APHAKIA:Advantages and Disadvantages of Aphakics Correction, Types of Aphakic

Correction, Problems of Newly Corrected Aphakics

Suraj Chhetri B.optometry16th Batch

PRESENTATION LAYOUT Introduction

Causes of aphakia

Optics of aphakia

Clinical features

Types of aphakic correction

Advantages and disadvantages of aphakic correction

Problems of newly corrected aphakics

INTRODUCTION• Literally Aphakia means absence of crystalline lens from the

eye

• Optically Aphakia means absence of lens from pupillary area and it does not takes part in refraction

CAUSES OF APHAKIA• Congenital absence of lens (rare condition)• Aphakia due to absorption of lens matter after trauma of eye

in children • Traumatic extrusion of lens from eye ( rare condition)• Posterior dislocation of lens in vitreous (traumatic or

spontaneous ) optical aphakia• Surgical aphakia occurring after removal of cataractus lens is

the most commonest presentation• Heritable disorder associated with dislocation of lens 1. Marfan’s syndrome2. Weill-marchesani syndrome3. Homocystinuria

Heritable disorders reported with subluxation of lens1. Alport’s syndrome2. Craniofacial dysostosis3. Aniridia4. Megalocornea

Ocular disorder which can lead to subluxation of lens5. Buphthalmos 6. Intraocular tumor7. Mature or hypermature cataract8. Exfoliation syndrome

OPTICS OF APHAKIA Can be discussed under following topics

1. Changes in cardinal points of the eye

2. Image formation in the aphakic eye

3. Visual acuity in aphakia

4. Accommodation in aphakia

5. Binocular single vision and aphakia

CHANGES IN CARDINAL DATA OF EYE • Power of eye reduce to 44 D , eye become high hyperopia

• Anterior focal point become 23.2 mm in front of the cornea

• Posterior focal point become 31 mm behind the cornea

• The two principle points are almost at the anterior surface of cornea

• Nodal points are very near to each other and located about 7.75mm behind the anterior surface of cornea

IMAGE FORMATION IN APHAKIA Average image magnification reported by different methods

of aphakia correction Spectacle : 33% Contact lens : 10% AC IOL : 2-5% PC IOL : 0 %( about)

VISUAL ACUITY IN APHAKIA• The large image falsifies the VA recorded on the Snellen’s

chart

• VA recorded theoretically better than the actual VA in terms of visual angle

• A vision of 6/9 with corrected aphakic eye should be equivalent to 6/12 of an emmetropic eye because of high magnification of spectacle (33%)

ACCOMMODATION IN APHAKIA

• Due to absence or loss of lens from its position accommodation become zero

BINOCULAR SINGLE VISISON AND APHAKIA

• Presence of aniseikonia is detrimental to the development of BSV ( < 5% aniseikonia )

• Monocular aphakic child , the major hindrance to development of BSV due to possibility of high aniseikonia

• Unilateral aphakic corrected with spectacle , usually develop annoying diplopia

• As soon as possible operation of another eye in bilateral cataract

• Because during this period as the eye dissociated convergence become poor , streopsis and even fusion may not be attained

OPTICAL DEFECT IN APHAKIA

• Acquired high hyperopia

• Against the rule astigmatism • Absence of accommodation

• Change in colour vision

• The amount of astigmatism is +1.00 D to +3.00D cyl * 180 But gradually diminishes

COLOUR VISION IN APHAKIA

• The crystalline lens constitute a barrier function separating anterior to posterior segment

• Complete intracapsular extraction of lens results in barrier deprivation syndrome , it cause disturbances of aqueous hydrodynamics and biochemical equilibrium

• Removal of lens increases the intensity of incident short wave-length of light

• These two consequences of cataract surgery may cause microcirculatory disturbance and structural changes in macular area of aphakia

• So acquired B-Y colour vision defect seen in aphakic patients 1

1( marion marre, Ernst marre and Dorothea fulle in dresden 1987)

CLINICAL FEATURES

• SYMPTOMS only symptom in aphakia is marked defective vision for far and

near

SIGNS

Most deep anterior chamber (iris has no support ) Iridodonesis i.e tremulousness of iris Pupil is jet black in colour Absence of 3rd and 4th purkinje‘s image Absence of lens from patellar fossa , best elucidated on slit

lamp examination Fundus examination show small hyperopic disc Retinoscpy reveals high hypermetropia Linear scar at the upper limbus if cataract surgery done

TREATMENT

Spectacles

Contact lens

IOL(intraocular lens)

