ASTIGMATISM, PRESBYOPIA AND APHAKIA

Resident(1st year)

National Academy of Medical Sciences

ASTIGMATISM

ASTIGMATISM

Greek; A-without, Stigmos-a point/a spot (Coined by William Whewell;1849) DefinitionIt is that condition of refraction wherein a point focus of light cannot be formed upon the retina.It occurs due to unequal refraction of light in different meridians.

Etiology of astigmatismCorneal AstigmatismCongenital-occurrence is almost invariable in small degrees -most commonly the vertical curve is greater than the horizontal curve( about 0.5D). This is called direct astigmatism.-physiological

Acquired astigmatism• Trauma to the cornea

including surgeries

• Pressure of swellings on the lid( chalazion or tumor)

• Pterygium

Corneal astigmatism contd…

Acquired astigmatism• Conical cornea/

keratoconus

• Corneal inflammation and ulceration and scars

Lenticular astigmatism

• Lenticonus

• Oblique placement of the lens

Lenticular astigmatism

• Traumatic subluxation of the lens

• Decentration of IOL

• Index astigmatism

Retinal astigmatism

• The posterior pole of the eye may be placed obliquely as when it bulges backwards in a staphyloma in high myopia.

Types of astigmatism

Regular astigmatism• The two principle meridians are at right angles and

are therefore susceptible to correction– With the rule astigmatism – Against the rule astigmatism – Oblique astigmatism

Irregular astigmatism• There are irregularities in the curvature of the

meridians so that no geometric figure is adhered to• Cannot be corrected adequately by spectacles

Optics of regular astigmatismSturm’s conoid

With-The-Rule (WTR) Astigmatism

• The two principal meridians are at right angle to each other with the vertical meridian being more curved than the horizontal i.e. the greatest refractive power is at 90˚(+/-20˚)

• Corrected by convex cylinder at 90˚(+/-20˚) or concave cylinder at 180 (+/-20˚)

Against-The-Rule (ATR) Astigmatism• The two principal meridians

are at right angle to each other with the horizontal meridian being more curved than the vertical i.e. the greatest refractive power is at 180˚(+/-20˚)

• Corrected by convex cylinder at 180 ˚(+/-20˚) or concave cylinder at 90 ˚(+/-20˚)

Oblique (OBL) Astigmatism

• The two principal meridia are not horizontal or vertical though they are at right angle to each other.

Bi-oblique Astigmatism

• The two priciple meridians are not at right angle to each other but are crossed obliquely.

30˚

100˚

Regular astigmatism can further be classified as

• Simple astigmatism • Compound astigmatism• Mixed astigmatism

Simple astigmatism.

• One of the foci falls upon the retina while the other falls infront of or behind the retina

Simple myopic astigmatism

Simple hyperemetropic astigmatism

Compound astigmatism• Neither of the foci

fall upon the retina but both are placed in-front of or behind the retina Compound myopic astigmatism

Compound hypermetropic astigmatism

Mixed astigmatism

• One focus is infront of the retina while the other is behind so that the refraction is hypermetropic in one direction and myopic in the other direction

Mixed astigmatism

Irregular astigmatism• The refraction in different

meridians are different.• Small degree occurs

physiologically owing to minute difference in the refractive index of the lens.(accentuated by cataract)

• Marked degree occurs in pathological conditions of cornea – Irregular healing of cornea

after trauma or inflammations– Conical cornea(keratoconus)

Symptoms of astigmatism

• Reduced Visual acuity• Peculiar vision• Asthenopia( more with hypermetropic

astigmatism)• Head tilt in oblique astigmatism in children• Half closure of the eyelids

Clinical Tests

• Visual acuity tests – distance and near• Autorefraction• Keratometry• Retinoscopy – Most reliable source of information for cylinder

power and axis• Monocular subjective refraction– Astigmatic fan test– Jackson cross cylinder

Keratometry

• It shows different corneal curvature in different meridians

Retinoscopy

• Shows different power in different meridians.

• If the streak passes exactly through the axis of the astigmatism, a sharply defined reflex band is seen which moves parallel to the band of the light outside the pupil.

• If it doesn’t pass exactly through the astigmatic axis, the reflex becomes poorly defined and tends to remain fixed in the axis producing a break in the alignment between the reflex in the pupil and the band outside it tending to lie intermediate between the latter and the true astigmatic axis

Cross-cylinders

• Used to refine the axis and the power of the cylinder.

