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24 Cards in this Set
- Front
- Back
- 3rd side (hint)
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With and against the rule astigmatism |
So called "with-the-rule" astigmatism refers to a cornea which is steeper in the vertical meridian. "Against-the-rule" astigmatism refers to a cornea which is steeper in the horizontal meridian. Oblique astigmatism covers the rest. |
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Is this against or with the rule astigmatism |
44.00 x 90, 43.00 x 180 with |
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With or against the rule astigmatism |
43.00 x 90, 44.00 x 180 against |
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Clinically, convergence insufficiency has the following characteristics: |
Exophoria at near that is greater than exophoria at distance Low AC/A ratio Receded near point of convergence Reduced fusional vergences |
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What is AC/A ratio |
The AC/A ratio is a measure of the amount of convergence for a given amount of accommodation. Normal ratio is 4.1-5.1 |
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What are the two methods for measuring AC/A ratio |
Heterophoria method: The level of accommodation is changed by changing the fixation distance of an accommodative target. The amount of convergence is measured for a given fixation distance. Lens gradient method: This method involves introducing plus or minus lenses with the fixation target at a fixed distance. The amount of convergence is measured for a given diopter lens power. |
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How to use riley prism |
This test uses the Risley prisms in the phoroptor. Both eyes are un-occluded with the distance or near manifest refractions in place. The target is an isolated letter on the Snellen chart on the line above the line of best visual acuity of the poorer seeing eye. The Risley prisms are placed before each eye, set up for horizontal power ("0"s at 90 degrees). See the section below on the Risley prisms. Instruct the patient to tell you if the letter becomes blurry, and when the letter becomes double. Slowly move the prism dials simultaneously inward to increase base-in prism until blurring occurs. Note the prism power (e.g. 2 D OD and 2 D OS = 4 D total) and continue adding prism power. When the images break into two, note the prism power again. From this point, slowly reduce the prism power before each eye until the image is single again (recovery). Record the total (OD plus OS) prism power for each position: blur, break, and recovery. Base-in prism tests NFV. Use base-out prism to test PFV. It is best to test NVF first, then PFV. Test at distance and at near (40 cm). The chart on the phoroptor near vision rod can be used for testing at near. |
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Riley prism info |
moving the arrow temporally dials in base-out prism, and moving the arrow nasally dials in base-in prism. |
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Near point of accommodation is measured how? |
Measure each eye separately by occluding the fellow eye Hold the card about 1 meter away from the eye and slowly move the target toward the eye. Instruct the patient to tell you when the letters are blurry and can no longer be read. When the patient reports blurring, stop moving and make a note of the distance (in centimeters) that the card is from the eye |
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Amplitude of accommodation |
The amplitude of accommodation is the inverse (reciprocal) of the near point expressed in meters. Let's look at an example:
Suppose you measure the near point of accommodation to be 14 cm. We must first convert this to meters by moving the decimal point two spaces to the left. Therefore, 14.0 centimeters becomes .14 meters. The reciprocal is found by dividing this number into 1. You might want to do this with your cellphone.
1/.14 = 7.14 diopters of accommodative ability
In practical terms, what does this tell us? It is widely assumed that the eye can only sustain half of its accommodative power for a substancial amount of time. In other words, our patient may only comfortably be able to accommodate 3.5 diopters (half of 7) for an hour of reading. Does this patient need an add power for normal reading distance? No, because we know from the lens power formula (D = 1 / f) that it takes 2.5 diopters to focus parallel light at a distance of 16 centimeters, which is considered to be normal reading distance.
There are tables that give you these numbers for "normal" people. For example, a normal 5 year old should have a near point at 6.3 cm, which translates to 16 diopters of amplitude. A normal 45 year old will have a near point of 28 cm, which translates to 3.5 diopters of amplitude. A normal 65 year old will havea near point of 200 cm, which translates to .5 diopters of amplitude.
Let's take a closer look at the 45 year old. The amplitude of accomodation is 3.5 diopters. Half of 3.5 is 1.75. We need 2.5 diopters of power to focus at 16 inches, so we subtract 1.75 from 2.5 to get the shortfall in power, which is .75 diopter. Thus our 45 year old would need an add of about +1.00, which is what many age/add tables will give you. |
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How to measure prism |
The patient fixes with both eyes on a distant target. A relatively low prism power is placed before one eye in a manner that allows the eye to still see the target. The apex of the prism points in the direction of the deviation. The alternate (cross) cover test is used to determine if there is still eye movement after the prism is in place. The power of the prism is gradually increased until there is no eye movement with the alternate cover test. This is the endpoint, and the diopter power of the final prism is a measure of the degree of deviation. |
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Maddox rod |
Maddox rod can be found in a trail lens set, as part of the auxiliary lens wheel on a phoropter, and on the end of some occluders. It is a group of cylinders (usually red) lined up side by side that makes a point light source appear as a line to the patient. The line runs perpendicular to the orientation of the rods. |
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Orientation of Maddox rod test |
If you want to look for or measure a horizontal deviation, place the MR (white or red) in the horizontal orientation before one eye. The fellow eye does not have a MR in place. Use the distance or near refractive correction for each eye.
