electrophysiology in ophthalmology

Presenter: Dr Vignesh ShenoyModerator : Dr. Susan D’souza

ELECTROPHYSIOLOGIC TESTS

Clinical electrophysiological tests are objective tests which allow assessment of nearly the entire length of visual pathway.

Electrophysiological tests:

Electroretinogram (ERG) Electrooculogram (EOG) Visually Evoked Potentials (VEP)

UsesTo locate the site of pathology in case of unexplained visual

loss

To document the extent of the pathology

To detect drug toxicity

To document the amount of ischaemic damage in case of vascular events

ELECRORETINOGRAM

ELECRORETINOGRAM (ERG)

ERG is an electric potential generated by retina in response to brief stimulus of light.

The ‘amplitude of ERG’ (amount of electric potential generated) is directly proportional to area of functioning retina stimulated.

BASIC PRINCIPLE OF ERGSudden illumination of retina.

Simultaneous activation of all the retinal cells to generate the current.

Currents generated by all the retinal cells mix, then pass through vitreous & extra cellular spaces.

High RPE resistance prevents summated current from passing posteriorly.

The small portion of the summated current which escapes through the cornea is recorded as ERG.

ERG WAVEFORMS

‘a wave’ : It’s a ‘negative’ (downward) wave & reflects photoreceptor function.

‘b wave’ : It is a ‘positive’ (upward)

wave & reflects bipolar cell activity.

‘Oscillatory potentials ‘: Small rippling currents produced by inner plexiform layer.

ERG RESPONSES

ERG has 4 distinct responses depending in stimulus strength:

Rod response(scotopic) stimulus strength less than standard flash stimulus

Maximal combined responsebright standard flash stimulus

Single flash cone response(photopic) Standard flash stimulus repeated at intervals of >0.5 sec

30 Hz flicker response Standard flash stimulus repeated at intervals of <0.5 sec

Dark adapated

Light adapted

ROD RESPONSE (Photopic ERG) :

Produced by dark adapting patient for 20 min. & then stimulating retina with dim light flash which is below cone threshold.

The resultant waveform has ‘prominent b (positive) wave ‘& no detectable ‘a (negative) wave’.

MAXIMAL COMBINED RESPONSE :

It is a larger waveform generated by using bright flash in dark adapted state which maximally stimulates both rods & cones.

It results in prominent ‘a (negative) wave & ‘b (positive) wave’ with ‘oscillatory potentials’ which are superimposed on ‘b wave’.

CONE RESPONSES(scotopic) :

‘Single flash response’ is obtained by maintaining the patient in light adapted state & stimulating the retina with bright white flash.

The rods are suppressed by light adaptation & do not contribute to the waveform.

With patient in light adapted state, a flickering stimulus at 30 Hz can also be used to filter rod response & measure cone response (30 Hz flicker response)

RECORDING OF ERG:

Active electrode

It’s the main electrode.

Recording electrodes are of various types

Hard contact lenses that covers sclera such as Burian-Allen electrode, Doran gold contact lens, Jet electrode(disposable)

Filament type electrode placed on lower lid include Gold foil electrode, DTL Fiber electrode and HK-Loop electrode

Reference electrodeThe silver chloride electrode.Placed on the patient’s forehead, it serves as the negative pole as it is placed closer to the electrically negative posterior pole of the eye.

Ground electrodeIt’s placed on the earlobe.

ELECTRODES USED IN ERG

Jet Electrode Gold Plated Electrode Skin Electrode

DTL Electrode HK Loops Burian Allen Electrode

Stimulus

The Ganzfeld bowl is large white bowl which is used to stimulate the retina during the recording of the ERG.It diffuses the light & allows equal stimulation of all parts of retina.

Recording & amplification

The elicited response is then recorded from the anterior corneal surface by the contact lens electrodeThe signal is then channelled through consecutive devices for pre-amplification, amplification & finally display.

SPECIALISED FORMS OF ERGBRIGHT FLASH ERG:Used for assessment of retinal function in ‘severely traumatized eye‘ or ‘eye with dense media opacity’ like dense VH, corneal opacity or advanced cataract.The flash used is about 10.000 times brighter than that used in standard ERG.In this procedure successive responses are obtained with flashes of increasing intensity, allowing the time for re-adaptation in between flashes.A non recordable flash ERG is an ominous sign for visual prognosis.

FOCAL ERG (fERG):

Used for detecting small focal lesions or pathologies which are missed by standard full field ERG.

A small stimulus of 4o size is projected on area of retina to be tested.

Due to light scattering & poor signal to noise ratio, this technique is mostly used in research setting than in clinical setting.

Clinical uses of fERG :

Early detection of cone dystrophy or macular disease before the fundus changes are evident.

Can differentiate between early macular & optic nerve pathology.

Can be used for evaluation of any type focal macular pathology.

MULTIFOCAL ERG (mfERG):

The stimuli consists densely arranged black or white hexagonal elements displayed on CRT monitor.

These hexagonal elements change from light to dark independently & this change results into recording of mfERG.

Based on retinal activity, the recorded mfERG appears in ‘topographic map form’ & also in ‘small ERG waveforms’ from various parts of retina.

PATTERN ERG (pERG) :

It mainly represents inner retinal activity (especially ganglion cell activity)

Useful in differentiating optic nerve disorders from macular disorders.

Unlike flash ERG, pattern ERG is a very small response.

Recorded with full correction of refractive errors as visualization of stimulus for extended time is essential for recording.

ERG IN CLINICAL CASES

DIABETIC RETINOPATHY :

In DR there is reduction in amplitude & delay of peak implicit times.

These changes are directly proportional to severity of retinopathy.

Amplitude of oscillatory potentials (OP) is a good predictor of progression of retinopathy from NPDR to PDR.

