MORE ABOUT EYES

Optical Corrections

Myopia This is often referred to as "short-sightedness" and is the condition where near objects are in focus and distant objects are blurred. The focal point of the eye is in front of the retina instead of directly on it. Thus the distance vision is blurry. The spectacle correction is with a "minus" powered lens e.g. -2.00DS. . A minus powered lens is thicker at the edge and thinner in the centre, it also tends to make objects seen through it as smaller.

Hyperopia Commonly referred to as "long-sightedness" and results in the inability to focus easily on objects close up. With this condition the image is focussed behind the retina instead of on it. For these people the effect varies depending on the strength of their prescription and age. The spectacle correction is with a "plus" powered lens e.g. +2.00DS. The lens is thicker in the centre and thinner at the edge, it also tends to make objects seen through it as bigger.

Presbyopia As people get older, the crystalline lens which is positioned behind the iris cannot flex as easily which inhibits the ability to focus at near. This is the reason why people with good vision when young require spectacles in middle or old age. The "near add" is the extra power needed to focus for near vision. As a result the near power in spectacles will make the distance vision blurry. The usual age of onset of this problem is 40 - 45 years, but eventually happens to everyone.

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Astigmatism This results when the cornea is not perfectly spherical, causing a portion of an object to be in focus and the remainder of the object to be out of focus. For these people the optical power required for clear vision has a different optical power in two different directions. It occurs in all age groups and affects both distance and near vision, and may occur with myopia or hyperopia.

Refraction - the refractive error is the optical correction for the eye to see it's clearest with no focussing involved (focus muscles relaxed). This may be plano (zero), or require positive or negative lenses, or a cylinder correction to obtain the clearest focus. This is measured with the patient viewing at 6m, or optical "distance".

Prescription - . This is taken from the refraction as the most comfortable lens correction for the patient. It may not equal the refraction, for example, if there is a significant difference between the lens strength of the 2 eyes, or if there is a large change from what the person has previously worn.
The spectacle prescription is taken from the refraction, whilst a contact lens prescription covers the fitting details of the lenses on the eye and is not arrived at until the contact lens performance on the eye has been assessed

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Anisometropia - when there is a significant difference in the refractive state between the 2 eyes resulting in different image sizes being formed on the retina, so that when the brain attempts to put the 2 images together to get 3D vision, it is not able to do so. The person is likely to feel uncomfortable, not be able to wear their spectacles, and may get double vision. An alternative is contact lenses, as when the optical correction is closer to the eye the retinal image difference in size is less.

Aphakia - when the lens inside the eye is absent. The lens of the eye has a strong optical power, so aphakics need to wear either strong spectacles (about +12D) or an equivalent contact lens. The lens of the eye is removed with cataract surgery, and usually an IOL (intra-ocular lens - see later) is inserted to provide the optical correction

Common Eye Diseases

Cataracts
As the eye ages the crystalline lens becomes less transparent. If it becomes opaque, then the vision becomes "foggy" with less detail seen (both distance and near). When cataract is the cause of reduced vision, new spectacles lenses will not restore clear vision. The treatment is surgery by an ophthalmologist; to whom we refer the person. Following surgery an IOL (intra-ocular lens) is usually implanted into the eye to replace the original lens. If a person has the beginnings of a cataract, "early cataract" then surgery may not be indicated and the optometrist is likely to review the condition regularly.

Age Related Macular Degeneration (ARM)
The macula is the most sensitive central part of the retina. It has a delicate blood supply. Changes in the blood supply to various parts of the body are common as age increases. When this happens to the retina, the macula is usually affected, called ARM. The vision changes are initially distortions to straight lines with a gradual loss of detail when viewed centrally. As the peripheral vision is not affected, a person with ARM may be visually handicapped, but rarely becomes totally blind.
For many ARM cases there is no treatment available. However any sudden changes to vision we must see urgently as it may be possible for this sudden change to be repaired by an ophthalmologist.

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Glaucoma
The eye is filled with fluid. The measurement of the pressure of this fluid is noted as IOP (Intra-Ocular Pressure). If the pressure of this fluid is too high for the eye, it will cause damage to the optic nerve head (entry/exit for the nerves of the retina). This gradual damage initially affects the peripheral vision and is not painful. If left untreated, glaucoma will damage all the nerves and lead to blindness. The damage is not reversible. The usual treatment for glaucoma is aimed at reducing the pressure by either drops, laser or surgery under the care of an ophthalmologist.
Another more rare type of glaucoma is "Acute Angle Closure Glaucoma" which has a sudden onset and is painful. Again - sudden changes in vision must be seen urgently.

Diabetes
Diabetes affects the blood circulation of the entire body. As the eye has a delicate and high proportion of the blood supply, it is prone to damage with diabetes. Diabetic people should have regular (usually annual) eye examinations, in Wellington most diabetic people have regular retinal photographs taken as part of the WIPA programme for diabetic managment, these referrals are made by the GP. The retinal photographs do not replace a general eye examination which is still recommended 2 yearly.

Red Eye
A red eye (or both eyes) has a wide range of causes from conjunctivitis to corneal ulcers to glaucoma. A person with a sudden onset of a red eye should be seen the same day, and if wearing contact lenses be advised to remove them and wear their spectacles.

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Conjunctivitis
The conjunctiva is the clear tissue overlying the sclera (the white part of the eye). When this becomes inflamed (infection usually) it is "Conjunctivitis". The causes of conjunctivitis are usually viral, bacterial or allergic and the treatment depends on the cause.

Keratoconus
In this condition the cornea develops a cone shape instead of the normal flat curve. This distorts and reduces the vision. If vision is not correctable with spectacles, a RGP contact lens is usually fitted. The contact lens reduces the distortion and provides better vision. There is a contact lens subsidy administered by the DHB for people with Keratoconus. This subsidy covers most of the costs associated with consultations and replacement contact lenses. If the cone develops in steepness beyond being able to be fitted with contact lenses the person may then require a corneal graft (donor cornea to replace their own distorted cornea) by an ophthalmologist.

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