ERG (Electroretinogram)

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.

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

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