Refractive corneal surgery

SPECTACLES• Optical principle is to correct the error by convex lens of

appropriate power • Spectacles prescription has been the most commonly

employed method of correcting aphakia in the past

Predicting the power of an aphakic lens

• Laurance suggested the power of spectacle lens

• Example : +2.00 D hyperopic before surgery required aphakic lens is approximately +12.00 D

• Retzlaff and karff described formula of predicting aphakic refraction

• Where ,L = axial length of eye , K = preoprative keratometry finding in diopters

Faphakic= +11.00 D + (1/2)Fpre-aphakic

Faphakia = 80.4 – 1.65 L -0.7 K

ADVANTAGES OF SPECTACLE CORRECTION

Easy

Safe

Inexpensive

DISADVANTAGES OF SPECTACLE CORRECTION

1. Increased retinal image size2. Decreased field of view3. Presence of ring scotoma4. Increased lens aberrations5. Motion of object in field of view6. Appearance of the wearer7. Increased ocular rotations 8. Demand on convergence

INCREASED RETINAL IMAGE SIZE• If correcting lens is placed at anterior focal point of the eye,

size of retinal image directly proportional to the anterior focal length and inversely proportional to the refracting power of the eye.

• Retinal image size ( aphakia/emmetropia)

• (23.23/17.05) = (58.64/43.05) = 1.36

• Or increased in retinal image size of 36%

• In practice lens place closer distance decreases the size of retinal image but in aphakia magnification (aprrox. 28%)

• In B/L aphakia greatly increased image size means that aphakic patients must adapt to new size- distance relationship

• In unilateral aphakia corrected with spectacles BSV impossible due to high magnification

• Sometimes aphakic patients achieve central visual acuity that exceeds the best VA obtained before surgery due to magnification of retinal image size

• Increased magnification may permit the prescribing of weaker reading addition

Familiar object not only appear to be much larger , they also appear to be much closer

DECREASED FIELD OF VIEW • Base to the center prismatic effect of high plus lens reduce

the size of a field of view• Peripheral field of view tanφ = y (E-F) where, φ =one half of angular field of view E = vergence of light at entrance pupil of eye y = one half of lens aperture

Central field of view tanθ= y ( S-F) Where S = vergence of light at center of rotation of eye F = power of the correcting lens

• With standard parameter lens of 12.50 D show loss of peripheral field of view only about 7% but loss of macular field about 17 %

PRESENCE OF RING SCOTOMA Base in prismatic effect of strong plus lens causes angular gap

in object space completely around the lens which is know as ring scotoma

center to the ring scotoma wearer has corrected vision

• Ring scotoma moves in opposite direction of the eye movement ( roving eye scotoma )

• Ring scotoma creates different problems especially from 2 - 10 feet ,which is generally in ordinary room

Factors effect on size and position of ring scotoma 1. Lens power 2. Vertex distance 3. Lens size4. Pupil size5. Lens thickness6. Base curve Except vertex distance and pupil size all other show

direct relation with size of ring scotoma

JACK-IN THE BOX PHENOMENON

• Object seems to jump in and out of field of view as it moves out of and into the ring scotoma

• Simply sudden disappearance and sharp reappearance of the object in visual space with ring scotoma

INCREASED OCULAR ROTATION• Angle of eye turning in changing fixation from one object

point to another increases in comparison to emmetropic. • Due to prismatic effect

INCREASED LENS ABERRATIONS

• Spherical and coma occurs only for large aperture so aphakic with small pupil are affected very little by these aberrations

• Off axis aberrations ,oblique astigmatism and curvature of image which can effect on patients visual acuity and contrast

• Tscherning ellipses limits up to -23.00D to + 8.00 D so it could not cover the aphakic patients

• So oblique astigmatism can eliminated by making at least one surface aspheric

• Pincushion distortion- change in magnification across the field of view of a lens. To control distortion in aphakic steeply curved lenses required

• Axial and transverse chromatic aberration can not be eliminated by single refractive medium

• Transverse chromatic aberration is more bothersome because image location and magnification differ for different wavelength ,which primarily the tangential meridian of oblique astigmatism

• Thus controlling the tangential error effect , transverse chromatic aberration can be attenuated

oblique astigmatism and curvature of image are the most important aberration and have been the main targets of

lens designer

MOTION OF OBJECT IN THE FIELD OF VIEW• When high plus error holds the eyes steady and moves head

to any object then marked reversed motion (against motion) of the field of view is experienced

