Chapter 10: Optics Of Ametropia Flashcards
What is ametropia
Condition where light does not fall on retina.
Ie myopia and hyperopia.
What is a staphyloma
Abnormal protrusion of the uveal tissue. If it is posterior, can cause myopia as the retina falls behind f2
What is the d ifference between axial, refraction and index myopia
Refractive aka index is when refractive index of the eye is too high.
Axial = eye is too long
Give an example of refractive hyperopia
Aphakia = no lens
Define manifest hypermetropia
The strongest plus lens (convex) which still allows a person to see distance images clearly (20/20) (the most plus that can be tolerated without blurring of vision and without cycloplegia)
Define latent hypermetropia
Hypermetropia which is masked by ciliary muscle tone and involuntary accommodation
(difference between the manifest hypermetropia and hypermetropia measured with cycloplegia.)
NB ciliary muscle contract- zonules relax and lens becomes more anterior and thicker= more refraction
Why is cycloplegic refraction important in children?
Due to latent hypermetropia (tone in ciliary body). Using cycloplegics gets rid of this tone, which will allow you to assess the true magnitude of refractive error
Define facultative hypermetropia
Hypermetropia that can be overcome by accommodation. (difference between the absolute and the manifest hypermetropia)
What is absolute hypermetropia
Hypermetropia that can’t be overcome by accomodation. (the least amount of plus lenses needed for clear distance vision without cycloplegia)
Define regular, oblique and irregular astigmatism
Regular = meridians are at 90 and 180 degrees
Oblique = meridians are not at 90 and 180, but are still perpendicular to one another
Irregular= Meridians are not at 90 degrees - cannot be corrected with glasses
Define simple, mixed and compound hyperopia/ myopia in astigmatic eye
Simple = one image falls and retina, the other doesn’t
Compounds= both images do not fall on the retina
Mixed = one falls behind and one falls infront of the retina ie, person is myopic and hypermetropic
Define Anisometropia
When refraction by each eye is different.
Large degrees are a significant cause of amblyopia
In someone with anisometropia, is amblyopia more likely to develop in myopia or hyperopia and why?
More likely in a hypermetropic person. Can happen with 1D disparity.
This is because accomodation is binocular, ie each eye cannot accomodate by a different amount. As a result, one eye always stays out of focus.
Less likely in myopia because near vision is clear. More likely to happen in high myope.
Nucleosclerosis of the lens in one eye results in
Anisometropia as the sclerotic nuclear increases the refractive power of the lens.
Patients who weren’t previously myopic struggle to handle full correction of the discrepancy
What simple test can be performed to see if a reduction in visual acuity is either caused by refractive error or retinal damage.
Pinhole test.
If acuity is improved irrespective of the type of error= refractive issues
If no improvement = retinal issue or neurological
If worse= macular issue
What is the range at which a pinhole test will improve vision
-4D to +4D
In reality, pinhole transmits a pencil of light, not single beam. As a result, high refractive indexes will result in a diffuse image on the retina
Describe how to use stenopaeic slit to assess astigmatism
- A plastic sheet with a slit that allows light through is placed on a trial lens.
- Slit is rotated until patient sees through the slit most clearly
- Spherical lenses are then added to further improve vision
- When optimal vision is found, the slight is rotated 90 degrees
- This will cause blurred vision if there is astigmatism
- Lenses are added to further fine tune vision until optimal vision is found
- Cyclindrical power is equal to difference between lens power in both axis.
- Cyclindiral axis = the axis in step 2.
Where is the far point of a hypermetropic eye
Behind the retina
For a lens to correct emmitropia, what needs to happen?
Far point of the eye meets focal point F2 of the lens
The distance between the far point of the eye and the principle plane is roughly equal to what?
The focal length of the correcting lens.
Define static refraction
Aka, ametropic error
Reciprocal of far point of the eye to principle point or focal length of the lens that is used to correct it.
Measured in dioptres
Define effective power of a lens
If the F2 of a lens coincides with the farpoint of the eye, then the image is focused on the retina.
However, in reality, the lens is held in glasses which means the far point and f2 are not aligned.
As a result, the power of the lens must be adjusted so that the FP meets F2 again.
For example in a hypermetropic eye, the image falls behind the retina, if the convex lens is moved away from the eye, the image is also brought forward causing myopia, which has increased its effective power.
As a result, a weaker plus lens is needed.
What happens to the effective power of a concave lens when it is moved away from the eye
It is reduced.
Therefore, in a myopic patient, the -lens power needs to be increased
F= power of lens
P= principle plane of eye
R= retina
Fp= far point
Little f = focal length
What happens to the effective power of a convex lens when it is moved away from the eye
It increases.
Therefore in a hyperope, the plus lens power needs to be reduced.
F= power of lens
P= principle plane of eye
R= retina
Fp= far point
Little f = focal length
If you move a lens forward or backward relative to an ametropic eye, how do u calculate the power required so that the image still hits the retina (ie, far point of eye coincides with second focal point)
F2 = 1/(f1-d)
Or
F2= F1/(1-dF1)
F1= original power of lens
F2= new power needed
f1= current focal length of the lens
D= distance by which the lens is moved by.
