ELECTROPHYSIOLOGICAL TESTS FOR VARIOUS OCCULAR DISORDERS AND THEIR

INTERPRETATION

Dr. Pragya raiPG 2

INTRODUCTION

• Electrical activity in the retina & visual pathway is the inherent property of the nervous tissues which remain electrically active at all times & the degree of activity alters with stimulation.

• These tests give a recording similar to ECG , that helps to know the functional integrity of various layer of retina and ant. visual pathway.

• Sometimes clinical examination of eye cannot explain the exact cause of decrease in vision.

• This test categorize the site of lesion in visual pathways ,from the various layers of retina to optic nerve and brain cortex.

• Help know the extent , seriousness and stage of disease.

• First described by Prof. E. D. Reymond who showed that cornea is electrically positive with respect to posterior pole of eye.

• In 1908 Einthoven & Jolly showed that a triphasic response could be produced by simple flash of light on retina.

Electrophysiological tests

• ELECTRO-OCULOGRAM(EOG)

• ELECTRORETINOGRAM(ERG)

• VISUAL EVOKED POTENTIAL(VEP)

The electro- oculogram

• EOG examines the function of the RPE and the interaction between the RPE and the photoreceptors

• Based on the measurement of resting potential of the eye which exists b/w the cornea (+ve) & back of the retina (-ve) during fully dark-adapted & fully light-adapted conditions

INDICATIONS• Retinitis pigmentosa

• Bests’ vitelliform macular dystrophy Choroideremia• Stargardt’s disease• ARMD• Choroidal melanomas• Drug toxicity against RPE• Gyrate atrophy• Leber congenital amaurosis• Metallosis bulbi• Vit A deficiency

TECHNIQUE OF RECORDING

Measurement & interpretation

• Normally the resting potential of the eye progressively decreases during dark adaptation reaching a dark trough in approx. 8 - 12 min.

• With subsequent light adaptation the amplitude starts rising & reaches to light peak in approx. 6-9 min.

Arden ratio

• The largest peak to trough amplitude in the light is divided by the smallest peak to trough amplitude in the dark.

• Ratio of light peak to dark adapted baseline is also acceptable EOG measure.

Arden ratio=LP/DT * 100• Normal light rise values are

185 or above• Abnormal values are below

165

Limitations of EOG

• Pt with poor central fixation .• Children • Infants• Uncooperative adults• Media opacities • Illumination levels

ELECTRORETINOGRAM

• There exists a potential difference of 1 mV b/w cornea and posterior pole of the eye.

• Known as the corneoretinal potential .

• Modification in the corneoretinal potential in response to brief flash light known as electrortinogram.

• Various techniques are in clinical use to asses the electrical response of retinal cell to light.

• Common of these is the full-field flash ERG.

• Others- pattern ,focal , and multifocal ERG.• It is the composite of electrical activity from

the photoreceptors, Muller cells & RPE.

FULL-FIELD FLASH ERG

• It is the mass response of retina to full-field luminance stimulation

• Reflects the function of photoreceptor and inner nuclear layers of the retina in response to light stimulation.

Technique of ERG recording

• Recording of ERG requires:- 1. Recording, reference & ground electrodes. 2. Ganzfeld bowl stimulator. 3. Signal averager & amplifier. 4. Display monitor & printer.

Patient preparation

• Dilate the pupils.• Contact lens electrodes or thin nylon fibre.• Darkroom .• 30 min for dark adapted and 10 min for light

adapted.• No FFA before test, if done dark adaptation for

1 hour

Application of electrodes

• Main electrodes-• Placed on cornea embedded

in contact lens after topical anaesthesia.

• Bipolar and non bipolar • Non bipolar reference

electrode should be positioned at the ipsilateral outer canthi

• Electrodes that contact the cornea recommended for full field recording.

• Reference electrode• silver chloride electrode • Placed on forehead or near each orbital rim

temporally.

• Ground electrode• Forehead or ear

The stimulus

• Stimulus is an illuminated bowl which projects diffuse light into all part of fundus.

• intensity and frequency of flashes should be variable.

RECORDING AND AMPLIFICATION OF THE RESPONSE

• The elicited response is then recorded from the anterior corneal surface by the contact lens electrode.

• The signal is then channeled through consecutive devices for pre-amplification, amplification & finally display.

Recording protocol

Full pupillary dilatation

30 minutes of dark adaptation

Rod response

Maximum combined response

Oscillatory potentials

10 minutes of light adaptation

single flash cone response

30 Hz flicker

Waveforms

• A WAVE- Negative wave .• Arise from photoreceptors

• B WAVE- Positive wave .• Arise from Muller cells.• Representing activity of bipolar cell layer

• Disturbed by a ripple of three or four small wavelets known as oscillatory potential.

• C WAVE- Prolonged positive wave with lower amplitude.

• Generated from the RPE in response to rod signals only.

• In order to derive clinical information from ERG recording ,it is essential to separate out the cone response from the rod response.

