Friedman optics/refraction Flashcards
pt unhappy with glasses
> 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
Image jump
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
Image displacement
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
Flat top - image jump and displacement
Flat top = minimizes image jump
Majority of people are myopic, and therefore flat top minimizes image displacement for these patients.
Bifocals:
Myopics: flat top, executive
Hyperopics: round top
Move optical center of add close to top of segment
Loupes
Absolute hyperopia
Minimum (non-cycloplegia) plus correction required for clear VA at distance
Manifest hyperopia
Maximum (non-cycloplegia) plus correction the eye can accept without blurring
Facultative hyperopia
Manifest hyperopia - absolute hyperopia
Latent hyperopia
Cycloplegia hyperopia - manifest hyperopia
Drugs causing Myopic shift:
Topamax
Sulfa
Tetracycline
Drugs causing Hyperopic shift:
Chloroquine
Phenothiazine
Anti-histamines
Marijuana
Moving lens (glasses)
Forward (towards nose) : more (+) sph
Backwards (towards eyes): more (-) sph
Tilting lens (glasses)
Plus lens: more (+) sph, more (+) cyl
Minus lens: more (-) sph, more (-) cyl
Axis in axis of tilt
Keratometer measures?
directly: reflecting power
Indirectly: radius of curvature
myopia assoc/w/
pigment dispersion syndrome spherophakia NS cataract myelinated NFL neonatal VH ROP
Image jump vs. image displacement - round top
MOST image jump
more image displacement in myopes than hyperopes
Image jump vs. image displacement - flat top
MINIMIZE image jump, less image image displacement (in myope than hyperope)
Image jump vs. image displacement - executive bifocals
larger area dedicated to near vision
no image jump b/c optical centers are at the top of the segment.
Astigmatic dial refraction STEPS (#1-4)
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.
Duochrome test
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)
How to MRx
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
Binocular balance
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)
Determine bifocal add
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
Example of determining bifocal add
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
Retinoscopy: against vs. with motion
Against motion: needs minus or head forward
With motion: needs plus or head backwards
Retinoscopy: far point location?
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)
Accommodative insufficiency - systemic processes
hypothyroidism anemia pregnancy nutritional deficiencies chronic illness
Accommodative insufficiency - exam
MRx/CRx
ocular alignment and motility
near and far point
accommodative and convergence amplitudes
Prism diopter
∆ = displacement measured in cm at a distance of 1m
1° is about ~2∆ if
How should glass prisms be held with the line of sight?
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)
How does a minus or plus lens affect the measurement of the tropia?
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
How to check the base curve of a lens
Use Geneva lens clock
Why does the base curve of a lens matter in glasses
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
Disadvantage of aphakic glasses
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
Image enlargement: aphakic glasses into contact lens or with IOL
7% enlargement with CL
2.5% with IOL
Kestenbaum’s rule
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
RGP - tight fitting RGP
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.
RGP: apical alignment
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
RGP: apical clearance
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
Power calculation for RGP?
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
RGP fit theory
“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
Technique for fit of SCL
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
Indirect ophthalmoscopy
Field of view is 25 degrees and magnification is 2-3x
Transverse
(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
Axial
magnification of depth (along the optical axis)
Axial magnification
Maxial = (Mtransverse)^2
Angular
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
Retinal magnification with a 20D lens
Ma = Deye/D lens = 60/20 = 3X
Pinhole size (optimal size)
= 1.25 mm (optimum size)
limited by diffraction if
Pinhole removes refractive error up to…
+/- 5 diopters (3 Diopters per Friedman)
but can reduce vision in eyes with retinal disorders (good for lenticular or corneal irregularities)
how does a pinhole work?
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
Myopic refractive surprise s/p CE/IOL
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
Piggyback IOL - correcting myopia or hyperopia
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.
