blur

All kinds of blurry

In my very first post I wrote a little about myopia, which is near-sightedness. Here we'll discuss all the other "-opias" and "-isms" that can make your vision blurry. They are generally referred to as "refractive errors".

A quick review of Myopia. It is due the eye being longer than average. So, rather than an image being focused right on the retina, it falls a little short. Of course, this gives us blurry vision for distant objects which can be corrected with "minus" prescription glasses and contacts, or laser surgery. Example of near-sighted prescription is: -3.25.

 

Hyperopia. This is what we call far-sightedness. As you would expect, a hyperopic person would have more trouble with near and less trouble with distance. This is a result of the eye being a little bit shorter, thus images fall behind the retina. This is corrected with "plus" prescriptions. Also can be corrected with laser.

 

Astigmatism. Is not affected by the length of the eye. Astigmatism is a result of the shape of the front of the eye (the cornea). With astigmatism, the cornea is not perfectly round, it is a little steeper in one direction than the other. The common analogy is a basketball compared to a football. So, light cannot be focused to one point on the retina. It is split by the different curves of the cornea. To correct this, the prescription needs two powers (one for each curve of the cornea). An example of this would be -1.50 -0.75 x180. The "-0.75" is the astigmatism part of the prescription and "x180" shows that this prescription needs to be placed at 180 degrees. Astigmatism can be corrected with glasses, contacts, and laser.

 

Presbyopia. The first three refractive errors are mostly hereditary. The last one, however, is purely age-related and happens to everyone. As we age, the accomodative system (system inside our eyes that helps us focus at near) slowly stops working. Eventually, as we make it through our 30s into our 40s, we notice that things up close are not as clear as they used to be. Often, patients will say that they need to hold reading material further away. And eventually no matter how far away you hold it, its just not clear enough! Correction usually involves reading glasses, bifocals, or multi-focal contact lenses. Laser surgery is not a very useful option because it does nothing to improve the accomodative system.

What does 20/20 Mean?

 

This week's topic is something I get asked almost everyday. Unfortunately, the answer is never as simple as it may seem. I will do my best to explain it here.

 

Simple definition: 20/20 is the size of letter that the average person should be able to see when fully corrected (assuming that there are no visual opacities or retinal problems). It is what we aim to achieve when correcting patients with contact lenses, glasses, and laser surgery.

 

More specifically: A 20/20 letter is exactly 8.726mm in height at a distance of 20 feet.

 

FAQ:

Q: Does 20/20 mean "perfect vision"?

A: No, this is a common misconception. There really isn't such a thing as perfect vision. The sharpest estimated human visual acuity is about 20/8. Which, with a little math, equates to a minuscule 3.49mm letter at 20 feet!

 

Q: How is the size of the letter determined?

A: The letter size for 20/20 is based on the average spacing of the light-sensing cells in the retina, thus the average eye's ability to discern spaces between letters

 

Q: Why 20 feet?

A: That is the approximate distance at which the internal focusing system of the eye (used for near vision) is relaxed thus allowing your eyes to effectively see into the distance.

 

Q: What if I can't see any of the letters without my glasses? Does that mean I'm Legally Blind?

A: Vision without correction does not really provide any significant information about your eyes (other than the fact that correction may be needed).

The definition of Legal Blindness in Canada and USA is 20/200 in the better eye with correction. Thats 10 times the size of a 20/20 letter.

 

Q: Do I need to see 20/20 to drive?

A: No. This may come as a surprise. But, the legal requirement for driving is 20/40 in one eye. That's double the size of a 20/20.

Why Do My Eyes Itch?

Allergie

- most obvious answer, but not necessarily most common

- signs: swollen/puffy eyelids, watery eyes, bumps under eyelids (seen by your doctor), diffuse and more intense itch

 

Blepharitis (blepharo- = eyelids, -itis= inflammation)

- so, inflammation/irritation of eyelids, very common

- could be due to mild bacterial infection or blockage of glands at the base of eyelashes

- signs: redness along eyelid margin, itchiness usually concentrated at eyelid margin or at the corners of your eyes but not as intense as with allergies, also watery eyes

Why Do My Eyes Twitch?

There are multiple known factors that can contribute, including:

- stress

- lack of sleep or change in sleeping pattern

- low calcium levels or other electrolyte imbalances

- fatigue

- dry eyes

- caffeine

- alcohol

The medical term for eyelid twitching is Myokemia

 

Treatment for eyelid twitching usually involves resolving the above factors. In some rare cases, a botox injection is needed to relax the small muscles around the eye that are over-active.