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Module 4 Section 2 |
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Module 4: |
Accommodation and Near Vision | ||||||||||||||||
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Section 2: |
Near Add Determination | ||||||||||||||||
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If a presbyope wants to be able to read with glasses on, an "add" power must be added to the distance correction to bring the near point within reading range. The add can be given as a multifocal (bifocal, trifocal, or progressive) or as a pair of readers. Some myopes have the option of reading without glasses. For example, a -2.00 D myope with a +2.00 add will have a plano reading correction. The determination of the "correct" add power depends more upon the refractionist’s experience than on any particular refractive technique. This is because there are so many factors that affect a person’s ability to read comfortably. Factors affecting reading ability 1 Pupil size — Smaller pupils increase the pinhole effect, which increases the depth of focus (depth of field). Some older patients on Pilocarpine can read without an add if the lighting is bright. 2 Lighting — Brightly lit print is easier to read. The contrast is improved and the reflected light may contract the pupil, improving the depth of field. 3 Print size and print distance — Larger print is easier to read, up to a point. The closer the reading material is held, the larger the print appears. 4 Task requirements — An accountant will need better near vision than a garbage man. 5 Secondary task considerations — A piano player may want to increase the near focal distance in one pair of glasses rather than buy two pair of glasses (one for reading music, the other for reading print). 6 Body type and position — Some people, particularly those with long arms, like to hold reading material farther away than others do. 7 Previous correction — Many people don’t tolerate an increase in add power greater than .75 D at any one time. 8 Personality — Some ladies may delay getting reading glasses because of cosmetic concerns. Some gentlemen delay getting readers, or delay increasing bifocal power, because it makes them seem "weaker". 9 Low ametropia — Uncorrected low hyperopes complain of reading difficulty at an earlier age than uncorrected low myopes. 10 Contact lens wearers — Because of the vertex distance disparity, a contact lens corrected myope may need an add with her contact lenses before she will need an add with her glasses. She may need a stronger add while wearing contact lenses as compared to wearing glasses. A contact lens corrected hyperope may need an add with her glasses before she needs an add with her contact lenses. She may need a stronger add with her glasses than she needs while wearing contact lenses. 11 Health and medications — Diabetics typically need an add at an earlier age and need a stronger add for a given age group. What’s missing from the above list of factors that affect a person’s ability to read comfortably? The most important factor of all; the patient’s age. We are all aware that, in general, the older a person becomes (usually beyond the age of 40) the more difficult it becomes for that person to read. Many books on refraction publish a table that correlates a person’s age with the add power that should be needed. It is amusing that many of the books include a table and then the author tells you to ignore the table because there are so many exceptions. As items 1-11 suggest, there are many factors, but there are more patients who fall within the ranges of the table than do not. The following table has been derived from my own experience. If you do enough refractions you will, perhaps subconsciously, derive a table of your own. If you are inexperienced, this is a good one to start with .
Because of the many factors affecting reading ability, the table is but a general guide. So how do you determine the correct add for any given individual? Read 5 books and you will get 5 variations on technique. I have used the following technique for many years. I use it because it is straight- forward, fast, and reasonably accurate.
The starting point for add determination is the manifest refraction. 1 Rotate the arm down so that the Roto-chart is within the patient’s view at a distance of 16 inches (normal reading distance) as measured on the arm. I like to use the chart that has a 20/200 letter at the top and a 20/20 line at the bottom. 2 Adjust the vergence levers (figure below) on the phoropter to an inward position. This lines up the patient’s eyes with the target at 16 inches.
