little four eyes

Edited on 26-Jan-10 to correct the errors that Dr. Bonilla-Warford pointed out (thanks Nate!) – Ann Z

Glasses prescriptions can be confusing – lots of numbers and abbreviations that aren’t necessarily easy to figure out, or at least, they weren’t when I first tried to understand Zoe’s prescription.  And in fact, I thought I understood what all the numbers meant, but after doing more research for this post, I found it to be far more complex than I’d originally thought. (On that note, if any eye doctor types read this and catch any mistakes, please, please let me know and I’ll correct them – thanks Nate!).

Before we get too far into what a prescription is, I thought it might be worthwhile to mention what a prescription isn’t.   It will not tell you:

• what your child’s diagnosis is.  You can tell whether glasses will correct for nearsightedness or farsightedness, but that doesn’t tell you what the cause of the vision problem is.
• if your child has amblyopia, it will not tell you what his or her actual visual acuity is.
• if your child has strabismus, it will not tell you how much his or her eyes are turning in or out.
• how well your child’s eyes work together, and whether or not he or she has stereoscopic vision.
• how advanced your child’s cataracts or glaucoma is, and how that affects their vision.
• anything that is not related to how glasses or contacts should be made in order to correct your child’s refractive errors (that is, due to the shape of the eye not being able to focus light correctly).

Okay, so back to the eyeglasses prescription then, and what it can tell you.  Prescriptions have a lot of different components, and I’ve seen them written a number of different ways, but they do have common elements at the core.  I’ll go into more detail on each piece, but I tried to put together something of a “cheat sheet” here:

Which eye are we talking about?

Since we’re nearly always looking at prescriptions for two eyes, you’ll nearly always see two sets of numbers, one for the left eye, and one for the right.  I’ve seen some prescriptions that label them “left” and “right” or “l” and “r”, which even I can figure out on my own.  But more often, I’ve seen “O.D.” and “O.S.”.  These are abbreviations for the Latin words for left eye and right eye.  For the record:

• O.D. : right eye
• O.S. : left eye
• O.U. (which I’ve never seen before, but found this information so figured I’d include it) : both eyes

Sphere

Sphere, often abbreviated as “sph” is the spherical refractive error, or nearsightedness or farsightedness.  It’s pretty much what I think of when I think of an eye glasses prescription.  Then again, I’ve never really had to deal with astigmatism (for me or Zoe).  The first part of this number will be a plus or minus sign:

• + : farsighted, or longsighted prescription: hyperopia.
• - : nearsighted, or shortsighted prescription: myopia.
• 0, Pl, or Plano : no error

How bad is the spherical prescription?

The number is in “diopters” but we don’t need to know too much about that (read about it on Wikipedia here), it’s a measure of how much the curvature of the eye  is off from normal.  Basically, the higher the number (ignoring the plus or minus), the worse the prescription.

Myopia (-)

• 0.00 to -3.00 : mild myopia
• -3.00 to -6.00 : moderate myopia
• -6.00 and higher : high myopia

Hyperopia (+)

• 0.00 to +2.25 : mild hyperopia
• +2.25 to +5.00 : moderate hyperopia
• +5.00 and higher : high hyperopia

Cylinder

Cylinder is the measure of astigmatism.  Astigmatism is when there’s an irregular shape to the cornea, often described as a football shape.  It causes blurriness at any distance.  There are two measurements that go along with astigmatism, the first, cylinder, is a measure of how severe the astigmatism is.

How bad is the cylinder prescription?

Like the spherical error, the cylinder number is measured in diopters.  The thing you want to pay attention to is the number.  It may be written as a plus or a minus, but that doesn’t actually make any difference in how bad the prescription is, ophthalmologists use a “+”, optometrists use a “-” (see Dr. Bonilla-Warford’s comment for more explanation of that).   The higher the number after the plus or minus, the more severe the astigmatism.

• 0.00 to 1.00 : mild astigmatism
• 1.00 to 2.00 : moderate astigmatism
• 2.00 to 3.00 : severe astigmatism
• 3.00 and higher : extreme astigmatism

Axis

The axis tells you whish way the astigmatism is oriented on your child's eye

If you think of astigmatism as a football shape, it makes sense that the football might be turned any direction.  The axis number then, tells you the orientation of the astigmatism.  The number is in degrees, it doesn’t have anything to do with how severe the astigmatism is, just how it is situated on your child’s eye.

Add

If your child needs bifocals, you will likely see a number here.  This tells you how the prescription should be changed for close up.  Let’s say your child has a regular glasses prescription of +3.00, if the add number is +1.00, then the near distance prescription will be +3.00 + 1.00, which equals +4.00 (3+1=4).  In the same vein, if your child is nearsighted, say -4.00, but has an add of +2.00, then the  near distance part of the bifocals will have a prescription of -4.00 + 2.00, which equals -2.00.

Sometimes, you don’t have an “Add” part of the prescription, and instead you’ll just see a prescriptions for distance vision and a separate prescription for near vision.

But what does that mean for how my child sees?

If you’re wondering what your child’s prescription means in terms of how they can see, there are a couple of places online where you can plug them in to see how blurry things appear.  Keep in mind that an eyeglasses prescription is not the final word in how well a child sees.  For instance, it will not tell you how your child’s weak eye is seeing if your child has amblyopia, or how your child might be seeing if he or she has cataracts or glaucoma.

Online vision simulators:

• Eyeland Web Tools – this site has one tool for myopia, and a different one for hyperopia, and one for astigmatism.  All three show you a scene that you might see while driving a car.  So hopefully not a scene that your child would necessarily see, but it still helped me visualize how the world might look with Zoe’s prescription.  Simulates prescriptions from +9 to -9 for the spherical error, and up to -8 for the astigmatism.
• Eli Billauer’s blur simulator – I’ve mentioned this site before on Little Four Eyes.  Enter your child’s prescription (sphere, cylinder and axis) for each eye, and it will show you two Snellen charts (the one’s with the big E at the top), one for each eye, that simulates how your child might see it.  Simulates prescriptions from +5 to -5 for the spherical error and +5 to -5 for astigmatism.
• Wolfram Alpha – this is a weird search engine that tries to answer your questions, and calculate equations that you enter.  You can enter your child’s eyeglasses prescription, and it will give you information about it, including a simulation of looking at the Snellen chart, and the far or near point for clear vision.  You will only see the Snellen chart if the simulator thinks one of the lines would be visible, which is +2.5 to -2.5.   You need to enter the prescription this way:

  OD +4.25 cyl 0.5 x 90

  (the number after the “x” is the axis for astigmatism, you can leave off the astigmatism numbers if they don’t apply).