What is it?
Refractive error is present when light from a distant object does not come to a clear focus on the retina.
Types of refractive error
There are four types of refractive error:
- Myopia or short-sightedness
- Hyperopia or long-sightedness
- Regular astigmatism
- Irregular astigmatism
Myopia is the medical term for near-sightedness. A myopic eye may be considered as having too steep a cornea or too long an axial length so that light from a distant object is brought to a focus not on, but in front of, the retina.
The medical term for far-sightedness. A hyperopic eye may be considered as having too flat a cornea or too short an axial length so that light entering the eye comes to a focus not on, but behind, the retina.
An eye with regular astigmatism has different optical power in two different planes. The simplest form to understand is where light from a vertical line comes to a focus in front of the retina whereas that from a horizontal line focuses behind the retina. The cause of astigmatism is usually an asymmetry of the corneal surface.
Irregular astigmatism, also known as “higher order aberration”, is non-symmetrical irregularity in the optical components of the eye, usually in the shape of the cornea.
How is it diagnosed?
Simple refractive error
Simple refractive errors such as myopia, hyperopia and regular astigmatism are diagnosed by optometrists and ophthalmologists using trial lenses.
Higher order optical aberrations
Irregular astigmatism is diagnosed using devices called aberrometers, such as the A-CAT and T-CAT scanners of the Allegretto Wavelight laser system.
Refractive error was traditionally treated by optometrists with glasses or contact lenses.
Many people now choose to have their refractive error treated surgically. There are three main types of surgery:
- Laser refractive surgery – Lasik and PRK
- Refractive lens exchange
- Phakic intraocular lenses
Laser refractive surgery
The excimer laser is used to reshape the cornea to reduce or eliminate refractive error. Dr McKellar performs laser refractive surgery at LaserVision eye centre.
Refractive lens exchange
Some refractive errors are best treated by what is essentially cataract surgery in patients with little or no cataract. The natural lens of the eye is replaced with an intraocular lens of an appropriate power in order to change the overall focus of the eye.
Ideal candidates for refractive lens exchange include:
- Patients with eye conditions or refractive errors that cannot be safely treated by laser
- Older patients who are likely to develop cataract in the near future
Phakic intraocular lenses
Phakic intraocular lenses (IOLs) are also known as implantable contact lenses. They are small lenses placed within the eye, either in front of or behind the pupil. Phakic IOLs can correct myopia, hyperopia and astigmatism.
Dr McKellar was the first eye specialist in New Zealand to implant both the Verisyse® and Cachet® phakic intraocular lenses. The surgical procedure takes about half an hour and is performed under local anaesthesia.
The advantages of phakic IOLs are:
- Large refractive errors not amenable to laser surgery can be treated
- The procedure is reversible
Disadvantages of phakic IOLs include:
- Greater cost than laser surgery
- Risks of intraocular surgery
- Glare and halos
- Potential, though rare, complications such as lens dislocation, cataract, iritis and glaucoma
FAQs and Links
What is the best way to correct refractive error?
The answer depends on many factors including the type and degree of your refractive error, your age and the overall health of your eye. Dr McKellar can advise after talking with you and examining your eyes.
I have been told my eyes are not suitable for laser surgery. What are my options?
Many people who are not able to have laser eye surgery can still have surgery to correct their refractive error. The most common solutions are a phakic IOL and a clear lens extraction.