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Anisometropia Md. Saiful Islam2nd Batch, B. Optom,Institute of Community Ophthalmology, University of Chittagong
AnisometropiaThe condition in which the total refractive power of two eyes is unequal.
There are 4 parts of Anisometropia all come from Greek. an-not, iso-equal, metr-measure, ops-eye.So literary means, the measurement of eye (refractive power) in both eye is not equal.
Terminology Isometropia : Total refactive power of two eye is equal.
Antimetropia : opposite refractive power in between eyes (one eye myopic & one eye hyperopic). Aniseikonia : An anomaly of binocular vision in which the retinal images are unequal in size or shape or both.
Tolerance of AnisometropiaDifference of 1D in two eyes cause a 2% difference in the size of the two retinal images
5% size difference / 2.5D - well tolerated
2.5-4D individual sensitivity
>4D not tolerated
Etiology of AnisometropiaCongenital & developmental A. Occurs due to differential growth of the two eyeballs.Acquired Anisometropia : occurs due to Uniocular Aphakia Implantation of IOL of wrong power Inadvertent surgical treatment of refractive error Trauma to the eye Keratoplasty in one eye
Classification of AnisometropiaAbsolute anisometropia : It is that condition in which the refractive power of two eyes is unequal.
Relative anisometropia : The total refraction of the two eyes can be equal, but the axial length may be different. This will lead to clear retinal image but a difference in the size of the retinal images.
Continued..According to etiology.. a) Congenital b) Acquired
Clinical type. a) Simple Anisometropia b) Compound Anisometropia c) Mixed Anisometropia d) Simple Astigmatic A. e) Compound Astigmatic A. f) Mixed Astigmatic A.
Signs & Symptoms of AnisometropiaAmblyopia Strabismus Diplopia Headaches Eye Strain Light Sensitivity Difficulty Reading Impaired Depth Perception.
Symptoms in Children If fellow eye is close to emmetropic, there may be asymptomatic.Tend to close or rub one eye.For very young, parents should note : Any preferential looking Headache Failure to reach developmental milestones specially with mobility.
Effects of Anisometropia Uncorrected Anisometropia : Status of VisionCorrected Anisometropia : On Accommodation On Vergence System On Retinal Image Size
Status of vision in Anisometropia There are 3 possibilities.Binocular Single Vision : present in small degree of anisometropia.
Uniocular Vision : When refractive error in one eye is of high degree.
Alternating Vision : occurs when one eye is hyperopic and other myopic , then hyperopic eye is used for distant vision and myopic for near.
Effects of A. on AccommodationAccording to Herings law of equal innervation to the ocular muscle that the two eyes accommodate equally.
In anisometropia because of two different power in two eyes, there is different amount of accommodation required for different fixation distance.
Correcting lens that are equally effective for the two eyes of an anisometrope for distance vision, are not equally effective for near vision.
Effects of A. on Vergence systemAs a result of differential prismatic effects that are present when the visual axes pass through points in the lenses other than the optical center.
Effects of A. on Retinal Image SizeHigh plus correction magnify retinal image.
High minus correction minify retinal image.
High astigmatic correction produce meridional difference in retinal image size.
Differential retinal image size(>5%) may cause diplopia.
Vertical ImbalanceThe differential prismatic effects are present at varying position of gaze, resulting from a difference in power between right & left eye, the differential prismatic effect induce is referred as Vertical Imbalance.
eg; Optical Correction : OD) 7.00D OS) 3.00D Resultant prismatic effect : 4.00 BD before the Rt eye.
Correction of Vertical ImbalanceContact LensesTwo pair of glassesLowering the distance optical centerRaising the segment heightDissimilar bifocal segmentsFresnel press on prismSlab off lens Compensated R segments
Knapps lawIf the ametropia is axial : When a correcting lens is placed before the eye that its second principle point coincides with the anterior focal point of an axially ametropic eye, the size of the retinal image will be the same as emmetropic.
If the ametropia is refractive : Uncorrected image size will be the same size as image size for a emmetrope. The spectacle can magnify or minify the image but the CLs are able to correct the error, yet leave the image size almost unchanged.
ContinuedAccording to Knapps law.
Axial ametropia should be corrected with spectacle lens. Refractive ametropia should be corrected with contact lenses.
Diagnostic tests Visual Acuity
Dry & Wet (Cycloplegic) Refraction
Biometry (Keratomtry/Topography & A-scan) Refractive/ Axial Anisometropia.
Measurement of Deviations.
State of Binocular Vision may be assessed by- FRIEND Test Worths Four Dot Test (WFDT)
TNO Test to assess stereopsis.
F, I, N - GREENR,E,D - RED
The patient wears red green goggles and is seated at a distance of 6m from the chartBinocular single vision- will read FRIEND at onceUniocular vision will read either FIN or REDAlternate vision will read FIN at one time and RED at other time
F R I E N D
WORTHS FOUR DOT TESTSees all four lights in absence of manifest squint- normal binocular visionARC- sees four lights in presence of manifest squintSees 2 red lights- LE suppressionSees 3 green lights- RE suppressionSees 2 red and 3 green alternately- alternate suppressionSees 5 lights(2red, 3 green)- diplopia
Treatment OptionsGlasses Contact LensesRefractive SurgeriesSome Specific Modalities
GlassesIn children(under the age of 12) prescribe full refractive difference regardless of age, presence of strabismus or not, degree of anisometropia.
The corrective spectacles can be tolerated up to a maximum difference of 4D,after that diplopia occurs.
So in children where best corrected visual acuity is required in both eyes, contact lenses are preferred
Continued In adult, The small degree of anisometropia should be corrected full & dioptric difference generally up to 4D according to patients tolerance.The higher degree of anisometropia should be under- corrected & preferred CLs.In adults with alternating vision the condition is usually left alone. If the patient is symptomatic & young, an attempt may be made to induce him to wear the full correction.
Anisometropic spectacles- In these spectacles margin of the stronger lens is made weaker, thus minimizing the annoyance of peripheral prismatic effect of conventional lenses
Advised for higher degrees of anisometropia and for children
Sequential management of anisometropic amblyopiaFull refractive correctionImprove alignment of the visual axes when needed- Added lens if- inaccurate or insufficient accommodation, high AC/A. Prism if- esophoria at distance(Base out), hyperphoria(Base down). Direct Occlusion (part time, 2-5h/day).Vision Therapy : Monocular- maximize monocular acuity. Binocular- improve binocular functions.
Other ModalitiesIntraocular lens (IOL) implantation for uniocular aphakia
Refractive corneal surgery for unilateral myopia,astigmatism,hypermetropia
Removal of Crystalline lens for unilateral very high myopia.
References System for Ophthalmic Dispensing Clifford W. Brooks, Irvin M. BorishClinical Optics Troy E. Fannin, Theodore GrosvenorAmerican Academy of Ophthalmology
Duke-Elders Practice of Refraction David AbramsInternet.
Thanks to All