The multifocal ERG (mfERG) can produce a topographic ERG map of central retinal cone-system function, which can help the clinician diagnose macular dysfunction and assess the extent of central retinal involvement in generalized retinal disease (Fig 3-3). The stimulus consists of multiple hexagons, smaller in the center than the periphery to reflect cone photoreceptor density, each of which flashes with a pseudorandom sequence. Cross-correlation techniques are used to calculate the small ERGs corresponding to each hexagon. Overall stimulus field size is usually approximately 50°. For patients with stable and accurate fixation (essential for obtaining technically satisfactory and clinically meaningful results), mfERG can objectively determine the spatial distribution of macular dysfunction. The mfERG is less sensitive than the pattern ERG for disorders such as cystoid macular edema, in which primary photoreceptor dysfunction is not the main pathophysiological feature; mfERG and PERG can provide complementary information. Clinicians have increasingly used mfERG in the diagnosis of hydroxychloroquine toxicity (Fig 3-4). There may be relative sparing of the response to the central foveal hexagon with involvement of the responses to the ring of surrounding hexagons; a ring analysis may be beneficial. However, it should be noted that recently it has been demonstrated that Asian patients may show an extra-macular pattern of damage, which would not be detected by mfERG.
Marmor MF, Kellner U, Lai TY, Melles RB, Mieler WF; American Academy of Ophthalmology. Recommendations on screening for chloroquine and hydroxychloroquine retinopathy (2016 revision). Ophthalmology. 2016;123(6):1386–1394.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.