Applications of ERG mf ERG EOG Mohammad Reza

Applications of ERG, mf. ERG & EOG • Mohammad Reza Akhlaghi MD 2

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 3

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• Two principal measures of the ERG waveform are taken: • 1) The amplitude • 2) the implicit 5

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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 7

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) 8

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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. 10

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. 11

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ERGs in retinitis pigmentosa-like diseases 13

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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. 16

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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. 18

Congenital stationary night blindness (CSNB) 19

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. • • 20

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. 21

Cone & Cone-Rod dystrophy 22

CRAO & Ophthalmic artery occlusion • ERG with no b-wave (negative ERG) • Ophthalmic artery occlusions usually result in unrecordable ERGs. 23

CRAO 24

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 25

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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 27

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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 29

Chloroquine retinopathy 30

Extinguished ERG a few disorders result in a completely. They include the following: 1) Leber’s congenital amaurosis 2) Severe retinitis pigmentosa 3) Retinal aplasia 4) Total detachment of retina 5) Ophthalmic artery occlusion 31

The multifocal ERG (mf. ERG) • 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 32

The multifocal ERG • The most important development in ERGs is the multifocal ERG (mf. ERG). • With this method one can record mf. ERGs from hundreds of retinal areas in a several minutes. • Small scotomas in retina can be mapped and degree of retinal dysfunction quantified. 33

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 ) 34

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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 38

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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. 41

• 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. 42

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