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Optometric Theory I

Terms in this set (309)

What is the snellen fraction and what does it mean?
The snellen fraction tells us "Test Distance / Distance at which the letter subtends 5 min of arc"
Numerator: test distance
Denominator: At what distance the letter subtends 5 min of arc
Test distance is 20ft or 6m
What does 5 minutes of arc mean?
5/60 of one degree
What does subtend mean and describe angular subtense
20/50 subtends 5 min of arc at 50ft
20/20 subtends 5 min of arc at 20ft
The bigger the distance (denominator) the bigger the letter
What is detection acuity?
Used for patients who cannot recognize the test letters, mainly babies
Use a patterned sheet to see if the babies can see the pattern - their eyes will naturally move to the patterned areas, usually striped
What are some different ways to perform resolution acuity?
Tumbling E test - 4 directions
Landolt C test - 8 directions
What are the differences between recognition acuity and resolution acuity?
Recognition acuity: must be able to recognize and read the letters being tested
Resolution acuity: Universally applicable to all patients as it does not require knowledge of a language or character
What is localization acuity?
Testing vernier acuity to see if the patient sees any space between lines.
Tests the smallest gap between the lines that the patient can resolve.
Can make it more challenging by separating the lines vertically
How is vernier acuity quantified?
The distance between the lines that the patient can identify and made into an angle. The patient will tell the doctor
Calculate the height of a letter. What is the vertical height of a 20/200 letter?
At 200ft, the angle is (5/60)°, but need to divide by 2 to get a right angle triangle.
Therefore at 200ft, the angle is (2.5/60)°.
tan (2.5/60) = x/200
x = 200 tan(2.5/60) = 0.145 ft = 1.745 in
Height of the letter = 2x = 3.49 in
Calculate the height of a letter. What is the vertical height of a 20/20 letter?
At 20ft, the angle is (5/60)°, but need to divide by 2 to get a right angle triangle.
Therefore at 20ft, the angle is (2.5/60)°.
tan (2.5/60) = x/20
x = 20 tan(2.5/60)
Height of the letter = 2x = 0.349in
A patient is viewing a 20/100 letter from the exam chair. What angle does it subtend at their eye?
The viewing distance is 20ft and the letter subtends 5' at 100ft.
tan(2.5/60) = x/100
2x = 1.75 in
tan(θ) = (1.75/2) / (20*12) = 0.0036 [multiply by 12 so that units are the same]
θ = 0.209° or 12.5 minutes
2θ = 25 minutes
A patient is viewing a 20/40 letter from the exam chair. What angle does it subtend at their eye?
10 min of arc
A 20/20 letter will subtend 5 min of arc, so just multiply by 2 and get 10 min
When viewed at a distance of 5ft, what angle does a 20/100 letter subtend at the patient's eye?
At 20/20, it is 5 min of arc at 20 ft
At 20/100, it would be 25 min of arc at 20ft
So at 20/100, it would be 100 min of arc at 5 ft
What is the pattern and relationship of distance and angle calculation?
Double distance = half angle
Half distance = double angle
When viewed at a distance of 10ft, what angle does a 20/40 letter subtend at the patient's eye?
At 20/20, it is 5 min of arc at 20ft.
A 20/40 letter will subtend 10 min of arc at 20 ft.
So at 10ft, a 20/40 letter will subtend 20 min of arc
In a vernier acuity test, the smallest separation between the vertical lines that a patient can resolve at a viewing distance of 40cm is 3mm. What is the arc at the eye?
tan(θ) = 1.5/400 = 0.00375
θ = 0.215° or 0.215 * 60 = 12.89'
2θ = 25.8'
How does a decimal equivalent work?
Uses Snellen ratio and makes it into a decimal
20/20 = 1
20/40 = 0.5
20/100 = 0.2
Calculate the vertical height of: (1) a 20/60 letter, (2) a 20/120 letter, and (3) a 20/400 letter.
When the patient is seated in the exam chair, what angle does (a) a 20/60 letter, (b) a 20/120 letter, (c) a 20/400 letter subtend at their eye?
You are standing on the corner of 42nd Street and viewing a building that is 256 feet tall from a distance of 100 feet. Calculate the angle subtended by the building at your eye if: (a) your eye is 6 feet off the ground and (b) you are lying on the ground?
What is minimum angle of resolution?
The angle that must be resolved for the patient to be able to identify the letter correctly.
How are snellen letters constructed?
Snellen letters are made on a 5 by 5 grid, therefore the angle subtended by each limb of the letter = 1 minute
How is minimum angle of resolution (MAR) calculated?
Reciprocal of the Snellen fraction
VA = 20/60; MAR = 60/20 = 3'
VA = 20/80; MAR = 80/20 = 4'
What is the MAR for 20/60? 20/80?
VA = 20/60 ; MAR = 60/20 = 3'
VA = 20/80 ; MAR = 80/20 = 4'
What is LogMAR?
Uses the logarithm of the MAR
For 20/200, what is the logMAR? 6/24? 6/6? 20/15?
20/200; MAR = 10 ; logMAR = 1.0
6/24; MAR = 4 ; logMAR = 0.60
6/6; MAR = 1 ; logMAR = 0.0
20/15; MAR = 0.75 ; logMAR = -0.12
How is the logMAR chart, how is it different from the Snellen chart, and how is it assessed?
0.00 is equivalent to 20/20
Every letter read correctly counts as -0.02 (no matter where on the chart it is)
If pt reads HNVHF, acuity is 0.30
if pt reads HNVXX, acuity is 0.36
if pt reads RP on the next line in addition to above, acuity is 0.32
What are the characteristics for the LogMAR chart?
1. Same number of letters on every line (5)
2. Each line is 0.10 logMAR less than the previous line
3. Space between each letter is equal to the width of the letter
4. Space between each line of letters is equal to the height of the letters in the lower line
5. Can be averaged, and can be used in statistical testing
What is normal VA?
20/10; Snellen forgot to multiply by 2
How do you calibrate a chart?
In a short room, you need to make sure that your decimal equivalents are correct
Example: 20/40 decimal equivalent at 12ft is 12/24
Adjust the physical size of the letter to make sure the decimal equivalents are correct
What would you record if the patient cannot read the top letter on the chart?
Move the patient closer to the chart or move the chart closer to the patient
Your exam lane is 14 ft long. When seated in the chair, the patient is unable to read the largest letter (labelled 20/200) on the chart. But they are just able to resolve this letter at a viewing distance of 8 ft. What is their VA?
test distance / distance at which the letter subtends 5 min of arc
New test distance = 8ft
The letter denoted 20/200 should more correctly be described at a 0.1 decimal equivalent letter. It is really a 14/140 letter (subtends 5 min of arc at 140ft)
Therefore, the correct VA is 8/140
If the patient cannot see anything after bringing the chart closer, what other methods would you use?
Hand motion (HM)
Light projection (Lproj)
Light perception (LP)

If none of these tests work, they have No Light Perception (NLP) and pt is basically blind
Why should finger counting not be used?
