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Natasha V. Nayak, George Hu, Jun Lin, James C. Tsai; Utility of the Isolated-Check Visual Evoked Potential Technique as a Glaucoma Monitoring Device. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5480.
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Structural damage to retinal ganglion cells (RGCs) precedes visual dysfunction in glaucoma, as measured by perimetry. The isolated-check visual evoked potential (icVEP) device can identify early glaucoma by assessing neural activity in the magnocellular pathway. It has not been evaluated whether the icVEP can detect RGCs functional changes due to intraocular pressure (IOP) lowering treatment. Our current study focuses on further accessing the clinical utility of the icVEP in the monitoring of glaucoma patients.
In order to increase the sensitivity of the icVEP for the purpose of glaucoma monitoring, we modified the bright contrast parameters (± 10% or 15% vs. +16%, 32%, or 100%) and number of 2-second test runs (8 vs. up to 16). Multivariate statistics were used to calculate signal-to-noise ratios (SNR) and signal-to-deviation ratios (SDR) to characterize magnocellular pathway activity. We recruited 2 groups of patients: (1) non-glaucoma subjects as control, and (2) diagnosed glaucoma patients. All icVEP data were obtained under the modified bright contrast condition.
Modified icVEP measurements were performed in non-glaucoma (control, n=4; 42-43 year-old; mean IOP=15 mmHg) and glaucoma (n=6; from 49-73 year-old; mean IOP = 19 mmHg) subjects. All controls had SNR values > 1, results consistent with previous data using the 15% bright contrast settings. In both the controls and glaucoma subjects, mean SNR and SDR values increased with increasing isolated-check contrast levels. At all three bright contrast levels, control subjects showed higher mean SNR (at 16%: normal: glaucoma = 1.37 : 1.07; at 32%: = 2.15 : 1.91, and at 100%, = 2.53 : 2.48) and higher mean SDR (at 16%: =1.21 : 0.84; at 32%: = 2.03 : 1.75, and at 100%: = 2.39 : 2.34) than glaucoma subjects. Thus, the values obtained at each contrast level were not significantly different. Calculating the proportion of the SNR and SDR of low vs. high contrast stimuli may be an additional method of assessing glaucoma progression since increasing contrast decreases the specificity by which the magnocellular pathway is stimulated.
Our modified protocol may expand the utility of icVEP as a glaucoma monitoring device. Further studies will hopefully provide greater understanding of the relationship between icVEP measurements and changes in IOP.
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