• Done after the cylindrical correction has been made.

• Combination of a negative and positive cylinder of equal strength (usually 0.5D) mounted at an angle of 90 degrees

• In practice combined into one piece of glass

Refinement of the axis• The cross-cylinder is placed with its axis at

45˚ to the axis of the cylinder in the trial frame( first with +0.5 cylinder and then with -0.5 cylinder or vive-versa) and the patient is asked to tell any change in the visual acuity.

• If the patient notices no difference then the correcting cylinder in the trial frame is correct.

• If visual improvement is attained by one or the other the correcting cylinder is turned slightly in the direction of the axis of the cylinder of the same denomination in the cross cylinder.

• It is repeated until the position of the trial cylinder is found at which rotation of the cross cylinder gives no alteration in distinctness.

Refinement of the power of the cylinder• The cross-cylinder is placed in

the same direction as the axis of the cylinder in the trial frame and then perpendicular to it.

• If the visual acuity is unimproved in either position the power of cylinder in the trial frame is correct.

• If visual acuity is improved in any of the positions then change should be made.

Astigmatic fan test(Maddox V test)• The patient is asked to look

at the figure and if any of the line is more distinct than the other then astigmatism is present.

• Rotating the V slightly to the direction of the blacker limb an intermediate position is reached when the two limbs of V are equally distinct. This gives the direction at right angles to the exact axis of the correcting cylinder.

Management

Optical correction • Spectacles or contact

lenses– Appropriate cylindrical

lens– Toric soft contact lenses

or toric rigid gas permeable contact lenses

Guidelines for optical correction

• Small astigmatism(0.5D or less) should be treated only if there is visual deterioration or asthenopic symptoms.

• High astigmatism should be fully corrected• Change in the axis of the lenses in patients used to the

previous axis should be done cautiously• New astigmatism correction in adults is not tolerated. In

such cases it is better to undercorrect and give full correction gradually.

• Bi-oblique astigmatism, mixed and high astigmatism are better treated by contact lenses

• Spherical component should also be corrected.

ManagementRefractive SurgeryIncisional correction of Astigmatism• Transverse keratotomy• Arcuate keratotomy incisions

are placed in the cornea at 7mm optical zone) to the steepest corneal meridian.

• Can correct upto 4-6D of astigmatism.

Limbal relaxing incisions• Incision is given at the

limbus• Correct -1 to -2D of

astigmatism.

Conductive keratoplasty• CK is a nonablative, collagen-shrinking

procedure based on the delivery of radiofrequency energy through a fine conducting tip that is inserted into the peripheral corneal stroma

• As the current flows through the tissue surrounding the tip, resistance to the current creates localized heat. Collagen lamellae in the area surrounding the tip shrink in a controlled fashion and form a column of denatured collagen. The shortening

of the collagen fibrils creates a band of tightening that increases the curvature of the central cornea.• Corrects <0.75D of astigmatism

Laser based corneal refractive procedure

• Photo refractive keratotomy(uses cylindrical photoablation pattern.)—can correct upto 3D of astigmatism

• Astigmatic LASIK can correct upto 5D

• Wavefront-guided and wavefront-optimised LASIK can correct– -10 to -12D of Myopic astigmatism– +6D of hyperopic astigmatism and– +5D of mixed astigmatism

Intraocular lens for correcting astigmatism

• Angle supported anterior chamber phakic IOL• Toric intraocular implant

Treatment of irregular astigmatism

• Spectacles though may not provide full correction some amount of correction can be attempted

• Contact lenses• Surgical correction by keratoplasty

PRESBYOPIA

Definition (Latin; old man’s eye/aged eye)

Presbyopia refers to the slow, normal, naturally occurring, age-related, irreversible reduction in maximal accommodative amplitude (i.e., recession of the near point) sufficient to cause symptoms of blur and ocular discomfort or asthenopia at the customary near working distance.

• first reported clinically between 40 and 45 years of age, with its peak onset between 42 and 44 years

• Its onset may occur any time from 38 to 48 years of age, depending on a variety of factors.

• From approximately age 52 years on, the prevalence of presbyopia is considered to be essentially 100%

Risk factors for development of presbyopia

Refractive error• Hyperopes have their near point considerably

further away than emmetropes(exhibit apparent relatively reduced accommodative amplitudes) and thus effectively become presbyopic a few years earlier than either myopes or emmetropes.