Hands are different shapes, colors, sizes
If they can count fingers, they can see a letter at some distance
What are the methods to record near VA? Describe each method
Standard near VA is tested at 40cm or 16in
1. Snellen: uses the same principles as distance chart. However, because testing is done at 16" (40cm), the numerator should be 16 or 40 instead of 20/20
2. Jaeger: letters range from J1 (small) to J20 (large). Since the dimensions of this chart are not standardized, it is of limited value
3. M notation: M denotes the distance in meters at which the letter subtends 5 minutes of arc. Thus, the M number corresponds to the denominator of the snellen fraction (expressed in meters)
When testing 40cm, if the smallest type which can be read is 0.5M, then acuity is 0.40/0.50
4. Point: one point equals 1/72 of an inch. The smallest size used when measuring near VA is generally N5.
What does the "point size" refer to?
Point size does not refer to the size of the letter. Rather, it refers to the "block" on which the letter is (was) mounted during printing. So 10 point means the block was 10/72 inches high
How is "point size" used for upper case letters?
All upper case letters are assumed to fill the block. So in the case of 10 point upper case letters, both the block and the letters are 10/72 inches high
How is "point size" used for lower case letters?
For lower case letters without ascenders or descenders, the height of the letter is assumed to be half the height of the block. Therefore, the size is 5/72 inches high.
What factors affect VA?
Blur, contrast, retinal eccentricity
What is a blur circle?
Rays are focused either behind or in front of the retina, resulting in a blurred retinal image. Because the retinal image is both blurred and circular, it is known as a blur circle
What happens to the blur circle if the degree of refractive error is greater?
The greater the degree of refractive error, (the further away from the retina the image is formed) the larger the blur circle
Can the image be resolved if the blur circles are overlapping? Are not overlapping?
Overlapping: no
Not overlapping: yes, but may be blurry
How can you improve VA without a lens? How does this work?
Using pinhole - reduces the diameter of the blur circle by constricting the light rays. Does not change the image distance
How is contrast calculated?
Contrast = (Lmax - Lmin)/(Lmax+Lmin)
L = Luminance
Contrast units = percentage
How much does contrast affect visual acuity?
Visual acuity declines with decreasing contrast
Up to 40% contrast, the relative acuity does not change much
Below 40% contrast = exponential decrease in acuity
What is retinal eccentricity?
Retinal eccentricity describes how far a point on the retina lies away from the fovea
Usually expressed as an angle
What is the fovea?
The most sensitive part of the retina
When we look directly at something, we usually place the image on the fovea
An image landing on the fovea is seen as "straight ahead"
What is the visual acuity dependent on retinal eccentricity?
Visual acuity declines rapidly as a function of retinal eccentricity
What are some important numbers for refraction?
42% of the adult population of the USA are myopic
Uncorrected refractive error is the second leading cause of blindness worldwide (#1 worldwide is cataracts)
What are spherical refractive errors?
Having the same refractive error in all directions (meridia).
myopia or hyperopia
What is a refractive mypoe? An axial myope?
Refractive: the light is being refracted/bent too much at the lens or cornea
Axial: when the axial length is too long
What is a refractive hyperope? An axial hyperope?
Refractive: the light is not being refracted enough
Axial: when the axial length is too short
How is lens power stated?
The power of a lens is stated as the reciprocal of the focal length in meters. Lens power is measured in diopters
Which side of the lens is negative? Positive?
Incident side is negative
Emergence side is positive
What is the equation for lens power and refraction?
(n'/l') - (n/l) = F
What is the far point of the eye?
The point conjugate with the retina of the unaccommodated eye.
"Pr" and "Mr"
What are conjugate points?
The positions of the object and image are called conjugate points.
In this diagram, T and T' are conjugate points. If the object lies at T, then the image falls at T'. Similarly, if the object is at T', then the image lies at T
What is the "object" in a lens system?
A point in space where the incident rays theoretically cross
What is "l"?
Distance of the object to the lens
What is the "image" in a lens system?
Where the "emergent" rays cross
Where is the far point of a myopic eye?
The far point of a myopic eye lies in front of the eye. The distance from the far point to the front of the eye = k meters
k = 1/refractive error in diopters
How do you calculate the refractive diopters using the far point?
1D myope: far point lies 1/1 = 1 meter in front of the eye
2D myope: far point lies 1/2 = 0.5 meter in front of the eye
5D myope: far point lies 1/5 = 0.2 meter in front of the eye
Where is the far point of an emmetropic eye?
For an emmetropic eye, the far point is at optical infinity
Where is the far point of a hyperopic eye?
Far point of a hyperopic eye lies behind the eye. The distance form the far point to the front of the eye = k meters, where k = 1/refractive error in diopters
How can you turn a hyperope into a myope?
Overcorrect them
How do you correct refractive errors?
Any refractive error can be corrected by a lens whose second focal point coincides with the patient's far point
Where does the image lie if the object is at infinity?
When the object is at infinity, the image lies at the second focal point.
Distance from the lens to the second focal point is called the secondary focal length of the lens (f')
Where does the object lie if the image is at infinity?
When the image is at infinity, the object lies at the primary focal point.
Distance from the lens to the primary focal point is called the primary focal point of the lens (f)
How do you correct a myope and a hyperope?
By using a lens whose second focal point coincides with the patient's far point
For multiple lenses, what is the object for the second refractive surface?
The image formed by the first refractive surface becomes the object for the second refractive surface
Where must the focal length be for a perfectly corrected lens?
We need the lens whose focal length is equal to the distance from the lens to the far point.
The power of the lens must equal 1/fc
fc = focal length
What is the back vertex distance (BVD)?
BVD = distance from the back surface of the lens to the front of the eye
What happens if you change the BVD?
Change BVD = change in power of the correcting lens
The far point lies 125mm in front of the eye. What Rx would you order for a bvd of (i) 15mm, (ii) 9mm, (iii) 0mm?
i. -(125-15) = -110mm. F = 1/-0.11 = -9.09
ii. -(125-9) = -116mm. F = 1/-0.116 = -8.62
iii. -(125-0) = -125mm. F = 1/-0.125 = -8.00
When should you specify BVD for a patient?
All the time
Why are hyperopes harder to correct?
Because of accommodation. Individuals under 50 years of age can accommodate to bring the light rays converging to the retina. Many people who are hyperopes do not know they are hyperopic
What is presbyopia?
Accommodation is harder, so near distance is harder to read
What must we make sure to do to refract a patient with hyperopia?
We want to make sure the far point is at the retina with NO ACCOMMODATION
What are some ways to stop a hyperope from accommodating?
Fogging: turn the patient into a myope by giving them too much plus lens power
Cyclopegic drugs: temporarily paralyze some or all of the patient's accommondation (dilation)
How do you know when you have turned a patient into a myope when fogging?
You know when you have turned the patient into a myope (fogged them) because their visual acuity will start to drop
What is total hyperopia?
When you get rid of all the accommodation with a cycloplegic drug and get the best VA
What is manifest hyperopia?
When you find the maximum plus lens that gives the best VA
What is latent hyperopia and how do you calculate it?
