Binocular Vision Lecture 5 Handout PDF

Binocular Vision Lecture 5 – Subjective assessment of vision Professor Phillip Buckhurst By the end of the session you will be able to: Understand how prisms are used in strabismus correction Preform a Maddox rod test Preform a Maddox Wing test The use of prisms in binocular vision What ha...

Binocular Vision Lecture 5 – Subjective assessment of vision Professor Phillip Buckhurst By the end of the session you will be able to: Understand how prisms are used in strabismus correction Preform a Maddox rod test Preform a Maddox Wing test The use of prisms in binocular vision What happens with a tropia In a tropia the eyes are not aligned and so the retinal image does not fall on the corresponding point of both retinas – E.g in the right eye the image is falling on the temporal retina whilst the image in the left eye is falling on the fovea Therefore unless there is some neural adaption (more on this later) the patient sees double Right eye nose Left eye Remember from optics Light is refracted towards the base of the prism Image appears deviated towards the apex Apex Virtual image Object Base What happens with a tropia You can use a prism to redirect the light so that the retinal image falls onto the corresponding position on both eyes – Eg in both eyes the retinal image falls onto the fovea Right eye nose Left eye  Prescribing correcting prism For correction of a strabismus you place the apex of the prism in the direction of the eye Point the image at the eye However in optometry we describe the direction of the prism by the direction of the base rather than the apex nose nose Exo = Base IN Eso = Base OUT  What happens in phoria We can use prisms to measure the size of a phoria – The difference between the eye position when the eyes are in there associated (active) position and the dissociated (passive) position Without Prism nose nose Associated position Dissociated position Associate position ≠ Dissociated position  What happens in phoria You introduce prism infront of the eyes until the associated eye position is equal to the dissociated eye position With Prism nose nose Associated position Dissociated position Associate position = Dissociated position What direction would we place the prism in? nose L R What direction would we place the prism in? nose L R  Measuring the size of a deviation in prism dioptres Prism bar cover test Maddox rod Maddox wing Note: Fixation disparity measures aligning prism and not dissociation Maddox rod and madox wing Allow you to measure the distance between the associated and dissociated positions These tests are used to quantify heterophorias Usefull for vertical phorias < 3∆ that may be missed during the cover test and yet can still cause symptoms These are DISSOCIATED TESTS Both Maddox rod and maddox wing do not allow fusion – Therefore the eyes move to their passive positions (dissociated positions) Maddox Rod The Maddox rod Uses a lens which distorts an image beyond recognition – Image so distorted that fusion cannot occur Distortion caused by small planoconvex cylinders on a disc of coloured glass (generally red) A spot light seen through the disc is distorted into a streak of light at right angles to axis of grooves Used for quantifying distance phorias Procedure The maddox rod is used at the end of the refraction once the final distance prescription has been determined and before accommodation or near addition is assessed Place the MR infront of the right eye and turn off the lights Turn on the spotlight on the test chart Note: for computerized test charts you need to use a horizontal or vertical red line as they do not have a spot of light sufficient for the test To check for any horizontal phorias Place the MR in front of RE, grooves horizontal Ask “Can you see a spot of light and a vertical red line” Ask “Is the red line to the right, to the left or going straight through the spot?” Note: If the Px only sees the spot or only sees the red line then they may be suppressing (more on this later) “The red line is going through the spot” Orthophoria – the dissociated eye position matches the associated eye position “The red line is going through the spot” The eyes are dissociated Both eyes are still pointing at the spotlight The red line seen (by the right eye) overlaps the spot of light (seen by the left eye) Right eye sees the red line nose Left eye sees the spot  Left eye sees Right eye sees spotlight “The red line is to red line the right of the spot” A horizontal phoria is present We need to use prisms to align the line with the spot light nose L R  Left eye sees Right eye sees spotlight Use prisms to align red line the spot with the line Remember the with a prism the image moves towards the apex Place a prism infront of the right eye with the apex pointing to the left Apex pointing to the left to move the line to the spotlight This is base _________ nose L R  Left eye sees Right eye sees spotlight Or you could red line move the spotlight Place a prism infront of the left eye with the apex pointing to the right Apex pointing to the right to move the spotlight to the line This is still base ________ nose L R More complex explanation The eyes are dissociated There is an esophoria and the right eye has wondered inwards The image of the red line is now falling on nasal retina The image of the spot and the line are now out of alignment and prisms are needed to realign them Uncrossed disparity – If the retinal image falls on the nasal retina then our brain thinks that the object is on the temporal visual field (red line on the right of the spot) Right eye sees the red line MR image which falls on nasal retina nose Left eye sees the spot More complex explanation A base out prism is used to move the image of the red line into alignment with the image of the spot light Base out prism Right eye sees the red line image which has been MR moved into alignment nose Left eye sees the spot  Right eye sees Left eye sees red line “The red line is to spotlight the left of the spot” A