Prism Dr. Meenank. B

M.S. Ophthalmology (post-graduate )

ASRAM medical college

Refractive surface – The two refractive surfaces of a refractive prism inclined at the apical angle

Axis – line bisecting the refractive angle

Reflecting surface – In some prism, internal ray hitting the 2nd surface is subjected to total internal reflection (reflecting prisms)

Apex – Tip of the prism where the two refractive surfaces meet

Base – bottom of the prism / side opposite to apex or optical angle. Helps in orientation of ophthalmic prisms

Prism is a portion of a refractive medium, bordered by 2 plane surfaces which are inclined at a finite angle

Prism – Refraction

Angle of deviation – Angle between the incident and the emergent rays

D = (n-1)α

Refractive angle / apical angle - angle between two refractive surfaces ‘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

Image formed – virtual, erect and displaced towards the apex

Power of a prism – Amount of light deviation produced by the prism

Prism Dioptres ‘∆’

1 ∆ = 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

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 : 40∆ Prentice if held at Frontal plane - 32∆

Prentice Position

Minimal Angle Deviation – Plastic, more common

Line of sight makes equal angle with both surface

Difficult to maintain in clinical practice

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

Minimal Deviation

Stacking of prism’s

Glass prism – max – 40 ∆

Plastic prism – max – 50 ∆

For more than this ‘stacking’ – combining 2 prisms is done – but never combine linearly cause they give more effect

Eg – 50∆ + 3∆= 58∆ ( additive error)

So, to dec. this error prism is held before both eyes

For a V and H dev. Prism can be combined

Measuring strabismus with glasses

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 +/- 5∆

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

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 (𝑎2 + 𝑏2 = c²)

While prescribing always mention the base and the meridian

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

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

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

5∆, 10∆, 15 ∆ 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

Types of Prisms in 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 ∆, V-15 ∆

Clin

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ass

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of

Pri

sm’s

Relieving

Inverse

Yoked

Sector (or) Regional

Corrective

Over corrective

Rotating

Slab off

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

Diagnosis

Investigations of BSV

Prism reflex test

Prism 4∆ 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)

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

Investigations of BSV

• Prism reflex test

• Prism 4∆ 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 10∆, 20∆

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

Prism 4∆ test

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

Fusional Vergence Amplitude

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

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(12∆)-SV-BI(3∆)-SV @ 40 cm

Failure – less than 15 cpm

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

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 )

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

Direction of prism base 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

Variables in measurement'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

Measurement of 9 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

Simultaneous Prism Cover 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)

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

Prism Reflection Test / 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

Assessment of Torsion

Maddox double prism

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

.• Maddox double prism

• Double Maddox rod

Double Maddox rod

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

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

Test for ocular 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

Prism adaptation Test

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

Progressive prism compensation

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

Pre-Op Prism Adaptation

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 8∆ – no sensory fusion

Prism adaptation responder – deviation stabilized at 8∆ and peripheral fusion

Prism adaptation non-responder – exo deviation / stable angle with no evidence of

sensory fusion / angle built up more than 60∆

Test for Retinal Correspondence

Vertical Prism Red Filter Test

Used to detect ARC from NRC in patients with suppression by placing 15∆ 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

.• Vertical Prism Red Filter Test

• Investigation of suppression

Investigation of suppression

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

Post-Op Diplopia Test (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

Management

Prisms may be 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

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

Inverse Prisms for 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

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

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 – 20∆ ET @ Dv

10∆ 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

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 esobecomes an optical exo

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

Aphakia and Pseudophakia 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 4∆ vertical prism will cause diplopia

BO prism for the ill eye and when focused to eliminate diplopia malingering is ruled out

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 eyewith full prism correction + atropinization + Nv correction with + 3 DS

Field Defects

Prisms can be used in the management of visual field defects - m.chemianopia'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

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

Prisms in ophthalmic instruments

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

Thank you