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EW NEWS & OPINION 18 November 2015 eye conditions like retinitis pigmen- tosa that might contribute to low- light issues, she said. While in the study investigators placed large filter overlays over the testing monitor, Dr. Lin said that practically speaking this can be cum- bersome. "It's a very large sheet and it can wrinkle," she said. However, a lot of neutral density filters are designed for cameras and come in small camera lens sizes. She recom- mended holding one of these over the patient's eye rather than putting a large filter over the screen. Dr. Lin hopes that practitioners come away from the study with a clearer understanding of the impor- tance of mesopic testing. "I think the take-home message would be very similar to what other research- ers have found regarding mesopic visual acuity—that it is an important measure and that photopic visual acuity does not tell the whole story of a patient's real visual function," she said. "I think for the future it would be good to have more research on mesopic visual acuity and for more doctors to adopt this as part of their assessment for patients." EW Editors' note: Dr. Lin has no financial interests related to this article. Contact information Lin: rlin@ketchum.edu by Maxine Lipner EyeWorld Senior Contributing Writer at the lowest light level. At the 0.75 mesopic level, twilight vision was determined to be reduced by about 2.5 lines. At both the 0.75 and 0.38 mesopic levels there were significant and repeatable decreases in visual acuity. "I was surprised to find that the repeatability was good even at the lower light levels; one of the concerns was whether these results would be repeatable if we were getting reduced acuity," Dr. Lin said. Under mesopic conditions, change in pupil size and aberrations were found to be unrelated to diminish- ment of visual acuity, and accom- modative error only had a small effect. In the clinic From a clinical perspective, Dr. Lin thinks that mesopic acuity can be important to take into account. "Especially for refractive surgeons who are doing cataract surgery, it can be valuable to measure contrast sensitivity or mesopic visual acuity both before and after the surgery," she said. "If a patient comes in and is having difficulty, if you only have pre- or post-measures of high-con- trast visual acuity, it is hard to say whether overall visual function has improved or gotten worse." Data from the study, which included healthy subjects who had a mean age of 24.7, could be used as normative numbers for comparing to young patients with contribute to the decrease in acuity associated with twilight vision. In addition, investigators wanted to assess benchmarks for measuring vision in mesopic conditions. "One of the standards that has been used is 3 candelas per meter squared," Dr. Lin said, adding that they wanted to test if this was actually a low enough level since this is still fairly illumi- nated. Mesopic testing For the study, which involved 43 normal subjects, mesopic acuity was tested on a monitor at 3 different levels—3, 0.75, and 0.38 candelas per meter squared. In addition, the patient's photopic acuity was tested at a level of 94 candelas per meter squared. Investigators attained the 3 candela per meter squared light level by reducing the monitor's luminance. For the two lowest light levels they placed additional filter overlays over the monitor's screen. After testing at all of these levels, investigators determined that there was a statistically significant decrease between the acuity at the photopic level and those measured at mesopic light levels, Dr. Lin reported. Study results showed that visual acuity decreased significantly with each progressive reduction in illumination. Investigators found that acuity, which measured 20/16 in photopic conditions, dropped approximately 3.5 lines when tested Identifying patients with "night myopia" T wilight vision, also known as night myopia, where patients may have difficulty seeing in dim light, can be an all too common prob- lem, especially among older indi- viduals, according to Rachelle J. Lin, OD, assistant professor, Marshall B. Ketchum University, Fullerton, Calif. "A lot of older patients can have difficulty with night driving under mesopic or scotopic conditions," Dr. Lin said. Unfortunately, there has not been a standardized approach to testing twilight vision. However, a study published in the May 2015 issue of Optometry & Vision Science indicates that using filters to dim the light level of standard visual acuity testing to a level 100 times lower can offer reliable results, Dr. Lin said. While there have been a lot of studies looking at standard high-contrast acuity, this does not give the whole picture of the patient's overall visual function. "There are plenty of patients with good high-contrast visual acuity who complain of night driving dys- function or who can't read well in low light conditions," Dr. Lin said. Spurred by this, investigators set out to determine whether accommoda- tive error, higher-order aberrations and changes in pupil size could Measuring twilight vision Simulation of a mesopic image used for twilight vision visual acuity testing Photopic visual acuity image, which was tested at a level of 94 candelas per meter squared Source (all): Rachelle J. Lin, OD

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