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Prism
- 1. Prism Dr. Meenank.B M.S. Ophthalmology (post-graduate ) ASRAM medical college
- 2. Prism is aportion of a refractive medium, bordered by 2 plane surfaces which are inclined at a finite angle Apex – Tip of the prism where the two refractive surfaces meet Refractive surface – The two refractive surfaces of a refractive prism inclined at the apical angle Reflecting surface – In some prism, internal ray hitting the 2nd surface is subjected to total internal reflection (reflecting prisms) Axis – line bisecting the refractive angle Base – bottom of the prism / side opposite to apex or optical angle. Helps in orientation of ophthalmic prisms
- 3. Prism – Refraction Refractive angle / apical angle - angle between two refractive surfaces ‘α’ Angle of deviation – Angle between the incident and the emergent rays D = (n-1)α Obeying Snell's Law of refraction light passing through the prism is deviated to the base Angle of deviation - D = (n-1)α ; where n- Refractive index, α – refractive angle Thus, D = α/2 RI of glass – 1.5 Factors responsible for ‘D’ Wavelength - The angle of minimum deviation is smaller for longer wavelengths , so red deviates less and vice-versa Material of prism - directly proportional with refractive index Angle of prism – directly proportional Angle of incidence – forma a ‘U” shaped curve
- 4. Image formed– virtual, erect and displaced towards the apex Power of a prism – Amount of light deviation produced by the prism Prism Dioptres ‘Δ’ 1 Δ = displacement of image towards the by 1cm kept at 1m distance Centrad ‘▼’ 1 ▼ = displacement of image towards the by 1cm kept at 1m distance along an arc
- 5. Prism Position’s Ophthalmic prism's – plastic / glass and amount of deviation depends on position in which they are held 3 common position – 1. Prentice Position 2. Minimal Angle Deviation 3. Frontal Plane Position Prentice Position – glass prisms . Most common Requires the patients line of sight to strike the rear end of the prism @ 90˚ Small error – large deviation Eg : 40Δ Prentice if held at Frontal plane - 32Δ Prentice Position
- 6. Minimal AngleDeviation – Plastic, more common Line of sight makes equal angle with both surface Difficult to maintain in clinical practice Minimal Deviation Frontal Plane Position – Prism is held in parallel to the frontal plane Holding a prism in frontal plane – very nearly produces MAD Error’s Small error b/w – F.P and MAD Large error b/w – P.P and F.P Plastic > glass Frontal Plane Position
- 7. Stacking of prism’s Glass prism – max – 40 Δ Plastic prism – max – 50 Δ For more than this ‘stacking’ – combining 2 prisms is done – but never combine linearly cause they give more effect Eg – 50Δ + 3Δ= 58Δ ( additive error) So, to dec. this error prism is held before both eyes For a V and H dev. Prism can be combined
- 8. Measuring strabismus withglasses Maximum additive error occur even when prism is held correct, krimsky test normal, and other subjective test Both the line’s of sight cannot pass through the optical center of the spectacle – glasses producing prismatic effect cause deviation which is measured from infront of glasses, due to peripheral prismatic effect More common with +/- 5Δ
- 9. Prismatic Effect Spherical lens Spherical lens behave like prisms in all quadrants except the center The refractive angle between the lens and the surface inc. at the edges viz inc. prismatic effect Cylinder lens Cylindrical lenses have no power along the axis meridian, hence the cylinder can exert no prismatic effect along its axis. The power of a cylinder lies at right angles to its axis, i.e. along its power meridian, So a cylinder exerts prismatic effect only at right angle to its axis Most commonly if correction is not equal in both eyes High myopes Aphakia
- 10. Vector Addition The correction in both vertical and horizontal planes can be achieved by one strong oblique prism summed by vector addition or by calculating using Pythagoras's theorem (푎2 + 푏2 = c²) While prescribing always mention the base and the meridian
