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Oral boards > Friedman optics/refraction > Flashcards

Friedman optics/refraction Flashcards

1
Q

pt unhappy with glasses

A

> 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

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2
Q

Image jump

A

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

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3
Q

Image displacement

A

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

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4
Q

Flat top - image jump and displacement

A

Flat top = minimizes image jump

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

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5
Q

Bifocals:

A

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

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6
Q

Absolute hyperopia

A

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

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7
Q

Manifest hyperopia

A

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

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8
Q

Facultative hyperopia

A

Manifest hyperopia - absolute hyperopia

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9
Q

Latent hyperopia

A

Cycloplegia hyperopia - manifest hyperopia

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10
Q

Drugs causing Myopic shift:

A

Topamax
Sulfa
Tetracycline

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11
Q

Drugs causing Hyperopic shift:

A

Chloroquine
Phenothiazine
Anti-histamines
Marijuana

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12
Q

Moving lens (glasses)

A

Forward (towards nose) : more (+) sph

Backwards (towards eyes): more (-) sph

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13
Q

Tilting lens (glasses)

A

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

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14
Q

Keratometer measures?

A

directly: reflecting power
Indirectly: radius of curvature

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15
Q

myopia assoc/w/

A 
pigment dispersion syndrome
spherophakia
NS cataract
myelinated NFL
neonatal VH
ROP
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16
Q

Image jump vs. image displacement - round top

A

MOST image jump

more image displacement in myopes than hyperopes

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17
Q

Image jump vs. image displacement - flat top

A

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

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18
Q

Image jump vs. image displacement - executive bifocals

A

larger area dedicated to near vision

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

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19
Q

Astigmatic dial refraction STEPS (#1-4)

A

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.

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20
Q

Duochrome test

A

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)

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21
Q

How to MRx

A

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

22
Q

Binocular balance

A

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)

23
Q

Determine bifocal add

A

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

24
Q

Example of determining bifocal add

A

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

25
Q

Retinoscopy: against vs. with motion

A

Against motion: needs minus or head forward

With motion: needs plus or head backwards

26
Q

Retinoscopy: far point location?

A

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)

27
Q

Accommodative insufficiency - systemic processes

A 
hypothyroidism
anemia
pregnancy
nutritional deficiencies
chronic illness
28
Q

Accommodative insufficiency - exam

A

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

29
Q

Prism diopter

A

∆ = displacement measured in cm at a distance of 1m

1° is about ~2∆ if

30
Q

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

A

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)

31
Q

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

A 
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

32
Q

How to check the base curve of a lens

A

Use Geneva lens clock

33
Q

Why does the base curve of a lens matter in glasses

A

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

34
Q

Disadvantage of aphakic glasses

A

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

35
Q

Image enlargement: aphakic glasses into contact lens or with IOL

A

7% enlargement with CL

2.5% with IOL

36
Q

Kestenbaum’s rule

A

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

37
Q

RGP - tight fitting RGP

A

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.
38
Q

RGP: apical alignment

A

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

39
Q

RGP: apical clearance

A

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

40
Q

Power calculation for RGP?

A

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

41
Q

RGP fit theory

A

“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

42
Q

Technique for fit of SCL

A

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

43
Q

Indirect ophthalmoscopy

A

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

44
Q

Transverse

A

(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

45
Q

Axial

A

magnification of depth (along the optical axis)

Axial magnification
Maxial = (Mtransverse)^2

46
Q

Angular

A

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

47
Q

Retinal magnification with a 20D lens

A

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

48
Q

Pinhole size (optimal size)

A

= 1.25 mm (optimum size)

limited by diffraction if

49
Q

Pinhole removes refractive error up to…

A

+/- 5 diopters (3 Diopters per Friedman)

but can reduce vision in eyes with retinal disorders (good for lenticular or corneal irregularities)

50
Q

how does a pinhole work?

A

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

51
Q

Myopic refractive surprise s/p CE/IOL

A

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

52
Q

Piggyback IOL - correcting myopia or hyperopia

A

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.

Oral boards (13 decks)

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