Latent hyperopia = hyperopia masked by accommodation
Latent hyperopia = Total hyperopia - Manifest hyperopia
Example: you perform a maximum plus to best VA refraction and get a finding of +5.00 (Manifest). You instill a cycloplegic agent and re-examine the patient. You now get a finding of +7.50 (Total). So there was 2.50D of latent hyperopia, or the patient was accommodating 2.50D during the precycloplegic measurement
What do you do when you do not want to dilate a patient?
Fog the patient for about 30 minutes and find maximum plus lens that gives the best VA
What is facultative hyperopia?
The difference between the result found under fog and the maximum plus lens that gives the best VA
Facultative hyperopia = Hyperopia under fog - Hyperopia before fogging
What is absolute hyperopia?
The amount of hyperopia that exceeds the maximum accommodation.
For example, a +8.00 hyperope with a maximum accommodation of 5D and 3D of absolute hyperopia. This patient, with an absolute hyperopia of 3D, does not usually accommodate to compensate for their refractive error since even with maximum accommodation, the target will still be blurred
What is astigmatism?
Astigmatism is a refractive condition where different refractive errors exists in different meridian of the eye. It is specified as the difference between the two meridian
How do you calculate for astigmatism?
Difference between the power of one meridian and the power of the other meridian
How do you correct astigmatism?
Corrected by a cylindrical (cyl) lens that has maximum power in one meridian, and zero power 90 degrees away from the direction of maximum power
What is cylinder axis?
The direction of the zero (additional) power is called the cylinder axis. The maximum power of the cylinder lens lies 90 degrees from the axis
What are some rules for the cylinder axis?
The horizontal meridian is always 180 (never zero)
The vertical meridian is always 90
Only numbers between 1 and 180 are used
Never write a degree sign
How do you use a combination of sphere and cylinder?
Correct one meridian with a spherical lens (a lens that has the same power in all directions.
Correct the other meridian with the difference with the axis at the meridian of the spherical lens
Consider a patient who has 3D of myopia in the vertical meridian and 1D of hyperopia in the horizontal direction. The vertical and horizontal meridia will be corrected with -3.00D and +1.00D, respectively
Correct horizontal meridian of +1.00D
Correct vertical meridian of -4.00D at axis of 180 (because horizontal meridian has 0.00D)
+1.00-4.00x180 (minus cyl)
or
Correct vertical meridian of -3.00D
Correct horizontal meridian of +4.00D because -3.00D made the initial +1.00D horizontal meridian more hyperopic by 3.00D
Axis is 90 because vertical meridian is zero
-3.00+4.00x090 (plus cyl)
How do you transpose?
The new sphere is the sum of the original sphere and the original cylinder.
The new cylinder is flipped from the original cylinder.
The new axis is 90 degrees from the original cylinder in between 0 and 180 degrees
Transpose -2.00-4.00x060
New Sphere: -2.00 + -4.00 = -6.00
New Cylinder: -4.00 -> +4.00
New Axis: 60+90 = 150
What are some rules on writing prescriptions?
OD is always recorded before OS
Plus and minus signs must be shown
Do not place a degree sign after the cylinder axis
Always place at least one zero before the decimal point
Always record two digits after the decimal point
Never write a D for diopters
Eighth diopters are recorded rounded down (0.125 - 0.12)
It is not essential to write 3 numbers for the axis
What is refractive ametropia?
A refractive hyperopic eye is hyperopic because it is underpowered, but it has a normal axial length
A refractive myopic eye is myopic because it is overpowered, but it has a normal axial length
What are the assumptions to be made for an eye?
Emmetropic eye has a power of 60D and an axial length of 22.22mm
n = 1.333
How do you calculate eye power?
If an emmetropic eye has a power of 60D, then a 3D refractive hyperopic eye will be less than 60. 60 - 3 = 57D
If an emmetropic eye has a power of 60D, then a 3D myopic eye will be more than 60. 60 + 3 = 63D
How do you calculate axial length?
An axial hyperopic eye will have an axial length shorter than normal.
An axial myopic eye will have an axial length longer than normal
What is the axial length of a 2D myopic eye?
(1.333/l') - (1000/-500) = 60
l' = 22.98mm
This makes sense because myopic eye is longer
What is the axial length of a 4D axial hyperopic eye?
(1.333/l') - (1000/250) = 60
l' = 20.83mm
This makes sense because hyperopic eye is shorter
An eye is fully corrected by +12.00-3.00x090 at bvd 10mm. What is the required power if the bvd was changed to 15mm?
Do calculations for each meridian
This lens is +12.00 at 090 and +9.00 at 180
Do bvd calculations with these Rx
The sphere is always the most plus (or least minus) power
The axis is given by the direction of the most plus
+11.32-2.71x090
"the sphere power changes and the cyl power changes but the axis does not change"
What is the normal range of bifocals ADD?
+0.50 to +4.00
What are prisms?
Prisms bend light, but they don't change the vergence of the beam
How is light deviated in a prism?
The light is deviated towards the prism base and the image is displaced towards the prism apex
What is prism diopter?
1 prism diopter is a displacement of 1cm at viewing distance of 1 meter
How do you calculate the displacement of prisms?
Displacement (cm) = prism power x viewing distance in meters
What are the two types of prism?
Ground (worked) prism: this has the same magnitude of prism all over the lens
Prism by decentration: the amount of prism varies across the lens
What is DBC?
The distance between the optical center of one eye to the OC of the other.
Same as PD
If the image is displaced down in a lensometer, the patient has a ___________ lens
Base down prism
If the image is displaced temporally in the right lens in a lensometer, the paitnet has a?
Base out prism
What is prentice's rule?
P = c x F
P = prism by decentration
c = decentration in centimeters
F = power of the lens (in D)
What is OC?
The optical center is the point on the lens where the is zero prism by decentration
The DBC is the distance between the optical centers of the OD and OS lens
The PD is the distance between the right and left pupils
The line where the 2 triangles meet is considered to be the optical center of the lens:
- Plus lens: two triangles with the bases against each other
- Minus lens: two triangles with the apex against each other
What is CD?
The near centration distance at the spectacle plane
Gets smaller if spectacle plane extended outwards
How is PD recorded?
PD (mm) / near CD
example: 64/60
Near CD is approximately 3-4mm shorter
What is the center of rotation?
Usually 13mm behind the cornea, the point in the globe where the eye is rotated
How do you calculate near CD using values given with PD?
Similar triangle method
How far is the pupil from the cornea, usually?
5mm
What must you remember with PD?
PD is the distance between the pupils, not the corneal plane
How do you calculate total prism?
Base in and Base in: add together
Base out and Base out: add together
Base in and Base out cancel each other out
Base up/down + Base up/down = difference
- Example: 2 BU OD, 3 BU OS = 1BU Total (same effect as 1 BU OS or 1BD OD)
- Vertical prisms are described by the relative separation of the two images
What happens when there are two prisms in front of one eye?
When vertical prisms are in front of the same eye, simple add them together
How could you cancel ground prism on the lensometer by decentration?
Go opposite
If 2 BU, go 2 BD to center in lensometer
What is retinoscopy?