- 11. Types of Prisms Ground prism’s – Permanently incorporating the prism into glasses by decentring the present spherical lens or by mounting on them Fresnel prism (or) Wafer prisms – Series of plastic Prisms of 1mm Originally developed by French physicist Augustin-Jean Fresnel for lighthouses. Small prisms stacked to give an effect of a large prism to over come wt., aberration, and achieve higher power Apical angle determines the strength
- 12. Viewing through Prism Prisms – Dec. image quality (or) distort Field of Vn – elongated at apex compressed at base Components of prismatic distortion 1. Horizontal magnification 2. Curvature of vertical line 3. Asymmetrical horizontal lines 4. Vertical magnification 5. Change in vertical magnification with horizontal angulation
- 13. Viewing through Fresnel Poor optical quality, low resolution and contrast due to use of plastic Prismatic Distortion – low H and V magnification but more curvature on V and H in Fresnel 5Δ, 10Δ, 15 Δ prisms Fitted by cutting the membrane to the shape of the lens and placing ‘em on the inner surface of the lens under water High myopes – outer side Commercially available in plastic of different powers from 1 – 20 giving a PD of 40 Adv – cost effective, easy to use, light wt., flexible Dis-adv - glare, chromatic aberration, vision decline Press on prism (3M) – in changing strabismus in thyroid disease
- 14. Types of Prismsin Clinical Practice 1. Loose Prisms 2. Prisms Bars 3. Trial Prisms 4. Fresnel Prisms 5. Rotating Prisms – Introduced into instruments 6. Risley Double Prisms – 2 rotating Prisms of same strength on a rotating frame 7. Vari-prisms – Single hand-held instrument where prisms power can be changed by rotating the two glasses – H -90 Δ, V-15 Δ Clinical classification of Prism’s Relieving Inverse Yoked Sector (or) Regional Corrective Over corrective Rotating Slab off
- 15. Uses of Prism Magnitude of prisms are used in orthoptic settings, mainly for diagnosis and management Diagnosis Conformation of BSV Measurement of angle of deviation To correct angle of deviation Management Eso/ exo deviation of concomitant / incomitant Amblyopia, Nystagmus, Malingering, Visual Field defects
- 16. Diagnosis Investigations ofBSV Prism reflex test Prism 4Δ test Prism Fusional Vergence Amplitude Vergence facility Measurement of Angle of Deviation Prism alt. Cover test Measurement of 9 positional gazes Simultaneous Prism Cover Test Maddox Rods Prism Reflection Test / Krimsky Test Assessment of Torsion Maddox double prism Double Maddox rod Correction of Deviation Test for ocular symptoms Assessment of potential BSV Prism adaptation Test Progressive prism compensation Pre-Op Prism Adaptation Test for Retinal Correspondence Vertical Prism Red Filter Test Investigation of suppression Post-Op Diplopia Test (PODT)
- 17. Binocular Single Vision Binocular Single Vision Basis for testing and investigating for BSV is checking for immediate response induced by prisms In presence of normal BSV – adaptive response occurs on cont. viewing Vergence adaptation in normal BSV Normal BSV + V/H Prism in front of one/ both eyes – displaces image away from fovea causing deviation Normal BSV – latent deviation gradually reduced proving BSV more faster for BO than in BI Adaption occurs at 1sec in BO and break up on removal of stimulus
- 18. Investigations of BSV • Prism reflex test • Prism 4Δ test • Prism Fusional Vergence Amplitude • Vergence facility Investigations of BSV with Prism Prism Reflex Test Can be used to test BSV infants (4 – 6 months) BO infront of one eye while other eye is fixating (33cm), observe eye movement Most commonly done with 10Δ, 20Δ 1. Normal BSV with motor fusion – eye with BO displaces image temporally – diplopia – refixation with fellow eye – Herrings law 2. Prism over suppressed eye – initial shift will not be appreciated / no movement 3. Prism over non-suppressed eye – versional movement but, no fusinoal movement 4. Exophoria one/both eyes becoming divergent on prism intro – alt. fixation, unable to fuse-insufficient fusinoal Vergence
