Anisometropia and Aniseikonia PDF

OPT3054: OCULAR OPTICS II ANISOMETROPIA & ANISEIKONIA MISS INDIRA MADHAVAN BOptom (Honours) UKM, MHSc (Optometry) UKM Course Content Anisometropia Aniseikonia Symptoms Spectacle magnification Com...

OPT3054: OCULAR OPTICS II ANISOMETROPIA & ANISEIKONIA MISS INDIRA MADHAVAN BOptom (Honours) UKM, MHSc (Optometry) UKM Course Content Anisometropia Aniseikonia Symptoms Spectacle magnification Comparison of spectacle and contact lens magnification Relative spectacle magnification Prescribing to eliminate/minimize induced aniseikonia Indication of the presence of aniseikonia Measurement of image size differences Avoiding/minimizing induced aniseikonia Prescribing and lens design 2 Anisometropia 3  Anisometropia is considered to Anisometropia exist when the SER of the two eyes differs by 1.00 D or more. Uncorrected Corrected Anisometropia of this amount or greater can be responsible for Accommodation Children Adult several problems, both in the system uncorrected state and in the corrected state Functional amblyopia No binocular vision Vergence system Binocular vision Retinal image problem size 4 Uncorrected anisometropia Eyes may accommodate, in any given situation, so that the retinal image of one eye/of the other is in focus/so that the two retinal images are more/less equally out of focus For children underage of 5/6 years, may lead to the development of: Functional amblyopia Binocular vision problems. If older child/adult: Normal binocular vision seldom exists 5  Accommodative system Lens effectivity differs for different fixation distances Lenses which are equally effective for the two eyes of an anisometrope for distance Corrected vision are not equally effective for near vision Vergence system Anisometropia Horizontal PE Vertical PE Relationship between the retinal image sizes for the two eyes 6 Horizontal PE When lenses of unequal powers are worn, any amount of movement of the visual axes away from the OC of the lenses brings about a prismatic imbalance. The amount and direction of the imbalance can easily be calculated with the aid of Prentice's rule. EXAMPLE If a patient wears the prescription RE +3.00 D LE + 1.00 D centered for the distance PD, two visual axes pass through points in the lenses 20 mm to the L of the OC 20 mm to the R of the OC 7  This shows that when horizontal conjugate movements are made, the visual system must somehow compensate for the PE. The resultant PE is shared equally by the two eyes. However, no correction needed as eyes capable of horizontal FV for large amplitude PFV to overcome BO prism NFV to overcome BI prism 8 Vertical PE Eyes are capable of vertical fusional movements of only a small amplitude Vertical PE which differ for the two eyes are likely to cause symptoms of eyestrain When a corrected anisometrope either elevates or lowers the visual axes, vertical PE occurs Most likely to become a problem when the visual axes are lowered for reading Induce: BU: for the more hyperopic eye of an anisometropic hyperope BD: for the more myopic eye of an anisometropic myope 9  Only those bifocal and multifocal wearer will have significant PE as they need to read through a level that is approximately 10 mm below the distance OC, not single vision wearer Amplitude of vertical fusional movements is very small For moderate amounts of anisometropia, the vertical imbalance can be overcome by means of the small vertical fusional movements However, if the anisometropic patients doing extensive near work, evaluation of an induced vertical imbalance is crucial 10  Management to compensate for differential vertical PE at the reading level: 1. Lowering the distance OC 2. Prescribing single-vision lenses for reading only 3. Prescribing dissimilar bifocal segments for the two eyes 4. Prescribing compensated bifocal segments 5. Prescribing prism segments 6. Prescribing a "slab-off” lens 7. Prescribing a Fresnel Press-on prism 8. Prescribing contact lenses 11 Aniseikonia 12  Spectacle correction of anisometropia cause: Aniseikonia Unequal image sizes for the two eyes Symmetrical Asymmetrical Disturbances in binocular vision Overall Meridional One meridian All directions Aniseikonia is defined as the Vertical Barrel relative differences in the sizes/shapes of the ocular Horizontal Pincushion images of the two eyes. Types of image size differences: Oblique Symmetrical Differences Asymmetrical Differences 13 Symmetrical Differences Overall Image of one eye is increased/decreased in size equally in all meridians as compared to the other