Ophthalmology: Correction of refractive errors

Basic optics and refraction

Refractive errors are the commonest ocular problem worldwide. In order to understand the aims of cataract surgery, refractive surgery and strabismus surgery, you should understand the basic refractive errors. With the increasing ageing population, presbyopia and its correction by glasses or refractive surgery are becoming bigger issues that need
addressing.

Aims

• Refractive errors:
  ○ What is longsightedness?
  ○ What is shortsightedness?
  ○ What is astigmatism?

• Accommodative changes with age.
  This chapter summarizes the basic optics of the eye and defines refraction.

Refraction by the eye

• The ability of the eye to bend light rays.
• Determined by the refractive media (cornea and lens) plus the axial length of the eye.
• Refraction is calculated in dioptres (D) (e.g. 1D is the power of a convergent lens to focus parallel light at its focal point (f) 1 m behind the lens).
• The total refractive power of an emmetropic eye (normal length) is approximately 58D, of which 43D is contributed by the cornea and 15D by the lens, the aqueous and the vitreous.

Refraction techniques

These are techniques for testing the refraction of the eye.

Subjective refraction

The patient distinguishes between the effects of various lenses on the visibility of letters on the Snellen and LogMar charts.

Objective refraction

This includes examination with the ophthalmoscope, the retinoscope or various types of autorefractors.

Retinoscopy

• This technique is particularly useful in testing children under 7 years for glasses.
• In children under 7 years old, retinoscopy must be done with cycloplegia (drops inserted to temporarily paralyse the ciliary body and inhibit accommodation) in order to obtain an accurate refraction.
• A retinoscope is an instrument used to assess the objective refraction of the eye. A bright streak of light is shone through the pupil and is seen as a red reflex reflected from the retina.
• The retinoscope streak is moved gently, and the direction of the light reflex from the retina is observed.
• By placing a series of plus or minus lenses in front of the patient’s eye, the observer can calculate whether the patient is short-sighted (myopic) or longsighted (hypermetropic) and measure the amount of astigmatism that needs correcting.

Accommodation

The ability of the eye to focus clearly on an object at any distance is due to the elasticity of the lens. The far point is the furthest distance away at which an object can be seen clearly. In order to see a near object clearly, the ciliary muscle must relax (a parasympathetic reaction), enabling the lens to become fatter and bend (refract) the light rays more, so that they are in focus on the retina. The nearest point that the eye can see clearly with maximum accommodation in force is called the near point. The distance between these two points is the range of accommodation.

Refractive errors

For correction of refractive errors,

• Hypermetropia: longsightedness. Patient can see clearly in the distance but not near.
  Optics: the focal point is behind the retina. The converging rays that fall on the retina produce a blurred image.
  Cause: the axial length is too short.
  Correction: convex (plus) glasses

• Myopia: short-sightedness. Patients can see clearly close up, but their distance vision is blurred.
  Optics: the focal point is in front of the retina. Divergent light rays falling on the retina produce a blurred image.
  Cause: most commonly, excessive axial length (axial myopia); rarely due to too great refractive power (e.g. cataract refractive myopia).
  Correction: concave (minus) glasses.

• Astigmatism: part of the image in one plane is out of focus due to unequal refraction.
  Optics: the parallel incoming rays deform and do not focus at a single point, causing a blurred retinal image.
  Cause: corneal curvature.
  Correction: cylinders (toric lenses), corneal surgery or laser surgery.

• Presbyopia: gradual loss of focusing power. The subject is usually over 45 years old and cannot see clearly to read near type. They progressively hold the type further and further away until their arms no longer reach far enough.
  Optics: there is a normal loss of accommodative range with increasing age, due to a decline of lens elasticity.
  Cause: stiffening of lens and weaking of the ciliary body muscle.
  Correction: the reading correction (plus sphere) is added to the distance correction.

Glasses, contact lenses and low-vision aids

Although many people are having corneal refractive surgery to correct their refractive error, glasses and contact lenses remain the first choice for the majority. Low-vision aids can help both young adults in employment with inherited macular disorders, and older people with age-related macular degeneration.