- 19. Prism 4Δtest Most commonly used for Bifoveal BSV By displacing the image small amount a central suppression scotoma extending to this amount can be detected But, if the degree of suppression scotoma is smaller than the degree of movement then we get a normal response Test performed ( 33cm / 6m) – BO prism intro and observe for patterned movement, check for both eyes Micro-exo – BI
- 20. Fusional VergenceAmplitude Vergence movement compensated for phoria and keep the eyes aligned on target Exotropia – uses convergence ( strongest Vergence Improves with exercise) Esotropia – uses divergence (weakest Vergence, no sig imp. With exercise ) Strength of verg. measured in P.D. – Fusional Vergence Amplitude Test – induce small dev. That can be fused – inc. dev until blur point – then inc. until break point (Using – Risley prisms / Prism Bars ) Near convergence amplitude – 40cm, start with 4 PD and inc. The greatest prism where patient can fuse is Fusional Vergence Amplitude Normal Fusinoal Vergence Amplitude's Vergence Distance (6m) Distance (40cm) Convergence 20 – 25 PD 30 – 35 PD Divergence 6 – 8 PD 8 – 10 PD Vertical Vergence 2 -3 PD 2 – 3 PD
- 21. Vergence facility Prisms used to induce convergence and divergence alt. to asses the ability of fusional Vergence system to cope with change in demand 1 cpm – SV-BO(12Δ)-SV-BI(3Δ)-SV @ 40 cm Failure – less than 15 cpm
- 22. Measurement of Angleof Deviation • Prism alt. Cover test • Measurement of 9 positional gazes • Simultaneous Prism Cover Test • Maddox Rods • Prism Reflection Test / Krimsky Test Measurement of Angle of Deviation Prism Alternate Cover Test Amount of prism needed to neutralize the full deviation tropia and any latent phoria Used to measure deviation in anticipation of strab. Surgery Can be done using – prism bars/ lose prism Fixation ( 33cm/ 6m) cover test performed to detect the direction of deviation and suitable prisms are placed In manifest deviation prisms can be placed over normal / deviating eye Eye without prism – fixating eye ( primary position )
- 23. Test –1st perform alt cover test to know the size of deviation – prism over one eye to neutralize the dev. – alt cover test with prism – any residual movement ↑∕↓ prism with one eye covered In incomitant – angle may be measured by fixating one eye and performing alt. cover test with prisms until reversal of deviation is seen ( ensuring total angle deviation ) and dec. until no movement during alt. cover test Should never stack prisms for higher powers In V and H deviations – two prisms held with higher power closer to eye
- 24. Direction of prismbase for correction of deviation Deviation Prism in front of Right eye Prism in front of Left eye Esophoria / tropia BO BO Exophoria / tropia BI BI Rt hyperphoria / tropia BD BU Rt hypophoria / tropia BU BD Lt hyperphoria / tropia BU BD Lt hypophoria / tropia BD BU Alt. hyperphoria / tropia BD BD Alt. hypophoria / tropia BU BU
- 25. Variables inmeasurement's 1. Poor control of accommodation – use targets at visual thresh-hold 2. Variable working distance – most common @ 40 cm (or) 1/3 m 3. Tonic fusion not suspended – seen in intermittent exo / accommodative eso – dissociate BSV by prolong occlusion on prism alt. cover test 4. Physiological Redress fixation movement's – in large deviations – deviation corrected causes over-shooting of the fixated eye Solution – allow peripheral vision of the occluded eye Take point of neutralization as a point where redress = refixation movements, and dec. until best neutralization occurs 5. Incomitant deviation ( A,V patterns, lateral gaze, face turn, head tilt, chin elevation/ depression) – will change measurements if incomitant Solution – control head position for 1 position and cardinal gazes 6. Poor vision – always conduct under full correction If with sensory starb/ 20/400 – use krimsky method
- 26. Measurement of9 positional gazes The positions of gaze are usually measured with the patient fixing on a distance target Nv(33cm) / Dv( 6m) allowing only head movement (no tilt) With refraction corrected and prism on frontal plane position all 9 position are checked In deviation with head tilt – base should be parallel with lateral wall for horizontal and floor / roof for vertical deviation