• This is due to prismatic( base in ) in the direction of head movement

• This motion often referred to as swim• swim can be avoided by moving the eyes from one fixation

point to another while the head stationary but this may produce distortion

• Best procedure is to turn head slowly so that head and eye moves slowly

APPEARANCE• Apparent enlargement of eye and their unusual appearance

behind bulbous , thick lens

• General awkwardness of the patients bearing and movement ( both impression originate from magnification and distortion

effect of the lens )• lenticular lens provide good optical performance in central part

but poor cosmetic appearance ( bull’s eye or fried egg) due to high magnification confined to central portion of the lens

DEMANDS OF CONVERGENCE• In ordinary condition, convergence of visual axis towards near

fixation point creates base out prismatic effect

• So while reading eye must converge more than low power spectacle . This can place strain to positive fusional vergence and may lead to discomfort

• This effect can be reduced by placing distance center closer together than the patient’s distance PD

• Or by incorporate base in prism with aphakic lens

CONTACT LENSES• Aphakic contact lens is not very common in practice

• It is treatment of choice who are unsuitable for IOL implantation or who have experienced a failed IOL implants

• Common in unilateral aphakia

• Contact lens provide less image magnification (about 5-7%) which make possibility of BSV in uniocular aphakia – Provide BSV only where fellow eye has useful vision• Can achieve only 140 seconds of arc (titmus)• 80% with intermittent suppression 2– 2( Guilin et al. 1980)

GENERAL PROCEDURE• 6-10 weeks post surgical to permit healing/ settling of power • Find any contraindication for CL fit• Spectacle Refraction • Measurement of Ocular dimension– Keratometry – Pupil size, shape, position – Eyelids integrity – CL fitting • Over refraction (astigmatism / near addition)• BSV

– After care

CONTACT LENS OPTIONS FOR APHAKIA• Hydrogel lenses : cases with low astigmatism or failed RGP

lens wearers• RGP lenses : cases with high corneal astigmatism and corneal

distortion• Silicone elastomer : cases with low astigmatism and pediatric

aphakic patients • Scleral lenses : patients with significant corneal distortion • Tinted lenses :to attenuate bright light and offer some UV

protection• DW or EW contact lenses based on manual dexterity . EW is

best suited to poor lens handling patients

PEDRIATIC CONTACT LENS FITTING IMPORTANT AND STRONG NECESSITY

• Time of fitting

• Length of wearing

• Lens material

• Fitting

FITTING TECHNIQUE RGP lenses

• Keratometry based technique

Record post-operative keratometry Start trial lens 0.5 – 1 mm flatter Centration and movement is determine Study flourescein pattern

o Minimal apical clearance is the goal

Retinoscopy over the lens Calculate final lens prescription

FITTING TECHNIQUE RGP Lens

Diagnostic lens fitting: experience based Insert the diagnostic lens Observe centration and movement Fluorescein pattern study Central posterior area steeped until small apical air bubble

appear Final trial lens flatter by 0.5-1 mm Push up test Retinoscopy over lens Calculate final lens power

FITTING TECHNIQUE Soft lens

Limit the feasibility of frequent lens handling

Pediatric soft lens parameter o BC : 7.5- 7.8 mmo TD : 13.7mmo Power : + 20.00 D to 35.00 D

• Fitting procedure , same as adult contact lens fitting• Younger the child and higher the power , steeper is the lens • Diagnostic lens fitting technique

FITTING SILICONE ELASTOMERS• B & L silsoft • Parameters Material : elastoflicon Water content : 0.2% FDA material group I Oxygen permeability (DK) : 340 Oxygen transmission : 70 BC : 7.5 , 7.7 , 7.9 , 8.1 , 8.3 mm TD : 11.30 mm , 12.50 mm Power : +12.00 D to +20.00 D (in +1.00 D steps) +23.00 D to + 32.00 D ( in +3.00 D steps) Back optic zone : 7 mm Central thickness : 0.51mm to 0.71 mm

FITTING PROCEDURE• Examine that eye are ready to fit with contact lens

• Determine the parameter of the first trial lens Corneal diameter Keratometry (optional) Base curve selection : should be one step flatter than the

steepest lens available As per age starting base curve

< 1 year = 7.5 mm 1-2 year = 7.7 mm > 2 year = 7.9 mm

FITTING PROCEDURE• Approximate power = add + 2.00 to 3.00 D more power in

mean refractive error

Starting point could be + 30.00 D for 6 month child + 27.00 D for 1 year child +23.00 D for 2 year child +21.00 D for 3 year child

ADVANTAGE OF CONTACT LENS IN APHAKIA• Image magnification is 7-8 % and this

magnification is usually tolerated by thepatients

• Increase in the visual field. ( both central and peripheral)• In monocular cases diplopia is usually

absent with retention binocularity.• Cosmetically it is well accepted.

