Readers' Forum

Using Spectacle Blur Refraction to Troubleshoot RGP Lenses

readers' forum

Using Spectacle Blur Refraction to Troubleshoot RGP Lenses

November 2000

At times it seems we all get caught up with new technology like corneal mapping. It's good to have, but I believe we can find out what is going on under a rigid gas permeable lens by going back to some of the basics without using the topographer. Spectacle blur refraction is an easy test you can use to quickly determine what a contact lens has done to alter the corneal surface.

Spectacle blur refraction compares the original baseline data from the earliest known spectacle Rx before contact lens wear (if the patient is wearing contact lenses, I suggest a clean starting point), to the spectacle Rx caused by wearing the lens.

Calculating the Condition

We can easily discern contact lens fatigue or warpage, corneal edema, corneal flattening, corneal molding or progressive myopia.

Step 1: Convert the original spectacle Rx into the equivalent sphere component for future comparison. (Remember the spherical equivalent is half the cylinder added to the sphere power.) So ­2.25-1.00x180 would have a spherical equivalent of ­2.75. Since this ­2.75 equivalent sphere is now our baseline, any deviation from this is spectacle blur.

Step 2: If the patient is wearing lenses, perform the spectacle refraction after the lenses have been worn for at least four hours.

Step 3: Do an overrefraction with sphere and cylinder. If the overrefraction at dispensing was plano, then any deviation from plano tells us that:

  • With additional plus, the base curve has flattened.
  • With additional cylinder, the base curve has warped.
  • With additional minus, the base curve has steepened (rare).

Step 4: The spectacle refraction is the new refraction immediately after you remove the lens. Compare this refraction to the original data before contact lens wear, then convert this to the equivalent sphere:

  • If you need additional minus compared to the original spherical equivalent, this indicates corneal edema. (Original equivalent sphere was ­2.75D, spectacle refraction now of ­3.00-0.50x180 now becomes an equivalent sphere of ­3.25D.)
  • If you need additional plus after converting to the equivalent sphere, then corneal flattening is occurring. (Original equivalent sphere was ­2.75D. The new spectacle refraction is ­1.00 -1.50x180; therefore, we now have an equivalent sphere of ­1.75D.)
  • If you find no change in the equivalent sphere, but the sphere and cylinder are different, you have corneal molding. (Original refraction of ­2.25-1.00x180 yielding a ­2.75D equivalent sphere, and now it is ­1.75-2.00x180. The equivalent sphere is still ­2.75D, then no change is necessary).

Calculating a Remedy

For corneal edema, change the base curve to a 0.50D flatter base for every 1.00D of change found. If the original equivalent sphere is ­1.50D and now is ­2.50D, you should make your new base curve 0.50D flatter. With corneal flattening, make your base curve steeper, again changing the base curve 0.50D for every 1.00D of change in the equivalent sphere. For corneal molding, no change in the curve is necessary.

One other possible result is progressive myopia. When you find additional minus power with the contact lens in place and the same amount of minus power after removing the contact lens, then the fit is all right, but you must adjust the power. This is a result of progressive myopia.

Use spectacle blur refraction as a fitting guide to diagnose potential problems before they become patient complaints. 

Dr. Hettiger, a graduate of the Illinois College of Optometry, has been in private practice since 1982 in Flora, IL. He specializes in and has given lectures on contact lenses.