Simple, inexpensive method of determining the patient's refractive error
What is subjective vs objective?
objective: does not require any response on the part of the patient (ALWAYS BETTER)
subjective: requires the patient to make judgements and respond to questions, such as which looks better, 1 or 2
What is "reflex"?
A red reflection that you can see inside the pupil
What are the different movements that are possible for the reflex in the retinoscope?
With movement, against movement, or no movement (neutral)
What determines the direction of reflex in a retinoscope?
Direction of reflex movement depends on the position of the patient's far point
What kind of movement would you see if the far point is behind the patient's eye or behind the retinoscope mirror?
If the patient's far point lies either behind their eye or behind the retinoscope mirror, then you will see WITH movement
What kind of movement would you see if the far point is in between the patient's eye and the retinoscope?
If the patient's far point lies between their eye and the retinoscope mirror, then you will see AGAINST movement
What kind of movement would you see if the far point coincides with the retinoscope mirror?
If the patient's far point coincides with the retinoscope mirror, you will see a neutral reflex
If you see with movement, how do you change the lens in a retinoscope?
With movement = move into plus direction
If you see against movement, how do you change the lens in a retinoscope?
Against movement = move into minus direction
What is bracketing?
Start off iwth big steps and then go down to smaller steps
What is the distance form the retinoscope to the patient called and how far is it?
Working distance = distance from the retinoscope to the patient
The neutralizing lens is not the same as the correcting lens
50cm: -2.00D
67cm: -1.50D
If the working distance is 50cm and the determined retinoscope reading is +4.50D, what is their final corrected power?
+2.50D
Mr. Thomas has -3.25 lens in place after working up retinoscopy. You performed the retinoscopy at a working distance of 67cm. What is his actual prescription?
-4.75D
What is the calculating equation for retinoscopy?
Correcting lens = neutralizing lens - working distance
You observe a neutral reflex at a working distance of 50cm through a -4.50 lens. What is the Rx?
-6.50
You observe a neutral reflex at a working distance of 67cm through a +2.00D lens. What is the Rx?
+0.50D
How do you do retinoscopy for astigmatism?
Do retinoscopy along each of the principal meridia
What is the benefit to retinoscopy vs autorefractors
Retinoscopes are cheaper and can control accommodation much better than autorefractors
If the far point is in between the patient and the retinoscope, what could the patient be? Hyperope, myope, or emmetrope?
Myope
If the far point is behind the patient or behind the retinoscope, what could the patient be?
Hyperope, myope, or emmetrope
What is the aim of keratometry?
To measure the power and astigmatism of the anterior corneal surface, which is the most powerful refractive element in the eye
How much power do most corneas have?
40 and 45D which contributes to 60-75% of the eye's total refractive power
Why is keratometry important?
To determine the site of ocular astigmatism (corneal, noncorneal)
When fitting contact lenses
When ocular refraction is difficult
To determine if ametropia is axial or refractive
To examine corneal health
How are keratometry findings recroded?
As the power and orientation of the principal meridia (directions of maximum and minimum power) or in terms of the correcting cylinder:
For 42.00M 45 / 43.75 M 135,
i) add +1.75 to the 45 M to make the power 43.75
ii) add -1.75 to the 135 meridian to make the power 42.00
So the astigmatism can be corrected using a -1.75 cylinder at x045
The power in the axis meridian here is 42.00, so this can be written as: -1.75 x045 AM 42.00
How do you identify if the astigmatism is corneal or non-corneal?
For a finding of 42.00 M 45 / 43.75 M 135 indicates that there is 1.75D of corneal astigmatism. If the Rx for this eye is -3.00-1.75x045, ALL of the astigmatism in the eye must be at the cornea.
For a finding of 43.00sph keratometry and an overall refraction of +2.00-2.00x090: the eye has 2D of astigmatism, but none of this is corneal. Therefore, the astigmatism must be noncorneal.
What is with-the-rule astigmatism and against-the-rule astigmatism?
With-the-rule: the vertical meridian (+-20) has the highest dioptric power. This can be corrected by a minus cyl axis of 180 (+-20)
Against-the-rule: the horizontal meridian (+-20) has the highest dioptric power. This can be corrected by a minus cyl axis 90 (+-20)
What is oblique astigmatism?
Astigmatism that does not meet the definition of with or against the rule. Maximum power along 35 degrees or corrected by a minus cyl axis 120 degrees is termed oblique astigmatism
How does with or against the rule chagne by age?
Below 45 years of age, with the rule astig is more common
Over 45 years of age, against the rule astig is more common
What is irregular astigmatism?
Astig where the principal meridia are NOT 90 degrees apart.
Can occur due to corneal irregularities and has been associated with refractive surgery. Irregular astigmatism cannot be corrected with spectacles, but may be helped with rigid contact lenses to provide a smooth anterior surface
How do you determine non-corneal astigmatism?
Ocular (total)astigmatism = Corneal astig + non-corneal astig
Example. Refraction: +3.00-1.00x180; KEratometry: -1.00x180 AM 42.00
The refraction indicates the ocular astig which is -1.00D of wtr.
Keratometry indicates the corneal astig which is 1.00D of wtr.
Ocular (total) astig = Corneal astig + non-corneal astig; so non-corneal astig = zero
What is the general rule of astigmatism when it comes to wtr and atr?
wtr and atr astig cancel each other out
3D wtr and a lens with 1D atr = 2D wtr
Example. Refraction: +2.00sph; keratometry -1.50 x 090 AM 41.50
Ocular astig = zero
Corneal astig = 1.50 atr
Therefore, non-corneal astig must be 1.50D wtr
What is Javal's Rule?
OA = 1.25*CA + 0.50atr
-2.00 x 175 AM 43.00. Use Javal's rule
CA = 2.00 wtr
therefore, 1.25*2.00wtr + 0.50 atr = 2.00 wtr
How can I estimate if the refractive error is axial or refractive?
If Rx = -9.00sph, we would expect high corneal power. If K = 53, most likely refractive. However, if K = 42, this pt is most likely an axial myope.
If Rx = +-12.00, we would expect high corneal power. If K = 35, most likely refractive. However, if K = 42, this pt is most likely axial hyperope
How can you measure the health of the cornea?
clear, crisp reflected "mires" circles
Sphere = round
Astigmatism = oval
unhealthy cornea = blurred mires
How do mires change in dry eye patients?
They clear up when pt blinks
How do you convert radius and power to each other?
F = (n' - n)/r
Is curvature or power more important?
Depends on what you are using it for
Power if you want tot know how much corneal astig is present
Radius if you are fitting contact lenses
Which part of the cornea does the keratometer actually measure?
Measures the annulus around the corneal apex
How does corneal topography work?
For a more thorough estimation of corneal shape - uses a large number of points on the corneal surface
What are corneal topographers used for?
Fitting CL, especially RGPs
Keratoconis
Orthokeratology
Pre and post-refractive surgery
IOL calculations pre-cataract surgery
What is the point of subjective refraction?
To determine a prescription through which the patient can see comfortably
To assess the patient's blur discrimination
Because you are not refracting an eyeball - it has someone on the end of it
When you cannot perform objective testing due to media opacities or pinhole pupils
How do the rays focus at the focal line?
