Overview
This lecture covers the anatomy and optical role of the lens, the causes and types of aphakia, optical consequences, complications, and disadvantages of correcting aphakia with spectacles.
Anatomy and Function of the Lens
- The lens is a transparent, biconvex structure located between the iris and vitreous chamber.
- Provides about 35% of the eye's refractive power; the cornea provides 65%.
- The lens aids accommodation (focus adjustment) and absorbs harmful UV light below 350 nm.
Aphakia: Definition and Types
- Aphakia is the absence of the lens from the eye, either removed or dislocated.
- Types: Surgical aphakia (lens surgically removed), traumatic aphakia (lens lost due to injury), congenital aphakia (lens absent from birth), and optical aphakia (lens displaced from the pupillary area).
- Surgical aphakia may follow cataract surgery, especially in infants to prevent amblyopia.
- Congenital aphakia may be primary (failure of lens placode) or secondary (resorption after formation).
Optical Changes in Aphakia
- In aphakia, only the cornea refracts light (44 D); lens (16 D) is missing, causing hypermetropia (farsightedness).
- The loss of the lens increases the eye’s focal length and reduces total refractive power.
- Astigmatism may occur post-surgery due to corneal incisions or suturing, creating "against the rule" or "with the rule" astigmatism.
Clinical Features and Complications
- Symptoms: Defective vision (both near and far) due to high hypermetropia and loss of accommodation.
- Signs: Deep anterior chamber, iridodonesis (iris tremor), jet-black pupil, reduced Purkinje images (only 2 seen), limbal incision/scar, hypermetropic optic disc on fundus exam.
- Long-term risks: Retinal detachment, glaucoma, corneal decompensation, and vitreous degeneration.
Optical Correction of Aphakia
- Correction options: High-plus spectacles, contact lenses, intraocular lenses (IOLs), or corneal refractive surgery.
- IOLs and contact lenses are standard; spectacles are less favored due to drawbacks.
Problems with Spectacle Correction
- High-plus (+10 D) spectacles cause about 33% image magnification.
- Relative spectacle magnification is higher with glasses (1.36) than with contact lenses (1.1) or IOLs (~1).
- Aniseikonia (image size difference between two eyes) leads to diplopia.
- Spherical and chromatic aberrations distort perception and color fringes.
- Prismatic effects induce peripheral ring scotomas (roving ring scotoma) and “Jack in the Box” phenomenon.
- Glasses are heavy, uncomfortable, prone to slipping, and have cosmetic issues.
- Lenticular (fried egg) lenses reduce weight but also narrow the field of view.
- UV protection is necessary due to increased UV exposure.
Key Terms & Definitions
- Aphakia — absence of the natural lens from the eye.
- Iridodonesis — trembling of the iris due to lack of lens support.
- Aniseikonia — difference in image size between the eyes.
- Astigmatism — unequal corneal curvature causing blurred vision.
- Spectacle Magnification — ratio of corrected to uncorrected image size in one patient.
- Relative Spectacle Magnification (RSM) — ratio of corrected image size to normal image size.
- Ring Scotoma — a circular area of visual field loss caused by spectacle edge effects.
- Jack in the Box Phenomenon — objects suddenly enter/exit vision due to shifting scotoma.
Action Items / Next Steps
- Ensure proper refraction for each aphakic patient before prescribing glasses.
- Advise patients on the need for UV-protective lenses.
- Prepare for next session on contact lenses and IOL correction for aphakia.