Electrophysiology

Question 1

What is the elctrooculogram?

Answer 1

TRans-pigment epithelial potential generated by separation of ionic gradients across the RPE maintained by tight junctions

Question 2

What is electroreinogram

Answer 2

Response from all retinal layers

Question 3

What is the pattern ERG

Answer 3

Response after processing in the bipolar cell layer

Question 4

What is the visual evoked potential

Answer 4

Records electrical activity from the occipital cortex following presentation of a light stimulus to the retina

Question 5

How does the EOG work?

Answer 5

Records the electrical dipole ocurring between the cornea and retina - reverses in direction when the eye moves from side to side

Measurements are taken between electrodes at outer and inner canthi in the dark for 20 mins then in the light

Amplitude of signal reaches a minimum in dark (dark trough) and should increase markedly during light adaptation to maximal value (light peak).

Critical value is the ratio between light peak and dark trough (Arden ratio) should be >180%

Question 6

What is the use of the EOG

Answer 6

Distinguishing between localised and diffuse retinal disease and for establishing retinal integrity in the presence of opaque media.
It is a function of RPE activity

Question 7

What affects the ERG?

Answer 7

Intensity, duration, wavelength and pattern of the stimulus and the level of light-dark adaptation ofthe retina

Assessment of photoreceptor, bipolar cell function in photopic and scotopic conditions

Question 8

What are the components of ERG

Answer 8

Negative a wave - hyperpolarisation in the photoreceptor innersegments

a1 from cones
a2 from rods

Positive b wave - bipolar cells directly or indirectly spread to Muller cells

b1 cone dominated bipolar cell
b2 rod dominated bipolar cells

Question 9

What is wave amplitude on ERG

Answer 9

Trough of a wave to b wave peak

Question 10

What changes in ERG in retinitis pigmentosa?

Answer 10

Primarily affects rods so will affect the scotopic ERG ( very dim white light or blue flash used in dark - response purely generated by rods)

Question 11

What changes in ERG in retinoschisis and congential night blindness

Answer 11

Rod component of b wave is missing producing a negative ERG

Question 12

What changes in ERG in CRAO/CRVO?

Answer 12

b wave lost, a wave spared
As photoreceptors as supplied by choroidal circulation

Question 13

What is the pattern ERG used for?

Answer 13

To distinguish retinpathy from maculopathy

Stimulus is reversing checkerboard

P50 - positive 50ms produced by damage to inner retina, macula,and ganglion cells

N95 - negative 95ms optic nerve damage

Amplited/unit area of pattern ERG is less dominated by the fovea - abnormal in variety of local macular conditions and early retinal degeneration

Question 14

What does an abnormal ERG in a pt with ocular htn indicate

Answer 14

May be developing glaucoma N95 abnormality

Question 15

What is the visual evoked potential

Answer 15

Measure of the repsonse of the occipital cortex to visual stimulation - using scalp electrodes

Stimuli are reversing checkerboard or diffuse flash

Question 16

What are requirements for VEP

Answer 16

Pupillary dilation is CI
Correction of refractive error is necessary

Question 17

What are components of the VEP?

Answer 17

P100 - positive at 100ms - latency of this is measured - optic nerve demyelination produces a delay and can cause a reduction in amplitude

N70 and N135
Amblyopic eye will give a normal flash VEP and and abnormal pattern VEP

Eye disease will acuse latency and reduced amplitude of P100

Question 18

What do delated pattern VEP indicate?

Answer 18

MAcular disease

Question 19

what does latency delay indicate?

Answer 19

Optic nerve disease