Vertical focal line: At the point where the horizontal rays focus, the image has height but no width (vertical line image)
Horizontal focal line: at the point where the vertical rays focus, the image has width but no height (horizontal line image)
What is the circle of least confusion (COLC)?
The dioptric midpoint between the two focal lines
How do different lenses move focal lines?
Plus lenses move focal lines anteriorly
Minus lenses move focal lines posteriorly
(think how lens moves the focal length when you give a plus lens or minus lens)
What are the steps of subjective refraction?
1) Find the best sphere
- The minimum minus lens or minimum plus lens that gives the best visual acuity
2) Cylinder refinement
- JCC: consists of plus and minus cylinders of equal magnitude, oritented with their axes 90 degrees apart
- Red dots are the minus cylinder axis (where the minus cylinder IS NOT)
- Plus dots are the plus cylinder axis (where the plus cylinder IS NOT)
- have the patient focus on either: one line above the best VA so far, two lines above the best VA so far, or Verhoeff rings
Set JCC so that the axes lie 45 degrees away from the cyl axis determined by retinoscopy
Flip JCC while giving pt options, and turn the axis towards where they like the red dots
What is the interval or sturm?
The dioptric distance between the focal lines
How much sphere do you change for every cyl?
-X/2 sphere for every X diopter cyl change
Why do you change the sphere power depending on how much you change the cyl?
Because you want to keep the COLC on the retina
In what direction do minus lenses move the focal line?
Minus lenses move the focal line backwards, thus cyl only moves the front focal line backwards
Determining the axis allows us to only move the front focal line, which then also moves the COLC
The sphere moves everything uniformly - plus lens moves everything forward and minus lens moves everything backwards
What are some issues with JCC?
Requires patients to remember the earlier presentation
Rectilinear targets are not optimal for keeping COLC on the retina (Verhoff rings best)
inconvenient to change power of JCC on phoropter (can use handheld JCCs over phoropter)
What do you need to do for fan charts?
Need to start by placing the back focal line on the retina
Start by finding best sphere and putting COLC on retina
Estimate how much astigmatism is required with VA
Add plus sphere that equals half the estimated cyl
Remove the cylinder
Show the fan chart: If they all look equal, patient has no astigmatism or the COLC is on the retina
The clearest line on the fan is the same as the orientation of the focal line nearest the retina (from the patient's perspective)
Show patients "blocks" that show a bunch of their clearest lines and their least clearest line
Add cyl along that axis so that the two blocks are equal
Determine how much astigmatism correction is required with VA
How does VA and spherical refractive error relate?
After ret: +3.00-1.50x060; after best sphere: +3.50-1.50x060 (VA = 20/30)
Estimated astig still uncorrected = 1.00D
So, add +0.50 to the sphere
Now in phoropter: _4.00-1.59x060
After removing the cyl: +4.00 in phoropter
Example: Rx: -0.50-2.00x030; which focal line is closest to the retina?
compound myopic astigmatism
This patient is 0.50D myopic along the 30 meridian and 2.50D myopic along the 120 meridian
Thus, the least ametropic meridian is the 30 meridian
so the 120 focal line is closest to the retina
Therefore, the line that they will report as being clearest is going to be 60 meridian (180-120)
Example: The patient reports that the vertical line appears clearest
The pt is telling you that the focal line nearest the retina (from their perspective) = 90
Therefore, the focal line nearest to the retina (from the doctor's perspective) = 90
Therefore, the meridian nearest the retina (from the doctor's perspective) = 180
Example: The patient reports that the 30 line appears clearest
So the focal line nearest the retina from the pt's perspective = 30
So the focal line nearest the retina from the doctor's perspective = 180-30 = 150
So the meridian nearest the retina from the doctor's perspective = 150-90 = 60
After using the block, how do you tell if astigmatism is corrected?
Move the sphere (the entire internal of sturm) forward
If the COLC is on the retina, then moving the sphere would make on focal line nearer on the focal line than the other and one meridian of the block look clearer than the other
What is second sphere refinement?
Refog and unfog the patient to find the maximum plus (or minimum minus) lens that gives the patient their best possible VA
Do not stop at 20/20 unless that is the absolute best the patient is capable of. Otherwise you are giving them below average acuity
What are some check tests to perform to see if overminusing?
+0.50 sphere: turn the patient into a 0.50 myope and show them the lowest line they can see. They should have no hesitation in telling you that their vision has gotten worse
+1.00 sphere: turn the patient into a 1.00D myope and measure their VA. Their VA should be in between 20/40 and 20/60
Duochrome: blue or green is refracted more than red light (blue bends best)
What is the point of binocular balancing?
To ensure that the accommodative stimulus in each eye is equal
What is phoria?
In most individuals, the eyes are in different positions under monocular versus binocular viewing conditions. If patient has exophoria, they need to converge to see forward => convergent accommodation
What happens in exophoria
When switch from monocular viewing to binocular viewing, the eyes have to converge => convergent accommodation
What is successive alternate occlusion (SAO)
1. Fog each eye by +1.00D
2. Alternately cover each eye and ask pt which eye is clearer
3. Add more fog to the less blurred eye until 2 eyes appear equally blurry
4. Reduce fog OU until get to maximum plus lens that gives best VA
This is technically not binocular balancing
What is vertical prism dissociation?
1. fog each eye by +1.00
2. Add prism
3. Add more fog to the less blurred eye until equal
4. Find max plus lens
This method is not binocularly fused
What is Humphriss immediate contrast?
1. Fog one eye. Pt views duochrome chart and responds based on unfogged eye
2. Test right eye with left eye fogged - reach an endpoint
3. Repeat with left
Best method because pt is binocular and is fusing the target
What is the septum (turville infinity balance)
Use a septum to separate the visual fields of the two eyes
1) fog each eye by +1.00D
2) ask patient which side of the line is clearer
3) add more fog until both sides are equally blurry
Good for binocular balancing but hard to get septum into the middle of the room unless there is a mirror
What is the polaroid technique (vectograph)?
Polaroid filters only allow transmission of light through one plane
There are 45 or 135 polaroid filters on phoropters
A 45 degree light is transmitted through the 45 degree polaroid but is blocked by the 135.
The filter that blocks the light will see the letter (black space on a white background)
The filter that transmits the light will not see the letter (white pace on a white background)
What is a polarized duochrome?
The duochrome is polarized so that the left two circles are only seen by the right eye, while the right two circles are only seen by the left eye
Fastest way to do it because no fogging is needed
Adjust to the same duochrome endpoints
What is accommodation?
A change in the refractive power of the crystalline lens, resulting from contraction of the ciliary muscle
Contraction of the ciliary muscle reduces the tension in the zonular fibers, allowing the lens to bulge forward.
What id ED?
Equatorial diameter: decreases when accommodate
What is anterior and posterior pole?
Anterior pole: Anterior tip of the lens - pushes forward when accommodate (radius decreases)
Posterior pole: posterior tip of the lens - radius decreases only slightly because of vitreous
Distance from anterior to posterior pole increases
How do you calculate the stimulus to accommodation?
