By Dr Raghuram Y.S. MD (Ay) & Dr Manasa, B.A.M.S
When the image of the object does not form on the retina the vision is blurred or deformed. This type of visual disorder is called ametropia. It is a common condition. Due to irregularity of the corneal curve vision is deformed, both at long and short distance.
Therefore ametropia is a condition in which a refractive error prevents forming of images on the retina. All forms of ametropia can be corrected by proper and regular use of glasses or contact lenses.
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Emmetropia means ‘optically normal eye’. It is defined as ‘state of refraction wherein the parallel rays of light coming from infinity are focused at the sensitive layer of retina with accommodation being at rest’. It is an optimal state of vision. Here a faraway object is in sharp focus on the retina while the lens is completely relaxed. Therefore an emmetropic eye has perfect vision and doesn’t require corrective lenses.
Mechanism of ametropia – Ametropia is also defined as improper bending of light by the eye which results in a blurred image. It is a condition of refractive error. These are vision disorders in which the parallel rays of light falling on the eye are not focused in the sensitive layer of retina. This means to say that due to refractive error the light is either focused behind or in front of the retina in one or both the meridians.
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Table of Contents
Eye fatigue or tiredness in eyes (Asthenopia)
Difficulty in seeing close up things
Difficulty in reading
Crossing of eyes (esotropia)
The person tries to avoid doing works involving seeing close objects
Lack of concentration
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1. Myopia – Nearsightedness
Here the parallel rays of light are not focused on the retina. They are focused in front of the retina when the accommodation is relaxed. These people will have poor distance vision. This means to say that they cannot see the objects placed at a far-off distance. The vision often becomes worse in dim light or at night. This condition usually starts early in life, typically increase between 12 and 30 years of age.
2. Hyperopia / Hypermetropia – Farsightedness
It is the refractive error wherein the parallel rays of light are focused behind the retina instead of getting focused on it with accommodation relaxed. This condition can be inherited. In case of mild form of hyperopia the vision is difficult or blurred for near things but is clear for far off things. On the contrary in moderate and high hyperopia people complain blurred vision to both near as well as far of things. These people will also have asthenia symptoms due to sustained accommodative efforts. Hyperopia in children may also cause crossed eye. The symptoms of this condition would get worse with age.
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It is a type of refractive error wherein the parallel rays of light just enter the eye while the accommodation is relaxed and do not come to a single point focused on or near the retina. This consequently leads to distortion of the image. This condition causes blurred vision at all the distances and for all objects. The symptoms herein do not change very much with progression of age.
4. Presbyopia – loss of near vision with age
Many do not consider this condition amongst the types of ametropia. It is a vision problem related to old age. In fact, it is not a refractive error like the above mentioned 3 conditions. It is a physiological insufficiency of accommodation of vision which occurs due to natural ageing changes. This leads to progressive fall in near vision. This is also defined as loss of accommodative ability with age. The above said 3 conditions might affect only some people but Presbyopia occurs in almost all people as they get old since it occurs due to hardening of lens. Hardening of lenses with age makes it difficult to accommodate and with advanced ageing and disease process makes it impossible to accommodate. It is generally noticeable after 40-45 years of age and slowly gets worse until about 60 years. Usually, the main complaint of patients with presbyopia is difficulty in near vision. They find it difficult to read smaller prints and fonts and also find it difficult to thread the needle. This condition is also associated with asthenia after reading or doing any near task.
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Diagnose of refractive errors is done by conducting routine eye examination in a clinical setup. Also diagnosed through vision screening of the population at large. This is commonly carried out on school children.
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Uncorrected Myopia might cause one or more of the below mentioned complications –
Eye infections due to microbes introduced into the eyes while rubbing of eyes
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These conditions can be treated promptly by appropriate refractive correction. They should be diagnosed and corrected as soon as possible. If they are not promptly treated, they may cause permanent vision problems. High refractive error in children may lead to amblyopia, retinal problems and progress to cause blindness.
Commonly used eyeglasses in ametropia
Convex / plus lenses – for Hyperopia
Concave / Minus lenses – for Myopia
Cylinder lenses / sphero-cylinder / Toric lenses – for Astigmatism
Corrective contact lenses
Contact lens for Myopia and Hyperopia
Ortho-k – reshaping the cornea temporarily by using overnight contact lens – to decrease refractive errors
Toric soft contact lens and toricrigid gas permeable contact lens for astigmatism
Laser in-situ keratomileusis (LASIK)
Photorefractive Keratectomy (PRK)
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In a cross-sectional study in a sub-urban municipality in Ghana a total of 208 children were studied for ametropia. The prevalence of uncorrected refractive error among the pupils was high. It was henceforth recommended that periodic eye screening in school children should be carried out in such low-income settings and public health programs on childhood refractive error should be increased in such settings. (read more)
Myopia genetics – Identification of myopia susceptibility genes will provide insight into the molecular basis of this eye disorder. It will also identify pathways involved in eye growth and development. This effort may lead to effective therapies to treat or prevent this common eye condition.
A study was conducted to establish a relationship between retinal dysfunction and high ametropia. High metropia in children who were being investigated for poor vision is associated with a high rate of retinal electrophysiological abnormalities. This association is not found at low refractive errors. To rule out associated retinal pathology, electrophysiological testing should be considered in cases of high ametropia in childhood. (read more)
A disease of vision named as ‘dhumadarshi’ explained by Master Sushruta has often been compared to ametropia.