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Preclinical Optometry > Subjective refraction: BVS, JCC, F&B, BB > Flashcards

Subjective refraction: BVS, JCC, F&B, BB Flashcards

1
Q

Define subjective refraction

A

Refraction dependent on patient responses and ability to discern changes in clarity

Consists of:

  1. Spherical error determination
  2. Astig error determination
  3. Balancing refraction
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2
Q

How many lines does 0.25DS and 0.50DC correspond to?

A

1 line on the VA chart

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3
Q

Trial frame advantages

A

A: Near addition
Px with low vision, or have poor subjective responses.
Px with high RE (vertex distance)
Children

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4
Q

Phoropter advantages

A

A: Quicker, comfort, lenses cleaner, WOW factor

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5
Q

What is the BVS?

A

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

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6
Q

What is the COLC?

A

Circle of Least Confusion

The dioptric midpoint between the anterior and posterior focal lines in astigmatism.

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7
Q

Plus/minus method

A

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”.

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8
Q

+1.00 check

A

+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.

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9
Q

Duochrome chart

A

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.

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10
Q

Explain ‘plus to 6/6 refraction’

A

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.

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11
Q

How do you perform F&B?

A
  1. BVS - place COLC no the retina
  2. Apply appropriate amount of fog
    If -1.00DC suspected, fog = +0.50DS + 1/2 suspected cyl
  3. Direct Px to fan chart - “do any of the limbs on that fan stand out/darker than the others?”
  4. 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.
  5. 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).
  6. “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.
  7. Redirect attn to V, check still equal.
  8. Add +0.50DS and check all limbs on fan are now equal in clarity/blur as well.
  9. Change to VA letter chart, and remove fog in 0.25DS steps until best VA achieved.
  10. End point check applied to complete up to “monocular Rx + EPC”
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12
Q

What is ‘with the rule’ astigmatism?

A

Vertical meridian is steepest (like football lying on its side)

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13
Q

What is “against the rule’ astigmatism?

A

Horizontal meridian is steepest (like football on its tip)

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14
Q

Is F&B or JCC the test of choice?

What are the advantages/disadv. of each test?

A

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

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15
Q

What is the fan chart used for?

A

Location of the approximate axis of the posterior focal line in an astigmatic patient

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16
Q

What is the locator V chevron used for?

A

Refinement of posterior focal line axis found with the fan chart

17
Q

What is the corrector block used for in F&B?

A

Determine the power of the correcting cylinder required for an astigmatic patient

18
Q

What is the point of fogging?

A

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.

19
Q

If all lines on the fan chart appear equally clear…

A

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

20
Q

If patient sees multiple lines on the fan chart…

A

indicates meridian of posterior focal line lines in the middle,
also more likely that Px has a lower cyl

21
Q

What is the design of the X-cyl?

A

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

22
Q

Explain JCC.

A
  1. BVS found first, COLC placed on retina and remains on retina throughout test
  2. 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
  3. 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.
  4. 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).
  5. 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).
  6. Recheck axis, if change - recheck power & recheck axis.
23
Q

What is the effect of power on axis accuracy?

A

Higher powers require more accuracy with axis (greater visual impact if you are slightly off axis).

24
Q

Other targets used with X-cyl

A

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

25
Q

What other tests can you employ to determine astigmatism if JCC and F&B fail?

A

Retinoscopy

Stenopaic slit (meridional refraction): determine BVS, fog by +1.00DS, rotate slit until maximum acuity found and do spherical refraction with this slit in place. Repeat in other principal meridian.

Keratometry: Javal’s rule

The Simultan test: displays 2 X-cyls side by side

26
Q

For JCC, what to do if circular target is indistinguishable?

A

use circular letters that are 2 lines above monocular BVS

27
Q

What is binocular balancing?

A

Balancing the accommodation between the 2 eyes (not balancing the visual acuity)

Accommodation is a consensual response, imbalance can lead to symptoms such as asthenopia.

28
Q

When would you use PH?

A

If VA ≤ 6/9:
If improvement: uncorrected RE, paracentral media opacity
No improve: amblyopia, pathology

29
Q

Why are some patients uncertain about what lens gives clearest vision?

A

Smaller pupils increase depth of focus which allows clarity over a small range of distances (no finite focus end point).

30
Q

When do you skip BB?

A

Px who have no functional binocular vision

No active accommodation

31
Q

What is successive alternate occlusion?

A
  • crude and insensitive
  • non binocular
  • only used if acuities are equal
  • Px to read smallest line with BE
  • flip occluder from one eye to the other, comparing vision between two eyes
  • use lenses to equalise vision / +0.25DS to clearer eye
  • doesn’t allow Px to have resting accommodative state

V: +0.50DS BE to fog, compare relative blur using 6/9.5 line

32
Q

Vertical prism dissociation

A
  • insensitive
  • non binocular, unnatural
  • vertical dissociation with 2-3^ BU in one eye and 2-3^BD in other eye
  • 6/9.5 line: identify which eye views top/bottom image
  • +0.25DS in clearer eye until equal
  • if equal clarity not obtainable, give max + consistent with best vision

V: +0.50DS BE to fog, compare relative blur using 6/9.5
- unequal acuities: use duochrome chart (but req. 6-8^)

33
Q

Humphriss fogging technique

A
  • physiological septum technique
  • test of choice
  1. occlude one eye, fog other eye
  2. with BE, determine if +0.25DS addition is clearer, just the same or worse
  3. leave if same & repeat, take out if worse
  4. fog the other eye before unfogging the original eye
  5. repeat #2 and 3

Allows leave + for a few seconds to allow for relaxation of accommodation

Technique suspends foveal vision, but allows paracentral + peripheral vision to act as binocular lock

34
Q

Septum and polaroid techniques

A
  • only if acuities are equal
  • most sensitive tests
  • most natural binoc viewing condition

SEPTUM
- well controlled blocking

POLAROID

  • polaroid filters in front of BE, allowing diff plane polarised light in each eye (dissociated images)
  • 6/9.5, add +0.25DS to clearer eye until equal

V: +0.50DS BE to fog, compare relative blur using 6/9.5

35
Q

What is binocular addition?

A

Binocular balancing allows balancing of accommodation (but not nec. relaxed), therefore binocular additions ensures full relaxation of accommodation.

36
Q

What is binocular refraction?

A

Refraction under binocular conditions throughout subjective refractive routine.

More natural, more stable accommodation
Negates need for BB.
Useful for young hyperopes & latent hyperopes.

37
Q

What are the advantages of Humphriss fogging?

A

Equalises accommodation between 2 eyes as well as relaxing accommodation (little need to do BA after this)

Simple for Px & optom

Quick to complete, accurate

Works for equal and unequal VA between eyes

38
Q

Prescribing new glasses

Parameters for new/updating prescriptions

A

Ideal change ±0.50-0.75DS
Warn Px with new anisometropic or cyl axis change scripts
Prescribe when vision falls below legal limit or uncorrected RE is bothersome
Near Rx of +0.75DS in better eye, not earlier

Preclinical Optometry (14 decks)

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