- 27. Simultaneous PrismCover Test Objective method which is used to measure the tropia component of the monofixation syndrome with superadded heterophoria ( angle inc. on disassociation ) seen in small angle deviation Test – Performed by first estimating the size of the tropia with corneal light reflex testing. Appropriate prism on deviating eye; cover – non-deviating eye Remove prism and cover simultaneously – note difference – inc. prism if needed until there's no movement of eye behind the prism eye ( prism strength = manifestation)
- 28. Maddox Rods Subjective method to asses the angle of deviation by prisms Used to detect horizontal, vertical and torsional deviation Most dissociating test cause both eyes see totally different image Has a wash-board appearance which are stacks of multiple high power plus cylinder lens ( m.c. red colored) Spotlight viewed thgh Maddox as line @ 90˚ to groves Single Maddox test – for H and V deviation Test - When placed over the deviating eye the resultant displaced line relative to light are seen by fixating eye,- can be neutralized by prism Vertical lines for H deviation Horizontal lines for V deviation Light pass thgh lines – orthophoric / harmonious ARC Doesn't distinguish b/w tropia and phoria – has to be aligned first
- 29. Prism ReflectionTest / Krimsky Test Objective method of measuring the angle of manifest deviation Aim – equalize corneal reflex so they appear symmetrical Indication – To estimate deviation in uncooperative and sensory / poor Vn ( 20/400 ↑ ) patients Hirschberg corneal light reflex mixed with prisms for measuring strabismus Test – Neutralize one eye with appropriate prism – elicitate Hirschberg's on an accommodative target with pentorch – ↑∕↓ prism until reflex is symmetrical Prism on fixating eye with tropia – version movement of both eyes to the apex causing deviation of light Prism on non-fixating eye with tropia - eye directly moves the light reflex to the centre of the pupil without a version shift Any eye can be used except in restriction and paresis Here, measure 1˚ deviation – prism on limited rotation 2˚ deviation – prism on eye with duction Alt. prism may be place over the other eye ( deviating) until the image moves in
- 30. Assessment of Torsion Maddox double prism . • Maddox double prism • Double Maddox rod Only a qualitative test and cannot differentiate between phoria and tropia Test - Two 3 0r 4 PD base to base prism mounted on a trial frame bisecting the pupillary axis horizontally causing monocular diplopia Prism eye – parallel lines with vertical spacing Binocular view – parallel lines if, no torsion Intermediate oblique line appear in presence of torsion
- 31. Double Maddoxrod Measure the angle of torsion and more accurately but, only in primary position Test – two different colored Maddox rods are place in trial frame in horizontal Small vertical prism of 4PD is placed to separate the lines Patients is asked to rotate until the lines are parallel Result – measuring the angle will give the torsion angle In large vertical deviation – prisms can be used to bring the image near
- 32. Correction of Deviation In heterophoria – Aid to detect if symptoms are ocular or not In manifest deviations – If normal / abr. BSV is present Type of retinal correspondence Degree of surgery required Out come of Sx if BSV not present Correction may be in the form of test / temporary wear using Fresnel for a short interval Correction of Deviation • Test for ocular symptoms • Assessment of potential BSV • Prism adaptation Test • Progressive prism compensation • Pre-Op Prism Adaptation
- 33. Test forocular symptoms To determine that weather the symptoms experienced by the patients are in consistence with the findings Monocular occlusion by removing the effort of controlling the heterophoria relieve the symptoms But – field of Vn is dec. and loss of stereopsis So, Fresnel prisms are used to correct the angle and relieving of symptoms if this effort was the cause Assessment of potential BSV Correction of angle will enable to regain BSV if present After correction of angle and BSV obtained a cover test is performed which suggest If no manifest deviation – NRC and BSV Manifest deviation – abnormal BSV