Stimulus to accommodation = required power - unaccommodated power
What does the power of the eye need to be to focus the near target 250mm on the retina assuming normal conditions?
Stimulus to accommodation = required power - unaccommodated power
4D
What does the power of the eye need to be to focus the near target 250mm on the retina for a 2D myope?
64 - 62 = stimulus to accommodation = 2D
For a 4D myope, what is the stimulus of accommodation for a target at 250mm on the retina?
64 - 64 = stimulus to accommodation - 0D
What are the types of accommodation?
Tonic accommodation
Convergent accommodation
Blur driven accommodation
Proximal accommodation
What is tonic accommodation?
Caused by underlying tonus of the ciliary smooth muscle which never fully relaxes
Mean value = 0.50D
What is convergent accommodation?
When eyes converge towards each other, this stimulates convergent accommodation (CA)
Typically, 1 meter angle (~6PrD) of convergence will stimulate 0.40D of accommodation ot give a convergent accommodation to convergence ratio of 0.40D/6PrD
What is blur-driven accommodation?
Natural brain driven accommodation to clear retinal blur
What is proximal accommodation?
Caused by "awareness of nearness" of an object
Does accommodative response usually correspond with accommodative stimulus?
No, accommodative response is usually less than the accommodative stimulus except for accommodative stimulus less than 1D
Response is often greater in far targets than they are in near targets
What is lead accommodation and lag accommodation?
Lead: Accommodative Response > Accommodative Stimulus
Lag: Accommodative Response < Accommodative Stimulus
What is depth of focus?
For a GIVEN object distance. The distance in diopters through which the image-receiving surface can be moved without detriment to the quality of the image, or for a fixed image receiving surface, the greatest focusing error consistent with this requirement
What is depth of field?
The range of object distances expressed in diopters within which visual acuity does not deteriorate
What is hyperfocal distance?
The location of the point conjugate with the retina when the distal of the depth of field lies at optical infinity
Emmetropic pt = beyond infinity to 4m away without accommodation
Hyperfocal pt = infinity to 2m away without accommodation
VA is the same for the two
Basically doing maximum plus the best VA
If you are working in a 4m room and get max plus best VA at +3.00-1.00x090. What lenses would you prescribe for a hyperfocal distance refraction?
Since max plus to best VA places the distal end of the depth of field at the chart of 4m away, an addition -0.25 sph must be added to get it to "infinity" of 6m away
Minus spheres move DOF ______ the eye
away from
Plus spheres move the DOF _______ the eye
towarsd
Accommodation moves the DOF ________ the eye
towards
What is the minimum accommodative response?
The first time the target will appear clear when accommodating- where the proximal edge of the DOF coincides with the target
You perform an HDR on a patient. What is (i) the minimum and (ii) the maximum that they could accommodate to see a target clearly at a distance of 20cm (assume a DOF of +-0.40D), and HDR places the distal edge of the DOF at infinity
Accommodative lag! Person wants to accommodate 5D but they only really need 4.2D
A patient is emmetropic. What is the maximum that they could accommodate to see a target clearly at a distance of 20cm (+-0.40D DOF)?
Can the environment influence the axial length of the eye?
Yes, hyperopic retinal defocus (when rays focus behind the retina) can stimulate axial elongation
What is the best myopia control treatment available right now?
Atropine (1%)
Myopia was slowed by 80% - if anything became more hyperopic
When stopped, the rate of myopia progression becomes faster than what it would be without starting the treatment in the first place
What is the advantage of 0.01% atropine?
No "rebound effect"
No need for bifocal after use
But, there are studies that say 0.01% is not significant
How does atropine work?
Atropine may affect dopamine neurotransmitter release which could influence retinal signals that control eye growth
Reduces effects over the years
What is peripheral refraction?
Visual signals from the peripheral retina can impact emmetropization at the fovea and possibly the development of refractive error
Study shows that not having a fovea does not matter, so it suggests that peripheral retinal defocus might be more significant
What is orthokeratology?
Flattens cornea using rigid contact lens
What are cycloplegic drugs?
Anti-muscarinic agents which block the muscarinic actions of acetylcholine (parasympathetic nervous system) in the iris and ciliary body. Accordingly, they produce cycloplegia (loss of accommodative function) and mydriasis (pupillary dilation)
What are some common cycloplegic drugs?
Atropine
Homatropine
Scopolamine
Cyclopentolate (0.5% and 1.0%) - most common
Tropicamide (0.5% and 1%) - most common
When do you do cycloplegic refraction?
Patient unable to fixate steadily (young children)
Ocular deviations, particularly esotropia, which appear to be accommodative in origin
Much (>1D) more plus power (or less minus power) found on retinoscopy than subjective refraction
Amplitude of accommodation does not fall within normal limits for the patient's age
Symptoms of apparent refractive origin which are not eliminated by the present prescription (transient blurring, occasional horizontal diplopia, headaches after near-work)
Apparent amblyopia or an eye failing to reach its expected level of VA
Latent hyperopia - accommodation to compensate for hyperopia
Psuedomyopia (accommodative spasm) - problems with relaxing accommodation
What are the advantages of cycloplegic refraction?
Makes latent refractive errors manifest
Allows refractive assessment without any accommodative fluctuations
Avoids off-axis retinoscopy errors
Allows dilated fundus examination
What are the disadvantages of cycloplegic refraction?
Difference between shape of crystalline lens under cycloplegia and that under natural viewing conditions
Increased aberrations from dilated pupil on retinoscopy, essential to consider only the central portion of the reflex
Mydriasis may induce angle closure leading to increased IOP
With atropine, possibility of systemic poisoning in young children
Persistent cycloplegia and mydriasis will produce photophobia, possible decrease in distance acuity (especially in hyperopes), decreased near function which may interfere with ability to do daily work
Potential for adverse reaction (allergy, etc) to the instillation of a pharmacological agent
Difficulty in determining what to prescribe following cycloplegic refraction
What is adequate cycloplegia?
An amplitude of accommodation of less than 2D
What is the typical time of onset and duration of different cycloplegic agents?
What are some pre-dilation precautions that you should consider before cyclopleging?
History
- family history of glaucoma
- previously dilated? And any complications?
Tonometry - IOP should be measured both before and after dilation
Assessment of the angle
What is the timing of drug instillation for cycloplegic eyedrops?
Teens and adults: two drops 5 minutes apart
What are some post-exam discussions to be made with the patient?
How long before I can read again
How long before my pupils are normal size
How long before I can drive, operate machinery, do my job
Use of sunglasses
What is the fatal systemic dose of atropine in infants?
10mg-20mg
One drop of atropine sulphate contains around 0.5mg of atropine
A 3g tube of 1% atropine eye ointment contains 30mg of atropine, three times the possible fatal systemic dose for an infant
What is dynamic retinoscopy?
To measure the magnitude of the accommodative response
If an emmetropic patient viewing a target at a viewing distance of 33cm, the stimulus to accommodation is 3.00D. If the patient acoommodates 2.50D, the point conjugate with the retina will lie where? For 67cm working distance, will you see against motion or with motion?
