Link between pupil diameter, VEP may aid early glaucoma diagnosis
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Researchers discovered a significant relationship between pupil diameters and visual evoked potential among patients with early stages of glaucomatous optic neuropathy, providing a potential method for early detection.
“In the present study, visual evoked potentials [VEPs] were measured in patients with early-stage glaucomatous optic neuropathy, and the results were compared with pupil size measurements made under photopic conditions,” Mehmed Ugur Isik, MD, from Kastamonu University in Turkey, and colleagues wrote. “We assessed whether pupillometry can be used to gauge glaucomatous damage and used for glaucoma screening.”
Isik and colleagues analyzed and compared the right eyes of 24 patients with newly diagnosed open-angle glaucoma and 30 controls.
Photopic pupil diameters at three luminance levels (60, 100 and 130 cd/m2) were significantly larger in patients compared with controls. The researchers also found significant correlations between photopic pupil diameters and the latency of the VEP waveforms, particularly for the timing of P100.
While there was no significant correlation between photopic pupil diameters and mean deviation values; N135 times correlated with mean deviation and pattern standard deviation values, particularly for the P100 time. Additionally, P100 times correlated with pattern standard deviation values.
All photopic pupil diameters indicated that they were markers for early-stage disease at luminance level 1 (P = .01), level 2 (P = .008) and level 3 (P = .004).
Diameters less than 2.76 mm at luminance level 1 may signal glaucoma with a sensitivity of 75% and a specificity of 54%. Diameters less than 2.54 at level 2 can be accepted with sensitivity of 75% and specificity of 64%, and values less than 2.56 mm at level 3 can be accepted with sensitivity of 63% and specificity of 50%.
“Our results suggest that measuring pupil diameter may provide an additional objective method to detect and monitor glaucoma,” Isik and colleagues wrote. “However, pupil diameter should be evaluated in larger populations of healthy individuals and in different categories of glaucoma patients. In addition, factors that do not relate to optic nerve damage, such as mechanical properties of the iris, systemic conditions or medications, may result in variable responses in pupillary light responses.”
Perspective
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Given that glaucoma is a disease of the afferent visual system, it’s no surprise that the pupillary light reflex is impaired in patients with glaucomatous optic neuropathy. However, whether pupil testing facilitates earlier and more objective detection of a condition that often remains undiagnosed until it is quite advanced is another question altogether.
In this retrospective analysis, the authors note that photopic pupil diameters are statistically significantly larger in patients with “early” glaucoma, with an average mean deviation of –3.66 dB on 24-2 automated visual field (AVF) assessment, than in healthy controls. This asymmetry is commensurate with prolonged latency and decreased amplitude of VEP waveforms, which, in turn, are correlated with AVF metrics of functional loss.
That being said, the logarithmic scale of AVF assessment means that a 3 dB loss of sensitivity equates to a 50% loss of retinal ganglion cells, calling into question our definition of “early” disease. While pupil assessment is intriguing, at present, objective measures of structural damage (including conventional OCT assessment of retinal nerve fiber layer and macular retinal ganglion cell thickness and OCT-angiography measurement of parapapillary and parafoveal vessel density) and functional loss (VEP and OCT-A flow indices) provide more clinical value in what is truly early (that is, “pre-perimetric”) glaucoma.
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