3 Adjust the light from the lamp so that the chart is evenly and diffusely illuminated (no glare). 4 Make sure both eye ports in the phoropter are open. This is a binocular test. Instruct the patient to look at the smaller letters toward the bottom of the chart, unless the patient has low vision. 5 Begin simultaneously adding plus sphere power to each eye .25 D at a time. If the patient has never had a reading correction (probably a 40-45 year old), ask him to read the 20/20 line before you begin adding plus power. If he can easily read the 20/20 line, he probably does not need an add yet. Here is an instance where the age/diopter table comes into use. If the patient is 40 to 50 years old, you will want to add the plus power slowly, because you do not expect the end point to be over +1.75. If the patient is over 50, you can click in the first 1.00 D quickly, and then slow down, because you expect the endpoint to be somewhere between +1.50 and +2.50. 6 At this point it is very important to listen to the patient’s comments as you are adding plus power. If the patient is not voluntarily commenting (most do), ask her if her vision is improving as you add the plus power .25D (one click) at a time. Continue adding plus as long as the patient reports definite improvement in her vision. When you reach the point at which an added click "looks about the same", back up to the last click at which improvement was seen. This is the endpoint. The endpoint add power is the endpoint (near) sphere power minus the starting point sphere (MR) power. Be sure that you write down your MR results before you begin add determination, so you don’t get lost. Example: Your patient reads well at 16 inches with -4.00+1.00 x 90. The distance reading was -6.00+1.00 x 90. You went from -6.00 to -4.00, therefore the add power is +2.00. If you think you may have gone too far in adding plus power, back up .50 D from your endpoint and test the patient’s vision. If she can easily read the 20/20 line, you probably did give too much. Make the lesser power your endpoint and take away another .50 D and retest the vision to confirm. After you gain some experience you will be on-the-money 95% of the time just by using the patient’s initial response as a guide. 7 Compare your endpoint to the age/diopter table. If the measured add power does not fall within the range for the patient’s age on the table, you may want to repeat the measurement for confirmation; unless you already have an explanation of why the patient would be out of the range. Most mistakes in the add power result from giving too much power. If in doubt, go with the lesser power. 8 This endpoint add power you have measured is not necessarily the add power that will be prescribed. This should be the add power that the patient needs to comfortably read at 16" (normal reading distance). If you know the patient likes to read at a distance closer than 16", you will need to add .25 to .75 D to the endpoint add. If you know the patient likes to read farther away than 16", you will need to subtract .25 to .75 D from the endpoint add. If you know the exact reading distance ahead of time (some patients actually measure it) you can use this distance at the beginning of the measurement process, instead of 16". Many patients are unsure of their reading distance. It is helpful if you observe the patient reading a near card or a magazine prior to beginning the manifest refraction. It also helps if you are the person who takes the history and checks the distance and near vision. Many problems with prescribed add powers in non-progressive multifocals (such as flat-top bifocals and trifocals) are due to the add being too strong. A strong add will bring the near point too close and reduce the range of focus. Many problems with the add power in progressive lenses stem from the add power being too weak. Some refractionists automatically add .25 D to the determined add power if the lenses will be progressive. This is because the maximum add power for a progressive lens is at the very bottom, nasal portion of the lens. Patients can find a "weaker" add point in the progressive lens as they look through a higher point in the progressive lens, but many patients can not comfortably look through the very bottom of the lens where the maximum add power is. Some progressive lens manufacturers say they have solved this problem. I think progressives are better than they used to be, but the problem persists. The proof can be found in patient complaints and in lensometer readings at different points in a progressive lens. So why not add an additional .50 or .75 D to the add of every progressive Rx? The problem is that, generally speaking, the greater the add power is in a progressive lens, the greater the peripheral distortion is. Don’t get me wrong. I think progressive lenses are the best choice for most presbyopes. A good progressive such as the Varilux Comfort, that is fit well, generally does not need an extra boost to the add power. When I first started doing refractions on patients with IOLs, I thought the add determination was a no-brainer. These folks don’t have a natural lens. They cannot accommodate! All I have to do is slide the Rotochart to the reading distance on the rod and read off the diopter power on the rod! In fact, I don’t even have to do that. All I have to do is write down a +2.50 add for every pseudophake who reads at 16". Right? Wrong! I don’t know the strange optics involved, but it doesn’t always work out that way. Some of these patients only need a +2.25 or +2.00 add to read well at 16". It is a good idea to measure these patients just as you would a patient with a natural lens. |
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This technique makes use of the
Rotochart that comes with most phoropters. The Rotochart is a card
with a window that is mounted on a 28 inch rod that extends from the front of
the phoropter. Six different charts can be rotated into the window on the front
and six on the back of the card, using a wheel at the bottom of the card.
The card can slide along the bar
to any distance from the patient’s eyes, up to about 28 inches.
The bar is graduated in centimeters, inches, and diopters. Please note that the
diopters refer to the add power needed to focus at that particular distance for
a patient who has no accommodative ability.