3. -optically reduces the demand to the controlling fusional
vergence system for bifixation of the target.
- Rx is less than the angle of deviation.
-The base is Rxed opposite to the direction of the deviation.
- Moves light closer to the fovea.
-If the prism moves the image into the range of fusion,
the patient can verge the eyes to obtain fusion.
-This is Rxed for intermittent strabismics and phorias in
certain instances.
- The most commonly Rxed prism.
4. -Optically eliminates the oculomotor deviation.
- Rx prism equal to the magnitude of the
objective angle.
- The residual vergence demand is zero.
- Rx the base opposite to the direction of the
deviation.
- In certain cases, sensory fusion can occur
without the need for any fusional vergence if
there's no ARC or deep suppression.
- don't give patients with ARC or deep
suppression corrective prism.
5.
-optically changes the direction of the deviation.
-the power of the prism is greater than the
magnitude of the deviation.
- Example : 20 ∆ XT give 25 – 30 ∆ BI
-In such a position on a cover test ,you see the
deviation reverse in terms of direction.
This kind of like when you get reversal on ACT.
An eso becomes an optical exo.
-Used on certain instances to disrupt ARC through a
specific technique.
6.
-You put the base in the same direction as the deviation and
optically increase the demand to the controlling vergence
system.
Eso give BI
Exo give BO
-only used for a phoria patient in the later stages of strab VT.
Ex) Give BO to an exophoria to increase the convergence demand for
bifixation.
-the patient has to use vergence to control his deviation and also to meet
the demand of the prism.
-a passive technique.
-commonly used for VT (ex: read through prism).
-some people Rx in glasses~ less common
-to eliminate EF and ARC
7. Used when there is poor prognosis for a functional cure and
the patient doesn't want surgery or surgery isn't indicated.
makes the eye look better.
ex) ET give BI
You'll see a dual effect if the patient has ARC:
The observe sees the eye moved temporally when looking
through the prism.
The patient will make anomalous motor fusion movements.
If you were to give a 20∆ET with HARC 5∆BO of relieving
prism:
The image is shifted but the patient wants to maintain the
HARC so he makes a convergence movement of 5∆
On a CT, you'll measure 25∆ET.
This is called prism adaptation or eating prism and ARC
patient are notorious for this.
8. Never Rx relieving and correcting prism for ARC
patients because they will "eat it" and it will make the
deviation look worse.
If you give a 20∆ ET with HARC7∆ BI, the observer will
see the eye shifted out and to maintain the HRAC, the
patient will diverge the amount of the prism 7∆so the ET
will look smaller
For a 20-30∆ ET with HARC and a poor prognosis, a
8∆prism is a good first lens.
You put the prism over the strabismic eye.
If you get patient<20∆,you make them a noncosmetic
ET.
These patient rarely complain of diplopia.
9.
-Optically moves the retinal images of a fixed target
in a parallel direction toward the base and moves
the light toward the base and shows the target
toward the apex.
-Both of the eyes move in the same direction.
Example ) BASE RIGHT prism ( BO OD & BI OS )
BASE LEFT prism ( BI OD & BO OS )
BASE DOWN prism ( OU )
10. more : in low vision
-Give prism so you shift the patient's world over so that they're
never looking where they're missing vision.
Ex) A little girl with a Duane's Syndrome OS (13y.o.)
-abduction deficit
-she presented with a small head turn
-If you straighten her head or she looks into L gaze, she sees
double.
We could have given her base L prism if she would have had a
significant head turn.
The prism would shift the world over to the R and move the
eyes from L gaze so she won't have to turn her head
11. To move the eyes into the null position.
Null position = position where the frequency and
magnitude of the nystagmus is dampened or
eliminated.
when you Rx prism with non concomitant
deviations to get fusion ,you put all or most of
the prism in front of the paretic eye.
Typically we just split prism equally between the two
eyes for cosmetic purposes.
12. Ex) 10 ∆ Right ET & has a RLR paresis
when the prism is before the OS, the light is bent toward the
base and the image is projected toward the apex. Initially, the
first movement is going to be a version inward by the left eye.
BY Herring's Low , the right eye will move outward and then
there will be a fusional vergence movement.
since you are forcing the OD to move into the field of gaze of
the eye to move.
If you were to put the prism over the paretic eye, you would
get a version and then a vergence .the version would be
away from the DAF of the paretic muscle so you would need
less prism. Less prism means less distortion, etc.
13.
A method to change the sensory input for
constant strab to precipitate a change from
ARC to NRC.
Take a Fresnel prism and cut it round. For one
week ,the patient wears it BO and then you
rotate it to BU, then BI, then BD.
Take prism and change the base.
- to break down ARC.
-This is a disruptive prism technique.
14.
Different amounts of prism are needed in
different fields of gaze or for different
distances.
ex) 20∆ET at distance and 10∆ET at near
Only put the prism on a portion of the lens.
ex)only need prism in R gaze.
15.
Think what type of Prism you are going to give the patient.
If the patient doesn't have normal sensory fusion or has
ARC or suppression ,don't Rx relieving prism or corrective
prism right away.
Prism to break down ARC or suppression:
Over corrective
Inverse (disruptive)
Rotating
Regional Prism describes where you are mounting the
Prism.
you can Rx corrective regional relieving Prism.
you aren't putting Prism over the entire lens.
16. These are only guidelines.
1) BO useful for distance ET with NRC
don't give convergence excess patients BO,
an add works Better
BI more useful for XT than with exophoria because exophoria
are so easy to train.
2) Rx Relieving prism when normal sensory fusion is present (NRC &
no suppression) and normal binocularity can be sustained
3) Do not Rx Relieving prism if ARC is present (except when using
inverse prism cosmetically).
17. 4) Do not Rx vertical relieving prism for secondary vertical
deviations or DVD's.
Primary it's the deviation and is present all the time. The
vertical deviation is still present when you eliminate the
horizontal deviation.
Secondary the vertical deviation is not present when the
horizontal deviation is gone.
Many people with intermittent diplopia can relate to this
because when they are diplopic, the 2 images are separated
horizontally with a little bit of vertical misalignment.
18. ex) when the eye moves out 25∆, it's in the DAF or vertically
acting muscles ( SR & IR ). If the actions of the SR & IR
aren't exactly symmetrical, there will be a vertical
deviation.
On a secondary vertical deviation, when the eyes are
straight , the patient will never show a vertical deviation on
any test (Phoria, Torrington, Disparometer)
5) Rx neutralizing (corrective) prism in the initial Rx when
there's NRC and shallow suppression / amblyopia.
6) Nonconcomitant deviations: put all or most of the prism in
front of the paretic eye if you are Rxing for fusion.
.
19. 7) Whenever you Rx prism, watch for prism adaptation even if
you think your patient has NRC and shallow or no
suppression. You
don't want the deviation to get larger
8) Consider cosmetic
20. 1-30∆each eye (available in 1-10∆,12∆,15∆,20∆,25∆,30∆)
Quick
Less expensive($10-12/ prism vs . replacing an entire lens
with ground in prism). Good if you need to replace and
change the prism.
Can rotate
You can rotate it a pinch to neutralize a little bit of
vertical.
Decreases VA and contrast sensitivity
The more prism ,the more blur.
You can give it right away to alleviate symptoms.
