29 - BV alignment

Question 1

What’s Phi phenomena

Answer 1

Focusing on an object during a cover test will cause it to move during each shift of the cover paddle assuming you have a phoria/tropia. This is subjectively interpreted as movement of the object.

Question 2

How to get the most accurate result from objective alternating cover test?

Answer 2

Bracketing.
You would test their eye movements by inducing a base in and base out prism. If both prisms cause equal and opposite eye movements, then they’re ortho. If not, try to find the two spots where it causes the smallest but still noticeable movements and that both prisms give equal movements (but opposite). Their actual deviation is middle of these two ends.

Question 3

Describe Von Graefe technique.
Purpose:
Test Distance:
Room Lighting:
Equipment:
Target:
Steps:

Answer 3

Purpose: Measure dissociated phoria. Typically done post-refraction to see if changed.
Test Distance: 6m or 40cm
Room Lighting: Bright
Equipment: Phoropter + Risley Prisms
Target: Single line of letters 1 – 2 lines larger than VA of worse eye.
Steps:
- Place 15BI one eye and 6BD over other.
- When measuring horizontal deviation, we use vertical single lines and figure out which image is being seen by which eye. In our case, if 15BI is LE and 6BD is RE, then LE sees left down image and RE sees right up image. We adjust the base amount in LE (which has horizontal prism) until it matches with the RE image (top right). Final prism over LE is their horizontal deviation.
- With vertical deviation, use horizontal lines. Move the iamge seen by the eye with the veritcal prism until it subjectively matches with the other eye’s image. In our case, RE has 6BDR (vertical prism) so we move the right image down by reducing it’s prism. Final prism in RE is their vertical deviation.

Question 4

What’s Maddox Rod?
Purpose:
Test distance:
Room Lighting:
Equipment:
Target:
Method:

Answer 4

Purpose: Measure dissociated deviation
Test distance: 6m (Thorington card used at 40cm)
Room Lighting: Dim/dark
Equipment: Maddox rod lens + pentorch + prisms
Target: Pentorch (non-accommodative)

Method:
- Maddox is a high cyl lens made by using a lot of prisms (similar to fresnel lenses). Patient sees streak of light instead of spot of light and streak direction is perpendicular to grooves of maddox.
- Cover one eye w/ maddox (horizontal grooves)
- Shine pentorch and ask where the streak of light is (seen by maddox’d eye) relative to spot of light (seen by uncovered eye).
- Add prism to maddox’d eye until streak and spot are aligned. Prism = horizontal deviation.
- Repeat with vertical grooved maddox rod lens for vertical deviation.

Question 5

Describe Thorington Card

Answer 5

Small card that has numbers and letters on diagonal. Same procedure as maddox rod lenses except instead of adding prism, you can ask which letter/number the streak of light is hitting. Interestingly, can work on strabismus if suppression isn’t strong.

Question 6

Describe Howell card.
Purpose:
Test distance:
Room Lighting:
Equipment:
Target:

Answer 6

Purpose: Measure dissociated horizontal phoria (does require good VA).
Test distance: 40cm or 6m (depends on card)
Room Lighting: Bright
Equipment: Howell card, vertical prism (around 6 prism usually)
Target: Howell card.
Method:
- Place BD prism over one eye (e.g. RE).
- Patient will have RE image above LE image. Ask patient which number the top ruler is above when aligning the zero from the top ruler to a number on the bottom ruler.
- Ruler itself has even numbers on left and odd numbers on right. Double check which side of the ruler the top ruler is pointing at.
- For example, if it points to 4, top ruler has moved left. This means RE seeing image further left because it’s turned out so exo.

Question 7

Describe maddox wing
Purpose:
Test distance:
Room Lighting:
Equipment:
Method:

Answer 7

Purpose: Measure dissociated phoria at near
Test distance: 33cm
Room Lighting: Bright
Equipment: Maddox wing

Method:
- Maddox wing has septum which separates foveal lines.
- For horizontal, LE sees white numbers, RE sees white arrow. Ask patient which number the arrow points to. Maddox wing itself tells you prism size and deviation type depending on number patient sees.
- For vertical, LE sees red numbers, RE sees red arrow. Ask which number the red arrow points to.
- For cyclotorsion, RE sees white numbers + white line, LE sees red string that forms the arrow. Ask patient to align string so it’s parallel with the white line. No movement = already parallel to patient = no cyclotorsion.