Gauging Myopia Improvement — Snellen Chart and Beyond

Myopia rehabilitation won’t make much sense unless we can somehow keep track of our refractive status. Without any knowledge about how to gauge myopia improvement, any reasonable person will eventually want to give up. As with any other long-term project, the ability to track progresses can help us get on the right tracks, and provides the ultimate incentives to carry on with our effort. In what follows, we present:

  • Basic approaches to measure our refraction
  • The 5ft Visual Acuity Test
  • A list of symptoms associated with myopia improvement

Measuring Refraction — Basic Approaches

There are at least 3 approaches to measure one’s refractive status:

  • Trial-Lens Approach: This approach mimics what a phoropter does in a clinical setting. It consists of finding the weakest lens that allows one to read the 20/20 line on the Snellen Chart (i.e., recognizing half of the letters on the 20ft line), while being 20 feet away from the Chart, under a brightly-lit background. The prescription of that weakest lens would then give one’s dioptric value. For example, if after shining some light on your Snellen, you can read the 20ft line at 20ft, with a -3.5D lens (but not with a -3.25D lens on), then for most practical purposes, the dioptric value of your myopia would be -3.5D,

For the record, the 20/20 line corresponds to the 6/6 line in the metric system.

  • Visual-Acuity Approach: This approach consists of finding the smallest readable line (i.e., the line of which half of letters are readable), at 20 feet away from the Snellen Chart. One’s visual acuity is then derived from the number on that line. To illustrate, if you stand at 20ft away from the Chart, and manage to read half of the letters on the “60ft” line (but not the “50ft” line), then it means that what you just read at 20ft, an “emmetropic” person can read at 60ft (equiv., your visual acuity is 20/60).

Of course, you can also try the far-point approach with any prescription on. For example, if with a -3D on, your measured far point is 0.5m, then your net myopia would be -2D, which means your myopia  is -5D.

  • Far-Point Approach: The far point (of accommodation) is the longest distance at which one can still see an image clearly. The far-point approach then simply consists of measuring the far point (in meter), and taking the reciprocal value in order to convert it to a dioptric value. For instance, if you have a poster with very small letters on the wall, and you do the measurements and figure out you can read the letters at 0.2m (but not 0.21m), then your diopter would be $latex \frac{1}{0.2}$ = -5D, for most practical purposes.

The 5ft Visual Acuity Test

While the previous 3 refraction-measurement approaches are conceptually simple, they are not without drawbacks either:

  • While the Trial-Lens Approach measures one’s diopter directly, it requires one to find the right lens that does the job, which introduces an element of trial-and-error and requires one to purchase multiple lenses of slightly different diopters. For people with higher myopia, this is not exactly budget-friendly.
  • Traditionally, the Trial-Lens Approach and the Visual-Acuity Approach are both carried out at 20 feet (~6 meters). This usually would require putting on a fairly-strong minus prescription that could temporarily cause blurry vision (i.e., a symptom of lens-induced-myopia), thereby interfering with the result of the measurements.
  • On the other hand, the Far-Point Approach requires slightly more time and effort, and might not be accurately carry out for the higher myopes. In addition, measuring refraction at closer distances (e,g,, < 1 meter) can also cause temporary blurriness (i.e., a symptom of near-stress-induced-myopia), which would again interfere with the result of the measurements.

Although there is no perfect solution, the following approach, which we dub the 5ft Visual Acuity Test, mitigates the above concerns and can be carried out rather easily as follows:

1) Print this Snellen Chart (I-see.org’s Classic Eye Chart, p.3) and paste it on a wall.

(avoid rescaling the document, or using other related features such as “fit-to-page” when printing. Preferably, print on a thick paper and ensure the paper is pasted flat against the wall to prevent future “paper wrinkles”)

2) Stand at 5 feet away from the Chart, and find your smallest readable line.

(if no letter is readable with naked eyes, use a minimal prescription that, preferably, allows you to read up to the “13ft” line)

For example, you put on a -3D pair of glasses, turn on all the lights around your Chart. You then proceed to stay at 5 feet away from the Chart, and notice that you can read half of the letters on the 10ft line (but not the line underneath it). What this means, is that while you can read this line at 5ft, an “emmetropic” person can do so at 10ft (equiv., your visual acuity is 5/10, with a -3D on).

Of course, 5/10 corresponds to the standard acuity measure of 20/40. (the easy conversion is one of the rationals behind reading at 5ft — just multiply both numbers by 4).

In addition to discouraging the use of minus lens and preventing the effects of near-stress-induced myopia during the measuring procedure, the 5ft Visual Acuity Test also forces one to read smaller letters, rendering the letter-recognition skill not-so-useful ;). The Chart will then take over, allowing for relatively-finer measurements (i.e., as opposed to the usual 20/40 or 20/50)

Feel free to gather both monocular (i.e., measurements for individual eyes) and binocular measurements (i.e., measurements obtained with both eyes open) together and compare the results — This can be done regardless the measuring approach being employed.

However, since visual acuity is greatly affected by one’s depth of focus, any measurement procedure should be carried out under bright lighting condition. Also, the measuring procedures should be carried out consistently, to ensure the accuracy of the measurements (e.g., rinse the eyes every morning, carry out the measuring procedures two hours after waking and before any near work, etc.).

Symptoms of Myopia Improvement

Once we learn to measure our refractive status, we can take it one step further by learning to recognize the symptoms associated with myopia improvement. By freeing ourselves from requiring access to eye chart/tape measure, we can start to observe changes in refraction — in real-time.

Symptoms Associated with Changes in Anterior Optics

  • Anterior Tingling: A numb, tingling, “spicy” sensation in the anterior part of the eye. Some describes it as the sensation of having a thin layer of ice applied to the iris. It is most likely the sign of pupil dilation and ciliary relaxation.
  • Light/Contrast Sensitivity: Perceived images become brighter and clearer, possibly due to retinal over-excitation. If the sensitivity is sufficiently strong, it can lead to tears in the eyes.
  • Ocular Burst: A twitching, “tumor-like” sensation inside the eye, as if a bubble bursts, followed by a subtle distance clearing. This can occur when the anterior optics (e.g., cornea, aqueous humor, crystalline lens) undergoes changes in shape.
  • Ocular Weightlessness: A sensation of the eye becoming lighter, as if it’s no longer in the eye socket as one blinks.
  • Watery Vision: An impression of images being fluid, as if they have been projected onto the surface of a lake.
  • Monocular Multiple Vision: A juxtaposition of fainter images (i.e., ghost images) over a clearer image, in a single eye. In the myopia rehabilitation community, this is commonly experienced as double vision, and is to be distinguished from blurry vision. Monocular multiple vision is a sign of unequal refractive powers across different meridians of the eye, and can have corneal, lenticular or axial origins.

Symptoms Associated with Vitreous Shortening

  • Anterior Pressure/Posterior Pressure Elimination: A loss of pressure emanating from the posterior part of the eye), as if the ocular globe shifts towards the cornea. This could be triggered by choroidal and scleral thickening, which in turn shifts the retinal planes forward, ultimately reducing the eye’s refractive power.
  • Near Clearing/Unfogging: Increased clarity or fog reduction in the near-and-middle distance ranges. In terns of measurement, this translates into a temporary increase in far point, and appears to be a symptom of retinal-planes forwarding.

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About Tom @ The Sustainabilitist

An earthling interested in keeping the system alive so that it can thrive in prosperity. An organic by-product with an intense desire to learn, promote, denounce and criticize via a non-organic platform.

2 Comments

  1. I like the concept of self-empowered successful prevention. I vote for your success. Otis

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