aphakia and pseudophakia
TRANSCRIPT
APHAKIA AND PSEUDOPHAKIA
PRESENTER- DR. SHWETA
MODERATOR- DR. ANUBHA MITTAL
DEFINITION
Absence of crystalline lens.
Lens is absent from the pupillary line and does not take part in refraction.
CAUSES
Congenital absence of lens. Surgical aphakia. Aphakia due to absorption of lens matter. Traumatic extrusion of lens. Posterior dislocation of lens.
OPTICS OF APHAKIA
CHANGES IN CARDINAL DATA OF EYE Eye becomes highly hypermetropic. Total power of eye +44 D. Anterior focal point becomes 23.2 mm in front of cornea. The posterior focal point is about 31mm behind the cornea. The two principal points are almost at the anterior surface of cornea. Nodal points are located about 7.75 mm behind the anterior surface of
cornea.
ACCOMODATION IN APHAKIA
Total loss of accommodation. Glasses for near and distance are required.
BINOCULAR VISION Aniseikonia of 5% is compatible with binocular vision In aphakia aneisokonia is detrimental to development of normal
binocular vision
CLINICAL FEATURES Defective vision for near and far.
SIGNS Limbal scar Anterior chamber is deeper Iridodonesis Pupil – jet black Purkinje’s image test- 2 images Fundus examination- hypermetropic small disc Retinoscopy- high hypermetropia
TREATMENT By convex lens.
MODALITIES FOR CORRECTING APHAKIA Spectacles Contact lens IOL Refractive corneal surgery
BY SPECTACLES Most common +10 D is used Cylindrical lens required in surgically induced aphakia Additionally +3 to +4 D for near vision
ADVANTAGES Cheap Easy and safe method
DISADVANTAGES[1] IMAGE MAGNIFICATION Image size depends on axial length and keratometry reading
1D of convex power leads to about 3 % magnification of image, thus 10 D = 30%
Difference of image size between the two eyes of about 7 % is tolerable
beside that give rise to diplopia i.e., two images of one object are seen one small (from normal eye) and other larger (from aphakic eye).
Objects appear larger they appear falsely closer than reality, and this leads to physical in-coordination.
Not useful in unilateral aphakics
[2] ROVING RING SCOTOMA
Edge of a convex lens acts as a prism
Higher the power of the convex lens the greater is the prism angle (alpha). Light falling on the prism bends towards its base by an angle alpha/2 .
Aphakic spectacles, the angle alpha being large, the light falling at the edge of the lens bends towards the center of the lens (base of prism)
And does not reach the pupil and is, therefore, not seen.
Resulting in an area of the visual field which is not visible to the patient, or scotoma.
And because the edge of the lens is present all around the lens like a ring, so it gives rise to a ring shaped scotoma.
Position of this scotoma is not fixed in the visual field because the eye keeps moving (or roving) in relation to the aphakic spectacle.
JACK-IN-THE-BOX-PHENOMENON
[3] PIN CUSHION AFFECT
Magnification of image is more at the periphery of the lens due to prismEffect, objects appear stretched out (large, nearer, elongated in radial direction) at the corners like a pin-cushion.
Moving objects appear to be faster
Straight lines become curves
4) RESTRICTED VISUAL FIELD Both monocular and binocular vision is restricted 50° all around
5) COLOUR VISION PT may complain of colored hue Due to absence of natural filter of crystalline lens
6) COSMETIC BLEMISH Eyes appear larger Seen more in young aphakics
7)THICK AND HEAVY GLASSES
CONTACT LENS
ADVANTAGES
Less magnification of image No chromatic aberrations No prismatic affect Wider field of vision Cosmetically acceptable Better for uniocular aphakics
DISADVANTAGES
More cost Cumbersome to wear-both for young and old age Corneal complications may be associated
INTRAOCULAR LENS IMPLANTATION
Secondary IOLs- PCIOL IRIS CLAW LENS
SFIOL
REFRACTIVE CORNEAL SURGERY[1] KERATOPHAKIA
A lenticule prepared from donor cornea is placed between the lamellae of the patient’s cornea
[2] EPIKERATOPHAKIA
A lenticule prepared from donor cornea is stitched over the surface of patient’s cornea after removing the epithelium
PSEUDOPHAKIA Condition of aphakia when corrected with implantation of IOL is called
as pseudophakia.
Also called Artiphakia
CALCULATION OF IOL POWER
Most common method is SRK I (Sanders Retzlaff and Kraff) by regression formula
P = A-0.9k-2.5L
P = power of IOLA = constantL = axial length of eyeball
SRK IIBased on axial length, A constant is modified as
If L is <20 mm - A +3
20-20.99 mm - A+2
21-21.99 mm - A+1
22-24.50 mm - A
>24.50 mm - A-1.5
MODIFIED SRK-IIA constant is modified as-
If L is <20 mm - A +1.5
20-21 mm - A +1.0
21-22 mm – A + 0.5
22-24.5 mm- A
24.5-26 mm – A -1.0
>26 mm - A -1.5
SRK/T – Retinal thickness is also measured.
more accurate for long eyes i.e. >26mm.
Holladay- used in eye with AL 24.5 – 26mm.
Hoffer q – best for short eyes i.e AL <19mm.
REFRACTIVE STATUS OF PSEUDOPHAKIC EYE Emmetropia Consecutive myopia Consecutive hypermetropia Astigmatism
EMMETROPIA It is produced when the power of the IOL
implanted is exact. It is the most ideal situation. Such patients require plus glasses for near vision
CONSECUTIVE MYOPIA
occurs when IOL implanted overcorrects the refraction of eye.
Such patients require glasses to correct myopia for distance vision.
May or may not require glasses for near vision.
CONSECUTIVE HYPERMETROPIA
occurs when underpower IOL is implanted
such patients require plus glasses for distance vision and additional +2 to +3 D for near vision.
ASTIGMATISM
Varying degree of surgery- induced astigmatism is also present
SIGNS OF PSEUDOPHAKIA Surgical limbal / corneal scar Anterior chamber is deep Mild iridodonesis Purkinje image test – 4 images Pupil is blackish with shining reflex
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