Electrodiagnostic Tests in Ophthalmology

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

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