ERG, EOG and VEP Made Easy: One Test per Level
Last updated: October 2026
Electrophysiology reports look like a page of squiggles and numbers. The trick is to remember that each test listens to one level of the visual pathway. Ask which level is abnormal and the diagnosis usually follows.
Four rules
1. One test, one levelEOG = RPE. Full-field ERG = the whole retina (rods and cones). Pattern ERG = macula (P50) and ganglion cells (N95). Multifocal ERG = a map of central cone function. VEP = the whole path to the visual cortex, dominated by the macula.
2. a-wave = photoreceptors, b-wave = the next cellThe negative a-wave comes from photoreceptors; the positive b-wave mainly from ON-bipolar and Müller cells. If the a-wave is normal but the b-wave small (negative ERG), the problem is after the photoreceptors.
3. Separate rods and cones with light and speedIn the dark with a dim flash only rods respond. Background light saturates rods, so light-adapted responses are cones. Rods cannot follow a 30 Hz flicker, so flicker is pure cone.
4. The full-field ERG misses small lesionsA normal full-field ERG does not rule out macular disease: the macula is only a small part of the retina. Use pattern or multifocal ERG for the centre.
The ISCEV full-field ERG, step by step
| Step | Condition | What responds | What it tells you |
|---|---|---|---|
| DA 0.01 | Dark-adapted, dim flash | Rods only (b-wave, no a-wave) | Rod pathway |
| DA 3.0 (and DA 10) | Dark-adapted, bright flash | Rods + cones | a-wave = photoreceptors; b-wave = inner retina. Look at b/a ratio |
| DA oscillatory potentials | Filtered from the bright flash | Amacrine cells (inner retina) | Reduced in diabetic retinopathy, ischaemia |
| LA 3.0 | Light-adapted, flash | Cones | Cone system |
| LA 30 Hz flicker | Light-adapted, flicker | Cones only | Most sensitive cone test; delayed in early disease |
Causes of a negative ERG
- X-linked retinoschisis (with foveal schisis in a boy)
- Congenital stationary night blindness: complete (ON-bipolar defect, e.g. NYX) and incomplete (CACNA1F)
- Central retinal artery occlusion or ischaemic CRVO: inner retina dies, photoreceptors (choroid-fed) survive
- Melanoma-associated retinopathy (antibodies against ON-bipolar cells)
- Quinine toxicity, juvenile Batten disease, some birdshot chorioretinopathy
Memory hook: Negative ERG: “Schisis, Night blindness, Artery, Melanoma, Quinine” — SNAM-Q.
EOG: the RPE test
- The resting potential falls in the dark (dark trough) and rises in the light (light peak). The Arden ratio is light peak ÷ dark trough.
- Normal is roughly 1.8 or more (labs differ); below about 1.5 is clearly abnormal.
- Best vitelliform macular dystrophy (BEST1): abnormal EOG with a normal ERG, even in carriers with a normal fundus. This is the classic exam pairing.
- In widespread photoreceptor disease (RP) the EOG is also low, but then the ERG is abnormal too, so the EOG adds little.
Pattern ERG and VEP: macula or nerve?
| Finding | Meaning | Example |
|---|---|---|
| PERG P50 reduced | Macular dysfunction | Stargardt, cone dystrophy, macular oedema |
| PERG P50 normal, N95 reduced | Ganglion cell or optic nerve | Glaucoma, optic atrophy, LHON |
| VEP P100 delayed, amplitude near normal | Demyelination | Optic neuritis, MS (stays delayed after vision recovers) |
| VEP amplitude reduced, latency near normal | Axonal loss, compression, ischaemia | NAION, compressive neuropathy |
| Abnormal VEP + abnormal PERG P50 | Macula may be the cause, not the nerve | Check OCT and mfERG before blaming the nerve |
| Normal pattern VEP to small checks | Good acuity is likely | Suspected functional (non-organic) visual loss |
Common patterns in one table
| Condition | Full-field ERG | Other tests |
|---|---|---|
| Retinitis pigmentosa | Rods worse than cones; delayed flicker; extinguished late | EOG low; mfERG keeps a central island |
| Cone / cone–rod dystrophy | Light-adapted and flicker reduced (rods later) | PERG and mfERG abnormal |
| Achromatopsia | Cone responses absent; rods normal | Photophobia, nystagmus from infancy |
| Leber congenital amaurosis | Extinguished in infancy | Nystagmus, poor fixation, oculodigital sign |
| Stargardt (ABCA4) | Group 1 normal; group 2 cones down; group 3 rods and cones down | PERG P50 reduced in all groups |
| Best disease | Normal | EOG Arden ratio low |
| X-linked retinoschisis | Negative ERG | Foveal schisis on OCT |
| CSNB | Negative ERG (most types) | Night blindness, myopia, nystagmus |
| Hydroxychloroquine toxicity | Normal until late | mfERG: paracentral ring reduction |
| Cancer-associated retinopathy | Rods and cones reduced | Antirecoverin antibodies; MAR gives a negative ERG instead |
| Vigabatrin | Cone flicker reduced | Field constriction; monitor |
| Siderosis | Progressively reduced b-wave | Retained iron foreign body |
Where residents slip
- Accepting a normal full-field ERG as 'retina normal': the macula can still be diseased.
- Forgetting the EOG in a child with a vitelliform lesion: Best disease has a low Arden ratio with a normal ERG.
- Calling a negative ERG 'severe retinal degeneration': the photoreceptors are working; look for schisis, CSNB, CRAO or MAR.
- Blaming the optic nerve for an abnormal VEP without checking the macula (PERG P50, OCT).
- Using a pattern VEP in a patient who cannot fix: use a flash VEP for babies and very poor vision.
Test yourself: flip cards
Quick quiz
Related: Electrophysiology reference, Retina hub, Retinal layers and OCT made easy, Visual pathway made easy.
Key sources
- Robson AG, Frishman LJ, Grigg J, et al. ISCEV Standard for full-field clinical electroretinography (2022 update). Doc Ophthalmol. 2022;144(3):165-177
- Constable PA, Bach M, Frishman LJ, et al. ISCEV Standard for clinical electro-oculography (2017 update). Doc Ophthalmol. 2017;134(1):1-9
- Odom JV, Bach M, Brigell M, et al. ISCEV standard for clinical visual evoked potentials (2016 update). Doc Ophthalmol. 2016;133(1):1-9
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