Chapter 11: Presbyopia Flashcards
In infancy, a child’s eye is capable of how many dioptres of accomodation
14D
After the age of 60, how many dioptres of accomodation does a patient usually have
1 or less D
For a patient to comfortably focus on a near object, how much accomodation must they have left in reserve
1/3rd
Define presbyopia
A patient experiencing difficulty and discomfort focusing on near objects due to not having 1/3 of their accommodative reserve left.
Age related
At what age does presbyopia usually occur?
40-45
Define presbyopic correction
Adding convex lens to enable patient to see more comfortably
What variables do we need in order to work out the needed presbyopic correction
-Near point of the eye
-Desired working distance
Eg if a person has a near point of 33cm , they have an accommodative range of 3D.
To be comfortable, they should only use 2/3 of their range which is 2D.
If they want to focus on an object that is 25cm away. They need 4 dioptres of power.
To be comfortable, the eye can use 2 of its dioptres. Therefore we need to add a convex lens that will a further 2 dioptres, leaving 1/3rd of the eyes accommodative range free.
Why do patients with uncorrected hypermetropia develop presbyopia sooner then myopic patients
A hyperopic person of 3D needs to exert 3D of accomodation just to see an object at infinity (parallel rays focus on the retina). Ie has become rounder just to see at infinity
To see an object at 25cm, they need to further exert another 4D totalling 7D (4+3). For a person to be comfortable and not presbyopic, they need to have 1/3rd of their accommodative reserve free which isn’t feasible for a 45 year old with a accommodative range of 4D.
Meanwhile in a myope of 3D has a far point of 33cm. Ie lens is still flat.
To see an object that is 25cm away, they only need to use 1D which is less than 1/3rd of the accommodative amplitude (4-3=1D)
Amplitude = near point in D - far point in D
Place the refractive states
Emmetropia, myopia and hyperopia in order of who develops presbyopia first
1) Uncorrected hyperopia
2) Emmetropia
3)uncorrected myopia- myopic patients of >4D will never need reading glasses
How to safeguard against prescribing too strong a presbyopic correction
Make sure patient can read N5 from their normal reading distance and N8 at arms length with the prescription
What are split (franklin bifocals)
What are cemented bifocals
Attaching supplementary wafer lens to a distance lens of same refractive index. They are cemented together by UV epoxy resin which has superseded Canada balsam resin
What is a fused bifocal lens
A button made of flint glass is fused to a lens made of crown glass using heat.
The refractive index can be adjust by refractive index of the near add and the curvature of the distance portion
What is the drawback of fused bifocals
Near segment can be effected by chromatic aberration
What are solid bifocals?
Aka executive
Single piece of plastic or glass, the near segment is created by changing the curvature of the from or back of the distance segment.
Define distance visual point and near visual point
Shortened to DVP and NVP, the position on a lens which corresponds to the visual axis of the eye.
Where is the NVP relative to the DVP
8mm inferior and 2mm nasal.
It is usually a tangent of the limbus
Method used to ensure that children with converges excess esotropia use the near segment for all close tasks
The edge of the near segment should be at the lower margin of the pupil
Define prismatic jump in bifocal lenses
If the optical centre of a prism coincides with the DVP and NVP, when looking through the peripheries of an optical centre (axial rays but still parallel), you experience prismatic effects due to decentration. The prismatic effect suddenly changes when you look from the DVP to NVP causing a jump
Describe the prismatic effect seen at the NVP of a bifocal lens.
For cemented/ fused lenses you get a prismatic effect due to decentration of the base lens (for distance vision) as you are looking through its periphery.
Usually the optical centre of the near segment matches the NVP so doesn’t add any prismatic effects when looking through it. In this situation, the prismatic effect is only caused by distance segment.
NB: bottom of the distance lens acts as a base up prism, while top of near lens acts as a base down lens
How can we reduce prismatic jump in bifocals
Design the lens so that the optical centre of each segment lie close to the junction between the segments.
Alternatively, you can add a base up prism to the near add which counteracts the base down effect. This is known as prism control.
Why do patients experience oblique astigmatism when wearing bifocals and how do we mitigate this?
The near visual axis does not correspond to the near optical axis.
To mitigate this, add a pantoscopic tilt, so that the axes are more perpendicular, resulting In less aberration.
NB: optical axis is a line passing through the centre of curvature of a lens or spherical mirror and parallel to the axis of symmetry. Oblique astigmatism occurs when non axial rays pass through a medium
What is a progressive Addition lens
Trifocal lens, where there is a corridor between distance and near segment to see intermediate object. This avoids the prismatic jump seen in bifocals.
Name a downside to progressive addition lenses
To either side of the progression corridor, aberration and astigmatism increases
Why are SOFT PALS better tolerated
Soft progressive addition lenses have a smaller near and distance segment. This results in a longer transition corridor. As a result, there is less aberration on either side of the corridor.
