Anisometropia and Aniseikonia Flashcards
What is an uncorrected refractive error where the two eyes differ significantly ; impossible to have sharp focus on both retinas at the same time; can lead to suppression and amblyopia
anisometropia
having anisometropia can cause what to happen to the image
image size differences; vertical imbalance in the reading position and horizontal Vergence problems
what condition exists when the spherical equivalent refraction of the two eyes differs by 1.00D or more
anisometropia
what are some possible problems when correcting anisometropic refractive errors
accommodative, Vergence, and image size
this states that both eyes will always respond with equal amounts of accomodation
Herings Law of Equal Innervation ( accomodation is esp. a prob when one eye is myopic and one eye hyperopic
what can happen when the pt looks away from the optical centers of the spectacle lenses
can induce prism ( Vergence problem)
what is the typical reading position from straight ahead viewing
10 mm down
every 1.00D of aniso will generate ___prism diopter of vertical imbalance
1 prism diopter
how many prism diopters of vertical imbalance can be tolerated without asthenopia
1 prism diopter
what is a major big problem when correcting anisometropic refractive errors
image size; the greater the image size diff between the two eyes, the less chance of comfortable binocular vision
anisometropia can occur secondary due to ….
axial diff between the two eyes and refractive diff between the eyes ( corneal, lenticular)
retinal image size in uncorrected ametropia can be two things…
axial or refractive
what is the relative diff in the size and/or shapes of the ocular images of the two eyes
aniseikonia
aniseikonia can be broken down into what two size differences
symmetrical and asymmetrical
this type of aniseikonia size diff is meridional; the ocular image of one eye is increased or decreased in size in one meridian compared to the other eye
symmetrical
this type of aniseikonia size diff is a progressive increase or decrease in the image size in one meridian or all directions from the visual axis
asymmetrical
pts can develop what from aniseikonia
asthenopia
to produce a single image, images formed by the two retinas must undergo_______; the more alike the two images, the easier it is to fuse
sensory fusion
symptoms of this disorder including “ eyes feel funny”, eyestrain, HA, photophobia, nervousness
asthenopia secondary to aniseikonia
what is astenopia
eye strain
with asthenopia , if symptoms are relieved by occluding one eye , _________should be suspected
aniseikonia
how do we determine the amt of aniseikonia in a spectacle Rx
using the spectacle magnification formula
what is the spectacle mag formula
SM = retinal image size in the corrected eye / retinal image size in the uncorrected eye
what determines the factor by which initial image size is magnified or minified
spectacle mag
T or F: Spectacle Mag is usually stated as a %
T
What is the % equation of spectacle Mag
%SM= (SM-1) (100)
what two factors compose Spectacle Mag
Shape factor and Power Factor
SM= SF x PF
What is the Spectacle Mag equation with SF and PF included
see slide 19; slide 20-23 for example
Spectacle Mag will always be greater than 1 for
plus lens power; true for Cls and Gls
spectacle mag will always be less than 1 for a
minus lens power; true for Cls and Gls
how do you calculate the change in SM with a change in vertex distance going from Gls to Cls
F(s)/F(c); examples slides 26-27
How do you calculate the change in SM with a change in vertex distance going from CLs to Gls
F(c)/ F(s)
with vertex distance is the direction negative or positive if the lens moves towards the eye
positive; away from the eye is negative
what is the vertex distance ( downstream) equation
F= F(c)/ 1-dF(c)
what is shape factor dependent upon
base curve, lens material, and lens thickness ; slide 30 for equation
what is power factor dependent on
lens power and vertex distance; slide 31 for equation
how can we get the Spectacle Mag as close to one as possible
if the lens were infinitely thin , the SF would be 1 (t= 0) ; if the lens were placed as close to the entrance pupil of the eye as possible, the power factor would be 1 (h=0)
this rule says that if two lenses have the same back vertex power, are made of the same material, have the same thickness, and are the same distance from the eye, the lens with the more curved front surface ( usually the steeper BC ) will have the greater spec Mag
Base Curve Rule
how can we correct aniseikonia without compromising central VAs
“classic Gls only option”; manipulate the SF ( change the front surface power , change the thickness); manipulate the PF ( change vertex distance)
what does it mean when a lens is spherocylindrical
the SM will vary in the two principal meridians; the retinal image will be larger in the direction of the axis of the minus cyl
for Gls, for each 1.00D of cyl diff between the two principle meridians, there is a _____ % diff in SM
1.5
For Cls, for each 1.00 D of cyl diff between the two meridians , there is about a ___% diff in SM
0.3
which component of SM has little impact
SF
patients that have difficulty adapting to the SM dif in the two principle meridians due to large amts of cyl will find ___ easier to adapt to bc of the 5 fold decrease in SM between the principle meridians
CLs
T or F: the size of the retinal image changes with the axial length of the eye
T ( axial anisometropia)
this is when the size of the retinal image is determined by the distance between the centers of the blur circles formed at P and Q; size of the retinal image is the same for myopia , hyperopia, and emmetropia
refractive anisometropia
this law is when a correcting lens is so placed before the eye that its second principal plane coincides with the anterior focal point of an axially ametropic eye, the size of the retinal image will be the same as though the eye were emmetropic
Knapp’s Law
in Knapps Law, the desired change in the image size will occur in axial ametropia with ______ correction
Gls
in Knapps Law, minimal cange in the image size will occur in refractice ametropia with __ correction
Cls; look at slide 42 for example
when trying to figure out if anisometropia is axial or refractive; if the K’s are the same what does that mean
possible diff in axial length and minimal size changes can be experienced with Gls
when the Ks are not the same, the anisometropia can be accounted for as ______
refractive ametropia ; min size changes in Cls correction
for ametropia _____ than +/- 4.00 refractive components within emmetropic ranges
less
for ametropia ____ than +/- 4.00 D axial length almost always beyond emmetropic norms
greater
What does Sorsby say about low refractive errors
likely to be a combination of axial and refractive ametropia
What does Sorsby say about high refractive errors
likely to be an axial ametropia
image size diff greater than ____ % will result in loss of binocularity
5%
which rule is based upon the observation of a relationship between the degree of anisometropia and the degree of aniseikonia
” The Rule”
What is the rule of thumb for aniseikonia
for every 1 .00 D diff between the eyes there is a 1 % image size difference
the higher the refractive error , the greater the _______
anisometropia; greater the error rate of the Rule of Thumb
what is the software product for measuring aniseikonia ; uses red/green glasses ; but it underestimates aniseikonia ( mainly in the horizontal direction)
Aniseikonia Inspector 3
these are lenses designed to correct aniseikonia ; this is a lens pair with curvatures and thicknesses chosen to produce a diff in image mag between the eyes to correct image size diff.
aniseikonic lenses
these lenses produce mag and can have dioptric power like a regular ophthalmic lens
aniseikonic lenses
If the Aniseikonic lens produces mag only its called ____
size lens
this lens is dissolved to solve aniseikonia ; effectively eliminate the diff in the size of images in each eye
Shaw Lens
This lens is a binocular, digital Spectacle Lens System ; attempts to compensate for both statis dynamic image size diffs
shaw lens
