Friedman optics/refraction

Question 1

pt unhappy with glasses

Answer 1

> check correct prescription: measure glasses w/lensometer to confirm
check ocular alignment of lenses
record VA

Check MRx, consider cycloplegic refraction
>pts with large levels of correction are particularly sensitive to small changes in glasses prescription (>0.50 D and/or >5 degrees axis rotation)
>vertex distance and base curve of the lenses

Question 2

Image jump

Answer 2

POSITION OF THE OPTICAL CENTER OF THE ADD SEGMENT:
produced by sudden introduction of prismatic power at the top of the bifocal segment. The object which the eye sees in the INFERIOR field when looking straight ahead suddenly jumps UPward when the eye turns DOWN to look at it (2/2 base-DOWN prismastic effect of the bifocal segment)
If the optical center of the segment is at the top of the segment, there is no image jump

Question 3

Image displacement

Answer 3

produced by the TOTAL prismatic power acting in the reading position (the total prismatic power of the lens plus the bifocal segment).
>minimized when the prismatic effect of the bifocal segment and distance lens are in opposite directions

Question 4

Flat top - image jump and displacement

Answer 4

Flat top = minimizes image jump

Majority of people are myopic, and therefore flat top minimizes image displacement for these patients.

Question 5

Bifocals:

Answer 5

Myopics: flat top, executive
Hyperopics: round top
Move optical center of add close to top of segment
Loupes

Question 6

Absolute hyperopia

Answer 6

Minimum (non-cycloplegia) plus correction required for clear VA at distance

Question 7

Manifest hyperopia

Answer 7

Maximum (non-cycloplegia) plus correction the eye can accept without blurring

Question 8

Facultative hyperopia

Answer 8

Manifest hyperopia - absolute hyperopia

Question 9

Latent hyperopia

Answer 9

Cycloplegia hyperopia - manifest hyperopia

Question 10

Drugs causing Myopic shift:

Answer 10

Topamax
Sulfa
Tetracycline

Question 11

Drugs causing Hyperopic shift:

Answer 11

Chloroquine
Phenothiazine
Anti-histamines
Marijuana

Question 12

Moving lens (glasses)

Answer 12

Forward (towards nose) : more (+) sph

Backwards (towards eyes): more (-) sph

Question 13

Tilting lens (glasses)

Answer 13

Plus lens: more (+) sph, more (+) cyl
Minus lens: more (-) sph, more (-) cyl
Axis in axis of tilt

Question 14

Keratometer measures?

Answer 14

directly: reflecting power
Indirectly: radius of curvature

Question 15

myopia assoc/w/

Answer 15

pigment dispersion syndrome
spherophakia
NS cataract
myelinated NFL
neonatal VH
ROP

Question 16

Image jump vs. image displacement - round top

Answer 16

MOST image jump

more image displacement in myopes than hyperopes

Question 17

Image jump vs. image displacement - flat top

Answer 17

MINIMIZE image jump, less image image displacement (in myope than hyperope)

Question 18

Image jump vs. image displacement - executive bifocals

Answer 18

larger area dedicated to near vision

no image jump b/c optical centers are at the top of the segment.

Question 19

Astigmatic dial refraction STEPS (#1-4)

Answer 19

1) fog the patient to 20/60 with plus sphere
2) ask patient which line of astigmatic dial looks darkest/sharpest
3) add minus cylinder perpendicular to the axis or plus cylinder parallel to the axis until all lines are equally sharp
4) reduce the sphere using snellen chart until vision is clearest.

If using plus cylinder phoropter, for every 0.50 diopter of cylinder you add, you must subtract 0.25 diopters of sphere.

Question 20

Duochrome test

Answer 20

The red and green filters usually used create a chromatic spherical difference of only 0.50 D, requiring visual acuity of 20/30 or better to distinguish a blur difference. Balance with the red-green test should always be approached from the fogged direction (red clearer) to minimize accommodation. Add minus sphere in 0.25 D steps.

Red side better, refraction = too hyperopic (ADD MINUS)
Green side better, refraction = too myopic (ADD PLUS)

Sphere is adjusted until the black letters on the red and green halves of the test chart are equally clear, indicating that the red rays are focused as far behind the retina as the green rays are focused in front. Yellow light, midway between the red and green, will then be in perfect focus on the retina, the optimum focus when viewing with white light.

