Applications of ERG, mfERG & EOG Mohammad Reza Akhlaghi MD

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

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

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

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)

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.

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.

ERGs in retinitis pigmentosa-like diseases

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.

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.

Congenital stationary night blindness (CSNB)

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.

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.

Cone & Cone-Rod dystrophy

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

CRAO

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

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

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

Chloroquine retinopathy

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

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

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.

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 )

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

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.

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.