• This can be easily achieved by following techniques:-

CONE ERG

• Photopic ERG refers to testing pt. in bright light • Typical cone response elicited by photopic ERG

shows a lower amplitude and shorter implicit time.

• 5 to 8 million cones are contributing.• Scotopic ERG recorded 20 minutes after dark

adaptation .• Large amplitude and longer implicit time.

• 6 to 8 million cones and 125 million rods contribute.

• Cones are capable of responding to flickering stimuli of up to 50 Hz, after which point individual responses are no longer recordable(critical flicker fusion).

• Rods do not respond to flickering stimulus of more than 10 to 15 Hz.

• Thus by using 30 Hz flicker stimulus, only the cone function can be recorded.

ROD ERG

• Dark adapted • Flash of very dim light or blue light.• Reduced amplitude and longer implicit time.• However, if a bright light stimulus is used in

the dark adapted state both the rods & cones will respond called mesopic ERG.

Time sequences • Latency:- it is the time interval b/w onset of

stimulus & the beginning of the a-wave response. Normally it’s 2 ms.

• Implicit time:- time from the onset of light stimulus until the maximum a-wave or b-wave response.

• Considering only a-wave and b-wave response the duration of ERG is less than 1/4th sec.

ISCEV

• International society for clinical electrophysiology of vision

• Standard protocol for ERG,EOG,VEP• 5 basic responses recommended by ISCEV.

Principle Responses demanded by ISCEV

• Scotopic rod response • Scotopic maximal combined response• Photopic single flash cone response• Photopic 30 Hz flicker response• Oscillatory potentials

Interpretation of ERG

• ERG is abnormal only if more than 30% to 40% of retina is affected

• A clinical correlation is necessary• Media opacities, non-dilated pupils &

nystagmus can cause an abnormal ERG• ERG reaches its adult value after the age of 2

yrs• ERG size is slightly larger in women than men

Abnormal ERG response

• Full field Ganezfeld graded as---• Supernormal response-• Amplitude is > two standard deviation above the

mean both a and b wave• Such response is seen in-• Subtotal circulatory disturbances of retina.• Early siderosis bulbi• Albinism.

• Subnormal response- potential < 2 standard deviations beneath the mean normal

• Indicates that a large area of retina is not functioning .• Such response seen in-• RP• Chloroquine and quinine toxicity.• RD• Vit. A deficiency , hypothyroidism ,

mucopolysaccharidosis, and anaemia.

• Extinguished response- complete absence of response .• Total failure of rod and cone function .• Such response seen in-• Advanced cases of RP• RD• Advanced siderosis bulbi.• Choroideremia• Leber’s congenital amaurosis.• Luetic chorioretinitis.

• Negative response- characterized by a large a- wave .

• Such resopnse seen in-• Gross disturbances of the retinal

circulation(arteriosclerosis) .• Gaint cell arteritis • CRAO• CRVO

CLINICAL APPLICATION OF ERG

• 1-DIAGNOSIS AND PROGNOSIS OF RETINAL DISORDER-

• Retinitis pigmentosa and other inherited retinal dystrophies-

• RP show a marked reduction in amplitude of ERG.There is relationship b/w area offunctioning retina and ERG.

• Leber’s amaurosis- important for primary diagnostic role.

• Under anaesthesia.• Other retinal degeneration- • Rod –cone dystrophy• Choroideremia.• Gyrate atrophy.• Pathological myopia • Other variants of RP.

• DIABETIC RETINOPATHY-• Oscillatory potential of the

ERG is selectively abolished.• RETINAL DETACHMENT-• Immediate reduction in size

of b-wave with loss of vision • Depends on the extent of

detachment• ERG guides surgical

prognosis.

• VASCULAR OCCLUSIONS OF THE RETINA-• In CRAO disappearance of oscillatory potential

with reduction of b-wave.• Less marked changes in CRVO.• Predicts development of neovascularization.• TOXIC AND DEFICIENCY STATES-• Drugs stored in pigment epithelium cause

alteration in response.

• Chloroquine toxicity may be detected in this way.

• Other drugs include quinine , lead , phenothiazines , indomethacin etc.

• 2) TO ASSESS RETINAL FUNCTION WHEN FUNDUS EXAMINATION IS NOT POSSIBLE

• Recorded with dense opacity ,dense cataract and vitreous haemorrhage.

Limitations of ERG

• Since the ERG measures only the mass response of the retina, isolated lesions like a hole, hemorrhage, a small patch of chorioretinitis or localized area of retinal detachment can not be detected by amplitude changes.

• Disorders involving ganglion cells (e.g. Tay sachs’ disease), optic nerve or striate cortex do not produce any ERG abnormality

FOCAL ERG

• Record macular action potential• Not in widespread use .

INDICATIONS

• stargardt disease• Macular dystrophy• RP• ARMD.• Macular hole

MULTIFOCAL ERG-

• Response is recorded to a scaled hexagonal pattern in photopic conditions.

• Some laboratories attempt to record scotopic mfERG also.

• Distinguish diseases of outer retina from ganglion cell or optic nerve.