Chromatic aberration occurs strongly in the human eye, with almost 3.00 D difference in the focus of the far ends of the visible spectrum (1.50 D is usually stated in textbooks):
Light of shorter wavelengths is refracted more than light of longer wavelengths (green is bent farther than red)

Question 21

How to MRx

Answer 21

1) start with current prescription - check monocular VA
2) refine axis of cylinder with Jackson cross cylinder, but must be at least 20/40 to use the 0.25 cross cylinder
3) determine correct axis –> determine sphere with cross cylinder
4) recheck sphere until best acuity achieved
5) Binocular balance

Question 22

Binocular balance

Answer 22

1) prism dissociation (3 prism diopters base UP over one eye and 3 prism diopters base DOWN over the other with a Risley prism)
2) balanced fogging (fog both eyes and alternate cover until equally fogged)
3) duochrome test (red-green balance both eyes)

Question 23

Determine bifocal add

Answer 23

1) Measure accommodation monocularly
2) Then measure accomodation binocularly
3) Use Prince rule (reading card with a rule calibrated in centimeters and diopters to measure amplitude of accomodation) can be used with the phoropter to determine the necessary accommodative requirements
>half of pt’s measured accommodative amplitude should be held in reserve to prevent asthenopia

Question 24

Example of determining bifocal add

Answer 24

patient wants to read at 40 cm (2.5 D)
>prince rule: measures 2.0 D of amplitude
>leave 1.0 D available to pt to prevent asthenoptic Sx: therefore, add power is 1.5 D ( = total amount of accommodation required to read 2.5 D -accommodation 1.0 D)
>measure accommodative range (near point to far point of accommodation).
>if range is too close, then reduce add in steps of 0.25 D until the correct range is found

Question 25

Retinoscopy: against vs. with motion

Answer 25

Against motion: needs minus or head forward | With motion: needs plus or head backwards

Question 26

Retinoscopy: far point location?

Answer 26

If far point between examiner and pt (myopic - see AGAINST motion). If far point behind the examiner (Hyperopia - see with motion) Don't forget to adjust for working distance (ADD reciprocal of working distance to final reading)

Question 27

Accommodative insufficiency - systemic processes

Answer 27

``` hypothyroidism anemia pregnancy nutritional deficiencies chronic illness ```

Question 28

Accommodative insufficiency - exam

Answer 28

MRx/CRx ocular alignment and motility near and far point accommodative and convergence amplitudes

Question 29

Prism diopter

Answer 29

∆ = displacement measured in cm at a distance of 1m | 1° is about ~2∆ if

Question 30

How should glass prisms be held with the line of sight?

Answer 30

Easy way to think about it: hold it the usual way; plastic prisms (usual ones) are parallel to the FACE Prentice position: Glass prisms should be held with the back surface PERPENDICULAR to the line of sight a.k.a visual axis (parallel to the eye), APEX of prism is pointed in direction the eye deviation or problem If you don't hold it this way, then you measure more deviation then what the patient actually has (therefore if you use these incorrect measurements, you will over-correct)

Question 31

How does a minus or plus lens affect the measurement of the tropia?

Answer 31

``` Minus measures MORE (minus lenses make the deviation appear larger) Plus lenses (decrease the measured deviation) ``` Prismatic effect of glasses on strabismic deviation: 2.5 D = % difference

Question 32

How to check the base curve of a lens

Answer 32

Use Geneva lens clock

Question 33

Why does the base curve of a lens matter in glasses

Answer 33

A change in the base curve can cause the felling of distortion and feeling of motion sickness when looking off center Increasing the base curve results in thicker lenses and more magnification

Question 34

Disadvantage of aphakic glasses

Answer 34

Image magnification of 25% altered depth perception pincushion distortion ring scotoma (prismatic effect at edge of lens causes visual field loss of 20%) "jack in the box" phenomenon - peripherally invisible objects sudden appear when gaze is shifted

Question 35

Image enlargement: aphakic glasses into contact lens or with IOL

Answer 35

7% enlargement with CL | 2.5% with IOL

Question 36

Kestenbaum’s rule

Answer 36

Estimate of reading add for low-vision patients Reciprocal of Snellen acuity i.e. 20/200 → 200/20 = +10 Check and refine with trial frames or a phoropter

Question 37

RGP - tight fitting RGP

Answer 37

Tight hard CL = decentered inferiorly, don't move when pt blinks. Can also see: central area of fluorescein and absence of peripheral fluorescein* Problem: Lens has base curve TOO STEEP for the cornea. need more FLAT base curve so you INCREASE the base curve (increase radius of curvature)--> makes lens more flat. lens is vaulted HIGHER over the cornea and fluorescein pools in the CENTER or, to decrease tightness, you can DECREASE the diameter * b/c in tight fitting RGP, tends to fit about 2 mm smaller than the corneal diameter. Cornea 2/2 unprotected by the lens, dries out (at the 3:00 and 9:00 position). Steep lenses dig into peripheral cornea and = absence of green.