• In the myopes develop presbyopia later in life and if he has the error of -4D, presbyopia will never develop

Ambient Temperature• With the eyeball being peripheral to the body

core, it may exhibit considerable surface temperature variations because of the influence of ambient temperature.

• There is an inverse relation between ambient temperature and age of onset of presbyopia

Factors contributing to the age related decrease in accomodation

• Decrease in modulus of elasticity of lens capsule—cannot effectively mold the underlying lens substance

• Increase in stiffness of the lens substance—energy required to deform the lens substance increases

• Increase in size/volume of lens—decreases the effectivity of lens capsule function

• Anterior shift of the equatorial zonular fibres due to inrease in the size of the lens

• Decrease in equatorial zonular fibres• Increase in the number of disulfide bond in the

lens substance—stabilises the collagen molecules within the capsule and the lens by the process of cross-linking—lens substance becomes rigid and capsule becomes less elastic

• Mechanical and anatomical changes in the ciliary muscle

• Decrease in choroidal elasticity

Amplitude of accomodation and age

• The amplitude of accommodation represents the maximal accommodative level, or closest near focusing response, that can be produced with maximal voluntary effort in the fully corrected eye.

• It is calculated as Accomodative amplitude(A)= diopteric power needed to see clearly at near point-diopteric power needed to see clearly at far point

At the age of 10 yrs Near point=7cm Far point=∞Hence, A= 100/7-1/∞ =14-0 =14D

At the age of 40 yrs Near point=25cm Far point=∞Hence, A=100/25-1/∞ =4-0 =4D

Increasing near point of accomodation with age

Amplitude of accomodation with age

Variation of amplitude of accomodation with age

Symptoms

• Reduced vision at customary near-work distance.• Drowsiness after short period of reading or near work• Reading materials must be held farther away• Asthenopia related to attempts at excessive accommodative

effort is reported. It may even lead to an accommodative spasm and pseudomyopia.

• Transient diplopia and variable esophoria may be experienced as a result of the increased accommodative response/effort and the consequent synkinetically overdriven accommodative convergence that may be difficult to control consistently using compensatory negative fusional vergence.":"

Near Acuity Tests

• Reading distance at 16 inches(40 cm)

Treatment • The treatment of presbyopia is to

provide the patient with convex lenses so that his accomodation is reinforced and his near point brought within a useful working distance.

• To do this– know the working distance of the

individual– estimate his refraction– determine the amplitude of

accomodation– supplement this by appropriate

strength of lens allowing him sufficient reserve of accomodation

• One-third of the amplitude of accomodation should be kept for reserve if the patient is to work comfortably

• The near correction should never be overcorrected. It should be such that the patient should be able to read the near vision chart satisfactory not only at his reading distance but also some 12-15cm further away.

Example

• Emmetropic patient, whose working distance is 25cm has his near point receded to 50cm

• His amplitude of accomodation is 2D• With 1/3rd kept for reserve, remaining

amplitude is 2/3rd i.e. 1.3D• To see at the distance of 25cm, he will require

amplitude of 4D• Hence the prebyopic lens he requires is 4-1.3=2.7D

Treatment

Optical correction• Spectacles– Single vision reading glasses– Multifocal lenses containing near Add• Bifocal lenses• Trifocal lenses• Progressive addition lenses

• Contact lenses– Single vision contact lenses with glasses– Monovision contact lenses– Bifocal and multifocal contact lenses– Modified monovision contact lenses

Correcting Presbyopia: Contact Lens Monovision

Dominant eye:mainly correctedfor distance

Non-dominant eye:mainly corrected fornear

59-67%Patients Tolerate

Brain merges two imagesto see near and far withoutglasses

Refractive surgery for presbyopia

Non-Accomodative treatment of presbyopia• Monovision• Conductive keratoplasty• Multifocal IOL implants• Custom or Multifocal Ablations• Corneal Inlays

Refractive surgery for presbyopia

Accomodative treatment of Presbyopia• Scleral surgery• Accomodating IOLs

Monovision by refractive surgeryIn this mild myopia is created in the near eye in the presbyopic or peripresbyopic population.Also called modified monovision.The best candidates for modified monovision are myopic patients over the age of 40

Conductive keratoplasty

• conductive keratoplasty (CK) is a nonablative, collagen-shrinking procedure approved for the correction of low levels of hyperopia(+0.75 to +3.25 D). In CK, radiofrequency (RF) energy is delivered through a fine conducting tip inserted into the peripheral corneal stroma in a circular pattern. The application of RF energy shrinks the collagen in the periphery, which steepens the central cornea and induces a myopic shift.

