Refractive Errors & methods of correction M. F. Al Fayez, MD, FRCS

2 Emmetropia Distance Vision: Parallel (distant images) light is refracted by the Cornea & the Lens Light is focused on the Fovea & images are clear Near Vision: Divergent (near images) light rays focus behind the retina The lens changes shape (more convex) to focus near images on the retina (accommodation)

3 Presbyopia (Decreasing Accommodation) The lens can not accommodate enough to focus near images The aging eye starts to lose its ability to accommodate Bifocals or reading glasses are required

4 Myopia (Nearsightedness) Parallel rays (distant images) are focused in front of the fovea The eye is too Long &/or Cornea is too Steep for it’s refractive capability Correcting Myopia Contact Lenses or Glasses A concave lens diverges parallel light rays The focal point moves back & distant images are clear

5 Myopia Nearsightedness Types: Axial Refractive Index Cassification: Simple (Mild, Moderate & High) Pathologic (>8D)

6 Hyperopia Distance Vision: Young farsighted people can use accommodation to focus distant objects They will eventually need distance & reading glasses as their accommodative potential decreases with age High Hyperopes: Full accommodation does not have enough power to focus distant images on the retina A Convex lens is required to converge light rays on the fovea Near Vision Hyperopes usually need glasses to read because they have used all their accommodative potential to correct their distance vision

7 Astigmatism The Cornea is Steep in one axis & Flat in the other Multiple focal points in the eye Images are blurred &/or distorted Correction: Sphero-cylinderical Lenses

- Surgical - Traumatic Aphakia

Methods of Correction 1. Spectacles 2. Contact Lenses 3. Corneal Refractive Surgery 4. Intra-ocular Lenses

1. Glasses (Spectacles) Advantages:. Safe. Effective. Can be changed. Can include UV protection. Relatively inexpensive. In some cases, only correction available Disadvantages:. Uncomfortable. May not offer best correction. May be inconvenient

2. Contact Lenses Advantages:. Cosmetically pleasing. Convenient. Safe. Effective Disadvantages:. Require Care. Possible complications (infection). Expensive. Nuisance factor

3. Corneal Refractive surgery. Principle.. Incisional: Radial Keratotomy Astigmatic Keratotomy. Laser: Photorefrative Keratectomy (PRK) Laser Assisted In-situ Keratomileusis (LASIK) Laser Sub-Epithelial Keratomileusis (LASEK)