PC - Subjective Refraction 1 - Week 4 Flashcards
How many lines of drop can be expected per 0.25 dioptre difference? What is the equivalent cyl dioptre?
1 line drop per 0.25 spherical dioptre difference
Cyl equivalent is 0.5 dioptres
Define subjective refraction.
The attempt to determine, by trial and error using the patient’s cooperation, the combination of lenses that will provide the best corrected visual acuity.
What are the three components to subjective refraction?
Spherical error correction
Astigmatic correction
Balancing the refraction
When carrying out subjective refraction, when is it best to use a phoropter vs trial frame?
Phoropter - distance vision
Trial frame - near vision
What are the benefits of using a phoropter and trial frame? Name 4 each.
Phoropter Quicker Easier Lens are clean Impressive to patients
TF Near addition Patients with low vision Patients with poor subjective response Patients with high vertex error Children
Define vertex distance, and the dioptre level it becomes significant.
The distance between the back surface of a lens, and the cornea. It is important in converting between contact lenses and regular glasses.
Significant at ±6.00D.
When assessing a patient, in what ± direction should a practitioner begin? Why is this so?
The + direction first
If blurry, then -
If vision is better with the + lens, leave it in to check for hyperopia.
If its the same or worse, remove it, and continue -
What dioptre should you begin with if a patient presents with 6/60?
In what dioptre steps should you use if they are now:
6/12
6/9-7.5
6/60 - Begin with 3.00D
6/12 - 0.50D steps
6/9-7.5 - 0.25D steps
When working in the negative dioptre direction, what should be done if the patient reports their vision is clearer?
If a higher power lens is presented and the patient says their vision is clearer, always ask them to read the next line.
A significant increase is 3 or more letters.
Backtrack if they aren’t able to.
Negative lenses are sometimes backtracked even though the patient reports clearer vision. Why is this so?
If the patient cannot read an additional 3 or more letters, the lens is backtracked as this is likely due to accommodation.
Define the +1.00 blur check.
What happens if overminused?
Once the lens required for 6/6 or 6/4.8 vision is attained, a +1.00 lens is added. This will mean a 4 line drop in VA, which can be confirmed with the patient.
Overminus - <4 line drop
Describe the duochrome method for assessing BVS, and how the wavelengths of light are responsible for this technique.
Red light has longer wavelengths and is focused behind the retina.
Green light has shorter wavelengths and is focused in front of the retina.
The patient is asked to focus on the smallest two letters they can clearly see, and asked which of the two, green or red side, is more clearer.
Clearer red means minus D needed
Clearer green means plus D needed
This is done until they report equal clarity
What is best vision sphere? How does this relate to the interval of sturn? Define this interval.
Spherical alone cannot fully correct a patient with astigmatism. BVS refers to the best possible correction spherical lens can provide to them.
Astigmatic patients have two refraction points. The distance between them is the interval of sturn.
The goal of BVS is to centre the interval on the retina.
When doing a +1.00 blurr check, what should be done if the patient has a <4 line drop?
Add +0.25 until 4 line drop is achieved.
Each additional +0.25D represents a -0.25 overshoot.
Define the circle of least confusion, and the equation used to centre it on the retina.
COLC - the midpoint between anterior and posterior focal points of an astigmatic eye.
It is equal to the spherical component plus half the cylindrical component.