eye. Meridional Image of one eye is increased/decreased symmetrically in one meridian, compared to that of the other eye. Vertical Horizontal Oblique 14 Asymmetrical Differences Increase/decrease in the image size in One meridian All directions Barrel distortions Pincushion 15 Symptoms Mimic those reported by under corrected/oculomotor imbalance The symptoms persist/have their onset when lenses prescribed to correct the ametropia and/or oculomotor imbalance Patients complain not so much about distortion of space but about how their eyes "feel." Asthenopia Headaches Photophobia Giddiness Nervousness Diagnostic test Monocular occlusion Diagnosed to be aniseikonia if the symptoms persist under monocular occlusion 16 Spectacle Magnification retinal image size in the corrected eye SM = retinal image size in the uncorrected eye Percentage SM: %SM = (SM - 1)(100) Angular magnification (AM): F1 = the power (F1A) of the front surface of the lens, Fv = the back vertex power (Fv) of the composite lens, t = the lens thickness, n = the index of refraction of the lens material, h = the distance from the back vertex of the lens to the entrance pupil of the eye (it is normally assumed that the distance from the front surface of the cornea to the entrance pupil of the eye is equal to 3 mm) 17  Example 1 (Minus Lens) Example 2 (Plus Lens) A patient require -2.00DS for BE. What will A patient require +2.00DS for BE. What will be SM if the: be SM if the: Front surface power: +8.00D Front surface power: +6.00D Centre thickness: 2.5 mm Centre thickness: 2.2 mm Index of refraction: 1.523 Index of refraction: 1.523 Vertex distance: 12 mm + 3 mm Vertex distance: 12 mm + 3 mm (assumption) (assumption) SM= 1 0.0025(8) 1 1− 1.523 1−0.015(+2) 1 1 SM= 1− 0.0022(6) 1.523 1−0.015(−2) = 1.044 = 0.98 %SM= (SM-1)(100) %SM= (SM-1)(100) = (SM-1)(100) = (SM-1)(100) = -2.0% (minified image) = +4.4% (magnified image) 18 Spectacle vs Contact Lens Magnification Magnification Magnified Minified Plus: CL→ Plus: Spectacles Spectacles →CL Minus: Minus: CL→ Spectacles →CL Spectacles 19 CL (Plus Lenses) Spectacles (Plus Lenses) A patient wears +14.00DS spectacles with a VD A patient wears +16.83DS CL. What will be the: of 12mm. What will be the: a) Spectacle power a) CL power b) SM b) SM a) FC = 1 1 = 1 1 = +14.00D +VD +0.012 1 1 FS 16.83 a) FC = 1 = 1 = +16.83D −VD −0.012 FS 14 FS +16.83 b) = = 1.202, FC +14.00 FS +14.00 FC = +16.83 = 0.832, b) %SM= (SM-1)(100) = (1.202-1)(100) %SM= (SM-1)(100) = +20.2% (magnified image) = (0.832-1)(100) = -16.8% (minified image) 20 CL (Minus Lenses) Spectacles (Minus Lenses) A patient wears -11.36DS spectacles with a VD A patient wears -10.00DS CL. What will be the: of 12mm. What will be the: a) Spectacle power a) CL power b) SM b) SM a) F S= 1 1 = 1 1 = -11.36D +VD +0.012 1 1 FC −10.00 a) FC = 1 = 1 = +10.00D −VD −0.012 FS −11.36 FC −10.00 b) = = 0.88, FS −11.36 FS −11.36 FC = −10.00 = 1.136, b) %SM= (SM-1)(100) = (0.88-1)(100) %SM= (SM-1)(100) = -12.0% (minified image) = (1.136-1)(100) = +13.6% (magnified image) 21 Relative Spectacle Magnification RSM is defined as the ratio of the retinal image size for a corrected ametropic eye to the standard emmetropic eye image size for a corrected ametropic eye RSM = image size for a standard emmetropic eye 𝑅𝑆𝑀R Magnification ratio = 𝑅𝑆𝑀I If the keratometer findings for BE are: Equal: Axial anisometropia Unequal: Refractive anisometropia 22 Management of Anisometropia Anisometropia Astigmatism Axial Refractive Spectacles Contact lenses 23 Anisometropia Astigmatism Reading lenses of equal power for To reduce monocular distortion presbyopic patients, if they have with no sacrifice in VA: not previously worn a full 1. Use as short a vertex distance as correction for the anisometropia possible and have not experienced To reduce monocular distortion discomfort with sacrifice in VA: 1. Reduction of the power of the cylinder, but at the correct axis. 2. If the cylinder axes are oblique, the axes can be rotated toward the 180° or 90° position, depending upon which is closer. 24 Prescribing and Lens Design Factors to consider for management: 1. The age of the patient 2. The nature of the previous correction 3. The patient's occupation and hobbies 4. The patient's temperament 5. The patient's interest in the appearance of the glasses 6. The cost of the correction 7. The nature of the patient's complaint, and the practitioner's assessment of the likelihood of partial or total relief 25 THANK YOU MISS INDIRA MADHAVAN BOptom (Honours) UKM, MHSc (Optometry) UKM 26