horizontal phoria is present We need to use prisms to align the line with the spot light nose L R  Right eye sees Left eye sees red line Use prisms to align spotlight the spot with the line Remember the with a prism the image moves towards the apex Place a prism infront of the right eye with the apex pointing to the right Apex pointing to the right to move the line to the spotlight This is base ________ nose L R  Right eye sees red line Left eye sees Or you could spotlight move the spotlight Place a prism infront of the left eye with the apex pointing to the left Apex pointing to the left to move the spotlight to the line This is still base ________ nose L R More complex explanation The eyes are dissociated There is an exophoria and the right eye has wondered outwards The image of the red line is now falling on temporal retina The image of the spot and the line are now out of alignment and prisms are needed to realign them Crossed disparity If the retinal image falls on the temporal retina then our brain thinks that the object is on the nasal visual field (red line to the left of the spot) Right eye sees the red line MR image which falls on temporal retina nose Left eye sees the spot More complex explanation A base IN prism is used to move the image of the red line into alignment with the image of the spot light Base IN prism Right eye sees the red line image which has been MR moved into alignment nose Left eye sees the spot To check for any vertical phorias Place the MR in front of RE, grooves vertical Can you see a spot of light and a horizontal red line Ask “Is the red line above, below or going straight through the spot?” Note: If the Px only sees the spot or only sees the red line then they may be suppressing (more on this later) “The red line is going through the spot” Orthophoria – the dissociated eye position matches the associated eye position  Right eye sees red line “The red line is above the spot” A vertical phoria is present You need to align the spot and the line using prisms Left eye sees spotlight nose L R  Right eye sees red line Use prisms to align the spot with the line Remember the with a prism the image moves towards the apex Place a prism infront of the right eye with the apex pointing down to move the Left eye sees line to the spotlight spotlight This is base ________ nose L R  Or you could Right eye sees move the red line spotlight Place a prism infront of the left eye with the apex pointing up Apex pointing up to move the spotlight to the line This is base ________ Left eye sees spotlight nose L R  Left eye sees spot light “The red line is below the spot” A vertical phoria is present You need to align the spot and the line using prisms Right eye sees red line nose L R  Left eye sees spot light Use prisms to align the spot with the line Remember the with a prism the image moves towards the apex Place a prism infront of the right eye with the apex pointing up to move the line to the spotlight Right eye sees red line This is base ________ nose L R  Left eye sees Or you could spot light move the spotlight Place a prism infront of the left eye with the apex pointing down Apex pointing down to move the spotlight to the line This is base ________ Right eye sees red line nose L R Maddox rod and tangent scale Occasionally the spot light on a test chart has a tangent scale surrounding it These are calibrated for prism dioptres They allow the measurement of phoria without the need for prism 4 3 2 1 1 2 3 4  Maddox rod test for Cyclophoria Use two Maddox rods Place the red Maddox rod infront of the right eye Place the green Maddox rod infront of the left eye Ensure that the grooves are vertical orthophoria – To create horizontal lines Px looks at spot light on chart Place a 4-6 diopter base DN prism incyclophoria infront of the right eye Px rotates MRs in trial frame until images appear parallel excyclophoria – angle of cyclophoria can be determined using the axis scale on the trial frame (degrees) Madox Rod Test limitations: – Spotlight is poor stimulus for accommodation – So XOP overestimated & SOP underestimated Still useful for vertical and cyclophorias Recording your findings Orthophoria = If prism required Record prismatic power = for alignment required and base direction The maddox Wing The Maddox wing Different objects to both eyes – fusion cannot occur Maddox wing designed to work at 30cm Px looks through two eye slit holes Septa separate views seen by RE & LE RE – sees arrows LE – sees tangent scales Used for quantifying near phorias Maddox wing procedure Explain the test to your patient Px wears appropriate correction – If pre-presbyopic use distance Rx and distance PD – If presbyopic use near Rx with near centration distance Get the patient to hold the Maddox Wing up to his/her eyes To measure horizontal phorias ask “Which number does the white arrow point to?” To measure vertical phorias ask “Which number does the red arrow point to?” To measure cyclophorias ask “Is the red bar horizontal?”  Horizontal phorias Ortho 0 Arrow points to zero odd Esophoria 0 Arrow points at odd number Exophoria even 0 Arrow points at even number Note: Values are given in prism dioptres  Vertical phorias Ortho horizontal red line straight through 0 spot even L/R hyperphoria Arrow points at even number 0 R/L hyperphoria Arrow points at odd 0 number odd  Note: Examiners view of Cyclophoria bar, Px sees it as straight Ortho red bar horizontal Incyclo red bar appears tilted, alter angle of red bar until it appears horizontal Read off degrees from scale Excyclo red bar appears tilted, alter angle of red bar until it appears horizontal Read off degrees from scale Clinical Notes Test limitations: Numbers and arrows are poor stimulus for accommodation So XOP overestimated & SOP underestimated Septa seen by both eyes so dissociation not full Only designed for standard PD Recording your findings Orthophoria = Record prismatic power If phoria present = required and type of phoria Binocular Vision Lecture 5 – Subjective assessment of vision Dr Phillip Buckhurst