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Routine Procedure Of Refraction



The second phase of the clinical refraction starts when the complete process of first phase is carried out and the goal has been set with all predictions done in the mind. By now the examiner knows how the visual system of the patient is behaving, patient’s chief symptoms and his visual demands. The tools andequipments used in this phase are:• Trial lens set and trial frame/phoropter• Snellen’s test chart• Cross grid test• Near duochrome test• Near test chart• Hole in the card.The total routine procedure is done in the following sequence:1. Dominant eye test2. Monocular distance subjective refraction3. Determination of near addition.


DOMINANT EYE TEST


The dictionary of visual science defines ocular dominance as, “The superiority of one eye over the other in some perceptual or motor task. The term is usually applied to those superiorities in function which are not based on a difference in visual acuity between the two eyes, or a dysfunction of the neuromuscularapparatus of one of the eyes.” This implies that dominant eye is not necessarily an eye with superior visual acuity, still it contributes most to the visual perception. The extension of the concept of dominant eye is:• It is the fixating eye in the binocular vision• It aims at the object• It leads in receiving the visual inputs. The concept ocular dominance may be studied in threedifferent categories:• Sensory dominance• Oculomotor dominance• Directional dominance. Sensory dominance may be because of difference in retinal images of the two eyes in terms of image clarity, color or brightness that may lead to some binocular interaction. Oculomotor dominance occurs when an eye does a better job of fixating on an object of regard. Directional dominance is the sighting dominance, i.e. the eye that is sighted at the target. Sighting dominant eye aims at the target and the non dominant eye follows and looks at the same target slightly at a different angle. The small difference provides us the required depth perception. A stable dominant eye is, therefore very important to hold the visual system steady at the fixating target. It leads in visual inputs in binocular vision. Eye dominancy is someone’s innate characteristics. If there is dominance conflict, then often attention jumps several words or even lines and hence affects comprehension. It leads to binocular instability. Human species are predominantly right eye dominant. Nearly 70% of the total population is believed to be right eye dominant. However, prevalence of left eye dominancy is also present simultaneously. Therefore, establishing which eye is dominant plays a vital part in binocular vision function. The dominance may be weak, strong or highly profound which may also have fairly good impact on the visual performance when the visual demand exceeds the ability of the visual system. Some patients may feel uncomfortable if their dominant eye is fractionally blurred, whereas a small residual refractive error in the nondominant eye may not give any discomfort. The concept of dominant eye is very important in case of monovision correction in presbyopia. Usually dominant eye is being corrected for distant vision and nondominant eye is corrected for near vision. It has also been seen that when light level changes from low to high as it happens when coming out from dark room to bright sunlight, the dominant eye closes leaving the nondominant eye responsible for vision. It is, therefore, useful not only clinically to know the dominant eye, but to remember throughout the procedure. The test for dominant eye can be done by a simple test called “Hole in thecard test”.
The test procedure is as under:• The subject is given a card with a small hole of 25mm and is instructed to hold it with both hands stretched out.• He is then instructed to view a distant object through the hole with both eyes open.• If binocular vision is present they tend to center the hole between their eyes.• When the observer looks at the patient’s eyes through the hole in the card from a distance, he will see his dominant eye. 


MONOCULAR DISTANCE SUBJECTIVE REFRACTION



Subjective refraction is a method of estimating the refractive error that is largely dependent upon the patient himself. Under this technique one lens is compared against another and the patient is asked to provide input on changes in visual acuity as demonstrated on the Distance Vision Test Chart. Therefore, the accuracy of the results largely depends upon the intelligence, cooperation and observational ability of the patient. The process is completed for each eye separately. Traditionally right eye is completed first and is followed by the left eye. The ultimate goal is to arrive at a combination of lenses that result in maximum visual acuity. Subjective refraction is initiated by selecting an initial trial lens which may be based on the following factors:1. Current lens prescription of the patient2. A set of lenses you decide based on your initial data collection3. It may be the results of autorefractometer or retinoscope.Once the initial trial lens is put in the trial frame, you ask the patient to look at the Distance Vision Test Chart through a lens power one step stronger or one step weaker and inform which one appears clearer to him. Depending upon the lens the patient chooses, you compare another combination of one step stronger or one step weaker. “Clearer” being the key word as in many cases neither lens choice is very clear. Do not worry even if there is a mistake in picking up the correct response as the next pair oflenses will take you back to where you were. Broadly speaking there are three methods which are mostcommonly followed in the clinical practice for subjective refraction:1. Manifest subjective refraction2. Fogging method of subjective refraction3. Trial and error method. 

