GCO- Aniseikonia PDF

Aniseikonia GEOMETRIC & CLINICAL OPTICS 1 Aniseikonia Different image size and/or shape between R & L eyes  Issue with anisometropia, in addition to anisophoria and vertical imbalance  Generally not a problem with longstanding anisometropia  Recent-onset is oft...

Aniseikonia GEOMETRIC & CLINICAL OPTICS 1 Aniseikonia Different image size and/or shape between R & L eyes  Issue with anisometropia, in addition to anisophoria and vertical imbalance  Generally not a problem with longstanding anisometropia  Recent-onset is often problematic (like anisophoria) 2 Aniseikonia How do we measure ocular/retinal image sizes?  Clinical Instrumentation- most clinicians don’t have or use  We calculate angular size of image using geometric optics and angular magnification Compare retinal image size (RIS) with correcting lens to RIS without correcting lens “Spectacle Magnification” 3 Spectacle Mag Comparing angular size of focused image (with correction) to angular size of blurred, unfocused image in same eye (uncorrected)  Entrance pupil is reference point for uncorrected eye (not nodal point) 4 Entrance Pupil Entrance pupil (EP)  Image of the aperture (pupil) as viewed from the front of the lens system  Apparent location of pupil as viewed from a position in front of the cornea  Incident rays passing through entrance pupil will completely pass through actual pupil after refraction by the cornea  EP is 3 mm behind the corneal plane 5 Spectacle Mag Imagine thick, correcting lens as 2 components: 1. Afocal telescope: solid glass or plastic  Index of refraction, BC, & CT  “afocal”: plano power, BVP = 0 2. Thin lens containing the Rx, located in front of eye (entrance pupil) 6 Spectacle Mag Base curve (F1) CT 7 Spectacle Mag SM = Shape Factor x Power Factor t = CT (in m)h = entrance pupil distance in meters (vd + 3 mm) n = index Fv’ = BVP F1 = BC 8 Example 1 A patient’s spectacle Rx is OU +3.00 DS, with the following parameters: n = 1.498 (CR-39) CT = 4.3 mm BC = +8.50 D vd = 13 mm Calculate the spectacle magnification for this Rx. 9 Example 1, cont. SM = 1.025 x 1.050 =1.076 Convert SM to %: %SM = (SM – 1)100 So, %SM = (1.076 – 1)100 = +7.6% 10 Example 1- Key Points When this patient puts on his +3.00 D OU glasses, the RIS is 7.6% larger than the blurred retinal image without glasses Plus lenses have SM > 1.0 and positive (+) %SM RIS is larger (magnified) with correction compared to uncorrected RIS 11 Example 2 A patient has a spectacle Rx of OU -2.00 DS with the following parameters: BC = +6.00 D CT = 2.2 mm n = 1.523 vd = 12 mm Calculate the spectacle magnification for this Rx. 12 Example 2, cont. SM = 1.009 x 0.971 = 0.980 %SM = (0.980 – 1)100 = -2.0% Note: power factor dominates SM 13 Example 2, Key Points Minus lenses have SM < 1.0 and a negative (-) %SM RIS is smaller (minified) with correction compared to uncorrected RIS Many myopes observe that “everything looks smaller” with their glasses on 14 SM vs. RSM Relative Spectacle Mag (RSM) is slightly different than Spectacle Mag (SM) RSM = RIS with correction / RIS in Schematic Emmetropic Eye RSM used to compare RIS in axial & refractive ametropia  Knapp’s Rule:  Axial anisometropia: spectacles minimize image size difference  Refractive anisometropia: contact lenses minimize image size difference  Not clinically significant; ignored! 15 Issues with SM formula It’s just plain ugly!! Can’t easily visualize relationships Cumbersome to use, even with calculator Isn’t there an easier way? Somebody please help!! 16 Help is on the way! But it’s not in most textbooks Approximate formula: 17 Approximate SM Formula Gives %SM directly t and h in mm (not m) Add SF & PF, not multiply Can see relationships easier! 15 ~ 1.50 index term 18 Return to Example 1 Rx: +3.00 DS OU n = 1.498 (CR-39) CT = 4.3 mm BC = +8.50 D vd = 13 mm %SM = 2.44 + 4.8 = +7.24% (vs. +7.6%) 19 Return to Example 2 Rx -2.00 DS OU BC = +6.00 D CT = 2.2 mm n = 1.523 vd = 12 mm %SM = 0.88 + (-3) = -2.12% (vs. -2.0%) 20 Take a closer look! Shape Factor Power Factor As t ↑, %SM ↑ For plus (+) lens: As h (vd) ↑, %SM ↑ As F1 ↑, %SM ↑ For minus (-) lens: As n ↑, %SM ↓ As h (vd) ↑, %SM ↓ 21 BC impacts VD! Plus lens Minus Lens Steepening BC will ↑ SM Steepening BC will ↑ SM Steepening BC will ↑ vd Steepening BC will ↑ vd ↑ vd will ↓ SM ↑ vd will ↑ SM Steepening BC will have Steepening BC provides less effect on SM, since ↑ extra ↑ in SM, due to ↑vd! from BC is offset by ↑vd 22 Example 3 If a lens with BVP -10.00 D is fit 3 mm closer to the eye, how much does SM change? Given BVP and change in vd, ignore Shape Factor! %SM = [-3 mm x (-10)] / 10 = +3% So, for minus lens: as vd ↓, RIS ↑ 23 Example 4 If a lens with a thickness of 3 mm is made with base curve that is 5.00 D steeper, how much does the SM