Refractive Errors

Overview

This refers to impaired vision due to an abnormal focal point of the eye. The cornea performs most of the refraction in the eye. Ideally, the corneal curvature of an eye must match the eye’s size. A mismatch leads to a refractive error. Refractive errors can often be improved with glasses or contact lenses.

All refractive errors in childhood are associated with an increased risk of amblyopia and should therefore be treated as soon as possible

Myopia (Nearsightedness)

The focal point (focused image) is in front of the retina in myopia. The eye is either too long or the cornea has too much curvature. This makes it difficult for patients to focus on far objects.

• Causes Refractive myopia Image is projected in front of the retina because the refractive system is too convex (converging) Axial myopia Image is projected in front of the retina because the A-P axis oF the eye is too long (and the projection point is in front of the retina) Acute myopia Occurs when there is increasing convexity of the lens e.g. Hyperglycemia in diabetes causes the lens to swell
• Signs and symptoms
  • Blurry vision (far worse than near)
  • Eye strain
  • Headache
  • Learning difficulties (due to difficulty reading the blackboard)
• Investigation
  • Optometric examination using a Snellen chart
• Treatment
  • Diverging (negative) lenses (glasses, contacts)
    • A negative lens diverges the light thus the cornea will correct the focal point and focus the image on the retina
  • Laser-assisted in situ keratomileusis (LASIK)

Hyperopia (Farsightedness)

The focal point (focused image) projects behind the retina in hyperopia. The eye is either too short or the cornea has too little curvature. The patient is therefore unable to focus on near objects.

• Causes Refractive hyperopia The image projects behind the retina because the refractive system is too concave (diverging) Axial hyperopia The image projects behind the retina because the A-P diameter of the eye is too short (the projection point is behind the retina)
• Signs and symptoms
  • Blurry vision (near worse than far)
  • Eye strain (worse because the ciliary muscles are actively contracting when trying to accommodate near objects)
  • Learning difficulties (writing, reading)
• Investigations
  • Optometric exam
• Treatment
  • Converging (positive) lenses (glasses, contacts)
    • The positive lens acts as a magnifying glass which focuses the light before it reaches the cornea which will correct the problem
  • LASIK

Presbyopia

Presbyopia is a progressive decrease in the ability to focus on near objects. The lens stiffens with age leading to difficulty focusing on near objects.

• Causes
  • Senescent loss of elasticity of the lens
  • Senescent decrease in the strength of ciliary muscle
• Signs and symptoms
  • Eye strain
  • Difficulty seeing in dim light
  • Difficulty reading small print or seeing near objects acutely
  • Patient tends to move objects farther away to see clearly
• Investigation
  • Optometric exam
• Treatment
  • Converging lenses for reading
  • PresbyLASIK

Astigmatism

Astigmatism results from an irregular curvature of the refractive system resulting in different projection (focal) points in the vertical and horizontal meridians (axes). This causes a distorted image to be projected to the retina (objects appear blurry).

Regular astigmatism: The vertical and horizontal meridians are perpendicular

Irregular astigmatism: Vertical and horizontal meridians are not perpendicular

• Signs and symptoms
  • Blurry vision
  • Eye strain
  • Headache
  • Learning difficulties
  • Fatigue
• Investigations
  • Optometric examinaiton
• Treatment
  • Eye glasses
  • Contact lenses
  • LASIK