- Relieving- Corrective- Over corrective- Inverse- Yolked- Rotating- Regional
-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.
-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 theres no ARC or deep suppression.- dont give patients with ARC or deep suppression corrective prism.
-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.
-You put the base in the same direction as the deviation and optically increase the demand to the controlling vergence system.Eso give BIExo 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
Used when there is poor prognosis for a functional cure and the patient doesnt want surgery or surgery isnt indicated.makes the eye look better.ex) ET give BIYoull 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, youll measure 25∆ET.This is called prism adaptation or eating prism and ARC patient are notorious for this.
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 smallerFor 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.
-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 )
more : in low vision-Give prism so you shift the patients world over so that theyre never looking where theyre missing vision.Ex) A little girl with a Duanes 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 wont have to turn her head
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.
Ex) 10 ∆ Right ET & has a RLR paresiswhen 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 Herrings 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.
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.
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.
Think what type of Prism you are going to give the patient. If the patient doesnt have normal sensory fusion or has ARC or suppression ,dont Rx relieving prism or corrective prism right away.Prism to break down ARC or suppression: Over corrective Inverse (disruptive) RotatingRegional Prism describes where you are mounting the Prism. you can Rx corrective regional relieving Prism. you arent putting Prism over the entire lens.
These are only guidelines.1) BO useful for distance ET with NRC dont 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 sustained3) Do not Rx Relieving prism if ARC is present (except when using inverse prism cosmetically).
4) Do not Rx vertical relieving prism for secondary vertical deviations or DVDs.Primary its 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.
ex) when the eye moves out 25∆, its in the DAF or vertically acting muscles ( SR & IR ). If the actions of the SR & IR arent 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 theres 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..
7) Whenever you Rx prism, watch for prism adaptation even if you think your patient has NRC and shallow or no suppression. Youdont want the deviation to get larger8) Consider cosmetic
1-30∆each eye (available in 1-10∆,12∆,15∆,20∆,25∆,30∆) Quick Less expensive($10-12/ prism vs . replacing an entire lenswith ground in prism). Good if you need to replace andchange the prism. Can rotate You can rotate it a pinch to neutralize a little bit ofvertical. Decreases VA and contrast sensitivity The more prism ,the more blur. You can give it right away to alleviate symptoms.