Read the title again and take some to think about it, it’s a pretty important question. As we’ve discussed on the RYV blog before, good vision can be a hard thing to define. It’s entirely possible to have 20/20 vision and still have some serious practical visual difficulties.
Nowhere is this more apparent than with mesopic, or “twilight”, vision – a measure of a person’s ability to see in low-light conditions. Many people with very poor twilight vision are more than capable of passing common tests for visual acuity. While this may seem like an innocuous problem, there are some real consequences behind missing these deficiencies.
Driving is the most obvious real-world example. The visual tests given to drivers renewing their licenses are often cursory and check acuity first and foremost, with peripheral vision, and occasionally other measurements only sometimes receiving attention. As mentioned, a person could be almost blind at night but if they take their test during the day, nothing would appear to disqualify them from receiving a new license. With that in mind, there’s an obvious need for a test to pick up on these deficiencies.
A team of Californian researchers may have found one. While methods of checking mesopic vision have existed for some time, they’ve largely called for a physician and a lab to actually take place. The new test, devised at Marshall B. Ketchum’s School of Optometry may represent the first standardized test suitable for use with large numbers of applicants.
Disorders affecting twilight vision are unfortunately common in the US, particularly among aging populations.
Cataracts are the most common cause. These filmy, light-obstructing formations are created when proteins, damaged by age and UV exposure, unravel and become opaque. The structures they form scatter incoming light, reducing overall visual acuity. They also cause particular problems in low light conditions.
First, cataracts rob sufferers of contrast sensitivity, or the ability to distinguish object from background based on visual cues such as differences in color, tone, and lighting. Reduced contrast sensitivity shouldn’t be confused with low visual acuity; people with very poor contrast vision can still perform well on acuity tests, as the letters on testing charts stand out very clearly from the background they’re placed on. However, they can encounter difficulty when trying to definitively locate objects, a problem that gets much worse in low-light conditions.
Second, cataracts are notorious for causing trouble with glare. Because of their scattering effect, cataracts can distort a source of glare and blind an individual. This is, again, a problem in low light, as sudden light sources, such as headlights, can immediately overwhelm a cataract patient.
Nutritional deficiencies can also impact low-light vision. Vitamin A is probably the best known, and while carrots might not give you superhuman night vision, a complete lack of them and other sources of vitamin A can quickly put your eyes in trouble, as the nutrient is vital to keeping retinal cells – which are responsible for perceiving light – healthy. A lack of zinc can cause similar problems, as the metal works alongside vitamin A to maintain retinal cells.
While all of the above problems are fully treatable, even easily treatable, there are some disorders that have more lasting effects. Age-related macular degeneration (AMD) affects all facets of vision, but like cataracts, also attacks cells responsible for contrast sensitivity. Glaucoma, similarly, decreases a sufferer’s ability to perceive important visual cues in low light.
Worsening twilight vision is also an early sign of retinitis pigmentosa. This genetic disorder causes rod cells – cells in the retina that are crucial in providing low-light, low-detail vision – to gradual degenerate. It’s most often seen in younger patients, and while symptoms generally stop before reaching total blindness, night vision is a very common, hard-to-treat consequence.
The Twilight Vision Study
Given the prevalence of some of these disorders, there’s likely a very large number of people who, while qualified under current vision testing standards, represent a risk to themselves and others if allowed to drive. The California team’s test may be a way to further screen these individuals for driving tests or other licensing that requires high visual acuity at all hours of the day.
The team, under Dr. Jason Ng, tested 43 healthy patients on their twilight vision. After first measuring vision under daylight conditions, researchers used light filters to expose participants to three levels of mesopic light. They then measured changes in vision, and also made sure to account for “other potential explanatory factors” for those changes, such as pupil increase in response to lower light.
The largest takeaway from the study may be the discovery of a level of light suitable for standardized mesopic vision testing. Out of the three levels of light used by researchers, it was the middle one that generated a “significant and repeatable” decrease in visual acuity, said Ng.
Maintaining Twilight Vision
We may not be seeing a standardized mesopic vision test for some time, but until we do, there’s plenty that you can do to maintain your own twilight vision. If you notice yourself having greater difficulty seeing well at night, the first step is to assess exactly how poor your vision is. Under some circumstances – although not desirable – it’s sometimes safest to curtail night activities, especially any that require driving or put you in danger of a fall.
From a day-to-day perspective, we recommend trying out some eye vitamins. A proven supplement, like the Ocu-Plus Formula, can help boost visual acuity in general and ensures that your eyes receive the nutrients they need to stay sharp at all light levels.
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