Many opticians around the country have shown great interest in learning the procedures involved but have been unable to take a course on the subject. This article is designed to introduce the subject to those interested, and to provide some continuing education to former refraction students.
Refraction is defined as the act of determining the focal condition (emmetropia or various ametropias) of the eye and its corrections by optical devices, usually spectacles or contact lenses (Keeney, et al, 1995, p. 254). Many opticians around the country have shown great interest in learning the procedures involved but have been unable to take a course on the subject. This article is designed to introduce the subject to those interested, and to provide some continuing education to former refraction students.
One caveat: reading this article will not make you a refractionist! There is a great deal of material to cover on refraction and this article is only an introduction to merely help you get started or revisit previously learned material. There is a wealth of additional information to be found on the Web and offered through online courses.
While it is understood that most of you are well grounded in anatomy and physiology, a bit of review would be beneficial. Light comes through the refractive media and comes to focus somewhere near the retina. There are four parts of the system that focus light. They are:
1. the cornea
3. the lens
Any abnormality in any one of the refractive media can cause a blurred retinal image. Students often argue that the retina is part of the refractive media, but can actually be thought of as the film in the human camera. It is the job of the refractionist to determine whether or not a blurred image is a refractive problem or a medical condition which should be referred to a medial practitioner.
Before beginning any refraction, it is important to do a thorough history on the patient. A history should include:
1. chief complaint
3. ocular history, including last exam
5. medical history
6. family medical history
8. current Rx (if any)
A great deal of information is derived from the history. Care must be taken to record the information derived from patient exactly as it is presented. Do not try to dress up the information with more technical terms. Patients may not always be saying what you might think.
If on completion of the history, you feel the patient may not be a candidate for refraction alone, but needs a complete exam, it is incumbent upon you to refer him or her to the appropriate care provider. Opticians are not eye doctors, and the health of the patient must always remain the paramount concern. For example, if a patient presents a complaint of itching and burning, he or she must be referred to a care provider. If a refraction is necessary, it can be done after the medical condition is treated.
There are a number of pre-test that can tell us a great deal. Included are:
1. pupil measurement
2. visual acuity with and without current Rx
3. pupiliary reflexes
4. ocular motility tests (broad H, etc.)
5. near point convergence
6. amplitude of accommodation
7. range of accommodation
8. cover tests
10. color vision screening
11. observation of the external adnexa
12. pin-hole acuity
I will not go into specific detail on these procedures, but I would like to call you attention to the pin-hole acuity test. As mentioned earlier is it imperative to know 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. Placing a pin-hole before the eye will cause 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 the patient needs to be referred.
There are many ways to find the refractive status of the eye. In the old days of refraction, everything was done totally on subjective response of the patient. Today, we still depend a great deal on subjective responses to help us arrive at the perfect neutralization.
A 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 is a numerical chart a numerical chart using the premise that each line away from emmetropia on the Snellen Chart represents approximately 0.25 - 0.50 diopters of ametropia. Someone who reads 20/40 on the Snellen Chart will have a rough ametropia of approximately 0.75 diopters. Eggers chart logic does not tell what ametropia, merely how much. From that information we can readily judge whether the patient is a myope or hyperope by utilizing test 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 the day.
Astigmatism can be detected 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. A dial that looks like the hand of 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 cleared or sharper, then there is astigmatism present. The axis can be determined by multiplying the lower numbered hand on the clock by 30. For example, if the patient reports the 2 and 8 o'clock positions are clearer, 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 a plano used for a number of tests. It features a set of red dots representing minus power and a set of white dots representing plus power. The usual JCC is +/- 0.25, but may be +/- 0.50. By placing one set of dots on the principle meridians, you can find the presence of astigmatism. It is difficult to adequately describe here. It needs actually should be seen to be understood properly.
Once a rough idea of the refractive error is determined, we must refine or fine tune our findings. To do that we again utilize the JCC, but this time at a 45 degree angle to the axis. By simply bracketing around the axis we can refine the cylinder power. You cannot find the correct power without first finding the axis. 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. Again this is very difficult to explain. If you have a phoropter at your disposal, you should take a look at it to gain a better understanding.
