Competency 4.1.1 Flashcards
1
Q
Definition of Anisometropia
A
- a difference between the refractive errors of the two eyes that is over 1.00D in SER size
2
Q
Causes of Anisometropia
A
- Patients regular refractive errors can be vastly different
- Can result from cataract surgery
- Aphakia
3
Q
Problems Due to Anisometropia
A
- 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
4
Q
Features of Anisometropia in Young Patient
A
- 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
5
Q
Treatment of Anisometropia in Young Patients
A
- is aimed at preventing amblyopia
- Full optical correction combined with penalisation of the dominant eye via patching is a common treatment method.
6
Q
What is Aniseikonia
A
- 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
7
Q
Thin Lens Spectacle Magnification
A
- 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
8
Q
Spectacle Magnification d in Formula
A
- In equation The ‘d’ is the BVD and an additional 3mm to account for the 3mm depth of AC
9
Q
Shape Factor in Spectacle Magnification
A
- 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
10
Q
Power Factor in Spectacle Magnification
A
- 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
11
Q
When Does Aniseikonia Cause a Problem
A
- 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.
12
Q
Options to Overcome Aniseikonia
A
- Reduce Shape Factor
- CLs
- Surgery
13
Q
Reduce Shape Factor to Aid in Aniseikonia
A
- Can be achieved by thinning the higher prescription lens and not the other lens
14
Q
Contact Lenses to Aid in Aniseikonia
A
- In contact lenses the distance from the eye is so small that it makes the magnification produced negligible, regardless of the prescription.
15
Q
Surgery to Aid in Aniseikonia
A
- 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.