Abnormal amplitude of OP indicate high risk of developing PDR.

RETINAL DETACHMENT (RD) & CENTRAL SEROUS RETINOPATHY (CSR) :

In RD & CSR there is significant reduction in ERG amplitude.

However there is no significant change seen in waveforms of ERG.

RETINOSCHISIS :

ERG in retinoschisis is typically characterized by marked decrease amplitude or absence of b wave.

RETINITIS PIGMENTOSA :

A full field ERG in RP shows marked reduction in both rod & cone signals although loss of rod signals is predominant.

There is significant reduction in amplitude of both a & b waves of ERG.

CRAO :In vascular occlusions like CRAO, ERG typically shows shows absent b wave.Ophthalmic artery occlusions usually results in unrecordable ERG.

CONE DYSTROPHY :

ERG in cone dystrophy shows good rod b-waves that are just slower.

The early cone response of the scotopic red flash ERG is missing.

The scotopic bright white ERG is fairly normal in appearance but with slow implicit times.

The 30 Hz flicker & photopic white ERGs which are dependent upon cones are very poor.

RETAINED IOFB :

A retained metallic FB like iron & copper shows changes in ERG early as well as late stages.

A characteristic change is b-wave amplitude is reduced by 50% or more as compared with normal eye.

No intervention finally results into an unrecordable ERG (Zero ERG)

ELECTROOCULOGRAM

It is recording of standing potential of the eye

The electrodes are placed at inner & outer canthus of the eye with reference electrode placed on forehead.

The patient is asked to look back & forth between a pair of fixation lights separated by 30o of visual angles on Ganzfeld globe.

Like ERG, EOG reflects activity of entire retina & used to evaluate combined photoreceptor-RPE activity.

As validity of results depends upon consistent tracking of fixation target over 30 min., this test is not suitable in unco-operative patients & children.

Also EOG depends upon a minimum degree of light adaptation so it is not reliable in patients with dense cataracts.

CORNEOFUNDAL POTENTIAL :

It is the source of voltage obtained in EOG & it renders the cornea positive by 0.006 to 0.010 V as compared with the back of the eye.

The corneofundal potential results from metabolic activity of RPE (mainly) as well as corneal & lens epithelium.

Contributions of corneal & lens epithelium are not photosensitive but that of RPE is, which is substantialy increased during light adaptation & decreased during dark adaptation.

For EOG to be normal, it requires as little as 20-25 % of normal functioning retina.

Thus abnormal EOG indicates a dense pathology involving entire retina.

ARDEN’S RATIO :

It is the ratio of ‘largest EOG amplitude during light adaptation’ (light peak) to ‘least amplitude during dark adaptation’ (dark trough).

Clinically normal value of this ratio is 1.85 or higher.

Values below 1.85 are considered subnormal & those below 1.30 are considered severely subnormal or extinguished.

EOG IN CLINICAL CASES

BEST’S DISEASE :

Abnormal EOG with normal ERG is a hallmark.

Other examples of ERG to EOG dissociation are :

Diffuse fundus flavimaculatous Pattern dystrophy of RPE eg. Butterfly Macular Dystrophy. Chloroquine retinopathy Metallosis bulbi

VISUALLY EVOKED POTENTIALS (VEP)

VISUALLY EVOKED POTENTIALS (VEP)

Also called as ‘visually evoked response (VER)’ or ‘cortical potentials’.

It is the electrical response of the brain to sudden appearance / disappearance / change of visual stimulus.

Like EEG, VEP is detected by placing surface electrodes at scalp which can be placed anywhere, but should always include posterior occipital area.

VEP ELECTRODESThe occipital electrode (Inion) lies near visual area thus called as reference electrode.

The vertex electrode is placed over non visual area which detects minimum activity in response to visual stimulation is called as active electrode.

The 3rd electrode is placed over forehead is called ground electrode.

The stimulus shown is a flash of light (diffuse light spot, annulus ) or patterned stimulus (illuminated checkerboard)

The stimuli are repetitively presented at random within a short period of time. Eg. 1 cycle/second for 100 seconds.

The standard flash VEP is characterized by positive wave (P1 or P100) which most commonly studied clinically & 2 negative waves (N1 or N75 & N2 or N135).

Amplitude of these waves depends upon size of the check, contrast & alteration frequencies of stimulus.

VEP Terminologies

Amplitude of VEP : Height of the potential of P100 wave. Predominantly affected in ischemic disorders.

Latency of VEP : Time from stimulus onset to peak of the response. Predominantly affected in demyelinating disorders.

APPLICATIONS OF VEP

Recording visual acuity in nonverbal patients. Macular function test. Screening and early diagnosis of Multiple Sclerosis. To identify optic nerve diseases, visual pathway abnormalities. Amblyopia : latency relatively spared, so VEP can be used to

monitor response to occlusion therapy. Detection of a malingerer. To detect color blindness : Using chromatic patterned light stimuli.

VEP IN CLINICAL CASES

TOXIC & COMPRESSIVE OPTIC NEUROPATHY :

Following 2 changes are seen : Decreased amplitude of P100 wave. Increase in latency period.

Decreased amplitude of P100 is more predominant than increased latency period.

MULTIPLE SCLEROSIS :

Abnormalities in VEP are bilateral & seen 90 % of cases irrespective of visual symptoms.

In MS, increase in latency period is more predominant than decrease in P100 amplitude..

OPTIC NEURITIS :

In optic neuritis, VEP shows increased latency period &/or decreased amplitude as compared to normal eye.

These findings develop even before occurrence of visual symptoms & color defects.

In recovery stage, amplitude may return to normal but latency period continues to be decreased.

VEP IN OPTIC NEURITIS

NORMAL OPTIC NEURITIS

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