- 34. Prism adaptationTest Method for determining the amount of surgical correction in patients with partially accommodative esotropia Involves prescribing BO prisms for residual esotropia post full hypermetropic correction Patient is reviewed after 2-3 weeks and evaluated for any reminder deviation viz over correct if needed Fresnel prism placed over the normal eye 1. No manifest deviation with BSV – normal BSV 2. Angle remains same and no BSV – suppression and no BSV 3. Small manifestation with prism and anomalous BSV on testing – some form of BSV with manifest deviation 4. Similar size of manifest deviation with prism and test – ARC , patient has a desire to maintain to angle and any inc. in prism inc. deviation
- 35. Progressive prismcompensation In manifest strabismus when prism is placed over one eye fusinoal movement still occur which are comparatively slower than the normal response – anomalous movement induced by disparity Effect the Sx outcome if strong So, repeating test @ 2hrs is advice to check for anomalous movements
- 36. Pre-Op PrismAdaptation Aim –to obtain max angle of deviation pre-op and then aiming Sx correction at this angle to dec. under/over correction Rx amblyopia (6/12) + alt. prism cover test + Fresnel / split prism for angle correction R/a- 2wks or short duration of few hrs Test for sensory fusion and PCT on review More than 8Δ – no sensory fusion Prism adaptation responder – deviation stabilized at 8Δ and peripheral fusion Prism adaptation non-responder – exo deviation / stable angle with no evidence of sensory fusion / angle built up more than 60Δ
- 37. Test for RetinalCorrespondence Vertical Prism Red Filter Test . • Vertical Prism Red Filter Test • Investigation of suppression Used to detect ARC from NRC in patients with suppression by placing 15Δ red vertical prism over the deviated eye ARC – Two vertically displaced images with red over white The lights are vertically aligned cause the light in the deviated eye is over the pseudo-fovea to the true fovea of the normal eye NRC with central suppression scotoma – Two lights with V+H displacement Cause there is no pseudo-fovea and the center of reference is true fovea of each eye
- 38. Investigation ofsuppression Prisms can measure the areas of suppression by moving the image nasally / temporally / vertically / horizontally Diplopia will be appreciated if the image is out side the suppression area With prism – redirect the image into suppression areas when BSV is absent investigate possibility of post-Op diplopia in cases where there's no potential BSV
- 39. Post-Op DiplopiaTest (PODT) Used to an attempt to predict the intractable diplopia of post-Op, in cases of where BSV is potentially absent Angle of deviation is corrected gradually with prisms (Nv/Dv) and check for diplopia if present Which may occur with an under/full/ over correction
- 41. Management Prisms maybe used to restore BSV, weather or not diplopia is present (or) occasionally, to redirect the image on to a suppression area if potential BSV is not present Occasionally prisms are also used to separate diplopic image in absence on fusion potential or a separation area
- 42. Relieving Prisms Aim – stabilize sensory motor fusion Action – optically reduce demand on controlling fusional Vergence By moving light closer to fovea – moving it into foveal range – patient vergs – fusion obtained Rx – less than the angle of deviation Base – opposite to deviation Uses - intermittent strabismus, phorias
- 43. Inverse Prismsfor training / disruptive Aim – To increase fusional Vergence ability Action – Optically increases the demand for controlling fusional Vergence Base – Same direction as deviation Uses Training – used in phorias Disruptive – to eliminate ARC Inverse Prisms for cosmetic Indications – poor prognosis for functional care and doesn't want / not suitable for Sx Aim – make eye look better Base – same as the deviation
