Basic Procedures

Refraction is defined as “the act of determining the focal condition (emmetropia or various ametropias) of the eye and its correction by optical devices, usually spectacles or contact lenses” (Keeney, et al., 1995, p. 254). This article is designed to introduce the subject to those interested, and to provide some continuing education for others already involved in the process. While the process may seem confusing to some, once learned, it becomes almost second nature.

Pre-Tests

There are a number of pre-tests that can tell us a great deal. Included are:

1. pupil measurement

2. visual acuity with and without current Rx

3. pupillary reflexes

4. ocular motility tests (broad H test, etc.)

5. near point of convergence

6. range of accommodation

7. cover tests

8. stereopsis

9. color vision screening

10. observation of the external adnexa

11. pin-hole acuity

I will not go into specific detail about these procedures, but I do want to call your attention to the pin-hole acuity test. As mentioned earlier, it is imperative to recognize when to refer. The pin-hole acuity test will clearly indicate whether or not a refractive condition is present, or if the blurred image is caused by something else. As you recall from basic optics, central light rays come to focus at a different place than peripheral rays (commonly referred to as spherical aberration). Placing a pin-hole before the eye will cause a substantial improvement in visual acuity in someone with a moderate or greater refractive error. If a pin-hole shows no improvement, the error may not be refractive and needs to be referred.

Subjective Procedures

There are many ways to find the refractive status of the eye. In the old days of refraction, everything was done totally on the subjective response of the patient. Today, we still depend a great deal on those subjective responses to help us arrive at the perfect neutralization.

Refraction can be accomplished using entirely subjective means. Using a “guide” called Eggers Chart logic, one can gauge the rough amount of ametropia present, if any. Eggers Chart logic uses the premise that each line away from emmetropia on the Snellen chart represents approximately .25-.50 diopters of ametropia. Someone who reads 20/40 on the Snellen Chart will have a rough ametropia of approximately .75 diopters. Eggers Chart logic does not tell us what ametropia, merely how much. From that information we can readily judge whether the subject is a myope or hyperope by utilizing trial lenses.

The world would be a wonderful place if that were all there was to it, but something called astigmatism is around to mess up our day.

Astigmatism can be detected by using a couple of subjective techniques. The first we will talk about is the “clock dial.” This technique uses a plus lens to “fog” the patient to approximately 20/40, typically 3 clicks of plus power above best acuity with the “rough” sphere. A dial that looks like the hands on a clock is placed at 20 feet, and the patient is to report if one set of hands on the clock looks clearer. If all the hands on the clock are equal, no astigmatism exists; if one set of hands is clearer or sharper, then there is astigmatism present. The axis can be determined by multiplying the lower numbered hand on the clock by thirty. For example, if the patient reports the 2 and 8 o’clock positions to be clearest, then the axis would be 60 degrees.

We can also find astigmatism subjectively by utilizing the Jackson Crossed Cylinder on the phoropter. The JCC is a lens with a spherical equivalent of plano (-0.25/+0.25; -0.50/+0.50) used for a number of tests. It features a set of red dots, meaning minus power, and a set of white dots, plus power. By placing one of those sets of dots on the principal meridians, you can find the presence of astigmatism. It is difficult to adequately describe here; you need to see it and touch it to understand it, but for now, I want you to know it will work.

Once a rough idea of what refractive error is determined, we must refine, or “fine tune,” our findings. To do that, we again utilize the JCC, but this time the dots are positioned at a 45-degree angle to the axis. By simply bracketing around the axis, we can find the exact axis location. You cannot find the correct power without first finding the axis.

Once that is accomplished we move on to refine the power. This refinement is again accomplished by placing the dots on the JCC on top of the axis. By asking which looks better – red or white dots – we can easily find the right cylinder power. Red indicates a minus and white plus. Again, this is extremely difficult to get across in this fashion, but, if you have a phoropter at your disposal, you should take a look at it to gain a better understanding.

There is still one more thing we have to do before proceeding on to the other eye; we must make certain we are not over-minused; to give too much minus power can cause a problem with accommodation and convergence. Minus power will stimulate the accommodative reflex. We have a couple of different ways available to us to monocularly balance a patient. The first is the red-green or duochrome test. As you know, the red component in white light comes to focus at a different place than green. By showing the patient a 20/40 line and a colored slide with half the letter in green and half in red, we can determine if we are balanced. If the patient favors red, it indicates too much plus. If green is favored, too much minus. Either way we must adjust accordingly.

The second monocular balancing technique employs a three-click blue. We earlier presented Eggers Chart and described a 20/40 test line being approximately 0.75 diopters away from emmetropia. The same idea is employed here. If we dial in three “clicks” of plus power (each small movement of the large sphere wheel on the phoropter is 0.25) then the 20/40 line should be blurry. If it takes six clicks, then we have too much minus power. Go back three clicks, and you should be at the optimum monocular refraction. Remember, when doing refraction it is best to leave the patient at the maximum plus. MPMVA means Maximum Plus for Maximum Visual Acuity. That is a good acronym to remember.

Once we have completed the balancing procedures on the right eye, all of the same steps must be done for the left, from rough sphere to red-green. When they are accomplished, one final step remains: binocular balancing. This is simply accomplished by fogging the patient, and splitting the two images with a dissociating prism (Borish, Vol. 2, 1970, p. 753). There is a 6-diopter prism on the phoropter that will move the right image down. By looking at the two images simultaneously, the patient is asked if both images are equal, or if one is better than the other. If one is better, we add +0.25 to that better image and ask again. Usually this will correct the balance and the basic refraction is complete. Once the fog is removed, the data collection is complete. An additional step some refractionists do is to complete a binocular 3-click blur, just to be certain we are at MPMVA.

Next month we will continue with Objective Procedures and additional testing procedures.