A B C D E F G H I J K L M N O P R S T U V W

REFRACTIVE ERROR

What is a Refractive Error ?

Refraction is the bending of light rays to a point focus. Light is refracted by the eye to achieve focus by the curvature of the front of the eye (the cornea) and by an adjustable focal system; the crystalline lens of the eye. There are basically three types of refractive error: myopia (shortsight), hyperopia (long sighted) and astigmatism. In Myopia, the individual is able to see objects clearly close to but not in the distance. In Hyperopia, the individual can see well in the far distance but may have difficulty seeing near images clearly. Younger individuals with long sightedness can focus with the eye to see closely but this may result in a sense of strain and headache. In Astigmatism the image is not clearly focused as the eye bends the light in a way that is not uniform. The rays are bent more sharply on one plain compared to another plain. Astigmatism can be regular or irregular.

Amblyopia commonly occurs when one eye has normal focus and the fellow eye has a significant error, usually hyperopia or astigmatism. The dominant eye develops good vision and the second eye remains poor. If this is identified early generally by the age of 18 months correcting the error with glasses and then of course of patching (occlusion therapy) for the good eye may increase and maintain vision in the weaker eye. Regular monitoring is necessary until the child is eight years old. The visual system is then mature. Eye exercises or training the eye to see is not appropriate for refractive error in childhood. If there is a significant refractive error spectacle correction must be worn.

Presbyopia. Presbyopia is not really a refractive error but an inability to focus clearly on near objects. This occurs in middle age usually at about the age of 45. It is due to hardening of the crystalline lens of the eye. This means that the lens is unable to deform its shape to become "fatter" to focus on near objects. The lens becomes progressively more rigid with increasing age until the lens is essentially fixed focus by the age of about 70 years. Reading glasses correct the error. Exercises are not appropriate.

Why does Refractive Error occur ?

Myopia is generally when the eye is longer (bigger than normal). This is often inherited and certain racial groups have a higher incidence of myopia. Myopia developing in middle and later life is usually due to development of cataract which makes the lens more powerful at bending light. Hyperopia is inherited but is less common than myopia. People with a high degree of hyperopia have an eye that is smaller than normal. In later life they may have an increased risk of developing acute glaucoma. In childhood, hyperopia may be associated with a convergent squint (one or other eye turning inwards), particularly when looking at close up objects.

Astigmatism is usually caused by the shape of the front of the cornea. In typical irregular astigmatism which is also usually inherited, the eye rather than being spherical in shape (like a football), is shaped more like a rugby football. Certain corneal diseases cause the eye surface to become irregular causing astigmatism. Kerataconus (conical cornea) is one of these conditions. Corneal infection including herpes simplex and severe allergic eye disease can result in astigmatism.

Treatment Involved for Refractive Error

Correction of the refractive error of whatever type can generally be achieved with spectacles. High degrees of error particularly myopia and astigmatism however do not allow good corrected vision with glasses. This is particularly true of irregular astigmatism which occurs in kerataconus. Contact lenses correct refractive error and are different types. Hard or gas permeable which float on the surface of the eye on the tear film. Soft lenses allow oxygen to transmit through them due to the relatively high water content. These are applied directly to the surface and mould to the shape of the eye. There are a number of different types of soft lens. They can be used on a disposable daily basis, monthly disposable, extended wear or standard daily lens (usually changed annually). Hard or gas permeable lens are much more successful for correcting astigmatism or low toxic soft lenses can be used for lower degrees of irregular astigmatism. Surgery for correcting refractive error has developed over recent years. Radial keratotomy involves making radial cuts in the corneal surface to flatten the curvature to correct myopia. This procedure has been largely superseded by Laser Refractive Surgery. There are basically two forms of laser correction.

Photo-refractive keratectomy (PRK) involve treating the surface of the cornea with an Excimer laser which results in flattening of the cornea to correct myopia. The treatment is often painful for up to 48 hours after the procedure and is limited to the treatment of lower degrees of myopia generally up to minus 5 diopter. Individuals who have had this treatment previously for higher degrees of myopia have often regressed to become myopic again.

LASIK (laser insitu keratomileusis) involves lifting a small flap of corneal tissue and lasering onto the deeper cornea beneath the flat. The flap is usually fashioned with a microlathe. This method is more predictable and usually does not cause much discomfort. Both types of laser treatment have been well documented and researched over recent years. However there is a risk with any form of treatment of this type and the individual must be fully informed of the potential risk to eyesight.

After Treatment for Refractive Error

Spectacles. Generally there are no real problems with spectacles, however occasionally the positioning of the lens over the centre of the eye (centration) may be incorrect. This results in symptoms of discomfort, headache and often nausea. The Optometrist will correct this if identified. It is important to return to the Optometrist if the glasses feel unsatisfactory and insist that they are checked.

Contact lens. The most important consideration in contact lens wear is hygiene. Contact lenses must be sterilised according to instructions and not handled without thoroughly washing ones hands. Corneal infection can be very serious resulting in loss of vision if the routine of sterilisation is neglected. Extended wear lenses have a higher risk of corneal infection and for this reason many contact lens practitioners do not recommend their use. Allergy can develop to the solutions used to clean or sterilise the lens or indeed the material of the lens itself can cause allergy. This condition is called giant papillary conjunctivitis. The eye often becomes sticky and red and the lens uncomfortable to wear. Contact lenses must always be removed as soon as redness or discomfort occurs and not worn again until examined by a Contact Lens Practitioner or Eye Specialist. Corneal vascularisation is a condition when blood vessels grow into the edge of the cornea due to lack of oxygen. This occurs with over-wear of lenses or due to the lenses becoming clogged with protein due to a poor care regime.

If Refractive Error is Left Untreated

The developing visual system requires a clearly focussed image to develop properly.