Anisometropia

Md. Saiful Islam2nd Batch, B. Optom,

Institute of Community Ophthalmology, University of Chittagong

Anisometropia• The 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 Anisometropia

• Difference 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 Anisometropia• Congenital & developmental A. Occurs due to differential growth of the

two eyeballs.• Acquired Anisometropia : occurs due to – Θ Uniocular Aphakia Θ Implantation of IOL of wrong power Θ Inadvertent surgical treatment of refractive error Θ Trauma to the eye Θ Keratoplasty in one eye

Classification of Anisometropia

• Absolute 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 Anisometropia

• Amblyopia • 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 : Θ Any preferential looking Θ Headache Θ Failure to reach developmental milestones

specially with mobility.

Effects of Anisometropia

• Uncorrected Anisometropia : Θ Status of Vision• Corrected Anisometropia : Θ On Accommodation Θ On Vergence System Θ 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 Accommodation

• According to Hering’s 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 system

• As 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 Size

• High 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 Imbalance

• The 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 Imbalance

• Contact Lenses• Two pair of glasses• Lowering the distance optical center• Raising the segment height• Dissimilar bifocal segments• Fresnel press on prism• Slab off lens • Compensated R segments

Knapp’s law• If 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.

Continued…

• According to Knapp’s law….

Θ Axial ametropia should be corrected with spectacle lens.

Θ Refractive ametropia should be corrected with contact lenses.

Diagnostic tests • Visual Acuity

• Dry & Wet (Cycloplegic) Refraction

• Biometry (Keratomtry/Topography & A-scan) Θ Refractive/ Axial Anisometropia.

• Measurement of Deviations.

• State of Binocular Vision may be assessed by- Θ“FRIEND” Test Θ Worth’s Four Dot Test (WFDT)

• “TNO” Test to assess stereopsis.

“FRIEND” Test

F, I, N - GREENR,E,D - RED

1. The patient wears red green goggles and is seated at a distance of 6m from the chart

2. Binocular single vision- will read FRIEND at once3. Uniocular vision – will read either FIN or RED4. Alternate vision– will read FIN at one time and RED at

other time

F R I E N D

• WORTH’S FOUR DOT TEST Sees all four lights in

absence of manifest squint- normal binocular vision

ARC- sees four lights in presence of manifest squint

Sees 2 red lights- LE suppression

Sees 3 green lights- RE suppression

Sees 2 red and 3 green alternately- alternate suppression

Sees 5 lights(2red, 3 green)- diplopia

Treatment Options

• Glasses • Contact Lenses• Refractive Surgeries• Some Specific Modalities

Glasses

• In 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 patient’s 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

Table

Table

Contact Lenses

• Advised for higher degrees of anisometropia and for children

Sequential management of anisometropic amblyopia

• Full refractive correction• Improve alignment of the visual axes when needed- Θ Added lens if- inaccurate or insufficient

accommodation, high AC/A. Θ Prism if- esophoria at distance(Base out),

hyperphoria(Base down). • Direct Occlusion (part time, 2-5h/day).• Vision Therapy : Θ Monocular- maximize monocular acuity. Θ Binocular- improve binocular functions.

Other Modalities

• Intraocular 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. Borish

• Clinical Optics Troy E. Fannin, Theodore Grosvenor

• American Academy of Ophthalmology

• Duke-Elder’s Practice of Refraction David Abrams

• Internet.

Thanks to All