Eye Defects and Correction: Refractive Errors

Normal Refraction

• The total refractive power of a normal eye (emmetropic eye) is 60 Diopters (D), divided between the cornea (43D) and the lens (17D).
• An emmetropic eye is one in which parallel rays of light tend to focus directly on the retina when the eye is at rest.

Accommodation

• Accommodation is the contraction of the ciliary muscle to increase the curvature of the lens, increasing its refractive power to visualize objects closer than 6 meters.
• Amplitude of accommodation is the difference in the converging power of the eye between maximum accommodation and unaccommodated eye, which depends on the strength of contraction of the ciliary muscles and elasticity of the lens capsule.

Refractive Errors (Ametropia)

• There are three types of refractive errors: Hypermetropia, Myopia, and Astigmatism.

Myopia (Short-sightedness)

• Myopia is a refractive error where parallel rays of light come to a focus in front of the retina when the eye is at rest.
• Causes of myopia: increase in converging power of the eye or increase in AP diameter (> 24 mm).
• Symptoms: blurred distant vision, headache, fatigue, and nausea.
• Treatment: concave lenses, contact lenses, photorefractive surgery, and Implantable Collamer Lens (ICL).

Hypermetropia (Far-sightedness)

• Hypermetropia is a refractive error where parallel rays of light are brought to a focus behind the retina when the eye is at rest.
• Causes of hypermetropia: decrease in converging power of the eye or decrease in AP diameter (< 24 mm).
• Symptoms: blurred vision, eye strain, headache, fatigue, and nausea.
• Treatment: convex lenses, contact lenses, laser photorefractive surgery, and Implantable Collamer Lens (ICL).

Astigmatism

• Astigmatism is a refractive error where a point of focus of light cannot be formed upon the retina, resulting in two or more focal lines.
• Causes of astigmatism: unequal curvature of the cornea or lens, decentration of the lens, and index astigmatism.
• Treatment: cylindrical lenses, contact lenses, photorefractive laser surgery, toric Implantable Collamer Lens (ICL), and keratoplasty.

Presbyopia

• Presbyopia is a form of far-sightedness that makes it harder for people to read as they age due to loss of elasticity of the lens.
• Corrected by glasses with converging lenses.

LASIK Eye Surgery

• LASIK eye surgery is used to correct vision problems such as Myopia, Hyperopia, and Astigmatism.
• The procedure involves reshaping the cornea using an excimer laser to focus light directly on the retina.