Pathophysiology - Refractive Errors

REFRACTIVE ERRORS

Emmetropia (normal vision) – means that light rays are bent to focus on the retina

In refractive errors, vision is impaired because the light rays are not sharply focused on the retina

Refractive Errors include

1. Myopia – occurs in people with elongated eyeballs, because of the excessive length of the eye, light rays focus in the vitreous body before they reach the retina
2. Hyperopia – results when the eyeball is shorter than normal, causing the light rays to focus at a theoretical point behind the retina
3. Astigmatism – results from unequal curvatures in the shape of the cornea
4. Presbyopia – occurs because of degenerative changes. Caused by the gradual loss of elasticity of the lens, which leads to decreased ability to accommodate, or focus, for near vision. The loss of accommodation progresses gradually.

Diagnostic Evaluations

• Visual acuity – snellen chart
• Retinoscopy – vision of myopes improves when concave trial lenses correct the focusing power of the eyes
• Hypropes experience improvement when convex lenses are used

Treatment

• Eyeglasses or contact lenses
• Myopia – concave lenses
• Hyperopia – convex lenses
• Astigmatism – cylindrical lenses
• Prebyopia – prescription lenses

Surgical Management

• Incisional radial keratotomy (RK) – under local anesthesia, the cornea is reshaped by making incisions
• Myopia – the cornea is made flatter
• Hyperopia – more coned shape
• Complications- infection, increased glare from microscarring of the cornea
• Laser assisted situ keratomileusis (LASIK) – a corneal flap is created to expose the inner cornea
• Photo refractive keratectomy (PRK) – removal of the epithelial layer (top surface) of the cornea exposing the inner cornea curvature of the eye resulted
• Complications – dry eye syndrome, epithelial abrasion, infection