Multifocal Electroretinograms

Summary

The development of the multifocal electroretinogram (mfERG) is an important advance in the diagnosis and characterization of retinopathy. Multifocal electroretinograms are a mathematical average of an approximation of a b-wave. Software programs can derive ERGs from more than a hundred retinal areas in a few minutes per eye. Scotomas and retinal dysfunction can be mapped and quantified.

Abstract

A limitation of traditional full-field electroretinograms (ERG) for the diagnosis of retinopathy is lack of sensitivity. Generally, ERG results are normal unless more than approximately 20% of the retina is affected. In practical terms, a patient might be legally blind as a result of macular degeneration or other scotomas and still appear normal, according to traditional full field ERG. An important development in ERGs is the multifocal ERG (mfERG). Erich Sutter adapted the mathematical sequences called binary m-sequences enabling the isolation from a single electrical signal an electroretinogram representing less than each square millimeter of retina in response to a visual stimulus¹.

Results that are generated by mfERG appear similar to those generated by flash ERG. In contrast to flash ERG, which best generates data appropriate for whole-eye disorders. The basic mfERG result is based on the calculated mathematical average of an approximation of the positive deflection component of traditional ERG response, known as the b-wave¹. Multifocal ERG programs measure electrical activity from more than a hundred retinal areas per eye, in a few minutes. The enhanced spatial resolution enables scotomas and retinal dysfunction to be mapped and quantified.

In the protocol below, we describe the recording of mfERGs using a bipolar speculum contact lens.

Components of mfERG systems vary between manufacturers. For the presentation of visible stimulus, some suitable CRT monitors are available but most systems have adopted the use of flat-panel liquid crystal displays (LCD). The visual stimuli depicted here, were produced by a LCD microdisplay subtending 35 - 40 degrees horizontally and 30 - 35 degrees vertically of visual field, and calibrated to produce multifocal flash intensities of 2.7 cd s m^-2. Amplification was 50K. Lower and upper bandpass limits were 10 and 300 Hz. The software packages used were VERIS versions 5 and 6.

Protocol

1. Protocol text

Select a room with little electrical interference. A sink is convenient to wash your hands and to clean electrodes. An internet connection is also preferable to share data and for manufacturer of your system to update software or help solve problems.

1. Organize your dilating drops, balanced salt solution, topical anesthetic and cleaning products so that they are readily accessible.
2. Always place subjects in the same position and use the same room lighting for every subject.
3. After introducing yourself and the procedure, dilate the subject's eyes. Only mydriatic dilation is necessary. ....

Discussion

Traditionally, full-field and single point ERGs, which record from a limited area of the retina, have been used to diagnose and follow the progression of retinal disorders. A limitation of this approach is that these techniques are insensitive unless at least 20% of the retina is affected. Retinal trauma producing field defects produce abnormal mfERGs, consistent with visual field loss ⁵. In this way, multifocal ERG testing has opened new dimensions in diagnosis and progression of retinal disorders.