Nb: if concave lens is moved back, u need a stronger lens
I’d a convex lens is moved back, you need a weaker lens
Define relative spectical magnification
RSM= corrected ametropic image size/ emmitropic image size
What happens to the relative spectical magnification in an axial myope who wears contact lenses
RSM increases aka >1
What is the relative spectical magnification in someone who have REFRACTIVE ammetropia who wears glasses at the anterior focal point of the eye
Depends on type of lens
If hypermetropic, RSM is greater than unity aka>1
If myopic, RSM is less than unity aka<1
NB: anterior focal length = f1
In refractive ametropia, what happens to the relative spectical magnification when wearing contacts
Approaches unity aka 1
What happens to the relative spectical magnification in axial ametropia, if a lens is placed on the anterior focal point of the eye
It is at Unity aka 1
In axial ametropia, if the correcting lens is placed at the anterior focal point of the eye, the image size is the same as in emmetropia. The RSM is therefore unity. However, in axial myopia (Fig. 10.16), if the correcting lens is worn nearer to the eye than the anterior focal point, the image size is increased. The relative spectacle magnification is therefore greater than unity. Contact lenses in axial myopia thus have a magnifying effect
How to calculate relative spectical magnification from focal lengths in aphakic eye
Aphakic anterior focal length /emmetropic anterior focal length
Anterior focal length = F1.
What problem arises when wearing spectacles for aphakia
Need very high powered lenses, >10D:
- extremely heavy and can slip down. Increasing effective power
-More prone to spherical aberration.
-Due to prismatic effects also causes ring scotoma which can cause them to trip on unseen objects. When they move their head, scotoma move too, which can reveal unseen object, ie jack in the box phenomena.
- Straight lines appear curved unless looking through very small axial zone of lens
-Additionally, magnification is 1.33 compared to an emmetropic eye. This makes patients misjudge distances, thinking things are closer than they actually are. This is because angle substended at retina is larger
FYI, because everything is magnified, they perform better on Snellen chart, achieving 6/9
What methods can be used to combat the effects of a thick lens needed for an aphakic patient
- Use plastic lens which is lighter- but more easily scratched
-Use lenticular lens (periphery is just a carrier, you look through central zone) - use contacts or intraocular lenses.
Define aniseikonia and give an example of when it happens
When each eye sees an image that is bigger than the other eye.
When this happens a patient cannot fuse the images.
This happens when an aphakic patient wears glasses. The RSM of the aphakic eye with glasses is 1.33, which is one third larger than that seen by the normal eye.
How can you prevent aniseikonia in a patient with one phakic and one aphakic eye
Use intracoluar lens resulting in RSM of 1 or contacts, resulting in RSM of 1.1
Either way, images can be fused resulting in binocular vision.
What is a iseikonic lens
A lens that magnifies an object but has no refractive/ focusing powers. This happens by increasing the angle substended at the retina .
Cannot be used to correct aniseikonia in patient with aphakia and phakic eye as it can only magnify by 5%.
NB: aphakic eye with glasses causes 1.33 relative spectacle magnification
Why are iseikonic lenses falling out of fashion
-Expensive
-need to be custom made
-thick
-heavy
What is the SRK formula
Formula to calculate the power of an intraocular lens that will be implanted into an eye to achieve emmetropia:
P= A - 2.5(axial length of eye in mm) - 09.(keratometry reading in dioptres)
P= power
A= constant for the particular IOL to be implanted.
What is the SRK formula if you are not aiming for emmetropia
P= A - 2.5(axial length of eye in mm) - 09.(keratometry reading in dioptres) - D(R)
P= power
A= constant for the particular IOL to be implanted.
R= refractive condition
D= 1.25 if IOL power for emmetropia is >14D and 1 is the IOL needed for emmetropia is < or = 14D
What is the main issue with the SRK formula
Inaccurate for short axial lengths (<22mm) and long axial length (>24.5mm)
As a result, we have SRKII and SRK-T which are more accurate
How to measure the axial length of an eye to use in the SRK formula
A scan ultrasound
Ultrasound hits and reflects off of
1)posterior cornea
2)anterior lens
3)posterior lens
4)retina
5)sclera
Basically everything except the anterior cornea
True or false, A- scan can be used to calculate corneal thickness
True!
When calculating power of an iol, which value is more important to measure accurately, axial length or keratometry or the cornea
Axial length as it has a multiplier of 2.5 while keratometer reading is multiplied by 0.9
How do you reduce the chance of getting a wrong axial length measurement of the eye
Measure axial length of both eyes. You may have made an error if:
1) in the absence of Anisometropia , one eyes axial length is >0.5mm compared to the other
2) if predicted refractive state in first eye is off
How do you measure axial length in a patient with a staphyloma?
B Scan (2D uss scan), shows staphyloma as well as the axial length
When using an IOL what factors need to be taken into account when deciding on the best over all refractive state of the eye.
1) refractive state of the other eye, don’t want to cause anisometropia or anisokenia
2) Patients previous refractive state. Usually life long myopes don’t want to wear reading glasses and prefer concave distance correction
How do multifocal IOLs work
Part of the lens is for distance vision and part of the lens is for near vision.
Both images are formed at the same time.
Made of concentric zones of different powers.
Usually the plus lens aspect of the IOL is in the centre as near vision also involves pupil constriction.
However this can be an issue when looking at far objects in light conditions as distance part of the lens maybe blocked by pupil.
To overcome this, can have alternating near and far zones on the lens.
Can also use diffraction so that a close image is formed using constructive interference
What are the disadvantages of multifocal IOLs
Part of lens is for distance and part for close, sometimes you have intermediate distance too
Both images are formed at the same time.
When looking at a close object, the convex aspect of the lens forms the clearest image on the retina, while the concave lens forms a blurry image and vice versa for objects at infinity (convex is blurry and concave is clear).
As a result, a blur circle is formed and can cause glare when driving at night and seeing bright lights.
Image is not as clear and bright as Monofocal IOL
What is the power of a standard IOL
19D in air
15D when in the eye
What kind of eye would become emmetropic if the lens is removed
An axial myope requiring -18D to -20D glasses correction
NB: even though lens power is +15, the effective power of glasses needed is has to be less than -15 due it it being further away from the retina than the lens.
Degree of myopia is referred to by the spectacle power needed.
