How do you interpret an electroretinogram (ERG) result?

Interpreting Electroretinogram (ERG) Results

Full-field ERG is NOT recommended for screening or detecting early retinal toxicity, as it only shows abnormalities in very late disease stages; instead, multifocal ERG (mfERG) should be used when objective functional confirmation is needed, as it can detect localized parafoveal and extramacular dysfunction in early retinopathy. 1

Understanding ERG Types and Their Clinical Applications

Full-Field ERG: Limited Screening Value

• Full-field ERG is a global test of retinal function that records massed electrical potentials from the entire retina, consisting of an a-wave (originating from photoreceptors) and b-wave (originating from mid-retinal cells). 2, 3

• This test will only show abnormalities in very late chloroquine or hydroxychloroquine toxicity and is explicitly NOT recommended for screening purposes. 1

• Full-field ERG may be useful to judge the extent of damage beyond the macula once advanced toxicity is already established, but it cannot detect early changes that would allow intervention to prevent vision loss. 1

Multifocal ERG (mfERG): The Preferred Functional Test

When objective functional testing is needed, mfERG is the appropriate choice because it generates local electroretinogram responses topographically across the posterior pole. 1

Key Interpretation Principles for mfERG:

• Look for parafoveal or extramacular electroretinogram depression as evidence of early retinopathy, which can objectively confirm suspected visual field loss. 1

• Sensitivity is enhanced by comparing amplitudes between rings of responses about the center rather than evaluating absolute values alone. 1

• The mfERG has similar sensitivity to automated visual fields and serves as objective confirmation when subjective field testing shows questionable changes. 1

• The most frequent regions showing early damage are inferotemporal, corresponding to superonasal field defects, though this pattern is not absolute. 1

Clinical Context for ERG Interpretation

When to Order mfERG:

• Use mfERG when uncertain visual field changes need objective confirmation before diagnosing toxicity, as at least one objective test should confirm subjective findings unless toxic changes are advanced and obvious. 1

• Consider mfERG when internal controls are absent on other tests or when preanalytic issues are suspected. 1

• Order mfERG as part of the evaluation when SD-OCT shows very early or tenuous changes that require confirmation. 1

Important Caveats:

• mfERG requires proper well-calibrated equipment and experienced personnel to perform and interpret correctly, and is typically available only in large clinical centers and some specialty offices. 1

• The test measures the electrical field response, providing an objective functional assessment that complements structural imaging (SD-OCT) and subjective testing (visual fields). 1

Practical Interpretation Algorithm

For suspected retinal toxicity screening:

1. Primary screening should use automated visual fields and SD-OCT, not ERG testing. 1

2. If visual fields show suspicious but not definitive changes, order mfERG for objective confirmation rather than full-field ERG. 1

3. Interpret mfERG by looking for localized amplitude depression in characteristic patterns (parafoveal in non-Asian patients, extramacular in Asian patients). 1

4. Compare ring-to-ring amplitude ratios rather than relying solely on absolute amplitude values. 1

5. Correlate mfERG findings with SD-OCT and visual field results before making a definitive diagnosis of toxicity. 1