W10 Sphere Check Tests

What is the objective of subjective refraction?

Develop and refine a prescription through which the patient can see clearly

What are the different procedures to refine the sphere?

Plus/minus sphere and duochrome test

What is the purpose of refining the sphere?

Gives the most positive/minimum minus that provides maximum VA as well as minimising accommodation

What is the set up?

After completing ret, removing working distance lens and measuring vision, turn room lighting back on and occlude eye that’s not being tested

What is the effect of a plus/minus sphere in an emmetropia eye?

Plus lens moves image in front, negative lens moves image behind, the blur circle remains the same size

What is the effect of a plus/minus sphere in a myopic eye?

Negative lens is clearer, blur circles are different sizes

What is the effect of a plus/minus sphere in hyperopia?

Positive lens is cleaner, blur circles different sizes

What is the effect of increasing the minus power?

-0.25DS: same or smaller/darker

-0.50DS: smaller and darker/blurred, vision influenced by accommodation

What is the effect of increasing the positive power?

+0.25DS: Not as sharp

+0.50 DS: Blurred/fainter

What are reasons the patient would respond same/no difference at first presentation?

You already have the right lens (change rx power to demonstrate definite response)

Vision is too poor (increase lens power and repeat)

Small pupils (increase power and repeat, use pinhole)

How does the duochrome test work?

Whichever colour is focused nearest to the retina will be seen as clearest

Emmetrope = equal

Myope = red clearer

Hyperope = green clearer

Which power alteration do you use in the duochrome test?

Minus if red is clearer, positive if green is clearer

What are reasons px might respond same/no difference in the duochrome test?

Duochrome is balanced, rx too far out, small pupils, poor vision due to pathology (use plus/minus technique)

What are limitations of duochrome?

Won’t work if vision is less than 6/12, wont work is prescription is significantly incorrect, difference between clarity of red and green is reduced, red is bias to test as blue/green light is partially absorbed

What is the check test?

An alternative to duochrome/plus minus- add a +1.00DS over the current prescription and ask px if its clearer, blurrier or the same- if you remove the lens and the vision is worse, you’ve added too much plus

Which factors affect blur circle?

Reduced pupil size- reduced blur circle and vice versa (blurs back too quickly with large pupil)

What do results of the blur/check test indicate?

If vision with +1.00DS is better than 6/18 then blur circles smaller than expected (focal point behind retina) - too much minus/not enough plus

If VA with +1/00DS is worse than 6/18 then blur circle must be bigger (focal point in front of retina)- too much plus/not enough minus