• Also used in drug toxicity• Interpretation are still not standardized.

Pattern ERG

• Patterned stimulus -checkerboard -grating pattern• Contrast reversing

pattern with no overall change of luminance

• Display vary in size but not beyond 20 degree

• Allows both a measure of central retinal function and retinal ganglion cell function.

• Useful in electrophysiological differentiation b/w ON and macular dysfunction.

RECORDINGS AND MEASUREMENTS

• Recorded without mydriasis .• With refractive correction • With non contact lens electrodes.• Reference electrodes placed at ipsilateral

outer canthi.• Not on forehead or ear.• Binocular stimulation preferred ,except in

squint.

• The transient PERG has 2 main components :P50 at 50msec

• And P95 at 95 msec• P50 reflects macular function • P95 reflect ganglion cell function

Clinical uses

• Assess visual loss.(unknown etiology)• Visual loss due to macular photoreceptors and

inner retinal cells from diseases of ganglion cell and optic nerve

• Also helpful in drug toxicity.

VISUALLY EVOKED POTENTIAL

• VEP is an averaged and amplified record of action potentials in the visual cortex.

• VEP is macula dominated response.

• Only objective technique to assess clinically the functional state of visual system beyond the retinal ganglion cell.

TECHNIQUE OF RECORDING

TYPES OF VEP RECORDING

• 1-FLASH VEP- Recorded by using a calibrated intense diffuse light or stimulus.

• Used in- Corneal opacity • Cataract• Vitreous haemorrhage

• 2- PATTERN VEP- Recorded by some patterned stimulus displayed on TV screen (chess board).

• pattern appearance – black and white checker board presented in on –off sequence.

• Pattern reversal – in this pattern of stimulus changed.

RECORDING PROTOCOL FOR VEP

• FOR PATTERN VER- • Spectacle correction • 1 meter from screen • Other eye is patched • Focus on red dot on screen• It depends on form sense and thus give rough

estimate of visual acuity

• FOR FLASH VER- • Ganzfeld hemisphere bowl.

• Repeated 75 times for two run before the eye being tested.

NORMAL VERSUS ABNORMAL RECORD OF VEP

• PATTERN VER- Shows 3 wave .• 1st positive wave (peaks 70-100

ms).

• 2nd negative wave (100- 130 ms) .

• 3rd larger hyperpolarization (150 -200ms).

• Normally , amplitude is 10 -25 µv.

• < 10µV abnormal.• <3µV abolished.

• FLASH VER- complex compared to pattern VER.

• M – shaped curve 2 positive and 2 negative waves.

FACTORS INFLUENCING VEP

• 1- Stimulus. • 2-Position of electrodes.• 3-Age and sex .• 4- Attention of the pt to stimulus.• 5- Effect of diseases on VEP.

CLINICAL APPLICATION

Optic nerve disease1. Optic neuritis:- • Involved eye shows a

reduced amplitude & delay in transmission i.e. increased latency as compared to normal eye

• These changes occur even when there is no defect in the VA, color vision or field of vision.

• Following resolution, the amplitude of VER waveform may become normal, but the latency is almost always prolonged & is a permanent change.

2. Compressive optic nerve lesions:-• Usually associated with a reduction in the

amplitude of the VER without much changes in the latency

3.During orbital or neurosurgical procedures:-• A continuous record of the optic nerve

function in the form of VER is helpful in preventing inadvertent damage to the nerve during surgical manipulation.

Measurement of VA in infants, Mentally retarded & Aphasic pts

• VER is useful in assessing the integrity of macula & visual pathway.

• Pattern VER gives a rough estimate of VA objectively.

Malingering & hysterical blindness

• Pattern evoked VER amplitude & latency can be altered by voluntary changes in the fixation pattern or accommodation.

• The presence of a repeatable response from an eye in which only light perception is claimed indicates that pattern information is reaching the visual cortex & thus strongly suggests a functional component to the visual loss.

• A characteristic of hysterical response seems to be large variations in the response from the moment to moment.

• The first half of the test may produce an absent VER & 2nd half a normal VER.

Lateralization of defects in the visual pathway

• VER provides a useful information for localizing the defects in visual pathway in difficult cases e.g. children & non-cooperative elderly pts.

• Asymmetry of the amplitudes of VER recorded over each hemisphere imply a hemianopic visual pattern.

• The differentiation of tract lesion from that of optic radiation lesion is difficult.

• Decreased amplitude of VER recorded over the contralateral hemisphere, when each eye is stimulated separately indicates a bitemporal visual deficiency & may localize the site of chiasmal pathology

Unexplained visual loss• useful in general & in pts. with orbital/head

injury

Assessment of visual potential in pts. with opaque media

• like corneal opacities, dense cataract & vitreous hemorrhage.

Amblyopia• flash VER is normal but pattern

VER shows decrease in amplitude with relative sparing of latency

So pattern VER is used in the detection of amblyopia & in monitoring the effect of occlusion on the normal as well as the amblyopic eye, esp. in small children.

Glaucoma • helps in detecting central fields

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