Refractive Errors of the Eye

The rays of light are focused on the retina - the light sensitive layer of the eye. In refractive errors these rays are focused either in front of behind the retina or they form irregular focus.

What are the different types of Refractive Errors?

Myopia (Short-Sightedness): When the rays of light are focused in front of the retina. Need concave or – (minus) glasses for correction.

Hypermetropia (Far-Sightedness): When the rays of light behind the retina. It is corrected by convex or + (plus) lenses.

Astigmatism: Here the rays of light are focused in an irregular fashion due to uneven focusing power around the eye. This requires cylindrical lens oriented at a specific axis for its correction.

Presbyopia: It is not a true refractive error but a weakness of ability to focus for near objects. This is a normal aging phenomenon and manifests at about 40-45 years of age. It is corrected by using convex or + (plus) lenses / power which is ADDED to the distance power of the person.

	What is Pseudo-myopia?
	Prolonged near work in the form of reading or working on computers can lead to a state of spasm of inner eye muscles (ciliary muscles). This causes blurring of distant vision and gives a false appearance of myopia in the eye. When eyes are tested after dilating the eye this pseudo-myopia disappears. This (element of) myopia is not to be treated with glasses but rather by proper visual hygiene and regular eye exercises.

	What causes it?
	Most refractive errors are inherent / inherited conditions caused by following mechanisms: Axial: The eyeball is longer (in myopia) or shorter (in hypermetropia) than normal. Curvature: Abnormal curvature of cornea or lens of the eye. Index: Changes in the refractive index of the optical parts of the eye (as in cataract) Presbyopia: is caused by reduction in flexibility of the lens of the eye thereby reducing its ability to focus for near objects. This is part of normal aging phenomenon. Refractive errors are not caused by excessive reading, watching TV, working on computers or any dietary deficiencies.

	What are the symptoms & signs of refractive errors
	Often refractive errors (especially low numbers) do not cause any symptoms or signs and get picked up on routine eye examination. However, following are the common symptoms seen: Blurring of distance vision seen in myopia & significant astigmatism Blurring of near vision is noticed in Presbyopia and high Hypermetropia Eye-strain and discomfort seen commonly with hypermetropia & astigmatism Squint may be seen with high hypermetropia in children Redness, watering, pain in eyes, headache, etc. may occur with any of the refractive errors.

	How is it diagnosed?
	Refractive errors are diagnosed with the help of following tests: Visual Acuity Assessment: is measurement of vision or ability of eye to seen distant small objects with the help of special illuminated charts (Snellen’s Chart) or Projected Charts. The visual acuity is denoted by a fraction numerator / denominator. Where numerator denotes the distance (in meters or feet) at which the subject reads the chart and denominator is the farthest distance (in meters or feet) at which a normal person would be able to read the same line. Normal visual acuity is 6/5, 6/6 or 6/9. Refraction Assessment: This is the measurement of refractive error or spectacle power of the eye. This is done by the conventional method of Retinoscopy or by the automated instrument called Autorefractometer (the so called computerized vision testing). The number so obtained is verified by placing trial lenses in a frame worn on patient’s eyes – trial frame. In special situations it is required to instill some eye-drops in the patient’s eye to dilate the pupil and to relax the inner eye muscles (ciliary muscles) in order to measure the accurate refractive error – Cycloplegic Refraction. This is required especially in children and young adults or an unexpected increase in refractive error is observed or where the pupil is too small for accurate measurement. The patient is called once again after a few days for the final prescription of spectacles – Post-Cycloplegic Test.

	What is the treatment?
	The refractive errors are corrected with the help of: Spectacles The spectacles are the oldest, the commonest, the safest but not the cheapest way of correcting refractive error. The cumulative cost of spectacles to be used by a person in his lifespan can be exorbitant ! Contact Lenses Details about contact lenses are under the respective module Surgery

	What are the surgical options?
	Fortunately there are some dependable and permanent surgical treatments for refractive errors: Lasik ( this is the most commonly employed method ) Details about LASIK are under LASIK module Phakic IOLs or Implantable Contact Lenses Lens Replacement Surgery or Clear Lens Extraction There is no satisfactory solution for presbyopia till date; however, lens replacement surgery with implantation of multifocal IOLs is probably the best available option.