There is one more thing we must do before proceeding to the other eye; we must make certain we are not over-minused; too much minus power can cause problems with accommodation and convergence. We have a couple of different ways to monocularity balance. The first is the red-green or duochrome test. As you may know, the red component of white light come to focus in a different place that the green component. By showing the patient the 20/40 line through a colored slide with half the letter in green and half in red, we can determine if we are balanced. If the patient reports the letter in red to be blacker, sharper, or darker, too much plus has been employed. A favor for green means we have employed too much minus. The second monocular balancing technique employs a three-click blur. We presented the Eggers Chart earlier and described a 20/40 test line being approximately 0.75 D away from emmetropia. The same idea is employed here. If we dial in thee clicks if plus power (small movements on the large sphere wheel of the phoropter), the 20/40 line should be blurry. If it takes six clicks, we have too much minus power. Go back thee clicks, you should be at the optimum monocular refraction. Remember when doing a refraction, it is best to leave the patient at the maximum plus. A good acronym to remember is MPMVA - maximum plus for maximum visual acuity.
Once we have completed the balancing procedures on the right eye, the same procedures are performed on the left. Upon completion, one final step remains: binocular balancing. This is performed by splitting the two images with a disassociating prism (Borish, Vol. 2, 1970, p. 758). There is a 6 diopter prism on the phoropter that will move the right image down. The patient is asked if both images are equal while looking at them simultaneously. If one image is better, we add +0.25 to that side and ask again. Usually this will correct the balance and the refraction is complete. An additional stem some refractionists perform is a final three click blur, just to be certain we are at MPMVA.
We can also perform refractions using a variety of objective tests. Today, many offices utilize an autorefractor, which can provide a fairly accurate estimation of refractive error. While the autorefractor is a great machine, we will focus our attention on a simpler device; the streak retinoscope. The streak retinoscope is a device invented by Jack Copeland around 1920 (Corboy, 1989). While others had defined streak retinoscopy, Copeland's scope is the basis for all of today's equipment.
In streak retinoscopy, the refractionist sweeps across the pupil with the scope, watching the movement of the light in the eye from the scope. If the streak appears to be moving against the direction of the scope, minus lenses are employed. If the streak moves with the observer, plus lenses are required. If there is no apparent motion, neutrality has been reached. The streak will vary in different meridians if astigmatism is present. The procedure sounds simple here, and basically is, but it can be difficult to master. It is also important to remember when scoping the eye, the eye is "chained" to the scope. The refractionist is only about 67 centimeters from the eye. An extra -1.50 D must be added to the retinoscopy finding to move the focal plane to infinity, or the retinoscopy lens on the phoropter may be used. This is another topic that is difficult to explain and must be seen and done to fully understand. Once the objective procedure is completed, the subjective refinement procedures described earlier are employed.
The significance of objective procedures is evident with illiterate patients, children, or others who cannot subjectively respond. It is also much fasted than a strictly subjective procedure.
Finding the correct reading add is a difficult task. Most refraction errors come from improper add power. We will not attempt to discuss a great deal of theory here, but you should know that a patient can comfortably utilize - of their available amplitude of accommodation (the reciprocal of the near point). Amplitude diminishes with age. For example, researchers claim we have at age 10, between 11 and 14 diopters of accommodation amplitude, by age 40 it is between 4.5 and 5.5 (Borish, Vol. 1, 1970 pp. 169-170). It takes +2.50 D of accommodation to focus at 16 inches, normal reading distance. If we only have +5.00 available, it is easy to see why we may need bifocals at age 40. Unfortunately, all people are not the same. Some may need a +1.00 add at 40, while others may require a +1.25.
A fairly simple, but effective way to determine add power is to use the Eggers Chart for near. A rule of thumb that works well, states that at age 40 a +1.00 to +1.25 add will be required. For each 5 years after 40, and additional +.025 is required.
Always test subjectively. Ask the patients about their reading requirements. Some like to read at 20 inches while others prefer 14 inches. Computer use should be discussed and an approximation of the computer screen distance should be determined. Using the near point rod on the phoropter, it is relatively simple to check the range through the reading add. In these situations, some compromises might have to be made or specialty glasses for computer use may be necessary. Communication is extremely important in refraction. Discussing the patient's needs and expectations is the most important thing a beginning refractionist must learn.
Additional Testing Procedures
There are a multitude of functional test that would be performed at the end of the basic refraction that are beyond the scope of this article. Tests for phorias and tropias may be the topic of the next article on refraction.
Borish, Irvin M (1970). CLINICAL REFRACTION (3rd Edition). Professional Press, New York.
Warren G. McDonald, PhD