Optics 4 and 5
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Transcript of Optics 4 and 5
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Dr. Neetha K.I.R
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Accomodation
Presbyopia
Binocular optical defects
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The ability of the eye to increase its converging power while viewing a near object
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Radius of curvature :Anterior lens surface= 10mmPosterior lens surface =6mm
Contraction of ______ muscle Slackening of zonules Elasticity of the lens capsule
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Convergence Miosis
Synkinesis
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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
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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)
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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
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When near point recedes, amplitude of accommodation decreases
“age related inadequacy of accommodation is called presbyopia”
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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
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Difficulty in near vision: small print becomes indistinct
Accommodative asthenopia
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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
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Spectacles:Monofocals BifocalsTrifocalsProgressive lenses
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Contact lenses for presbyopia
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Surgical proceduresConductive keratoplasty: radiofrequency
energy is used to reshape the cornea of the non dominant eye
presbyLASIK
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Insufficiency Of AccommodationPremature sclerosis of the lensWeakness of ciliary muscleProdromal stage of glaucoma
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Paralysis of accommodationDrug induced Internal ophthalmoplegia: diphtheria,
syphilis, alcoholism, diabetes III rd nerve palsy: trauma, neoplasms,
inflammations
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Spasm of accommodationDrug inducedYoung patients; when the eyes are
subjected to too much of near work in unfavourable circumstances
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Condition in which the refractive status of the two eyes are unequal
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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
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Anisometropia of 2.5-4 D can be tolerated depending on patient’s sensitivity but > 4 D of anisometropia is not tolerated
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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
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Congenital / developmental: due to differential growth of the two eyeballs
Acquired anisometropia: may occur due to unilateral aphakia
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An anisometropia is never corrected by unequal contraction of the ciliary muscles
An optical correction is the only means of treatment
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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)
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Contact lenses:Higher degrees of anisometropiaVery useful in young children with high
anisometropia to prevent amblyopia
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IOL implantation: when unilateral aphakia
is the cause of anisometropia
Refractive corneal surgery:LASIK
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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
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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
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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
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Cortical aniseikonia: asymmetrical simultaneous perception in-spite of the equal sized images formed on the two retinae
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Symmetrical
Asymmetrical: with distortion
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AsymmetricalPincushion effectBarrel distortion
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Asthenopia Diplopia: when aniseikonia exceeds 5% Tendency to development of squint Difficulty in depth perception
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Contact lenses, IOL, refractive corneal surgery
Iseikonic/size lenses Retinal iseikonia: treat the cause Cortical aniseikonia: very difficult to
treat
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