Contrast sensitivity shows potential for measuring visual function loss in early AMD
A study suggests this user-friendly platform may be useful as a supplemental visual function measure in patients with AMD.
Contrast sensitivity measured with an active learning approach (Manifold Contrast Vision Meter, Adaptive Sensory Technology) may be a sensitive correlate of visual function in patients with dry age-related macular degeneration (AMD), said William C. Ou, BS.
A study using this method found that contrast sensitivity function measured in both standard and low luminance conditions decreased with increasing severity of dry AMD and was better in normal controls compared with the dry AMD subgroup representing the earliest stage of disease.
“AMD is a leading cause of vision loss in the elderly, although visual acuity tends to be spared until late in the disease course. However, patients with earlier stage AMD often have visual complaints, especially under low luminance conditions. Thus, there is a need to identify alternative measures of visual dysfunction that may better reflect the true degree of visual impairment,” said William Ou, researcher at the Retina Foundation of the Southwest, Dallas, TX.
“Our study suggests the tool we evaluated may be useful as a supplemental visual function measure in patients with AMD. However, while it appears to be promising, it remains unclear whether the metrics generated are predictive of AMD progression, and that is an important issue to investigate in further studies.”
Although contrast sensitivity has been considered a more sensitive indicator of visual dysfunction in early AMD compared with visual acuity, measuring contrast sensitivity with traditional methods is difficult outside of research settings. The method investigated in the present study uses an active learning approach to estimate a model of the contrast sensitivity function’s global shape. The algorithm, known as the quick contrast sensitivity function (qCSF), measures the full spatial contrast sensitivity function in just 5 to 10 minutes, which makes it practical for use in clinical settings.
For the testing, patients are asked to identify 25 consecutive optotype triplets varying in size and contrast. The software generates contrast sensitivities at individual spatial frequencies. Testing for the study was done under standard photopic conditions and then repeated at low luminance created using a 2.0 log neutral density filter.
A total of 65 patients with dry AMD and 23 age-matched controls participated in the study. The AMD patients had disease in at least one eye and a visual acuity of 20/80 or better. To investigate correlations between AMD severity and contrast sensitivity, the dry AMD patients were divided into three subgroups defined based on the presence of soft drusen only (ie., intermediate AMD n = 26), subretinal drusenoid deposits (n = 19), and geographic atrophy (n = 20).
A plot of the qCSF data from testing at standard photopic luminance showed decreasing function with increasing AMD severity across all spatial frequencies. Losses in contrast sensitivity were greatest at low-intermediate spatial frequencies (3 and 6 cycles per degree, equivalent to Snellen sizes of 20/200 and 20/100). “Our finding that the group with subretinal drusenoid deposits fell in between the intermediate AMD and geographic atrophy groups is consistent with previous studies that indicated patients with subretinal drusenoid deposits appear to be phenotypically distinct from patients with soft drusen alone,” said Ou.
The results from testing under low luminance conditions were similar although some significant floor effects were seen at the highest spatial frequencies.
“Nevertheless, the data showed good separation between the controls and the AMD severity groups at lower spatial frequencies,” Ou said.
To allow for a statistical analysis of the results, the area under the log CSF (AULCSF) was calculated. The AULCSF gives a single value for each contrast sensitivity function. Qualitatively, the data showed that the AULCSF decreased with increasing AMD severity under both standard photopic and low luminance conditions. Pairwise comparisons between AMD severity groups showed statistically significant differences for most but not all comparisons.
