VSP 2: Lec

Visual Electrophysiology

objective tests

allow assessment of almost the entire length of the visual pathway

psychophysical measures

how the patient percieves

dark adpation

color

contrast

ISCEV

international Society for Clinical Electrophysiology of Vision

established standards for ERG, EOG, and VEP

Reviewed every 3 years

Global standard

There is ongoing electrical activity in the retina at all imes because

its nervous tissue

Transduction

process by which light is converted to electrical energy by the PR into signals our brain can interpret

ERGs

electroretinograms

a group of electro-diagnostic tests to asses the integrity of the retina and its central connections

Types of ERGs

Full field

Multifocal ERG

Pattern ERG

Why are ERGs important

Recording of electrical responses of neurally active parts of the retina (functionally dissecting the retina)

objective assessment of retinal function

Non invasive

helps localize site of lesion

allows quantitative assessment of degree of malfucntion (long term prognosis)

Full Field Electroretinogram (ffERG)

Elicited by a flash stimulus

- not continuously present

- about 5 ms in duration

- evenly illuminates the entire retina

ffERG records

summed transient electrical responses (retinal potential) from the entire retina

ffERG flash is delivered in

a full field dome (Gonsfield)

ffERF flash elicits a

biphasic waveform which is recordable at the retina

Amplitude

amount of electrical response is proportional to area of functional retina stimulated

Basic principle of ERG

Flash of illumination on the retina --> simultaneous activation of all the retinal cells to generate the current --> currents generated by all the trinal cells mix that pass though vitroues --> high RPE resistance prevents summed current from passing posteriorly --> the small portion of the summated current which escapes through the cornea is recorded as ERg

Therefor the erg is

corneal measure of an electrical response

produced by the retina

when stimulated by sufficiently intense light

Full field ERG is also known as

standard ERG

Flash ERG

ffERG: potential is approx. ---in size

1 mV (relatively small)

ffERG: primary generated from the

outer retina

a-wave

1st initial large negative component

measures responses from photoreceptors outer segments

"late receptor potential"

reflecting general physiological health of the PR in the outer retina

b-wave

second corneal positive large wave 1st large positive wave

measures function of the inner layers of the retina

represents activity of the on-center bipolar cells (some say muller cells)

Amplitude of a wave

baseline to the negative trough of the a wave

amplitude of b wave

the trough of the a wave to the following peak of the b wave

Latency

the time between the onset of the flash stimulus and the beginning of the a wave response (about 2 ms)

Implicit Time of the a wave

(t)a from flash onset to the trough of the a wave

implicit time of the b wave

(t)b from the flash onsent to the peak of the b wave

Which is longer

implicit time?

Early receptor potential (ERP)

research

need to use very brighter stimulus to elicit

early, small, fast (1.5ms) biphasic potential

occrs before standard ERG responses

origins in photoreceptor outer segments - primarily the cones

c wave

prolonged postive wave

lower amplitude

generated from the RPE in response to rod signals only

not seen on clincally

not used clincally

d wave

small positive wave

reflects function of off bipolar cell s

not seen in standard clinical ERG tests

implicit time is measured from stimulus offset to peak of the d wave

Oscillatory Potentials (OPs)

3-4 small wavelts occur on the rising phase of the b wave

refelcts amarcrine cells in the inner plexiform layer

decreased in retinal ischemic disorders

Photopic Negative Respine

PhNR

measures retinal ganglion cell function/damage

occurs at about 80-90 msec

Which of the waves of the ERG is typically the smallest

Photopic a wave

Summary

Rods and cones

a wave

On bipolar cells

B wave

Pigmented epithelium

c wave

OFF bipolar cells

d wave

amacrine cells

OPS

Full field ERG uses

measures rod and cone generated retinal response

important in diagnosing numerous retinal disorders

findings must be correlated with other testinf (VF, OCT, Flourescein angiography)

ffERG disadvantages

no info on localization of a defect in retina

macula defects alone may not affect ffERG

What would happen if we dark adapted the eye first and then flashed a very dum light on the retina? Which PR would we see?

To obtain either a pure cone or rod response we can

change the lighting

or

use of temporal stimuli

Rods

large/slow response

high sensitivity

slow dark adpatation

Cones

small/fast response

low sensitivity

fast dark adaptation

Photopic a wave

smaller and occurs first

before a scotopic wave

Photopic b wave

smaller and occurs first

before scoptpic b wave

1. changing the lighting: rods respond to

dark adapted eye

stimulate with a dim single flashe

short wavelength

1. changing the lighting: cones respond to

light adapted eye

stimulate with a brighter single flas

middle wavelength stimulus

2. changing the temporal rate of the stimulus: scotopic system

repsonds slower

flickering stimuli of 10 to 15 Hz

2. changing the temporal rate of the stimulus: Photopic system

responds faster

responds to flickering stimuli up to0 50 HZ

if we use a flickering light that is too fast for the rods...

but okay for the cones. The result represents pure cone responses

and only the function of the cone will be recroded

PART 2

6 standard Full field ERG responses ISCEV

Scotopic (Dark adapted) recordings:

- rod only

- combined rod and cone response

- maximum combined rod and cone response

- Oscillartory potentials

Photopic (light adapted) recordings:

- single flash cone response

- 30 Hz flicker cone response

1. Scotopic (Dark adapted) recordings - rod only repsone

pt is dark adapted (scotopic)

a dim white or blue light is presented as a single flsh

only the rods are sensitive enough to respond

-slow positive response - only B wave!

indicates rod function in the peripheral retina

(a wave isnt seen bc too dim)

2. Scotopic Recordinfs - combine rod and cone responses/ maximum combined rod and cone response

brighter flash stimulus

both rods and cones response

large a and b wave

2. Scotopic recording - oscillatory potentials

high frequency wavelts on the ascending limb of the b wave

seen under bpth scotopic and photopic conditions

Photopic (light adapted) recording: cone responses can be done in two ways

1. light adapting the patient (rods are bleached)

2. use of a flickering stimulus (if we use 30 hz only cones will respond)

Photopic (light adapted) recording: Photopic single flash

light adapted pt

high intensity middle wavelength flashes

a and b waves are smaller - rapidly rise and fall

less implicit timme

Photopic (light adapted) recording: 30 Hz flicker cone respone

- repetitive stimuli of high intensity flashes (15 sweeps)

too fast for rods - CONEs only

faster the flicker - less response

in order the maximuze the siz eof the photobic B wave pf teh ERG, the stimulus should be what wavelength

550 nm

Which wave of the ERG is the largest

scotopic B wave

ERG is abnormal if

more than 30-40% of retina is affected

generally reflects mid -peripheral and peripheral retinal function

Why does the ERG represent mid-peripheral and peripheral retina more than central retina?

distribution of PR through the retina