Question 38

RGP: apical alignment

Answer 38

apical alignment: hard CL has the same base curve as the cornea. In ideal fit, contact lens has an apical alignment and the upper portion of the CL is under the upper eyelid. With blinking, the upper eyelid moves the CL so there is good tear exchange

Question 39

RGP: apical clearance

Answer 39

apical clearance: CL is steeper than the central cornea curvature --> therefore the lens is tighter than an apical alignment. If it is too tight, it will center within the interpalpebral fissure and have minimal movement with blinking. (can be advantage if patient has very large interpalpebral fissure where the lens would be extremely large if it were to rest partially under the upper eyelid

Question 40

Power calculation for RGP?

Answer 40

Prefer trial lens + over-refraction If not available: 1) Measure MRx + Keratometry (Ks) 2) Choose base curve for flatter K Usu. +0.5D steeper to form tear lens to prevent apical touch or tight lens 3) Convert MRx to (-) cyl & zero vertex, then disregard cyl (– cyl formed by tear). No need to correct for vertex if

Question 41

RGP fit theory

Answer 41

“SAM-FAP” steeper add (-); flatter add (+) Fit CL steeper than average cornea K measurement (forms plus tear meniscus) Therefore, need to subtract power (add minus) at end of power calculation For each diopter the base curve is made "steeper than K," subtract 1 D from the final lens power If the lens is fit flatter than K, than a minus tear meniscus is formed, so necessary to add plus power at end of claculation

Question 42

Technique for fit of SCL

Answer 42

spherical equivalent MRx corrected for vertex distance Base curve based on average Ks to evaluate fit, assess movement of the lens poor movement = lens is too TIGHT (steep) excessive movement = lens is too flat

Question 43

Indirect ophthalmoscopy

Answer 43

Field of view is 25 degrees and magnification is 2-3x

Question 44

Transverse

Answer 44

(linear or lateral) - magnification of image size (away from optical axis) Mtrans = image height / object height = image distance / object distance Use nodal point (17mm) when calculating retinal image height

Question 45

Axial

Answer 45

magnification of depth (along the optical axis) Axial magnification Maxial = (Mtransverse)^2

Question 46

Angular

Answer 46

magnification of angle subtended by an image with respect to an object. Used when the object or image size cannot be measured Ma = D/4, standardized to 25 cm (1/4 m) the near point of the average eye

Question 47

Retinal magnification with a 20D lens

Answer 47

Ma = Deye/D lens = 60/20 = 3X

Question 48

Pinhole size (optimal size)

Answer 48

= 1.25 mm (optimum size) | limited by diffraction if

Question 49

Pinhole removes refractive error up to...

Answer 49

+/- 5 diopters (3 Diopters per Friedman) | but can reduce vision in eyes with retinal disorders (good for lenticular or corneal irregularities)

Question 50

how does a pinhole work?

Answer 50

allows only the undeviated paraxial light rays to focus on the retina. Reduces refractive error and improves vision by increasing depth of focus but can be limited by difraction

Question 51

Myopic refractive surprise s/p CE/IOL

Answer 51

Wrong formula error in axial length measurement or keratometry or incorrect IOL calculation Sulcus placement without reducing IOL power Anterior displacement of IOL in bag from large capsulorrhexis Capsular distention/capsular block syndrome capsular contraction upside down placement of angulated IOL

Question 52

Piggyback IOL - correcting myopia or hyperopia

Answer 52

Myopia: same as myopic refractive error (i.e. -2.00 myope you place a -2.00 diopter IOL) For hyperopia, you use 1.5 x the hyperopic refractive error (i.e. a +2.00 D error requires a +3.00 D IOL) Can use Holladay R formula to calculate power of a piggyback lens.