Multifocal ERG and VEP responses and visual fields: comparing disease-related changes
Static visual perimetry and the multifocal technique both measure the local effects of diseases of the retina and optic tract. The purpose here is to relate the measures obtained from each technique and to describe this relationship in some diseases. It is important to measure both the implicit time and amplitude of the multifocal ERG (mERG) or multifocal VEP (mVEP) responses. Some diseases affect one measure of the responses but not the other. The comparison of either measure to local sensitivity changes measured with static perimetry (e.g. the Humphrey 24-2 and 30-2) presents a problem. Different stimulus displays are employed. Further, the multifocal responses are displayed with arbitrary spacing between the responses. One approach is to measure the amplitude and implicit time of the multifocal responses and display these values on the same coordinates as in the visual field plots. This allows a qualitative comparison of fields and multifocal responses on the same scale. A second approach involves modifying the Humphrey perimeter software so that the test spots are placed in the centers of the multifocal stimuli (e.g. the center of each hexagon of the mERG display). A third approach involves estimating the thresholds for the regions of the multifocal display by interpolating from values at the standard Humphrey locations. The second and third approaches produce a one-to-one mapping of the multifocal and field measures and allow a quantitative comparison between the two. The relationship between visual fields and multifocal responses, determined through one or more of these approaches, is different depending upon whether the disease primarily affects the outer retina (retinitis pigmentosa), ganglion cell (glaucoma), or optic nerve (ischemic optic neuropathy and optic neuritis).
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