“Refraction,” also known as “refractometry,” is one of the most fundamental and important procedures eye care professionals perform on patients. The eye may be considered functionally as an optical “instrument” whose role is to create a sharp image on the fovea after light rays have been refracted (i.e., bent) by the five interfaces in a normal eye: air-tear, tear-anterior cornea, posterior cornea-aqueous, aqueous-anterior crystalline lens, and posterior crystalline lens-vitreous. If this image is located on the fovea of an uncorrected eye, then vision will be sharp; if not, vision will be blurry. Allied health professionals can measure the power of lenses needed to move the sharp image, located elsewhere on the retina, on to the fovea.
Here, I will discuss the essential tips for refracting patients.
Absorb the basics
When viewing a distant object at least 20 feet (6 meters) away, a normal adult eye requires a total power of approximately +62 diopters (D) — note that it is PLUS power — in order for parallel light rays to refract appropriately and place a sharp image on the fovea (Table 1, page 16).
| DISTANCE1 VISION | |||
| Refractive State | Plus Power (D) | Refractive Error | Corrective Lens(Goal is to set total power at +62 D) |
| Emmetropia | +62 | None | None needed |
| Ametropia | > +62(same in all meridians) | Myopia (Eye is “strong,” i.e. has excess plus power) |
Minus SPH |
| < +62 (same in all meridians) |
Hyperopia (Eye is “weak,” i.e. has inadequate plus power) |
Plus SPH | |
| +62 in one meridian with others > +62 | Simple myopic astigmatism | Plus CYL technique: minus SPH combined with plus CYL Minus CYL technique: minus CYL |
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| +62 in one meridian with others < +62 | Simple hyperopic astigmatism | Plus CYL technique: plus CYL Minus CYL technique: plus SPH combined with minus CYL |
|
| All meridians different and > +62 | Compound myopic astigmatism | Plus CYL technique: minus SPH combined with plus CYL Minus CYL technique: minus SPH combined with minus CYL |
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| All meridians different and < +62 | Compound hyperopic astigmatism | Plus CYL technique: plus SPH combined with plus CYL Minus CYL technique: plus SPH combined with minus CYL |
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| All meridians different, some > +62, some < +62, and none +62 | Mixed astigmatism | Plus CYL technique: minus SPH combined with plus CYL Minus CYL technique: plus SPH combined with minus CYL |
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| 1. Viewing objects at 20 feet (6 meters) or farther; SPH stands for sphere; CYL stands for cylinder | |||
Go through the steps
The six steps of refraction and tips for improving the tech’s performance are described below (Table 2).
| STEP | GOAL | PROCESS | TIP(S) |
| Know each step. | Ask questions relevant to the goal. | Process is teamwork; proceed at patient’s pace. Keep changing choices, e.g., “1 or 2,” “3 or 4,” “5 or 6,” “A or B,” etc. |
Present entire eye chart; if using only one line then keep changing to smaller optotypes. Encourage blinking to maintain best vision. |
| 1.Determine SPH power (D) | To measure the greatest plus, or least minus, power for best patient vision | Determine anchor line (smallest line that patient can read) Add plus SPH first Add minus SPH only if vision improves |
Change SPH by 0.25 D to 0.50 D if BCVA is 20/60 or better Change SPH by 0.75 D or 1.0 D if BCVA is worse than 20/60 Do not accept “smaller” and “darker” responses from the patient Minus has to be earned. Only give increments of -0.25 D if more optotypes are read in a line or smaller ones in the next line. |
| Optional. Locate CYL axis | To determine the approximate location of CYL axis | Note: In plus CYL follow the white dot; in minus CYL follow the red dot. Add 0.25D CYL power Set axis mark to 180 degrees Flip choices to determine if patient prefers CYL power at 180, 45, 90, or 135 degrees |
This is the only time when CYL power is evaluated before CYL axis. Many practitioners leave CYL power at “0.00,” but that is less valid optically Axis dial is marked in 5-degree increments |
| 2.Refine CYL axis | Use JCC to fine tune axis | Note: In plus CYL rotate dial toward the white dot; in minus CYL toward the red dot Insert the JCC; turn knurled rings so they are parallel to the axis Flip JCC to determine if both choices are equally blurry Rotate axis knob in 10-degree increments; if the patient’s choice reverses, change in 5 then 2.5-degree increments |
CYL axis must be refined before CYL power Inform the patient that this step checks for astigmatism and images might be blurry. Changes will be made until both images appear equally blurry. Emphasize “sameness” not “sharpness” |
| 3.Refine CYL power (D) | To measure total power for best patient vision | Note: In plus CYL add power when the white dot is preferred and decrease plus CYL power when the red dot is preferred. In minus CYL add power when the red dot is preferred and decrease minus CYL when the white dot is preferred. Leave JCC in place and rotate the ring to make the “P” marking parallel to the axis Add CYL power in 0.25 D increments if the vision improves and decrease it if vision decreases For every +0.5 D CYL power increase, add -0.25 D SPH to preserve SE For every -0.5 D CYL power increase, add +0.25 D SPH to preserve SE Keep adding CYL power and maintaining SE until both choices appear the same or patient goes back and forth. |
CYL power must be refined after CYL axis Inform the patient that this step checks for astigmatism and might cause blurring, but it will be cleared at the end. Add as much CYL power as patient accepts. A licensed practitioner will decide how much to actually prescribe based on experience and established protocols. Do not adjust CYL power after JCC is removed |
| 4.Refine SPH power (D) | To fine tune the greatest plus, or least minus, power for best vision. | Same as step 1 | |
| 5.Duochrome (red-green) test | To correct excess plus or minus SPH power | Insert red-green filter Inquire if optotypes in red and green appear equally clear Red clearer = add minus SPH Green clearer = add plus SPH A pneumonic that may help: RAMGAP |
Ask if the optotypes (not the colors) are equally clear |
| 6.Balancing (fogging) | To ensure both eyes have equal accommodation | Binocular test (open OU) Add +1.0 D SPH OU Insert 6△BU prism (labeled, “6△U”) OD This creates an inferior OD image and superior OS image Ask patient if both images appear equally blurry Add +0.25 D SPH to the clearer image Remove the +1.0 D SPH added earlier |
An alternative way to complete this test: Each eye can be alternately covered with an occluder to compare the two images instead of using prisms. Test is best suited for hyperopes and presbyopes (especially first time) |
| Legend: SPH = sphere; CYL = cylinder; BCVA = best corrected visual acuity; JCC = Jackson Cross Cylinder lens; SE = spherical equivalent; BU = base up | |||
For a more detailed discussion on the concepts presented in this article, see the essential concepts of light, refraction, and lenses in Shukla (2009) and further tips for refracting patients in Shukla (2018). OP
REFERENCES:
- Shukla AV. Clinical Optics Primer for Ophthalmic Medical Personnel: A Guide to Laws, Formulae, Calculations, and Clinical Applications. SLACK Inc.; 2009.
- Shukla AV. Chapter 11: Retinoscopy and Refractometry. In: Ledford JK and Lens A, eds. Principles and Practice, in Ophthalmic Assisting: A Comprehensive Textbook. SLACK Inc.; 2018:179-211.
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