Subjective refraction: BVS, JCC, F&B, BB Flashcards
Define subjective refraction
Refraction dependent on patient responses and ability to discern changes in clarity
Consists of:
- Spherical error determination
- Astig error determination
- Balancing refraction
How many lines does 0.25DS and 0.50DC correspond to?
1 line on the VA chart
Trial frame advantages
A: Near addition
Px with low vision, or have poor subjective responses.
Px with high RE (vertex distance)
Children
Phoropter advantages
A: Quicker, comfort, lenses cleaner, WOW factor
What is the BVS?
Best Vision Sphere
Determination of the maximum plus spherical lens which provides Px with best VA
*spherical lens equivalent of spherocylindrical lens required to correct Px’s astigmatic RE if applicable.
SPH + 1/2 CYL
Places Px’s far point close to optical infinity and minimises Px’s accommodative response when doing so.
Places COLC on retina
What is the COLC?
Circle of Least Confusion
The dioptric midpoint between the anterior and posterior focal lines in astigmatism.
Plus/minus method
Add + lens in larger steps depending on VA “better, just the same or worse”
6/12 = ±0.50
If worse, add minus lenses in appropriate steps until “smaller and blacker” rather than “clearer”.
+1.00 check
+1.00DS is equivalent to 4 lines on the VA chart
Adding this lens in front of the BVS checks whether over-minusing has occurred.
If Px drops 4 lines (or even a little bit more), then no over-minusing has occurred.
If Px drops less than 4 lines, overminused, need to rechec BVS.
Duochrome chart
Uses eye’s axial chromatic aberration.
Green light (535nm) focuses in front of retina and red light (620nm) behind.
Therefore if green is clearer, need to add more +.
Red clearer, add more -.
Adjust until equally clear (not brighter).
Unsuitable for elderly, due to yellowing of lens.
Explain ‘plus to 6/6 refraction’
Used for young hyperopic Px
Do +1 blur check / until blurred vision and then work back down to 6/6.
Then re-check with +1 or duochrome.
How do you perform F&B?
- BVS - place COLC no the retina
- Apply appropriate amount of fog
If -1.00DC suspected, fog = +0.50DS + 1/2 suspected cyl - Direct Px to fan chart - “do any of the limbs on that fan stand out/darker than the others?”
- V chevron: axis refinement, move V so that both limbs point to the limb on the fan from #3. “Do either of the limbs stand out or is darker/clearer than the other?” Move V towards the blurred limb until they appear equal. Check for reversal.
- On phoropter/trial frame, add in -0.50DC at an axis 90deg to the direction the V is pointing to, also add +0.25DS for every -0.50DC added to keep COLC in front of the retina (maintain fog).
- “Does the block closer to the O or X appear darker/clearer?” Closer to O = add less cyl, X = add more cyl. Add -0.50DC & +0.25DS until reversal achieved. Add/remove 0.25DS as required. Adjust until equally clear.
Err on side with least correcting minus cyl. - Redirect attn to V, check still equal.
- Add +0.50DS and check all limbs on fan are now equal in clarity/blur as well.
- Change to VA letter chart, and remove fog in 0.25DS steps until best VA achieved.
- End point check applied to complete up to “monocular Rx + EPC”
What is ‘with the rule’ astigmatism?
Vertical meridian is steepest (like football lying on its side)
What is “against the rule’ astigmatism?
Horizontal meridian is steepest (like football on its tip)
Is F&B or JCC the test of choice?
What are the advantages/disadv. of each test?
JCC is the test of choice as it requires less targets to complete and is more efficient. (also COLC lies on retina)
F&B A: no comparison of 1 view after another, better for kids and elderly Px, better control of accommodation as both focal line are always in front of the retina, works for Px that are unsuitable for JCC
What is the fan chart used for?
Location of the approximate axis of the posterior focal line in an astigmatic patient
What is the locator V chevron used for?
Refinement of posterior focal line axis found with the fan chart
What is the corrector block used for in F&B?
Determine the power of the correcting cylinder required for an astigmatic patient
What is the point of fogging?
BVS places the COLC on the retina. Fogging moves both astigmatic foci forward so that the posterior focal line now remains in front of the retina (thus is clearer of the 2 foci).
Lines along this focal line appear clearer and allows determination of the posterior focal line axis.
If all lines on the fan chart appear equally clear…
Px may have no cyl (depends on their BVS VA, is it expected?)
Could be under fogged: add another +0.50DS to see if it makes a difference
Could be over fogged: slowly remove fog and see if it makes a difference.
*can check for reversal by adding in some cyl at a nominated axis
If patient sees multiple lines on the fan chart…
indicates meridian of posterior focal line lines in the middle,
also more likely that Px has a lower cyl
What is the design of the X-cyl?
Negative cyl crossed with a positive cyl, both aligned at orthogonal axes
Spherical equivalent is plano
Common powers: 0.25, 0.37, 0.50, 0.75 X-Cyl
If VA > 6/12, use ±0.25
Explain JCC.
- BVS found first, COLC placed on retina and remains on retina throughout test
- X-cyl chart turned on, Px asked to identify which view makes the circles rounder, clearer, more like circles.
If emmetropic: both views will be the same
Ametropic: one view preferred (one view reduces diameter of COLC)
Circles 6/12 equivalent - X-cyl presented with power axis at 90 / 180 degrees & 45 / 135 degrees. Position in which minus axis is preferred is noted for each presentation to determine 45 deg bracket for axis.
- Axis refinement: -0.50DC @ angle within 45 degree bracket obtained in #3 + 0.25DS added to prescription
VA > 6/9 = -0.50DC. Minus and positive cyl axes now straddle axis. Rotate axis to preferred minus view until both equally clear in 15deg steps until reversal (smaller steps). - Power determination: trial lens cyl now correctly aligned with Px’s axis of astigmatism
Prefers + view = less astig, reduce cyl
Prefers - view more = more astig, increase cyl until equally clear.
One view collapses IS, one view increases size of IS (inc. blur). - Recheck axis, if change - recheck power & recheck axis.
What is the effect of power on axis accuracy?
Higher powers require more accuracy with axis (greater visual impact if you are slightly off axis).
Other targets used with X-cyl
Letter charts: letter orientation can bias responses to one position of focal lines cf to another (if COLC minimally off retina)
Ring charts: symmetrical/no orientation preference
Dot targets: no orientation preference, less susceptible to optical distortions of X-cyl