• Treatment of presbyopia inn hyperopic and emmetropic individuals

Multifocal IOLs

• They are good options for patients undergoing cataract surgery.

ReZoom Multifocal IOL showing 5 concentric refractive zones

ReSTOR multifocal IOL with apodized diffractive changes in lens surface

Custom or multifocal ablations• Uses eximer laser to create a

multifocal cornea.

Corneal Inlays• Placing a biocompatible polymer

lens in the central cornea either beneath a LASIK flap or via a stromal tunnel.

• Inlays create near vision through different methods like change in corneal curvature, multifocality, pin-hole effect.

Accomodative treatment of presbyopia

Scleral surgery• Increase zonular tension

by weakening or altering the sclera over the ciliary body to allow for its passive expansion

• Consists of– Anterior ciliary

sclerotomy – Placement of scleral

expansion bands

Accomodating IOLs

• During ciliary muscle contraction, forward displacement of IOL led to the increase in effective IOL power and increase in near vision

Crystalens has flexible hinge in the haptic at the proximal end and a polyamide footplate at the distal end

APHAKIA

Definition :• A=without , phakia = lens• Absence of crystalline lens from the patellar

fossa

CAUSES

• Congenital absence of crystalline lens• Surgical aphakia after cataract extraction• Post-traumatic absorption of lens• Traumatic extrusion of lens• Posterior dislocation of lens

Optics of an aphakic eye• High hypermetropia• Total power of the eye reduced to +44 D• Aphakic eye consists of a curved surface, ie.

cornea , separating two media of different refractive indices, air (1) and aqueous and vitreous humor (1.33)

• Anterior focal point becomes 23.2 mm in front of the anterior surface of the cornea

• Posterior focal point lies 31 mm behind the anterior surface of the cornea

• Total loss of accomodation

CLINICAL FEATURES

• Symptoms: defective vision for near and far• Signs:- Limbal scar may be seen in case of surgical aphakia- Deep anterior chamber- Jet black pupil- Iridodonesis- Purkinje image test: 3rd and 4th images are absent- Fundus examination: small hypermetropic disc

TREATMENT

Optical treatment: • Spectacles: - Standard aphakic glass ~+12.5 D Advantages: safe, easy and inexpensive

method of correcting aphakia

Difficulties encountered in correcting aphakia with the use of spectacles:

- Image magnification of ~30% (cannot be used to correct unilateral aphakia as it will result in diplopia)

- Distance misjudgement leading to accidents

- Spherical aberrations resulting in “pin-cushion” effect

Jack-in-the-box phenomenon• Prismatic deviation occurring

at the periphery of a strong lens gives rise to a ring of blindness around the central field.( about 15˚extending from 50˚-65˚ from central fixation)

• When the eyes move, the circle of blindness also moves(in opposite direction) so that a person or and object appear and disappear like Jack in the box.

• Restricted field vision• Coloured vision due to

chromatic abberation• Cosmetic blemish• Cumbersome to use• Problem of near vision

Contact lenses

Advantages: - Less image magnification

(6-8%)- Elimination of aberrations

and prismatic effects of thick glasses

- Better field of vision- Cosmetically more

acceptable- Suitable for unilateral

aphakia

• Disadvantages of contact lenses:- Costly- Cumbersome to use esp in children and

elderly- Associated corneal complications

IOL implantation

• Best available method of correcting aphakia

• The commonest modality being employed nowadays.

Refractive surgery for aphakia

• Keratophakia: plus power lens is placed intrastromally to increase the curvature of anterior cornea.– Homoplastic(prepared from donor cornea)– Alloplastic( prepared from synthetic material)

• Epikeratophakia: a lenticule prepared from the donor cornea is sutured to the surface of patient’s cornea after removing the epithelium

References

• Duke-Elder’s Practice of Refraction(10th edition)• AK Khurana Theory and Practice of Optics and

Refraction• American Academy of Ophthalmology, Basic and

Clinical Science Course,Clinical Optics(2011-2012)• American Academy of Ophthalmology, Basic and

Clinical Science Course, Refractive Surgery (2012-2013)

• Borish’s Clinical Refraction• Ophthalmology -Myron Yanoff & Jay S.Duker

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