Hold a penlight, muscle light, or the beam from a direct ophthalmoscope at distance or at near. Do this in a dark or dimly illuminated room. The light source must be bright.
The patient will see the point source of light (white) with one eye and a vertical line (red or white) created by the MR in front of the other eye.
Let us assume that we are using a phoroptor and the horizontal red maddox rod (RMH on the wheel) is in place in front of the right eye.
Ask the patient if he sees a white dot and a red line. If the patient cannot see one or the other, then he may be suppressing the image and the Maddox rod test will not work on him. It may be that the point light source is not bright enough and/or the room illumination is not dim enough. Give the patient enough time to "find" both images.
If the patient sees both the point light source and the line, then ask if the line goes through the light, or if it appears to be to the left or to the right of the light.
If the red line appears to go through the white dot (as pictured below), then the patient has no horizontal deviation. |
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The Von Graefe Technique |
It is performed with the Risley prisms on the phoroptor.
The setup:
The patient looks through the photoptor with the distance or near Rx in place.
The target is an isolated (small box) letter on the Snellen chart one line above the BVA of the worst seeing eye.
12 diopters base-in prism is dialed into the R-prism in front of the right eye.
Six diopters base-up prism is dialed into the R-prism in front of the left eye. Dim the room lights or turn them off. The patient should see something like this:
Confirm that the patient sees double. If not, the patient may be suppressing, or not understand. Either way, the test cannot be completed without the perception of two images.
Testing Horizontal deviations:
Always test horizontal deviations first. This is controlled by the R-prism in front of the right eye.
Move the R-prism wheel slowly in one direction and ask the patient if the images are getting closer or farther apart horizontally. If they are going farther apart, move in the other direction.
Ask the patient to tell you when the images are lined up directly one above the other. Record the prism power and base direction on the R-prism at that point. This is the measure of the horizontal deviation. The end point should look like this to the patient:
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Hirshberg vs. Krimsky
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The Hirshberg technique uses the estimated displacement in millimeters to estimate the degree of deviation. The Krimsky technique uses loose prisms to move the deviated reflex back to the "normal" position as a means of measuring the deviation.
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When is amblyopia suspected |
Amblyopia may be suspected when there is a one line best-corrected acuity difference between the two otherwise normal eyes. The diagnosis is usually not official unless there is at least a two line difference. |
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What are the types of amblyopia? |
Strabismic, refractive,Anisometropic amblyopia results when each eye has a different uncorrected refractive error,Amblyopia ex anopsia (from disuse) results from disuse of the eye from a physical cause such as a congenital lid ptosis, or a congenital cataract |
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Slab off problems |
When looking through a bifocal, we are only concerned with the power of the lenses in the 90 degree meridian. Slab-off problems are really induced prism problems, with the focus on the 90 degree meridian. |
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What is the formula for induced prism |
Induce prism (in diopters) = Lens power (in diopters) x Displacement (in centimeters). |
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Assuming a slab-off is needed with induced prism of more that 1.5 D, does this glasses prescription need a slab-off? What is your induced prism calculation? Our patient is looking 8 mm below center into the bifocal. |
The right eye is looking into a base down prism, and the left eye is looking into a base up prism. Since the bases are opposite, the prismatic effect will be compounding (added together).
Both lenses are spheres, so we will not have to use an optical cross to find the power at 90. The decentration is given as 8 mm for each eye, which converts to .8 cm.
induced prism OD 2 x .8 = 1.6 OS 1 x .8 = .8 total induced prism is 1.6 + .8 = 2.4 PD
This is above our 1.5 PD threshold, so the patient would likely benefit from a slab off. The answer is "B". |
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Assuming a slab-off is needed with induced prism of more that 1.5 D, does this glasses prescription need a slab-off? What is your induced prism calculation? Our patient is looking 6 mm below center into the bifocal. OD -2.00 Sph OS +1.00 - 3.00 x 180 |
Since the OS lens power is a sphero-cylinder, we will use an optical cross to find the power in the 90 degree meridian. The first step is to transpose the prescription.
OS +100 - 3.00 x 180 transposes to -2.00 -3.00 x 90 |
Since both lenses have a -2.00 D power at 90, there is no need to draw the problem because the prismatic effect will be base down in both eyes (minus lenses) and will be mathematically perfectly canceling, as follows:
OD 2 x .6 = 1.2 OS 2 x .6 = 1.2
The effect is canceling, so the amounts are subtracted. Total effect = 1.2 - 1.2 = 0 PD
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What are common causes of corneal sensitivity? |
Some of the causes of decreased corneal sensation are diabetes, multiple sclerosis, viral infection, chemical burn, corneal surgery, abuse of topical anesthetics, fifth-nerve palsy, and severe dry eye. |
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There are three ways to measure corneal sensitivity:
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Cotton wisp test Cochet-Bonnet aesthesiometer Non-contact corneal aesthesiometer |
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Test |
Test |
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