There is a lag in accommodation
The point conjugate with the retina = where the eye is focused for the image to lie at the retina
You will see against motion because the point conjugate is between the retinoscope and the patient
In dynamic retinoscopy (Cross-nott retinoscopy), what does a neutral reflex mean?
When a neutral reflex has been found, the retinoscope coincides with the point conjugate with the retina.
Assuming a corrected eye (the most common condition), the reciprocal of the distance from the retinoscope to the eye (in meters) equal the accommodative response in diopters
If a neutral reflex is seen at 33cm for an uncorrected 2D myope, what is the accommodative response?
l = -330mm
l' = +22.22mm
so F = 63D
Unaccommodated F = 63D
Accommodated F = 63D
Therefore, accommodative response = 1D
Cross-Nott ret on uncorrrected 1D refractive hyperopic patient. The stimulus to accommodation is 4.50D and they have a 0.75D lag of accommodation. How far is the target from the patient and the retinoscope from the patient when a neutral reflex is observed?
Accommodative stimulus = required power of the eye - unaccommodated power of the eye
4.50 = required power - 59
Required power = 63.50D, so F = 63.5D
l' = -28.57cm
If AS = 4.50 with 0.75D lag, the AR = 3.75D
Power of unaccommodated eye = 59 + 3.75 = 62.75, so F = 62.75
l' = 22.22mm, so l = -36.36cm
How is AS and AR determined?
The accommodative stimulus is determined by the position of the target, and any uncorrected refractive error
The accommodative response is measured by the retinoscope position at neutral and any uncorrected refractive error
What is Bell retinoscopy?
The same, except you change the target position rather than the working distance
What is Sheard's retinoscopy?
Uses lenses to determine the accommodative error (lead or lag). Both the target and retinoscope remain fixed in the same location (frequently the target is mounted on the retinoscope)
The retinoscope and the fixation target are positioned at a viewing distance of 50cm. Thus, the stimulus to accommodation = 2.00D. Assume that the accommodative response = 1.50D. Where is the point conjugate with the retina? What reflex direction would you see? If a +0.50sph is introduced, what is the total refractive power and what is the reflex that you would see?
The point conjugate with the retina will lie 1/1.5 = 67cm in front of the patient's eye.
With
61.50 + 0.50 = 62.00D; neutral
What happens if you have a 1.50D accommodative stimulus and you continue to get neutral after adding plus power?
The eye will decrease accommodative stimulus every time there is an additional power. Therefore, you will continue to see a neutral reflex until all the accommodation is removed (excluding tonic accommodation, convergent accommodation, and proximal accommodation).
What is Low Dynamic Neutral (LDN)?
The lowest lens that gives a neutral reflex. Here, the LDN = +0.50sph (0.50 lag of accommodation)
LDN equals the accommodative error
plus LDN = lag; minus LDN = lead
Example: accommodative stimulus is 3.00D. The LDN = +0.75. What is the accommodative response?
Response = 3.00 - 0.75 = 2.25D
What is High Dynamic Neutral (HDN)?
The highest lens that gives a neutral reflex. HDN = +1.25
What does the difference between the HDN and LDN mean?
Shows how much the patient could relax their accommodation as lenses were being added.
A corrected patient is viewing a target at a distance of 50cm. LDN = +0.25 and HDN = +1.25. How much can the eye relax?
The initial accommdative response = 2.0 - 0.25 = 1.75
Patient was able to relax their accommodation by 1.25 - 0.25 = 1.00D as lenses are added
What is negative relative accommodation (NRA)?
The ability to relax accommodation while maintaining convergence on a target
NRA = HDN - LDN
What is positive relative accommodation?
The ability to increase accommodation while maintaining convergence on a target
What is Positive relative vergence?
The ability to increase vergence (convergence) while maintaining accommodation on a target
What is negative relative vergence?
The ability to decrease vergence (divergence) while maintaining accommodation on a target
What is Monocular Estimate Method (MEM)?
Lenses are interposed very briefly so that accommodation does not have time to respond to the change in stimulus. Solves the problem with sheard's retinoscopy where the accommodation responds to the change in added power. BUT it does not make a difference. Need 1/3 of a second only so very hard.
What are the three techniques of dynamic retinoscopy?
Cross-Nott retinoscopy: target remains stationary. Move the retinoscope to determine the accommodative response to a given stimulus
Bell retinoscopy: retinoscope remains stationary; move the target to determine the stimulus that gives the particular accommodative response
Sheard/MEM: Target and retinoscope remain stationary; introduce lenses to determine the response to a given stimulus
What is Mohindra's near retinoscopy?
Measure distance refractive error where you don't want to use cycloplegic. Performed at near (50cms) in a dark room. Have them fixate the ret for at least a brief period. There will be on accommodation because blurry light is the only thing you can see. However, babies have a lot of proximal accommodation, so this technique does not eliminate all accommodation. Therefore, an adjustment for both the working distance and accommodative tonus adds a value of -1.25D
What is the prevalence of myopia in adults?
1999-2004: 41.6% of adults
increasing dramatically
What is emmetropization?
Most newborns are hyperopic and that becomes more emmetropic
What is leptokurtotic distribution?
Shape peak distribution
Usually seen in the effect of emmetropization on refractive distribution from birth to the age of 6 to 8 years
How does emmetropization work?
Significant growth of the ocular components occurs during the first 3 years of life. To achieve emmetropia, the growth of these components must be coordinated (example: a longer axial length requires less corneal or crystalline lens power for emmetropia to be maintained)
For refractive error to occur, usually the axial length (specifically vitreous chamber) will be the change that occurs
What is the greatest cause of myopia?
Greatest risk: two myopic parents
What could possibly help prevent myopia progression?
Sunlight exposure
What happens with orthoK?
Central flattening and peripheral steepening
Peripheral steepening = peripheral myopia = slow myopia progression (by half)
What are the different ways to control myopia?
Atropine (most effective, but will bounce back very fast and you have to keep taking drops)
Soft multifocal contact lenses
orthokeratology
What are the possible mechanism for time spent outside?
Less accommodation in outdoor environments
Pupil constriction in brighter outdoors
Sunlight exposure mediating release of a retinal transmitter (dopamine?) which inhibits eye growth
When was myopia first discovered?
identified by Aristotle (384-322BC)
What are some refractive surgeries?
RK: Radial Keratometry
PRK
LASEK
LASIK
How does LASIK work?
Flattens the center of the cornea
This produces relative steepening in the corneal periphery, leading to relative peripheral myopia which would slow myopic progression
A microkeratome (laser) is used to create an epithelial flap which is then retracted
What is radial keratometry?
incisions that are made radially
If all of the incisions were not exactly even, then the patient would develop very large amounts of corneal astigmatism (sometimes >12D)
What is PRK?
A laser is used to reshape the cornea
The corneal epithelium is removed with a blade or rotary brush and new epithelium will subsequently grow back over the corneal surface
What is LASEK?
Same as PRK, except rather than removing the epithelium, this tissue is moved to the side and allows the laser to reshape the stromal head
Either alcohol or microkeratome is used to soften and move the epithelium
What are some complications from refractive surgeries?
