SAP testing may enhance glaucoma monitoring

[caption id="attachment_6893" align="alignnone" width="350"] Roger S Anderson PhD[/caption]   Individual modulation of the size, duration and luminance of stimuli used in standard automated perimetry (SAP) may enhance the monitoring of glaucomatous visual loss at different stages of the disease, according to Roger S Anderson PhD, DSc, FCOptom, University of Ulster, Coleraine, Northern Ireland. “In the future, instead of modulating the stimulus just in terms of luminance and size, we might modulate in terms of luminance, size and duration and in so doing increase the glaucoma signal, increase the dynamic range and possibly reduce the variability at the same time,” Prof Anderson told the 12th European Glaucoma Society Congress in Prague, Czech Republic. He noted that testing patients with SAP typically involves the use of a stimulus with a fixed size and duration and varying luminance. The stimulus targets, called the Goldmann sizes, are derived from kinetic perimetry. However, the stimulus duration and area of conventional SAP may be inadequate for identifying subtler types of visual field loss. “SAP has low sensitivity in early glaucoma, high variability in moderate glaucoma and insufficient dynamic range to monitor advanced visual field loss in advanced glaucoma,” he added. SPATIAL SUMMATION When presented with a Goldmann III stimulus, an eye with glaucoma will have one log unit change in the retinal ganglion cell number in the peripheral retina for every one log unit change in luminance threshold in the peripheral visual field. There will also be a three-to-four log unit of change in the retinal ganglion cell number in the central retina for every one log unit change in luminance threshold in the central visual field, Prof Anderson explained. He noted that, for a set of small stimulus areas there will be an inverse relationship between luminance and stimulus size at the threshold of visibility. That is, one log unit of change in area of the stimulus will have the same effect on its visibility as one log unit of change in luminance. The largest stimulus areas for which this holds true is called the area of complete spatial summation, or Ricco’s area. Prof Anderson noted that research he and his associates have conducted has shown that there is an increase in the Ricco’s area in eyes with early glaucoma. “We explained this by the visual system changing the area of complete spatial summation in response to glaucomatous damage, in order to maintain the number of retinal ganglion cells in that perceptive field. You start with a healthy eye, you lose ganglion cells and the visual system enlarges the perceptive field in order to maintain a constant number of ganglion cells in an attempt to preserve the signal-to-noise ratio,” he said. He added that when they used a stimulus in the central retina that is larger than the Ricco’s area like a Goldman III stimulus, there was only be a small glaucoma signal, whereas when using a smaller area stimulus, within Ricco’s area, the glaucoma signal will be much more pronounced. TEMPORAL SUMMATION Glaucomatous damage also appears to affect the temporal processing of visual stimuli, Prof Anderson noted. Similarly to spatial summation, when stimulus duration is plotted against luminance threshold there is a set of durations where duration and luminance have an inverse relationship, the limit of which is called the critical duration. He added that, in studies he and his associates have conducted, they found that when they used a Goldman III stimulus there was a small-to-moderate but significant increase in the critical duration or change in temporal summation in eyes with early glaucoma, and that the critical duration of the stimulus increased in tandem with total deviation values. Another finding was that when using a 200ms stimulus, as is typically used in SAP, there was little difference in luminance thresholds between normal and glaucomatous eyes. Roger S Anderson: rs.anderson@ulster.ac.uk

Tags: glaucoma