- 44. Yoked Prisms Aim – stabilizing binocular vision in non-concomitancy or dampen nystagmus Action – directs the eyes in specific gaze direction. Optically moves the retinal images of a fixed target in a parallel towards the base and moves the light towards the base and shows the target towards the apex - both eyes move in same direction Uses – gaze palsy , Duane’s Syndrome , nystagmus
- 45. Sector Prisms Aim – stabilize BSV in one / more gazes or distance Action – Reduce demand for controlling fusional Vergence in more then one gaze or distance Eg – 20Δ ET @ Dv 10Δ ET @ Nv Rotating Prisms A method to change sensory input for constant strab. To precipitate a change from ACR to NRC Fresnel prisms – 1 week BO then rotate BU , BI , BD Uses – disrupt ARC
- 46. Corrective Prisms Aim – to stabilize normal sensory fusion Action – Optically neutralize the demand for controlling fusional Vergence by elimination the oculo-motor deviation Rx – prism = magnitude of deviation Residual Vergence demand = 0 Base – Opposite to the deviation Over- corrective Prisms Aim - To disrupt ARC Action – Reverse the demand for controlling fusional Vergence and optically changing the direction of deviation Rx – Prism power > magnitude of deviation A deviation reverse is seen on cover test and ACT in these cases i.e. and eso becomes an optical exo
- 47. Slab-off Prisms An anisometeopic patient may experience – diplopia / asthenopia if the line of sight doesn’t pass thgh optical center of spectacle this is due to displacement induced by net prismatic effect Eg - +1.00 OS and – 3.00 OD, will have difficulty even at 1cm below from the optical center Solution slab-off Prisms (or) Bicentric Grinding Myopes- back ; hypermetropic – front Other – C.L , separate glasses for Nv and Dv, lowering optical center to an intermediate
- 48. Aphakia andPseudophakia following Cataract Surgery Diplopia post cataract Sx can been seen as a late complication which can be treated by prisms Most commonly seen with traumatic cataract due to torsion Malingering Prism Dissociation Test For malingering in monocular blindness Subjective correction with a 4Δ vertical prism will cause diplopia BO prism for the ill eye and when focused to eliminate diplopia malingering is ruled out
- 49. Amblyopia Can be use in diagnosis and treatment Diagnosis 10 Prism test ( vertical Prism test / induced tropia test ) Preverbal with straight eyes / small angle deviation – for accurate diag. Test – 10 to 15 BU/BD in front of one eye – induces vert. starb. And fixating presence can be known Inference 1. Spontaneous alteration 2. Hold well – Smooth / blink fixation by other eye by movements for atleast 5 sec. 3. Holds briefly – refixation delayed by 3 sec. 4. Hold momentarily – fixation maintained for 1 -2 sec. 5. Will not hold – refixation as soon as prism is remover Treatment – Rarely done when therapy fails Most commonly for amblyopia eccentric fixation by passive stimulation of amblyopic eye with full prism correction + atropinization + Nv correction with + 3 DS
- 50. Field Defects Prisms can be used in the management of visual field defects - m.c hemianopia's They expands the field of view in the direction of hemianopia's Object that falls on the edge of the scotoma of one eye is seen by the other eye 15 prism is placed over the effected eye with the base towards the defect, trimmed to be 2mm away from the mid-pupillary line, avoiding interference with central vision Can be used in stroke, field defects and visual neglect patients
- 51. Other uses Incorporated into C.L. for vertical diplopia For exercise – ti increase fusional convergence ARMD – to relocate retinal image to an area of preserved retinal function Bed ridden patients – in ankylosing spondylitis and other postural defect
- 52. Prisms in ophthalmicinstruments Reflecting and dove prism are used in almost all of the of the ophthalmic instrument and operating micro-scopes Using the property of total internal reflection prisms have basically replaced mirrors in SLB, microscopes, ect
- 53.