DISADVANTAGES OF CONTACT LENS IN APHAKIA

• Foreign body sensation

• Corneal complications

• Lens spoilage

• Spectacles may be required over contactlenses for reading

• High +Rx lenses more likely to decentered

ADVANTAGES OF HYDROGEL LENSES

• Offer good initial comfort

• Less prone to displacement

• Offer good centration

• Stable positioning of the optics over the eye during the blinking and eye excursions

• Stabilize promptly , after insertion

DISADVANTAGES OF HYDROGEL LENSES

• Poorer oxygen transmission • Aggravate existing dry eye problems • Greater care and maintenance needs • Higher overall costs• Limited parameter available • Handling can be more difficult • May discolor with topical medications • Uptake of preservatives

RGP LENS ADVANTAGE IN APHAKIC PATIENTS

• Provide optimum vision correction

• Fewer eye complications

• Easier lens care

• Customized designs possible

• Curves and BVP can be modified

IOL IMPLANTATION • IOL implantation in cataract ,has good outcome

• Difficult to implant IOL in congenital cataract due to small size of eyeball

• IOL power may hamper the emmetropization process due to inappropriate IOL size and power

• In unilateral cases, primary implantation is indicated as soon as the patient is fit for anesthesia, ideally between 2 and 3 months of age. The earlier the surgery is done, the better the chance that deep amblyopia can be overcome3

3(Yanoff & Duker: Ophthalmology, 3rd ed.)

In case of unilateral congenital cataract most studies suggested that surgery after 6 weeks of life is less likely to result good visual acuity3

3(systemic pediatric ophthalmology 3rd ed.)

In B/L congenital cataract difference between surgery of two eye must be less than 6 weeks

B/L congenital cataract is more common than unilateral

If major blood vessels can not be distinguished from central portion of the cataract significant visual deprivation can be expected from moderately size partial cataract 4

4 (systemic pediatric ophthalmology 3rd ed.)

• Rapid growth of eye ball up to 2 years of age makes difficult to chose power of IOL

• Ideally, the pediatric IOL should be available in diameters of the range 10.5–12mm

• The material from which the IOL is made must have a long track record of safety. PMMA( Polymethyl methacrylate )is currently the best material to be used for children5

5(Yanoff & Duker: Ophthalmology, 3rd ed)

IOL in pediatric age group

Some author prefer under correction by 30% to child below the age of 1 year

ADVANTAGES OF IOL IMPLANTAION

• Little image magnification

• No spherical and prismatic aberration

• Minimum or no aniseikonia

• Normal peripheral field

• Cosmetically well accepted

DISADVANTAGES OF IOL IMPLANTATION• Risks and complications may be moreo Infectionso Expulsive choroidal hemorrhage o Cystoid macular edemao Retinal detachmento Corneal edemao Dislocation of IOL ( child rubbing eye)o Posterior capsular opacities

• It needs specially qualified surgeons andsophisticated instrument.

• The cost is more

REFRACTIVE CORNEAL SURGERY IN APHAKIA• Refractive corneal surgery is under trial for correction of

aphakia . It includes

• Keratophakia : keratoplasty in which corneal tissue from a donor is frozen, shaped, and inserted into the cornea of a recipient

• Epikerakophakia : surgical procedure in which a donor cornea is transplanted to the anterior surface of the patient's cornea

• Hyperopic – laser assisted in situ keratomileusis : corrective eye surgery in which a flap of the corneal surface is raised and a thin layer of underlying tissue is removed using a laser

PARAMETERS OF APHAKIA LENSES

• Critical lens specification and fitting includes

Vertex distance Pantoscopic tilt Centration Lens weight and thickness Adjustment of the spectacles

VERTEX DISTANCE Effective power Fb = (Fa/1-dFa) Where d = vertex distance , As the vertex distance reduce though power of plus lens

become more powerful but image made small

Thus it is advice to place spectacles as much as close to eye to reduce vertex distance

PANTOSCOPIC TILT• Spectacle lens are tilted around horizontal axis to improve

their appearance on wearer’s face• Tilt is typically 8-10 degree• In aphakia to minimize the weight , optical center should fall

at the geometric center and the optic axis should pass through the center of rotation by altering the vertical position of the frame on the face

WEIGHT AND THICKNESS• Lens that correct aphakia are thick and heavy

• Weight can reduce by CR-39 plastics but thickness increased (lower index of refraction)

• Thickness reduce by high index glasses but weight remain heavy

• Lenticular aphakic lens reduces both thickness and weight but unattractive

• Weight and thickness can reduce by using symmetrical small eye sized frame and lenses with essentially knife- edge thickness

