Subjective Refraction- Duochrome Test Flashcards
Is this a monocular or binocular test
This is a monocular subjective test to minimise accommodation whilst the distance prescription is worn, which is especially important in myopes.
Why might a myope experience asthenopia
If a myope is overcorrected (prescription too minus), they are effectively rendered hypermetropic and may experience asthenopia (eye strain) due to prolonged accommodation.
What are the principles of the duochrome test?
relies on chromatic aberration, which is where white light, when refracted at an optical interface, is dispersed into its different colours (wavelengths).
Why are red and green light used for the duochrome test
their wavelength foci straddle yellow-green light by equal amounts (about 0.4 dioptres on either side), with green being deviated more than red, since red has the longer wavelength
How to perform the duochrome test?
After the JCC test, whilst the fellow eye is still fogged, ask the patient to look at the distant duochrome and ask if the circles/letters are clearer on the red, green, or about the same.
What to do if the patient prefers green on the duochrome test
add +0.25 sphere and repeat the question. Adding plus spheres should shift the preference from green to indifferent to red
Where are most myopes left?
On Red
For myopes where is it considered unacceptable to leave them?
For myopes, green is generally considered unacceptable, indifference (equal red and green) acceptable, and just on the red preferable. The reason why myopes should not be left on the green is that they will be accommodating, as the prescription is too minus (i.e. overcorrected, rendering them hypermetropic).
How to use the duochrome test for colour blind people
asked if the left or right (or upper or lower) rank is clearer, rather than the red or green rank.
What is the blur back test
a +1.00 sphere is added that should blur the acuity to 6/12. If the myope remains 6/6, the prescription is too minus (overcorrected) and this +1.00 spherical lens should be added to their prescription to remove their accommodation, whilst retaining distance acuity.
