The last part of the whole process is endpoint techniques which aim to refine the results of the complete test. Two clinical tests are usually performed for the purpose— one aim to refine the results monocularly and the other aim to ensure that the results of monocular subjective refraction works binocularly without
any problem.
The two tests are:
1. DUOCHROME TEST FOR SPHERICAL END POINT
Aim
The test is performed to refine the spherical correction or to determine the spherical end point of the refractive error after the subjective refraction is completed.
Illumination
The test has to be done under complete darkness as it reduces veiling luminance and dilates the pupil, thereby increases the chromatic aberration, making the test more effective.
Position
The patient should be asked to sit with face upright and results of subjective refraction should be in the trial frame in front of his eyes. The patient is sighting at distance chart.
Test Distance
The test has to be done at usual distance of 20 feet.
Principle
The test makes use of chromatic aberration present in the human eyes. Near the end point, an eye that is residually myopic by a small degree of spherical, sees black letters having a red background to be sharper and darker, with more defined borders. Letters on the green background appear slightly fuzzy and less dark with less defined border. An eye that is residually hyperopic by a small degree of the spherical power sees the letters on the green background sharp, darkened and more defined. Residually
emmetropic eye sees the letters on both sides of the chart to be equally sharp, dark and defined.
Color blindness does not invalidate the test because chromatic aberration is present even in color blind subject.
Critical Factor
It is important to control the accommodation by slight fogging. Add +0.25Dsph lens monocularly before asking the patient to notice the difference between letters of two backgrounds. The idea is to take the patient in a position where he reports that the letters on the red background appear to stand out better. This should occur in only one or two increments of plus sphere unless the eye is over minus.
Normative Data
The test is very sensitive to even a small change of 0.25DS. The plus sphere is reduced until letters on the both backgrounds appear equally distinct. Or the next reduction of only 0.25D makes letters on the green background more distinct. Ideally, spherical end point should be decided when the letters on the
red background are comparatively sharper.
Limitations
Although the test is very effective but its applications are also limited as may be seen in following cases:
• Test may be ineffective when the pupil is exceedingly small.
Red and green wavelengths of different charts may bring in disparity in results.
• Aging of bulb, faded or dirty charts may adversely affect.
• Yellowing of crystalline lens in cataract tends to reduce transmission of shorter wavelength, it may alter the patient’s preference for one color or visual acuity is not adequate to discern a difference.
• The instruction to the subject is to emphasize the sharpness,
darkness and definition of the black letters and not their background.
2. BINOCULAR BALANCING
Aim
The test is performed to equalize the stimulus to accommodation for the two eyes.
Test
The two most common procedures that are normally used are:
• Alternate Occlusion Test
• Prism Dissociated Test.
Illumination
The test has to be done under standard room lights.
Patient Position
The patient is asked to sit upright with monocular correction in the trial frame and both eyes open.
Test Distance
The test is performed at a distance of 20 feet.
Critical Factor
If balancing is not possible, leave the patient at a point which produces least difference. The dominant eye is left with little clearer vision.
Indications
The test is performed in the following cases:
- When visual acuity in both eyes is nearer to 20/20
- When both eyes visual acuity is equal
- The test is not effective for amblyopes.
Procedure of Alternate Occlusion Test
1. Ask the patient to keep both eyes open.
2. Begin with the results of monocular subjective refraction.
3. Ask the patient to look at a line three lines above BCVA, i.e. 20/50.
4. Fog both eyes with +0.75Dsph.
5. Alternately occlude each eye and ask the patient with which eye he sees better.
6. Add +0.25Dsph to the better eye and continue until each eye sees equally blur or clearer vision switches to other eye.
7. This is the end point.
8. In order to confirm, add +0.25Dsph before both eyes and repeat the test. If balance is correct, both eyes will lose equal amount of acuity.
9. Now both eyes are in perfect balance.
10. Subtract 0.75 Dsph from each eye.
11. Do this while the patient is viewing full chart with 20/20 at bottom.
Procedure of Prism Dissociated Test
1. Ask the patient to keep both eyes open.
2. Begin with the results of monocular subjective refraction.
3. Fog both eyes with 0.75Dsph or 1.00Dsph.
4. Isolate a line of letters (pref 20/40 or 20/50).
5. Put 3 prism dioptre base up over right eye and 3 prism diopter base down over left eye.
6. Make sure that the patient sees same line as two-one on top of other.
7. Instruct the patient to compare two lines.
8. Images of both the lines should be equally blurred.
9. Add +0.25Dsph to better eye and continue until each eye sees equal blur or clear vision switches to the other eye.
10. This is the end point.
11. Now both eyes are in perfect balance.
12. Subtract the original fogging lens power from each eye.
STOP AND THINK
The final results are arrived at, refinements are done and a suitable correction is being decided. Now it’s the time for the clinical judgment to prescribe the lenses and also suggest the suitable mode of correction. The examiner needs to keep in mind that over-minus will stimulate accommodation and over plus will
blur the vision. Another important fact he has to keep in mind that both health and the visual function problems will manifest themselves because of the change in lens prescription. Explain the status of the refractive system of the eye and prescribe the lens power. The examiner also needs to do cover test, near point of convergence test, ocular motility test, slit lamp examination and ophthalmoscopy so that he can make necessary referrals.
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