Dr. Neetha K.I.R

Accomodation

Presbyopia

Binocular optical defects

The ability of the eye to increase its converging power while viewing a near object

Radius of curvature :Anterior lens surface= 10mmPosterior lens surface =6mm

Contraction of ______ muscle Slackening of zonules Elasticity of the lens capsule

Convergence Miosis

Synkinesis

Far point or punctum remotum: the farthest point at which an object can be seen clearlyEmmetropiaHypermetropia Myopia

Near point or punctum proximum: the nearest point at which an object can be seen clearly when maximum accommodation is used

Range of accommodation:The distance between the far point and the

near point

Amplitude of accommodation:The difference in the refractivity of the eye at

rest and the fully accommodated eyeA = P-R P = refractive power of eye in fully

accommodated state (ie, reciprocal of near point in metres)

R = refractive power of the eye in unaccommodated state (ie, reciprocal of far point in metres)

As age advances… Lens becomes harder Ciliary muscle becomes weaker

It becomes more and more difficult to see near objects clearly

The near point recedes gradually

When near point recedes, amplitude of accommodation decreases

“age related inadequacy of accommodation is called presbyopia”

Infancy : 14D of accommodation (7 cm) 45 yr : 4 D 60 yr : 1 D

For comfortable accommodation, 1/3rd of accommodation should be kept in reserve

Emmetrope will start feeling the symptoms at ~ 40 yr age

Hypermetrope: earlier symptoms

Difficulty in near vision: small print becomes indistinct

Accommodative asthenopia

Near vision addition Convex lens

40 years- 1.0 D 45 years- 1.5 D 50 years- 2.0 D 55 years- 2.5 D 60 years- 3.0 D

Better to undercorrect than overcorrect

Spectacles:Monofocals BifocalsTrifocalsProgressive lenses

Contact lenses for presbyopia

Surgical proceduresConductive keratoplasty: radiofrequency

energy is used to reshape the cornea of the non dominant eye

presbyLASIK

Insufficiency Of AccommodationPremature sclerosis of the lensWeakness of ciliary muscleProdromal stage of glaucoma

Paralysis of accommodationDrug induced Internal ophthalmoplegia: diphtheria,

syphilis, alcoholism, diabetes III rd nerve palsy: trauma, neoplasms,

inflammations

Spasm of accommodationDrug inducedYoung patients; when the eyes are

subjected to too much of near work in unfavourable circumstances

Condition in which the refractive status of the two eyes are unequal

A difference of 1 D between the two eyes results in 2% difference in the size of retinal images of two eyes

A difference of 5% between the size of images from two eyes can be tolerated, i.e., 2.5 D of anisometropia can be tolerated

Anisometropia of 2.5-4 D can be tolerated depending on patient’s sensitivity but > 4 D of anisometropia is not tolerated

Binocular vision: smaller degrees of defect.

Alternating vision: eg. When one eye is emmetropic and other myopic

Greater difference between the refractive state of two eyes results in uniocular vision. In children, this can result in amblyopia

Congenital / developmental: due to differential growth of the two eyeballs

Acquired anisometropia: may occur due to unilateral aphakia

An anisometropia is never corrected by unequal contraction of the ciliary muscles

An optical correction is the only means of treatment

Spectacles: maximum difference of 4 D between the

two eyes. Beyond that, there will be diplopia due to

change in the size of the retinal image

In such cases, compromise has to be achieved by under correcting the more ametropic eye (maximum possible correction that does not result in diplopia)

Contact lenses:Higher degrees of anisometropiaVery useful in young children with high

anisometropia to prevent amblyopia

IOL implantation: when unilateral aphakia

is the cause of anisometropia

Refractive corneal surgery:LASIK

Mono-vision can be used in selective few patients who have one eye hypermetropic or emmetropic and the other eye myopic

one eye (dominant eye) can be used for distant vision and the other for near vision

Defined as a condition wherein the images projected to the visual cortex from the two retinae are unequal in size or shape

Upto 5% of aniseikonia is tolerated

Optical: due to inherent or acquired anisometropia of varying degree

Retinal aniseikonia: due to displacement of retinal elements due to retinal thickening, stretching or edema

Cortical aniseikonia: asymmetrical simultaneous perception in-spite of the equal sized images formed on the two retinae

Symmetrical

Asymmetrical: with distortion

AsymmetricalPincushion effectBarrel distortion

Asthenopia Diplopia: when aniseikonia exceeds 5% Tendency to development of squint Difficulty in depth perception

Contact lenses, IOL, refractive corneal surgery

Iseikonic/size lenses Retinal iseikonia: treat the cause Cortical aniseikonia: very difficult to

treat