change? Given CT and change in BC, can ignore Power Factor %SM = [(3 mm)(+5.00)] / 15 = +1% So, ↑ (steepening) the BC by 5.00 D, ↑RIS by 1% 24 Example 5 If a lens with a base curve of +10.00 D is made 3 mm thicker, how much does the SM change? Given BC and change in CT, can ignore Power Factor %SM = [(+3 mm) (+10)] / 15 = +2% So, ↑ CT by 3 mm produced an ↑ RIS by 2% (magnification) 25 Estimating aniseikonia Rule of thumb: 1% per diopter of anismetropia Rx OD -1.00 D, OS -4.00 D  Estimate 3% difference in SM between the eyes Clinical Objective:  Want to ↓ SM in most plus (least minus) lens  Want to ↑ SM in most minus (least plus) lens 26 Treating Aniseikonia Textbook (System of Ophthalmic Dispensing) has excellent stepwise approach: 1. If concerned but not certain, use “First Pass Method.” 2. If fairly certain, make “directionally correct magnification changes” to each lens. 3. Can estimate and change lens parameters. 4. Can measure (with instrumentation) and change lens parameters appropriately. 27 First Pass Method Easy to do, will not create problems even if aniseikonia is not significant.  Use frame with short vd & nose pads, if possible  Small eyesize (A) will ↓ vd  Use aspheric lenses for + powers to flatten BC  Use high index materials to thin the lens (↓ CT), especially + powers 28 Directionally Correct Changes Generally altering Shape Factor Lens modifications if:  Both lenses are plus (Box 21-1)  Both lenses are minus (Box 21-2)  One plus, one minus (Box 21-3) 29 30 31 32 What about CLs? Changing from specs to CLs is best illustration of the Power Factor!  Maximum change in vd, since vd = 0 Specs ↔ CLs will impact SM  Many patients notice this 33 CL Correction Consider Power Factor only  Specs: assume vd = 12 mm, so h = 15 mm  CLs: assume vd = 0 mm, so h = 3 mm Specs: %SM = (15 mm) Fv / 10 = 1.5 Fv CLs: % SM = (3 mm) Fv / 10 = 0.3 Fv So, SM with CLs is 1/5 (20%) that of specs 34 Example 1: CLs vs. Specs Spec Rx: +4.00 D (12 mm vd) CL Rx: +4.00 D (vd = 0)  Specs: %SM = [(15)(+4.00)] / 10 = +6% Compare SM for specs & CLs  CLs: %SM = [(3)(+4.00)] / 10 = +1.2% Both have magnified RIS, but CL has less magnification or approx. 20% relative minification 35 Example 2: CLs vs. Specs Spec Rx: -8.00 D (12 mm vd) CL Rx: -7.25 D (vd = 0)  Specs: %SM = [(15)(-8.00)] / 10 = -12% Compare SM for specs & CLs with power factor  CLs: %SM = [(3)(-7.25)] / 10 = -2.2% Both have minified RIS, but CL has less minification or approx. 20% relative magnification 36 Summary- CL Correction If Rx has plus power, changing from specs to CLs will ↓ SM  RIS is still magnified, but less magnification than with specs (relative minification) If Rx has minus power, changing from specs to CLs will ↑ SM  RIS is still minified, but less minification than with specs (relative magnification) Overall, CLs provide a more natural RIS that more closely resembles the RIS for an emmetrope. 37 What about astigmatism? Astigmatism creates “meridional magnification”  Different SM in each principal meridian of lens  Distorts shape of retinal image in each meridian  Retinal image is larger in most + (least –) meridian and smaller in least + (most -) meridian Patients with high cylinder Rx often report eyestrain, HA, poor tolerance of specs Can we do anything to help? 38 CL’s to the rescue (again)! Consider Power Factor only  Specs: assume vd = 12 mm, so h = 15 mm  CLs: assume vd = 0 mm, so h = 3 mm Specs: %SM = (15 mm) Fv / 10 = 1.5 Fv CLs: % SM = (3 mm) Fv / 10 = 0.3 Fv So, SM with CLs is 1/5 (20%) that of specs 39 Example 3:Meridional Mag A patient’s spectacle Rx is OD +4.00-3.00x090 (vd = 12 mm). Compare the meridional mag (MM) with spectacles & CLs. Specs +4.0 0 use Power Factor on each meridian 090: %SM = [(15)(+4.00)] / 10 = +6% +1. 00 180: %SM = [(15)(+1.00)] / 10 = +1.5% So, MM = 6 – 1.5 = 4.5% 40 Example 3, cont. CLs use Power Factor on each +4.0 0 meridian 090: %SM = [(3)(+4.00)] / 10 = +1.2% 180: %SM = [(3)(+1.00)] / 10 = +0.3% +1.00 So, MM = 1.2 – 0.3 = 0.9% (4.5%) 0.9 / 4.5 = 1/5 = 20% ↓ MM with CLs 41 Summary- Aniseikonia 1. Lenses have magnification (or minification) effects on the retinal image, in addition to changing vergence & prismatic effects. 2. Spectacle Mag (SM) = RIS with correction / RIS without correction 3. Exact formula and approximate formula (know both) 4. Shape factor and power factor; power factor predominates 3. Plus lenses increase RIS and minus lenses decrease RIS, compared to uncorrected RIS 4. CLs provide more natural RIS, similar to emmetropia (vd = 0) 5. For symptomatic patients, we alter shape factor (BC, CT, & n) as much as possible to minimize aniseikonia. These modifications can improve the visual comfort with glasses but negate the “corrective curve” lens design to minimize peripheral lens aberrations. 42