Refractive error is a defect in the eye’s ability to focus an image accurately onto the back of the eye. It is detected with a vision test called a refraction and is routinely performed during an eye examination from an Optometrist. There are four types of refractive error found in the human eye, which are:
“My book up close is in focus, but I can’t see Table Mountain clearly in the distance.”
Myopia is also known as short-sightedness or near-sightedness and comes from the Greek words “myops” meaning near-sighted and “ops” meaning eye. Myopia is a refractive error where the parallel rays of light entering the eye focuses in front of the retina causing a blurred image to form on the macula. This condition is mainly attributed to the elongation of the eyeball, specifically the posterior (back) part of the sclera, and/or the curvature of the cornea being too steep.
There are varying degrees of myopia ranging from low to high myopia. People who have high amounts of myopia, -6.00 Diopters and higher, are more at risk to develop associated pathology like floaters and retinal detachments and should have their eyes checked regularly.
“I can see Table Mountain clearly in the distance, but I struggle to read me book.”
Hyperopia is also known as hypermetropia, farsightedness or longsightedness and is derived from the Greek words “hyper” meaning over or beyond and “ops” meaning eye
It occurs when the length of the eyeball is too short for the cornea’s curvature, and the parallel rays of light entering the eye are brought to focus behind the retina causing a blurred image to form on the macula.
Symptoms of hyperopia include difficulty seeing near objects clearly, eye fatigue when reading and even blurred distance vision in severe cases of hyperopia. Hyperopia can occur at any age and is one of the main causes of infantile squint eyes when one or both eyes turn inward.
Hyperopia is often confused with another refractive error called Presbyopia, which also causes blurred near vision. Although most of the symptoms of presbyopia are similar to hyperopia, the origin of the refractive error is different and is present with all people from their 4th decade of life.
Young children’s eyes require a more specialised refraction called a cycloplegic refraction. During the cycloplegic refraction, special drops are used to relax the ciliary muscle of the eye which is used to accommodate or focus the crystalline lens on near objects. This is necessary as the accommodation process can mask hidden hyperopia, which if left uncorrected can lead to eye fatigue, poor reading, and school performance and in severe cases squints and [[amblyopia]].
“You have a rugby ball shaped eye.“
Astigmatism is derived from the Greek words “a” meaning without, “stigma” meaning a point, spot or mark and “ops” meaning eye.
Astigmatism is a refractive error where the parallel rays of light entering the eye cannot be focused clearly to a point on the retina because the curvature of the cornea and/or crystalline lens of the eye are not equal or spherical in all the meridians.
To understand astigmatism, imagine a cross drawn on the cornea. The curvature of the cornea on the one meridian (leg) of the cross will be different to the curvature of the cornea on the other meridian (leg) of the cross. The effect of this is when the eye focuses on an object in space the light rays traveling in on one meridian of the cross will focus on a different distance behind the eye than the light rays traveling in on the opposite meridian.
Cross Section of Astigmatic Eye
Astigmatism can be a stand alone refractive error which is called simple astigmatism or can be combined with myopia and hyperopia and is then referred to as compound astigmatism.
“My arms are getting too short”
Presbyopia is derived from the Greek words “presbys” meaning old person and “ops” meaning eye.
Presbyopia occurs when the nucleus of the crystalline lens in the eye starts to harden, a process known as lens sclerosis. The hardening of the lens material leads to a loss of the lens’ focusing flexibility (amplitude of accommodation) and as a result, is not capable of focusing the light rays entering the eye correctly on the retina when viewing near objects.
The loss of lens flexibility starts from childhood and eventually becomes noticeable around the age of 42 – 45. It is important to note that Presbyopia is not due to loss of muscle function. The ciliary muscle responsible for changing the lens shape is, in reality, exerting more force on the lens at age 45 than at age 25 to achieve the same near focus due to a more inflexible lens. It is the overexertion of the ciliary muscle that causes the eye fatigue which people over 40 experience when they do extended periods of near work. Other symptoms of Presbyopia include difficulty with reading small print or looking at small near objects, particularly in low light conditions, and blurred vision when changing focus between near and distance objects.
Presbyopia becomes less noticeable in bright light due to the iris closing to a pinhole aperture. The pinhole aperture of the iris results in an increased depth of focus, similar to a pinhole camera, which effectively reduces the focusing error of the eye allowing for a clearer image to form on the retina.
Treatment for Presbyopia consists of correcting the refractive error with reading-, bifocal- and multifocal spectacle lenses and contact lenses. Taking regular breaks from near and computer work and reading under bright light will also help to improve the symptoms experienced by Presbyopes.