POSITION OF OPTICAL CENTRE • Small error in PD produce significant effect in aphakic lens• Corneal reflection method prefer for monocular PD in

aphakia (irregular pupil due to surgery)

SPECTACLE ADJUSTMENT

Because of aphakic lens are heavy , they tend to slip down on the nose

Small misalignment and change in position introduce large error

SOME ADDITIONAL FACT ABOUT APHAKIC LENS DESIGN

• Aspheric front surface reduce the effects of aberration , weight ,thickness , magnification and size of ring scotoma by reducing power in periphery of lens

• One of the main advantage of aspheric front surface , it improve appearance of lens

• Steeper back surface curve has advantage of reducing distortion , lens reflections and sensitivity to difference to vertex distance

• Straight – top segment is relatively wide at the top and has little image jump as the line of sight enter the segment

• Increasing amount of oblique astigmatism induced as the visual axis move downward through the lens

• So bifocal segment of aphakic lenses should place as high as possible

High chance of UV radiation damage to aphakic retina, due to

In absence of lens ,increased amount of concentrate radiations energy receives by the retina

Coefficient of absorption of radiation by retina is greater for UV radiation than that for visible radiation

So it is essential to incorporate UV absorbing filter and aphakic patient also wish to have one of the light tint for general wear

such tint have advantage of improving the appearance of

lenses and reducing glare

DISPENSING-LENS SELECTION

• Aim of Lens selection– Reduce lens weight.– Reduce lens thickness– Reduce spectacle magnification– Increase field of view– Minimize lens aberration– Offer choice of optimum multifocal style and

position– Provide protection from glare and UV radiation.

LENS SELECTION FOR APHAKIA

– High index plastic

– Aspheric lenses

– Lenticular lenses – Light tints and UV protection

– Anti reflection coating

FRAME SELECTION FOR APHAKIC LENS

• Effective diameter of lens increases as the frame deviates from round or oval shape which is not more than 2 mm larger than eye size

• Frame difference greater than 9mm should be avoid because high plus lens tend to be thick on top and bottom edges which gave apparent look high plus lens even stronger

• Because field of view increased and magnification decreased as plus lens are moved closer to the eye , frame should hold closer as possible to the eye

• Adjustable nosepads become more necessity as they offer advantage of versatility in allowing modification of vertical position of the frame

• Comfort cable temples help to the glasses from slipping down the nose

• Frame with thick eyewires should be avoid because an eyewire may increase the width of the ring scotoma

UNILATERAL APHAKIA • Spectacle correction in unilateral aphakia have

following difficulties 1. Difference in size of retinal image of two eyes 2. Large prismatic effects encountered in the peripheral portion

of the lenses

• Magnification difference is so great that BSV impossible . This high magnification intolerable to the patient , so we have three options

i. Not correct the operated eyeii. Correct and occlude the un-operated eyeiii. Prescribe contact lens for aphakic eye

• Unilateral aphakia corrected by contact lens , the amount of residual aniseikonia (5-7%) makes normal BSV difficult .

Yet many unilateral aphakics appears to have comfortable vision with contact lens

• Residual aniseikonia in aphakia corrected by contact lens depends upon following factors

A. Refractive error before operation and its type ( axial or refractive )

B. Any alteration of power of cornea caused by the surgery C. Individual variations in the dimensions of the primary

refracting component D. Shape factor for the spectacle magnification formula

• Ogle, Burian and Bannon plotted residual aniseikonia (unilateral aphakia) on the basis of pre-cataract data, which summarized as

In high myopia before cataract onset( where axial or

refractive) , residual aniseikonia can be minimize by fitting contact lenses of the both eye

In high hyperopia before cataract onset (whether axial or refractive ) , residual aniseikonia can be minimize by fitting contact lens on the aphakic eye and spectacle lens on the un-operated eye

Enoch has suggested that BSV may be restored in unilateral aphakia by fitting the aphakic eye with reverse Galilean telescope which brings about minification of retinal image

The plus necessary for eyepiece of telescope is added to contact lens prescription for aphakic eye and minus power of telescope objective is supplied in the form of spectacle lens

PROBLEMS OF NEWLY CORRECTED APHAKICS

• Annoying diplopia if uniocular aphakia corrected with spectacles

• Due to high magnification familiar object not only appeared to be much larger , they also appeared to be much closer

• Peripheral distortion when aphakia correction with spectacle

• Asthenopia due to increase demand of convergence

• B-Y colour vision defect

• Appearance of ring scotoma in ordinary room condition

• Poor cosmetic appearance

REFERENCES