Aims

• Correction of refractive errors with glasses and contact lenses.
• Types of contact lenses.
• Use of low-vision aids.

Optical lenses

Spherical lens

This lens has an equal curvature in all meridians

• Concave (minus) lens:
  ○ Used to correct myopia.
  ○ Refracts light rays, making them more divergent.
  ○ Objects seen through a minus lens look smaller.

• Convex (plus) lens:
  ○ Used to correct hypermetropia, presbyopia and aphakia.
  ○ Refracts light rays to make them more convergent.
  ○ Objects seen through a plus lens look larger.

Toric (cylinder) lens

• Used to correct astigmatism.
• Shaped like a section through a rugby ball with one meridian more curved than the other (at right angles to each other).

Prisms

• A prism deviates light rays.
• Used to relieve diplopia by redirecting light onto the fovea.
• Fresnel prisms are temporary plastic prisms that are stuck onto the patient’s glasses to join diplopia (e.g. sixth cranial nerve palsy).

Contact lenses (CLs)

• CLs are superior in severe refractive errors and give better quality vision (e.g. correction of aphakia where an intraocular lens has not been placed is best corrected with a convex CL).
• Also used therapeutically in corneal disease as bandage CLs or as cosmetic lenses for the scarred cornea.

Indications for CLs

• Cosmetic (e.g. avoid glasses in low myopia).
• For sport (e.g. tennis and skiing for wider field).
• Severe refractive errors:
  ○ High myopia (e.g. >6D myopia): a patient with high myopia depends on contact lenses for visual acuity and a wider visual field.
  ○ Aphakic children without an intraocular lens post-congenital cataract surgery.
  ○ Irregular astigmatism (e.g. rigid contact lenses for corneal scarring and keratoconus). In the later stages, surgery (penetrating keratoplasty) may be required.

Types of CLs

Hard or rigid CLs

• Polymethylmethacrylate (PMMA).
• Poor oxygen transmission.

Soft CLs

• Hydroxymethylmethacrylate (HMMA).
• Better oxygen permeability but fragile.
• The most common type of contact lenses worn for simple refractive errors and as bandage contact lenses.

Disposable soft CLs

• Disposable lens are replaced daily, weekly or monthly.
• Disadvantage: higher infection rate (e.g. acanthamoeba).

Extended-wear soft CLs

• Risk of overwear syndrome.
• Complications more common.

Coloured or tinted CLs (soft or hard-rigid)

These are used for prosthetic purposes as hand-painted iris-coloured lenses:

• Cover corneal opacities, iris defects or cataracts in blind eyes.
• Prevent photophobia and improve vision in aniridia and albinism.

Rigid gas-permeable CLs

• Made from a mixture of hard and soft CL materials.
• Transmit oxygen much better than PMMA.
• Good for patients who are allergic to soft CLs

Scleral lenses

• A type of hard CL approximately 23 mm in diameter that bridges the corneoscleral junction.
• Used for prosthesis purposes and keratoconus

Low-vision aids

Important in the visual rehabilitation of patients with central visual field loss, especially macular degeneration

Text magnification

Optical magnification with magnifiers and telescopic glasses:

• Magnifiers increase the image at least 4× the normal size.
• Telescopic glasses increase the image size at least 8×, but cause constriction of the visual field.
• Closed-circuit television (CCTV) systems (Figure 9.3c) allow a magnification of 25× and are beneficial for patients with a high degree of vision loss. However, these are expensive!

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In patients with macular degeneration, fixation stability is often very poor and they have chaotic reading eye movements using a lot of searching with small saccades. This can be improved by encouraging non-foveal reading.

• Eccentric vision training.
• New methods of presenting text, such as electronic scrolled text (i.e. move text to them or serial presentation on a TV screen).
• High-tech magnification and image enhancement using a head-mounted CCTV with image processing of contrast enhancement (virtual reality).