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Visual electrophysiology: ERG, EOG and VEP

  • Examination

These tests record the electrical signals of the retina and visual pathway. They show where vision is lost when the eye looks normal.

Updated

See it in 3D: Retina · Macula · Optic nerve

What it is

The full-field electroretinogram (ERG) records the response of the whole retina to flashes, separating rod and cone function after dark and light adaptation. The pattern ERG reflects central retinal ganglion cells and the multifocal ERG maps cone function across the macula. The electro-oculogram (EOG) measures the standing potential of the retinal pigment epithelium. The visual evoked potential (VEP) records the response of the occipital cortex and tests the whole pathway, especially the optic nerve.

Normal values and scales

  • ERG a-wave = photoreceptors; b-wave = inner retina (bipolar and Müller cells).
  • Retinitis pigmentosa: reduced or absent rod responses, often before the fundus changes are obvious.
  • Electronegative ERG (b-wave smaller than the a-wave): X-linked retinoschisis, congenital stationary night blindness and some paraneoplastic retinopathies.
  • Best disease: abnormal EOG (low light-peak to dark-trough ratio) with a normal full-field ERG.
  • VEP latency is delayed in optic neuritis and demyelination, and often stays delayed after vision recovers.
  • Normal ERG and VEP in a patient reporting blindness support a non-organic cause.

How it is done

  1. Dilate the pupils for the full-field ERG (not for pattern tests) and dark-adapt for about twenty minutes before rod responses.
  2. Place a corneal electrode (thread or contact lens) or a skin electrode under the lower lid, plus reference and ground electrodes on the skin.
  3. Full-field ERG: dim and bright flashes in the dark, then flashes and flicker after light adaptation.
  4. EOG: skin electrodes beside each eye; the patient looks back and forth between two lights during dark and then light adaptation.
  5. VEP: an electrode over the occiput; each eye in turn, with correct glasses, watches a reversing checkerboard (a flash in infants).
  6. Compare the results with age-matched normal values recorded to international standards.

How to interpret it

  • Useful when vision is poor but the fundus and OCT look normal: early cone dystrophy, Stargardt disease, drug toxicity or optic nerve disease.
  • In infants with nystagmus or poor fixation, a flat ERG points to Leber congenital amaurosis; an asymmetric VEP across the hemispheres suggests albinism.
  • Multifocal ERG helps confirm hydroxychloroquine retinal toxicity when fields and OCT are borderline.

Next step / Initial management

  • Unexplained loss of vision with a normal exam and imaging: ophthalmology for electrophysiology before calling it non-organic.
  • Night blindness or a family history of retinal dystrophy: ophthalmology for ERG and genetic counselling.
  • Infant with nystagmus or not fixing and following: urgent paediatric ophthalmology, often with ERG and VEP.
  • Rapid loss of vision in a patient with cancer and a normal-looking fundus: consider paraneoplastic retinopathy and refer for ERG.
  • Suspected optic neuritis: VEP can support the diagnosis, but MRI and neurology decide the work-up for demyelination.

Test yourself

Think of the answer, then tap the question to see it.

Which cells produce the a-wave and the b-wave of the ERG?

The a-wave comes from the photoreceptors; the b-wave from the inner retina (bipolar and Müller cells).

Which disease typically has an abnormal EOG with a normal full-field ERG?

Best vitelliform macular dystrophy.

What VEP finding is typical after optic neuritis?

Delayed latency of the main positive wave, often persisting after recovery.

An infant has nystagmus and a flat ERG. Which diagnosis is suggested?

Leber congenital amaurosis.

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Educational content; not a clinical guideline and no substitute for assessment by a health professional. For sudden visual loss, severe eye pain, a chemical burn or an eye injury, go to an emergency department at once.

Sources: Full-Field Electroretinogram (StatPearls Publishing (NCBI Bookshelf), 2023); Multifocal Electroretinogram (StatPearls Publishing (NCBI Bookshelf), 2022); Pattern Electroretinogram (StatPearls Publishing (NCBI Bookshelf), 2023); Visual Evoked Potential (StatPearls Publishing (NCBI Bookshelf), 2023); Electrooculogram (EyeWiki (American Academy of Ophthalmology), 2025).

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