|Low-intensity red stimuli was found to be superior when it comes to detecting pupil defects. Photo: Caroline B. Pate, OD. Click image to enlarge.|
Color pupillography is a useful, objective and non-contact method for determining retinal function in certain retinal diseases. However, while retinal defects are precisely detectable by pupillography, inconsistent results are reported in the literature with regard to defects in the subsequent visual pathway via the optic nerve, optic tract and visual cortex.
Researchers in Germany decided to perform chromatic pupil campimetry to examine systematically how prechiasmal, chiasmal and post-chiasmal lesions along the visual pathway affect the respective pupillary responses to specific local monochromatic stimuli. In neuro-ophthalmologic disorders along the visual pathway, the team observed reduced maximal pupil constriction amplitudes in the affected visual fields of post-chiasmal lesions and found prolonged latencies in those of pre-chiasmal lesions with low-intensity red stimulation. They also found that the choice of stimulus characteristics was relevant in detecting defects in the pupillary pathway of impairment along the visual pathway, favoring red stimuli of low intensity over blue stimuli.
The study assessed three patient groups (10 subjects post-anterior ischemic optic neuropathy, six with chiasmal lesions and 12 with optic tract or occipital lobe lesions [tumor, ischemia]) using red (stimulus intensity 60cd/m2, stimulus wavelength 620nm full width at half maximum, irradiance 8.2x10-4W/m2, peak energy 14.765x10-3mW/m2), low-intensity red (stimulus intensity of 20cd/m2, irradiance 2.96x10-4W/m2, peak energy 5.692x10-3mW/m2) and blue (stimulus intensity 20cd/m2, stimulus wavelength 460nm full width at half maximum, irradiance 11.11x10-4W/m2, peak energy 31.2x10-3mW/m2) local stimuli within the central 30° of the visual field.
The researchers observed a statistically significant relative maximal constriction amplitude reduction in the affected areas of post-chiasmal lesions with red and low-intensity red stimulation. The reduction in the affected areas seemed more pronounced for low-intensity red stimulation (46.5% mean reduction compared with non-affected areas, 36% for red stimulation), but it was statistically not significant. In pre-chiasmal lesions, a statistically significant latency prolongation was possible in the affected areas with low-intensity red stimulation.
“Our data contribute to basic research and understanding of pupil regulation as well as to clinical applicability of pupillography,” the study authors concluded. “For pupillography in patients with pre-chiasmal anterior ischemic optic neuropathies, chiasmal lesions or hemianopia due to post-chiasmal lesions, red stimuli of low intensity seem favorable.”
They believe that this knowledge opens the door for further fundamental research in pupillary pathways and is important for future clinical application of pupillography in neuro-ophthalmologic patients.
Kelbsch C, Stingl K, Jung R, et al. How lesions at different locations along the visual pathway influence pupillary reactions to chromatic stimuli. Graefes Arch Clin Exp Ophthalmol. December 13, 2021. [Epub ahead of print].