Understanding Myopia with Presbyopia:
- Myopia is condition where total power of our eyes increase and lead to rays focus in front of the retina.
- The parallel rays coming from infinity focus in front of the retina when accommodation is at rest.
- If a patient is myopic as well as presbyopic then he/she will see clearer at near without correction as compared to emmetropic patient with presbyopia.
- Because in Myopia, rays focus in front while see at distance but rays focus on the retina or close to retina while see at near.
- If Myopic power is equal or close to presbyopic power, then without presbyopic correction two possibilities are there:
1. Without myopic correction patient will see near clear but will see blur at distance.
2. With myopic correction patient will see distance clear but near blur.
- Ideally, we should correct for both near and distance for clear visual acuity at near and distance.
But the question is “Should we give full correction or under-correction in myopic patient with presbyopia”
- Few things need to be considered:
- Patients demand.
- Amount of Power.
- Comfort Level with Previous Glass.
Patients demand:
- If a patient main concern is for distance work, then we have to give full correction for myopia and required presbyopic power.
Amount Power:
- If patient is having high degree of myopia, then we can go under-correction of myopia and minimum or no correction for presbyopia.
Comfort Level with Previous Glass:
- If patient is not comfortable with previous Bifocal and progressive, then we can follow under-correction for myopia and minimum correction for presbyopia (for bifocal or progressive) or no correction for near (Single Vision).
Now another question arises:
How under-correction in myopia will lead to minimum or no correction for presbyopia?
- Let’s take an example.
- A patient with OU -5.0DS for distance and OU +1.50DS for near. He was advised a bifocal, but he was not comfortable with that bifocal spectacle.
- Now if we separate near and distance power then:
- At Near patient will need: -5.0 + 1.50 or -3.50DS
- At Distance he will need: -5.0DS.
- This problem can be solved in two ways:
Technique-1: Distance correction -3.50 or -4.0DS with no Add
Technique-2: For Distance -4.50 or -4.25DS with near add +1.00 or +0.75DS.
Technique-1: Distance correction -3.50 or -4.0DS with no Add
- With a -3.50 or -4.0DS instead of -5.0DS, he will see little blur at distance but will see clear at near.
- Because with a -3.50DS lens, rays will focus in front of the retina but for near it will focus on the retina.
- Thus, he will see little blur at distance but will see clear at near.
Technique-2: For Distance -4.50 or -4.25DS with near add +1.00 or +0.75DS.
- In 2nd technique, if we give -4.50 or -4.25DS instead of -5.0D, then he will see almost 6/6 with both eyes but for near he will need 0.50 to 0.75D less addition power.
- Because a -4.50D lens will lead to focus the rays in front but very close to the retina.
- And for near, he will need a +1.0D lens instead of +1.50D as 0.50D is under-corrected for distance.
- Thus, an addition power of +0.75D or +1.0D instead of +1.50D may easily be accepted by him in a bifocal spectacle.
Conclusion:
- If patient’s profession requires fine distance visual acuity, then we have to give full correction.
- If patient’s profession requires fine near visual acuity, then we can go for no distance correction for very mild myopia (-0.25 to -1.0) or under correction for distance.
- If patient is having facing problem with previous spectacles (Bifocal or Progressive), under correction of myopia along with less addition power may be helpful.
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1 Comment
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