dominance Eye Test
Dominance Eye Test

Manifest Subjective Refraction


Usually the following sequential procedure is followed while doingmanifest subjective refraction:1. The first step is to find out the initial or working sphere. Put the sphere power in the trial frame and change at a step of 0.50D. Always put plus lens first and then minus lens. Obtain the best possible acuity by giving most plus or least minus. You may take the help of “Initial Duchrome Test” to find out initialor working sphere. Ask the patient to look from the green side to the red side and then back to green side and indicate which side has the clearer letters or both the sides are equally clear. Do not use the word “better”, “darker” or “brighter”. Red side clearer indicates the need of minus spherical and green sideclearer implies the need of plus spherical. Keep changing the spherical lens until both side’s letters are equally clearer. Introduce an additional 0.25D minus spherical power. Then the patient is into the green. Now check the visual acuity on the Snellen’s Chart. Remember that visual acuity of 20/20 or 6/6 may not be possible at this point as cylinder correction has not yet been refined. Now it’s time for astigmatic correction.
2. The second step is to ascertain the need for astigmatic correction. This is done in two steps— first the axis of cylinder is determined and then the amount of cylinder is measured. Use Jackson Cross Cylinder for the purpose.3. With initial spherical lens in front of the eye the patient may inform visual acuity of, say 20/30. Now ask the patient to look at a target which is slightly larger than the achieved acuity. In our example it may be a 20/50 line. Hold the JCC in front of the patient eye and ask him to concentrate on how things look. It is not necessary to read the line but to concentrate on how it looks. Then flip the JCC and ask the patient to compare the two and report which is better. Give enough time to the patient before flipping the JCC. If both the position looks equally clear at the very first time, it implies either you have found out the correct axis or there is no astigmatism. In case he likes either of two, you must rotate the JCC to realign theaxis. To do this, you must figure out what was the orientation of JCC that he likes better. Then rotate the JCC so that the axis you flipped around gets rotated by 15°–20° in the direction of the axis of the plus cylinder. Now repeat the process and ask the patient “which is better”. If he says, both look alike, itimplies you are straddling the axis. This is the endpoint of the test.4. The next step is to determine the amount of cylinder lens needed. In order to determine the amount of cylinder, the axis of the JCC has to be line up with the axis determined as above. Wait for a moment and then flip the JCC. If the patient truly needs the cylinder, he will appreciate one more than other. In order to be sure, flip the lens back to the one that is preferred. Now put +0.25 cylinder at the axis. Then repeat theprocedure. Then again ask the patient to see 20/50 line and ask which flip is better? Continue the process till both looks alike. Remember to reduce 0.25 sph for every 0.50 cylinder addition.5. Now refine the spherical lens by changing at a step of 0.25D. Stop at most plus or least minus when images are about the same. Note down the visual acuity.


Fogging Method of Subjective Refraction


Fogging method of subjective refraction allows controlling the accommodation. In case you follow the fogging method of refraction, start with higher plus than the results of objective method of refraction in case of hypermetropia and with low plus in case of low myopia and no plus lens is needed in case ofhigh myopia. Most commonly Fan and Block Test is used for the fogging method of refraction. The step by step procedure is as under:1. Occlude the left eye and put the fogging spherical trial lens in the trial frame in front of right eye as selected above and start reducing the fogging. While reducing fogging care must be taken to put the new plus lens first and then remove the one from the trial frame and for minus lens, remove the minus lens first and then put the new one.2. Obtain the best visual acuity using spherical lenses only, i.e. best vision sphere. It is assumed the best vision sphere will put the circle of least confusion on the retina and the patient will report the acuity of 6/9 or more.3. Now ask the patient to look at the Fan Chart for astigmatic correction and ask which line or group of lines appears clearest and darkest. This gives an approximate direction of the astigmatic error.4. If no lines appears to be black than the other, there are two possibilities— either the patient may not need any astigmatic correction or the eye is excessively fogged. In case the patient identifies any one line or group of lines as darker than all others, align the Maddox Arrow with the same.
5. Now direct the patient’s attention to Maddox Arrow. Ask him if both the limbs of the arrow are equally blurred. Rotate it away from the black limb towards the blur limb until both limbs appear equally blurred. This gives the axis of the astigmatic correction. Care must be taken to ensure that the patient head is upright.6. Then direct the patient’s attention to two blocks. The patient will report that one of the two blocks is clearer and darker than the other. Add minus cylinder at the appropriate axis until the second block becomes as clearer as the first one. If this is not quite possible, over add cylinder lens by –0.25Dto get the reversal. Once the reversal is achieved, you may reduce the additional cylinder lens added and go one step down. Remember it is always better to under correct than to overcorrect, leaving the initial block clearer than the other.7. Finally, with corrected sphere and cylinder ask the patient to look back to Snellen’s chart to refine the spherical. Do not continue reducing plus unless you are very sure that they really need it to see clearly. Similarly do not go on adding minus as well, it will not make the letters clearer, rather letters will become sharp black and small. The goal is to find “maximum plus to best acuity” or  “minimum minus to best acuity”.8. A simple fogging with +0.50Dsph and then reducing fogging at the step of 0.25 step may be applied. At this stage it is quite likely that unfogging another 0.25D may also yield 6/6 acuity. A clinical judgment based on patient’s past correction and his visual demands may help. 


