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.


Thanks for reading. Please feel free to pass this info on to anyone who may find it useful and don't hesitate to send me your questions and comments.

Mirko Cro Cop suffers serious eye injury

In recent days, while training for his heavyweight bout against Frank Mir at UFC 119, Mirko Cro Cop suffered a significant injury to his right eye. The picture below shows a gruesome image of what must a very painful eye.

MiddleEasy.com has an interview with UFC President Dana White discussing Cro Cop's situation. You'll see Dana mentions the doctors told him that the eye heals faster that any other part of the body. This is partially true. The cornea and the outer covering of the eye ball, the conjunctiva, heal very quickly (usually, a scratch or cut will heal within hours to a couple of days).

A corneal abrasion wouldn't lead to bleeding like you see in this image, so my guess would be a conjunctival laceration. Mirko was seen by multiple opthalmologists and cleared to fight at this weekend's event. For a guy like Cro Cop, its just a love tap :)

Laser Eye Surgery

Laser procedures are commonly used to treat refractive errors (ie. near- or far-sightedness and astigmatism), with the end goal of providing the patient with clear vision without glasses or contact lenses.

There are 2 major types of Laser Refractive Surgery. While the process is a little different, they both essentially flatten the cornea (for near-sighted) or steepen it (for far-sighted):

LASIK - Laser-ASsisted In-situ Keratomileusis

• The more recent of the 2 procedures
• Most common due to shorter recovery period after surgery (not necessarily better results)
• Requires the surgeon to make a thin "flap" of the surface of the cornea before using the laser
• The flap is replaced after the the surgery - effectively producing a natural bandage for the eye

PRK - PhotoRefractive Keratecomy

• The older of the two procedures
• Less common of the two due to longer healing period
• Still produces accurate results
• Usually used for patients with thinner corneas (not enough thickness to form flap before using laser)

FAQ:
Q: Do I still need to have regular eye exams if I can see perfectly after my surgery?
A: YES YES YES!!! Patients who have had laser surgery need regular eye exams more than the average population! Firstly, they have had surgery. Next, the majority of patients that have these procedures are near-sighted and the primary cause of near-sightedness is an increased length of the eye. This causes stretching of internal structures (such as the retina) and leaves these patients with a higher risk for retinal detachments. This does not change just because they no longer have to wear glasses!

Q: Is it safe?
A: Yes. The surgeons that do these procedures, do hundreds to thousands per year. Technology is improving daily to provide more accurate results. Complications occur in less than 0.5% of patients.

Q: Is my prescription too high?
A: Most likely not. Refractive errors as high as -12.00 can be successfully treated. Tiger Woods and Roberto Luongo were about -10.00 and -11.00 respectively before having their surgeries.

Q: What are the common side effects or complications?
A: The number one post-operative complaint is dry eyes. Halos around lights, decreased night vision, sensitivity to light, and recurrence of prescription are also well documented. Most surgeries are guaranteed for 5-10 years. After which, it is possible the patient will require glasses again or a "touch up" surgery.

Q: What are the alternatives?
A: Intacs and Ortho-K are 2 possible options. Intacs are very small corneal implants that create the same effect as laser surgery of flattening the cornea. Ortho-K are contacts that you wear while you sleep. Again, these have the effect of flattening the cornea. In the morning, you take them out and don't need to wear any glasses or contacts during the day. The benefits of these 2 alternatives is that they are both fully reversible. These are limited to lower prescriptions of approximately -1.00 to -4.00.

Thank you for reading. I hope you found this information useful. Please feel free to contact me with questions and comments.

Find me at Facebook.com/HarbirSianOD and Twitter/HarbirSianOD!

Why Rubbing Your Eyes is Bad for You

Everyone knows you're not supposed to. But why?

• Short-term:
• Rubbing your eyes when they're itchy or irritated can cause break down of the cells within the eyelids and on the surface of the eye ball. This releases more histamines which make the itchiness worse.
• Also, rubbing can cause breakage of small blood vessels in the eyelids which can result in dark circles forming around your eyes

• Long-term:
• consistent long-term rubbing of the eyes, especially in kids, can potentially lead to a condition called keratoconus
• Keratoconus is a condition where the front surface of the eye (the cornea) becomes thinner and thus loses its shape. This results in blurry vision for the patient, which cannot always be fully corrected with glasses or contact lenses.

Are Carrots Good for Your Eyes??

The answer is "Yes... However..."

• Carrots are high in Beta Carotene, which our body converts to Vitamin A
• Vitamin A is very important for the health of your eyes, helping the retina function more efficiently

However...

• Eating lots of carrots will not change/improve your glasses prescription

Find me at Facebook.com/HarbirSianOD and Twitter.com/HarbirSianOD for more info!

As usual, questions, comments, and suggestions are welcome.