?

protein found of dendrites of bipolar cells

What is the inheritance pattern of type 2 CSNB (incomplete form)?

X-linked

What causes type 2 CSNB?

mutation in gene that codes for specialized VGCC in PRs

Why is type 2 CSNB considered incomplete?

because only specialized VGCC in PRs are affected, normal VGCC still work, so PRs + BPC synapses are not completely impaired

What is more affected in type 2 CSNB, rods or cones?

rods are more affected but they are both impaired

What are the symptoms of CSNB?

night blindness

RE

strabismus

VA can be normal to severely reduced

abnormal ERG

What would the full field ERG of CSNB type 1 look like?

normal a-wave, no b-wave

What would the full field ERG of CSNB type 2 look like?

normal a-wave, very decreased b-wave

ASK IN CLASS

Where are horizontal cells located?

INL

How to horizontal cells respond to light?

hyperpolarize (sign conserving)

What type of potentials do horizontal cells generate?

graded potentials

What are the receptors on horizontal cells?

ionotropic AMPA/Kainate

horizontal cell response to dark? (steps)

1. PRs depolarize + release glutamate

2. glutamate binds AMPA/Kainate receptors on horizontal cell

3. AMPA/kinate channel opens, cations flow in

4. horizontal cell is depolarized

What is the limiting factor to a PRs receptive field?

width of the outer segment

How do horizontal cell responses change as light moves away from center of cell?

response decreases but is still present, even if light is 1mm away

Why do horizontal cells have such large receptive fields?

gap junctions pass the electric charge to neighboring cells

What is a receptive field?

the area in which stimulation leads to a response