Optometry Education

1. Anisometropia
Md. Saiful Islam
2nd Batch, B. Optom,
Institute of Community Ophthalmology,
University of Chittagong

2. 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.

4. 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.

5. 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

6. 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

7. 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.

8. 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.

9. Signs & Symptoms of Anisometropia
• Amblyopia
• Strabismus
• Diplopia
• Headaches
• Eye Strain
• Light Sensitivity
• Difficulty Reading
• Impaired Depth Perception.

10. 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.

11. Effects of Anisometropia
• Uncorrected Anisometropia :
Θ Status of Vision
• Corrected Anisometropia :
Θ On Accommodation
Θ On Vergence System
Θ On Retinal Image Size

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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

18. 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.

19. Continued…
• According to Knapp’s law….
Θ Axial ametropia should be corrected
with spectacle lens.
Θ Refractive ametropia should be
corrected with contact lenses.

20. 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.

21. “FRIEND” Test
F, I, N - GREEN
R,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 once
3. Uniocular vision – will read either FIN or RED
4. Alternate vision– will read FIN at one time and RED at
other time
F R I E N D

22. • 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

23. Treatment Options
• Glasses
• Contact Lenses
• Refractive Surgeries
• Some Specific Modalities

24. 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

25. 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.

26. • Anisometropic
spectacles-
In these spectacles
margin of the
stronger lens is
made weaker, thus
minimizing the
annoyance of
peripheral
prismatic effect of
conventional lenses

27. Table

28. Table

29. Contact Lenses
• Advised for higher degrees of anisometropia
and for children

30. 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.

31. 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.

32. 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.

33. Thanks to All