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Applications of ERG, mfERG & EOG

• Mohammad Reza Akhlaghi MD

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Applications of ERG

• The basic method of recording the electrical response is by stimulating the eye with a bright light source.

• The flash of light elicits a biphasic waveform recordable at the cornea.

• The two components that are most often measured are the a- and b-waves. The a-wave is the first large negative component, followed by the b-wave which is corneal positive and usually larger in amplitude

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• Two principal measures of the ERG waveform are taken:

• 1) The amplitude• 2) the implicit

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introduction

Origin of waveforms in ERG• The a-wave, photoreceptors • The b-wave inner layers of the retina,

including bipolar cells and the Muller cells • Oscillitatory potentials are thought to reflect

activity in amacrine cells

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Standard Type Of Responses

dark-adapted & light-adapted

• Rod response (dark-adapted)• Maximal combined response (dark-adapted)• Oscillatory potentials (dark-adapted)• Single-flash "cone response" (light-adapted)• 30-hertz (-Hz) flicker responses (light-adapted)

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Cautions

• density of cones & rods the fovea and macula, • 90% of cones lie beyond the macula in large

macular lesions ERG b-wave amplitude would be reduced only about 10%.

• ERG does not necessarily correlate with visual acuity, which is a function of the fovea.

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Applications

• The ERG is important for diagnosing and following retinal dystrophies and degenerations.

• Not a direct test of macular function• The ERG is also useful in assessing disorders of

dark adaptation, color vision, and visual acuity, and evaluation of hysteria or malingering.

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ERGs in retinitis pigmentosa-like diseases

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ERGs in retinitis pigmentosa-like diseases

• differential diagnosis of RP : Syphilis, particularly the congenital form, can mimic the fundus appearance of RP. In rubella and early stages of syphilis the ERG is usually normal or only slightly subnormal.

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Congenital stationary night blindness (CSNB)

Often seen with a normal appearing retina• Based on ERG there are two types. • Type 1: abnormal dim scotopic ERGs but

maintains oscillatory potentials. • Type 2: very abnormal dim scotopic ERG and

maximum response has a large a-wave and no b-wave (negative ERG). Oscillatory potentials are also missing.

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Congenital stationary night blindness (CSNB)

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Cone & Cone-Rod dystrophy

• Cone dystrophies : • inherited in all forms, • poor color vision & poor acuity. • Bulls eye appearance or diffuse pigmentation

in the macular area• Nystagmus and photophobia.

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Cone & Cone-Rod dystrophy

• Rod response is good but just slower. However, the early “cone” portion (bx) of the scotopic red flash ERG is missing.

• Maximum response is fairly normal but with slow implicit times.

• 30 Hz flicker and cone response are very poor.

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Cone & Cone-Rod dystrophy

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CRAO & Ophthalmic artery occlusion

• ERG with no b-wave (negative ERG)• Ophthalmic artery occlusions usually result in

unrecordable ERGs.

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CRAO

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X-linked juvenile retinoschisis

• A splitting or schisis in the central retina with a characteristic fundus appearance. They have poor acuity. The ERG has a specific abnormality showing a normal a-wave but no b-wave (negative ERG).

• The picture is similar to that recorded in CRAO and CSNB Type 2

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ERG in IOFB

• The ERG is useful to assess cases of retinal foreign bodies and trauma to estimate the extent of retinal dysfunction.

• In general if b-wave amplitudes are reduced 50% or greater compared to the fellow eye, it is unlikely that the retinal physiology will recover unless the foreign body is removed

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Chloroquine retinopathy

• A number of drugs given in high doses or for long periods of time can produce retinal degeneration with pigmentary changes. Chloroquine retinopathy shows as a characteristic “bulls eye” appearance of the macula. The full-field ERGs may become abnormal in these cases

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Chloroquine retinopathy

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Extinguished ERG

a few disorders result in a completely. They include the following:1) Leber’s congenital amaurosis2) Severe retinitis pigmentosa3) Retinal aplasia4) Total detachment of retina5) Ophthalmic artery occlusion

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The multifocal ERG (mfERG)

• limitation of full-field ERG is a mass response.

• Unless 20% or more of the retina is affected with a diseased state the ERGs are usually normal

• a legally blind person with macular degeneration, enlarged blind spot or other small central scotomas will have a normal full-field ERG

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The multifocal ERG

• The most important development in ERGs is the multifocal ERG (mfERG).

• With this method one can record mfERGs from hundreds of retinal areas in a several minutes.

• Small scotomas in retina can be mapped and degree of retinal dysfunction quantified.

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The multifocal ERG

• A topographic ERG map of the • Multifocal ERG tests cone-generated

responses that subtend 25° radially from fixation.

• In patients with stable and accurate fixation• Objective test for macular dysfunction (for

patients with decreased VA & NL funduce )

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ELECTROOCULOGRAM

• The electrooculogram measures the potential between cornea and Bruch’s membrane .

• Origin of the EOG: RPE , but requires both a normal RPE and normal mid-retinal function.

• Movement of the eye produces a shift of electrical potential.

• By attaching skin electrodes on both sides of an an eye the potential can be measured by having the subject move his or her eyes horizontally

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• The major limitation of the EOG as a clinical tool is that the origin and meaning of this electrical response are not well understood

• The relationship of the EOG to physiologic functions of the RPE is unclear because it does not correlate closely with either pigmentary changes in the RPE or visual function.

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• The most common use of the EOG nowadays is to confirm Best’s disease

• There is considerable variation in the fundus appearance in Best’s disease. In most cases of vitelliform macular dystrophy the EOG is reduced but the ERG normal.

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