ELECTRO-DIAGNOSTICTESTS

(ERG, EOG, VER)

Dr. Ankit M. Punjabi

DOMS (final year)

Dept. of Ophthalmology, KIMS Hospital

Bangalore, Karnataka, INDIA

Email: drankitalways@gmail.com

ERG

• Electric potential generated by retina in response to stimulation of light.

• First recorded by Frithiof Holmgren (1865)

• In humans by Dewar (1877)

• Extensive work thereafter by Riggs (1941)

ERG waves (a, b, c)

• ‘a’ is negative wave. Amplitude is from baseline to trough & implicit time is from onset of stimulus to trough of ‘a’ wave

• ‘b’ is large positive wave. Amplitude is trough of ‘a’ to peak of ‘b’ & implicit time is from onset of stimulus to peak of ‘b’

• ‘c’ is lower amplitude, prolonged +ve wave less imp

ERG

ERG

• ‘a’ originphotoreceptors

• ‘b’ originMuller’s cells + bipolar cells. Mainly from Muller’s in response to increase (ECF) K+ in bipolars

• ‘c’ origin RPE

• Oscillatory potentials (small wavelets on ascending limb of ‘b’) from amacrine cells

Physiologic basis of ERG

• a wave –

- Light falling – Hyperpolarisation

- Outer portion of photoreceptor – positive

- Inner portion - negative

- Blue dim flash - Rod ERG

- Bright red light - Cone ERG

Physiologic basis of ERG

• b wave-

- Muller cells – modified astrocytes

- No synaptic junction

- Respond to potassium concentration

- Change in membreane potential

- Cells provide b wave from rods and cones

- Oscillatory potential

Physiologic basis of ERG

• C wave –

- RPE – in response to rod signals only

- Direct contact of rod cells with RPE

Amplitude Implicit time

Recording protocol1. Full mydriasis

2. 30 min dark adaptation

3. Rod response / scotopic blue/dim white

4. Max. combined response / scotopic white

5. Oscillatory potentials

6. 10 min of light adaptation

7. Single flash cone response / photopic white flash

8. 30 Hz flicker

ERG recording

• Electrodes active, reference, ground

• Ganzfeld bowl stimulator

• Signal averager

• Amplifier

• Display monitor

• Printer

Factors influencing ERG

1. Stimulus –

- a wave increase in size

- b wave reaches maximum

- Shortening of latency of peaks

. Flickering light – cone response only

Factors influencing ERG

2. Recording equipment -

3. Dark adaptation –

- ERG increases in size

- b wave becomes slower

4. Age and sex –

- small ERG within hr of birth , declines in adults

- Larger in females than males

Cone Rod ERG• In light adaptation 6-8 million cones tested

• In dark- additional 125 million rods contribute

• In dark adaptation initial 6-8 min majority of response is from cones

• Orange-red stimulus cone + rod response

• White flicker at 30 Hz with intensity constant only cones respond. As the freq increases ‘b’ amplitude decreases

Separation of cone & rod ERG

• For clinically useful information

• Cone ERG flickering stimulus 30-70 Hz (rods upto 50 Hz)

• Rod ERG in dark adaptation / blue light

ERG recording

1. Normal Waveforms Rod response / scotopic blue / dim white are usually smoother, dome shaped. Initial –ve ‘a’ wave is not seen & is hidden by ‘b’. Longer implicit time. Only rods contribute

ERG recording

2. Max combined response / scotopic white flash / mesopic response is a deep ‘a’ wave with tall ‘b’. Longer implicit, larger amplitudes. Both rods & cones contribute

ERG recording3. Oscillatory potentials

4. Single flash cone response / photopic white flash small ‘a’ & ‘b’ waves. Waveforms are more peaked with shorter implicit & smaller amplitude. Cone function

5. 30 Hz flicker multiple peaked waveforms. Cone function

4 5

Clinical Applications 1. Diagnosis and prognosis of retinal

disorders –

a. Retinitis pigmentosa

b. Diabetic retinopathy

c. Retinal detachment

d. Vascular occlusions of retina

e. Toxic and deficiency status

Clinical Applications

2.To assess retinal function when fundus

examination is not possible

- Corneal opacities

- Dense cataract

- Vitreous haemorrhage

EOG

EOG

Measurement of resting potential of eye

Which exist between cornea and back of

the retina during fully light adapted and

Fully dark adapted conditions.

EOG

• First discovered by Du Bois-Raymond (1849)

• Riggs (1954) & Francois worked extensively

• Arden & Fojas discovered importance of ratio

• Records overall mass response only.

EOG recording

• Dilate (>3 mm)

• Skin electrodes near both canthi of BE

• Ground electrode at forehead. Lighted room

• 3 fixation lights 15o apart (dim, red)

• Looks left & right with 30o excursion at rate of 15—20 rotations per minute.

EOG recording

EOG recording

• Base line. Keep lights on for 5 min

• Turn off the lights. Record for 15 min in dark adapted state

• Turn on the lights. Record for 15 min in light adapted state

• Recordings sampled at 1 min intervals

• Response decreases progressively during dark adaptation

EOG

• Potentials decrease progressively reaching lowest value called ‘dark trough’ in 8-12 min

• Light insensitive part of EOG

• Switch on record in light adapted state

• Progressive increase in potential, peak is called ‘light peak’ in 6—9 min

• Light sensitive part of EOG

EOG

Arden’s ratio

• Light peak / dark trough X 100

• >180% Normal

• 165—180% Borderline

• <165% Subnormal

• Difference of >10% in BE is significant

• Good pt cooperation is required

A) Light sensitive – [ Light peak ]

- Contributed by rods and cones

B) Light insensitive – [ Dark trough ]

- Contributed by RPE , Photoreceptors

inner nuclear layer

2 components of EOG

EOG

Indications

1. Best dystrophy markedly reduced with Arden ratio is less than 120%

2. Butterfly pattern dystrophy

3. Chloroquine toxicity

4. Stargardt’s dystrophy

VisuallyEvoked

Potential (Response)VEP / VER

Visual evoked potential• Gross electrical signal generated at visual cortex

in response to visual stimuli

• Impulses carried to visual cortex via visual pathway

• Recorded by EEG

• It is the only objective technique to assess

clinical and functional state of visual syst.beyond

retinal ganglion cells.

Types of VEP

1. Pattern VEP (checker-board patterns on TV monitor)

2. Flash VEP (diffuse flash light for uncooperative subjects)

VEP

• Un-dilated pupils. Sit 1 meter from monitor

• Electrodes in midline at forehead, vertex & occipital lobes

• 2-3 different checker sizes are shown

• Recording is done

VEP

Normal waveform

• Pattern VEP has initial –ve (N1) +ve(P1)second –ve (N2) wave

• Positive wave – 70 100 ms

• Negative wave – 100 – 130 ms

• Positive wave - 150 –200 ms

• Flash VEP is complex. 2 positive & 2 negatives.

VEP Indicationsa) Un-explained visual loss

b) Optic neuritis

c) Multiple sclerosis

d) Compressive ON lesions

e) Cortical blindness

f) Amblyopia

g) Glaucoma

No one can drive you crazyunless

you give them the keys