Reduced BCVA
Reduced contrast sensitivity
Reduced corneal sensitivity
May now need reading glasses
Increased dry eye
Reduced night vision
Starbursts and halos around lights
Thinner cornea (safety risk)
Progression of myopia in school-aged children showed increased myopia during covid in ages 6-13. What are some issues with the study?
Non-cycloplegic findings: permanent myopia or accommodative aftereffects?
Axial length measurements not available
Are the changes due to excessive near work or lack of time outside?
But, there was weak (yet significant) correlations between myopia progression and either less outdoor time or more screen time
In a study where subjects were already using atropine, what happened?
average increase in spherical refraction was -0.73D and 0.46mm
Atropine couldnt keep up?
What can myopic shift be caused by?
Increased screen time (especially phones and tablets)
Reduced sunlight exposure
Limited distance fixation (indoors in small apartments), resulting in possible accommodative aftereffects and hyperopic peripheral defocus
Not getting regular eye examinations during lockdown
Insomnia or change in circadian rhythm
Stress and anxiety
Blue light from screens
Was there a significant association with sleep and myopia?
NO
What is the amplitude of accommodation?
The dioptric distance between the far point and the near point of accommodation
What is the far point?
Point conjugate with the retina of the unaccommodated eye
What is the near point?
point conjugate with the retina of the maximally accommodated eye (for objective amplitude), or closest distance that can be seen clearly (for subjective amplitude)
Calculate the amplitude of accommodation
Amplitude = Far point (D) - Near point (D)
An uncorrected 3D myope has a near point at 25cm
Amplitude = -3 - (-4) = 1D
What happens to amplitude of accommodation with an emmetropic patient?
Amplitude = near point in D
What are the methods for measuring amplitude of accommodation?
Push up
Push down
Push away
Minus lens
Objective techniques (dynamic ret, autorefractor)
What is the push up technique?
Advance a target towards the patient until they report the first, slight, sustained blur. This represents the near point (and if the patient is corrected, their amplitude)
Measured OD, OS, OU
What is the first, slight, sustained blur?
When the target just starts to go fuzzy around the edges, and the patient is not able to clear the letters (with encouragement). Note that they should still be able to read the letters
What happens to the near point and amplitude of accommodation if the target is bigger in the push up method?
Bigger the target, the bigger the amplitude of accommodation
A larger target gives a higher amplitude of accommodation because patients are less sensitive in noticing the first slight sustained blur, and so they respond later. It is not because the accommodate less. Ideally, we want to use the smallest target they can see at their near point. If too large, then the amplitude will be overestimated
What is the problem with the push up method?
If the patient's VA is 20/20, you are reading a letter bigger than their acuity
The problem is that the 20/20 line is very often the smallest target you have
What is the push down technique?
Same as for push-up, except the target is advanced towards the patient until it goes blurred, and then pushed down (away) from the patient until it becomes "absolutely clear"
This is used because push-up value probably is an over-estimate of the true amplitude.
BUT, push-down technique probably under-estimates the true amplitude.
Therefore, the average of push-up and push-down is probably a good value to use
What is the push-away amplitude?
Sometimes used with young children
Start with the target very close to the eye so that it is blurred
Push the target away from the eye until the child can recognize the target
What are some problems with push-away amplitude?
End point is when the subject can recognize the target, not first blur, so the target is probably pushed farther away than necessary.
Children have very high amplitudes, so they are being asked to jump from zero accommodation up to around 15D of accommodation. It's hard to make such a big jump in the response even for a young child
What is the minus lens amplitude (ML)?
Target is kept at 40cm (AS = 2.50D)
Minus lenses added over the distance Rx until patient reports the first slight sustained blur
Amplitude = 2.50 + amount of minus lens added before blur is reported
We are creating an image that is getting closer to the patient
How much does the AS increase with 1D of minus?
Each diopter of minus lens power increases the accommodative stimulus of 1D
What is the difference between push-up and minus lens techniques?
Push-up:
- Target gets bigger with increasing AS, so less sensitive to blur
- Increasing proximal accommodation
- Can be done monocularly and binocularly
- Better method if we want to get maximum accommodation
Minus lens:
- Target gets smaller with increasing AS = more sensitive to blur
- Proximal accommodation stays constant
- Only used monocularly (OD and OS, but not OU)
- Usually will get a lower accommodative value than push up
What is positive relative accommodation? (PRA)
An increase in accommodation while the vergence remains constant
What is the difference between subjective amplitude and objective amplitude?
Subjective: measures where the patient reports the closest distance that they can see clearly
Objective: measure the position of the point conjugate with the retina when the patient is exerting their maximum accommodation
An emmetropic patient can exert a maximum of 5D of accommodation. They have a DOF of +- 0.40D. Would subjective or objective have a higher accommodation?
The maximum AR = 5.0D
The closest distance the patient can see clearly = 5.40D
This would cause the subjective to be higher accommodation than objective, and the difference is 0.40D (half the depth of field)
Where is the objective amplitude of accommodation (maximum AR) here?
5.00D
When does the patient report first, slight, sustained blur?
Blur occurs when the difference between the accommodative stimulus and the accommodative response exceeds half the depth of focus of the eye
What are the average value of accommodation by age?
The amplitude is inflated by depth of field
What is another way you can solve for average amplitude of accommodation by age?
Minimum expected amplitude = 15 - 0.25 * age in years
Mean expected amplitude = 18.5 - 0.3 * age in years
Maximum expected amplitude = 25 - 0.4* age in years
What is the problem with duane's and hofestetter's equations?
They are based on AS rather than AR
What have we recently realized with accommodation in children?
Accommodative ability is less than what we thought and found in subjective refraction, and it would be better to correct young hyperopes early
What are the three ways to measure accommodative response?
Dynamic retinoscopy (Cross-knott retinoscopy)
Dynamic cross cylinder
Near duochrome
How does dynamic cross cylinder work?
A subjective test to quantify the lead or lag of accommodation
1. Correct the patient for distance
2. Introduce a JCC, negative axis vertical, over the distance Rx
3. Patient will se rectilinear target (horizontal and vertical lines)
If the patient has a LAG of accommodation, (AR<AS), the HFL will be closer to the retina than the VFl, and they will report that the horizontal lines on the target appear clearer than the vertical lines.
Therefore, you will add plus sphere until both lines become equal (COLC on the retina).
The amount of plus added to achieve equality equals the lag of accommodation
If the patient has a LEAD of accommodation, (AR>AS), the VFL will be closer to the retina than the HFL, and they will report that the vertical lines on the target appear clearer than the horizontal lines.
Minus spheres are added until the two sets of line appear equal (COLC on the retina)
The amount of minus added to achieve equality equals the lead of accommodation
How is the dynamic cross cylinder performed?
Binocularly (OU): sometimes called Fused cross cylinder
Monocularly: to get rid of convergence accommodation (never called fused cross cylinder)
What are some assumptions to be made for dynamic cross cylinder?
1. the patient doesn't change their accommodation while viewing the rectilinear target
2. The patient doesn't change their accommodation when lenses aren't added
Who should you use dynamic cross cylinder on?
Anybody older than 14 years of age
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