Trial and Error Method


Trial and error method of clinical refraction includes the application of different trial lenses to determine the spherical and the cylinder correction needed. Results of autorefractometer may be used as a starting guideline. The steps may follow the following order:1. Determine the maximum plus or minimum minus spherical lens that gives the maximum acuity. Care must be taken to explain the patient to understand the difference between “changes that makes the letters clearer and changes that makes the letters darker and smaller”.2. Then check for astigmatic correction. Find out the axis first by rotating a cylinder lens in the step of 5°–10° in either direction and asking the patient whether the acuity improves. The important factor is to decide the amount of cylinder to be used for the purpose. Usually when the patient needs lowcylinder correction, it is difficult for him to pick the correct position of clarity while rotation. Under this situation a stronger cylinder lens may be used.3. Once axis is determined, the amount of cylinder correction can be determined by changing the cylinder lens at the respective axis.



DETERMINATION OF NEAR ADDITION


Around the age of 40 years when a patient starts squinting at the near reading object, he has achieved middle age and is assumed to have presbyopia. A presbyopic patient needs “near addition” or “add” as a correction to read comfortably in addition to distance correction. The “add” is the positive spherical lens power that is added to the distance correction of an individual. Near addition is determined with both eyes open with target kept at a distance of either 40cm or habitual near working distance. A careful distance refraction provides the foundation for determining the management of presbyopia. The optical correction for presbyopia is the sum of the refractive correction for distance correction plus the power of the near addition. Different methods may be applied to perform the test for near addition. Most common of them are:1. Cross grid test2. Near duochrome test3. Trial and error method.


Cross Grid Test


• Use phoropter• Place the cross grid at a near point distance• Reduce room illumination• Put ±0.50D cross cylinder with minus axis vertically in front of both eyes with distance correction in place• Now ask the patient which set of lines are clear and sharper— Horizontal or Vertical• Expected answer : Horizontal lines• Add plus lens binocularly until vertical lines are clear and sharper• Then reduce plus to achieve equal clarity and sharpness
• That’s the end point.

 
Cross Grid
Cross Grid

Near Duochrome Test



• Ask the patient to hold the Near Duochrome Test in his hand with distance correction in place in the trial frame at a distance as decided.• Duochrome with black rings or letters may be used. Keep both eyes open and ask the patient which black rings are clearer or darker. Three possible responses can be expected:1. Either black rings on green background is clearer and darker which indicates the need for plus lens.2. Or, black rings on the red background is clearer and darker which indicates the need for reduction of plus power.3. Rings on both sides – red and green background appear similar. In a young patient with active accommodation the preference for red and green will be alternate indicating that there is no need for any near “add”. Uncorrected or undercorrected presbyopes will have marked green preference. The most important thing to note is that this test applies only for a particular fixation distance used during the test. If the test distance is moved towards the eyes about 5 cm, the green preference will again be dominantand if the test distance is taken away by 3cm from the fixation point, the red preference will become dominant. Once the near addition is determined by any of the above methods, put the proposed near addition lens in the trial frame in front of the distance correction, ask the patient to hold the reading text chart and make sure that he has fairly good visual performance at a range of distance of his normal near working distance.



Trial and Error Method


Near addition may be prescribed by trial and error method. In this process near addition is decided based on the age of the patient and the lenses are put in the trial frame to verify that vision is fairly good and the patient is comfortable at his usual working distance. One problem with this approach is that it assumesthat all individuals of same age have the same amplitude of accommodation, which is not the case in practice. The typical age table with correspondingly required near addition may be designed as under:

 
Trial and Error Method

THINK AND GO AHEAD



All clinical steps of the refraction procedure are completed and the examiner has landed to a result. During the process the results may be affected by any of the following factors:1. The patient may provide wrong information by accident, especially if he is tired, bored or have trouble understanding the examiner.2. Accommodation may fluctuate and mislead the results, especially with patients who have active accommodation.3. Patient may squint to see or he may read by memory to misguide the results. This implies that there is scope for the intellectual process where the examiner has to think and decide the next suitable step for verifying results so obtained to get into the final step of clinical procedure of refraction.

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