An electroretinogram (ERG) measures the electrical signals produced by the retina in response to light flashes. It evaluates the function of photoreceptors (rods and cones) and other retinal cells.

Key takeaways

  • Measures retinal function in response to light
  • Evaluates rods and cones (photoreceptors)
  • Helpful for unexplained vision loss
  • Diagnoses retinal dystrophies
  • May detect cancer-associated retinopathy

Questions about your diagnosis? Contact our office

Visual guide: ERG: record the retina responding to light
Educational schematic showing retinal light stimulation, electrical recording, and specialist interpretation of an ERG.
Read this visual as text
Light stimulates the retina: Flashes or patterns stimulate retinal cells, including rods and cones. Electrodes record responses: Eye or skin electrodes record signals during adapted-light conditions. Specialist interpretation: The recording helps assess retinal function; it does not give a diagnosis alone.
Educational schematic: erg: record the retina responding to light. Symbols summarize the article; they are not clinical images.

Why It's Done

  • Diagnose inherited retinal dystrophies (retinitis pigmentosa)
  • Evaluate unexplained vision or night vision loss
  • Detect cancer-associated retinopathy
  • Assess retinal toxicity from medications
  • Differentiate retinal from optic nerve disease

What to Expect

Full-Field ERG

Before:

  • Sit in dark room for 20-30 minutes (dark adaptation)
  • Dilating drops may be used

During:

  • Contact lens electrode or skin electrodes placed
  • Light flashes presented
  • Response recorded
  • Both dark-adapted and light-adapted testing
  • Takes 45-60 minutes

Multifocal ERG (mfERG)

  • Tests small areas of central retina
  • Hexagonal pattern of stimuli
  • Takes 30-45 minutes
  • More detailed central retinal map

Getting Home After Testing

If dilating drops are used, bring sunglasses and arrange a ride if your vision is blurred or light-sensitive. Wait until vision is clear and comfortable before driving. Ask the testing team when to expect the specialist's interpretation; the recording alone does not give a diagnosis.

What It Shows

Normal ERG

  • Good responses from rods (dim light) and cones (bright light)
  • Normal waveform shape and timing

Abnormal Findings

  • Reduced responses-retinal dysfunction
  • Absent rod responses-rod-cone dystrophy
  • Negative ERG-specific pattern suggests certain conditions

ERG vs VEP

ERG VEP
Tests retina Tests visual pathway to brain
Flash stimulus Pattern stimulus
Eye electrodes Scalp electrodes
Retinal disease Optic nerve/brain disease

References

  1. American Academy of Ophthalmology. Electroretinogram.
  2. National Eye Institute. Retinitis Pigmentosa.