Competency 4.1.1

Question 1

Definition of Anisometropia

Answer 1

• a difference between the refractive errors of the two eyes that is over 1.00D in SER size

Question 2

Causes of Anisometropia

Answer 2

• Patients regular refractive errors can be vastly different
• Can result from cataract surgery
• Aphakia

Question 3

Problems Due to Anisometropia

Answer 3

• Is significant in a young patient as can lead to amblyopia as the brain ignores the image from the more ametropic eye.
• In patients with large degrees of anisometropia the brain can be unable to fuse the images from each of the two eyes, resulting in diplopia.
• Different prescriptions can result in differential prism which can result in diplopia

Question 4

Features of Anisometropia in Young Patient

Answer 4

• Anisometropia presenting in young patients is very often axial and is commonly related to retinopathy of prematurity.
• It is more associated with myopic prescriptions and higher degrees of astigmatism.
• Anisometropia can arise from a young patient with a accommodative squint, as the deviating eye does not emmetropise similarly to the dominant eye.
• Associated with microstrabismus

Question 5

Treatment of Anisometropia in Young Patients

Answer 5

• is aimed at preventing amblyopia
• Full optical correction combined with penalisation of the dominant eye via patching is a common treatment method.

Question 6

What is Aniseikonia

Answer 6

• an inability from the brain to fuse the two images due to the effects of spectacle magnification
• there is a difference in retinal image size when a patient is wearing spectacles between the eyes

Question 7

Thin Lens Spectacle Magnification

Answer 7

• In this case only the power factor is accounted for
• A thin lens calculation can be used in a minus powered lens as the centre thickness will always be around 1.5mm, which combined with a RI of 1.5 will yield a t/n of around 1, therefore making the shape factor of 1.0 or near enough

Question 8

Spectacle Magnification d in Formula

Answer 8

• In equation The ‘d’ is the BVD and an additional 3mm to account for the 3mm depth of AC

Question 9

Shape Factor in Spectacle Magnification

Answer 9

• The shape factor is nearly always greater than 1.0, which indicates magnification as because ophthalmic spectacle lenses rarely have anything but convex anterior surface
• Spectacle magnification increases with an increase in front surface power or with an increase in lens thickness .
• SM decreases with an increase in the index of refraction of the lens material

Question 10

Power Factor in Spectacle Magnification

Answer 10

• Spectacle magnification varies directly with the back vertex power , with the retinal image size increasing with the increasing plus lens power and decreasing with the increasing minus lens power
• If plus lens move forward or closer to the eye decreasing the value of h , there is a decrease in retinal image size , whereas , when a minus lens is moved closer to the eye there is an increase in image size

Question 11

When Does Aniseikonia Cause a Problem

Answer 11

• problems only occur if the difference in the right and left spectacle magnifications is greater than about 5 per cent
• In aphakia the difference in spectacle magnifications can be as great as 30 per cent.
• Images formed in the two eyes do not fall on corresponding areas. Thus cortical cells are not driven binocularly.

Question 12

Options to Overcome Aniseikonia

Answer 12

• Reduce Shape Factor
• CLs
• Surgery

Question 13

Reduce Shape Factor to Aid in Aniseikonia

Answer 13

• Can be achieved by thinning the higher prescription lens and not the other lens

Question 14

Contact Lenses to Aid in Aniseikonia

Answer 14

• In contact lenses the distance from the eye is so small that it makes the magnification produced negligible, regardless of the prescription.

Question 15

Surgery to Aid in Aniseikonia

Answer 15

• In cases of aphakia, implantation of an IOL or AC IOL can result in the spectacle prescription required being equalled out, meaning the magnification difference is brought to tolerable levels.

Question 16

What is Differential Prism?

Answer 16

• Occurs in anisometropia when looking off axis
• The different powers between the eyes result in different value of induced prism when looking down through a bifocal segment
• Can cause asthenopia or diplopia

Question 17

When does Differential Prism Become An Issue?

Answer 17

• vertical differential prism of less than 1^ at the near visual points (NVPs) is unlikely to cause problems.
• some subjects with as much as 5^ of vertical differential prism at the NVPs experience no symptoms due to adaption
• Normal vertical fusional range is around 2 to 3^

Question 18

Methods For Overcoming Differential Prism

Answer 18

• Unequal segment sizes
• Slab-off lenses
• Franklin Split and Presto Lenses

Question 19

Unequal Segment Sizes to Overcome Differential Prism

Answer 19

• By putting a bigger segment in front of one eye compared to the other can overcome the relative prism created at N.
• Different segment sizes work better with a smaller differential prism but a larger near addition.

Question 20

Slab-Off Lenses in Overcoming Differential Prism

Answer 20

• We use slabbing off to induce extra prism into the least plus lens to match that of the higher plus power which will prevent any diplopia.
• The opposite is true of a reverse slab-off which is used on the least minus lens
• Can be used to correct for imbalance amounts ranging from 1.5D to 6D
• In great prism difference consider using slab off on one lens and reverse slab off on the other

Question 21

Franklin Split in Overcoming Differential Prism

Answer 21

• essentially moulds two single vision lenses into the one frame, appearing at one lens
• allows the patient to look through both OC’s whilst only wearing one pair of spectacles.

Question 22

Presto Lenses in Overcoming Differential Prism

Answer 22

• allows the patient to look through both OC’s whilst only wearing one pair of spectacles
• the BIF segment is cut out of the distance lens, and another lens is added into this space

Question 23

Possible Errors in a Prescription

Answer 23

• Out of date Rx
• Prescription over ±4.00DS with no BVD
• Optometrist name/signature missing
• One eye in + cyl and the other in – cyl
• Prism vertically split incorrectly, i.e BU in both eyes