REFRACTIVE ERRORS OF THE EYE

REFRACTIVE ERRORS

Refractive error/refraction error, is a problem with focusing light accurately on the retina. The most common types of refractive error are near-nearsightedness, farsightedness, astigmatism, and presbyopia.

Nearsightedness: When the optics are too powerful for the length of the eyeball one has myopia or nearsightedness. This can arise from a cornea or crystalline lens with too much curvature (refractive myopia) or an eyeball that is too long (axial myopia). Myopia can be corrected with a concave lens, which causes the divergence of light rays before they reach the cornea.

Farsightedness: When the optics are too weak for the length of the eyeball, one has hyperopia or farsightedness. This can arise from a cornea or crystalline lens with not enough curvature (refractive hyperopia) or an eyeball that is too short (axial hyperopia). This can be corrected with convex lenses, which cause light rays to converge prior to hitting the cornea.

Presbyopia: When the flexibility of the lens declines, typically due to age. The individual would experience difficulty in near vision, often relieved by reading glasses, bifocal, or progressive lenses.

Cylindrical errors cause astigmatism, when the optical power of the eye is too powerful or too weak across one meridian, such as if the corneal curvature tends towards a cylindrical shape. The angle between that meridian and the horizontal is known as the axis of the cylinder.

Astigmatism: A person with astigmatic refractive error sees lines of a particular orientation less clearly than lines at right angles to them. This defect can be corrected by refracting light more in one meridian than the other. Cylindrical lenses serve this purpose.

Other terminology include anisometropia, when the two eyes have unequal refractive power, and aniseikonia which is when the magnification power between the eyes differ.

Fig.1. Refractive Errors

EPIDEMIOLOGY and PROBLEM STATEMENT:

The number of people globally with refractive errors has been estimated at one to two billion. Rates vary between regions of the world with about 25% of Europeans and 80% of Asians affected. Nearsightedness is the most common disorder. Rates among adults are between 15-49% while rates among children are between 1.2-42%. Presbyopia affects most people over the age of 35. The number of people with refractive errors that have not been corrected Is huge and it is one of the most common causes of vision loss along with cataracts, macular degeneration, and vitamin A deficiency.

SYMPTOMS:

Nearsightedness results in far away objects being blurry, Farsightedness and Presbyopia result in close objects being blurry, and Astigmatism causes objects to appear stretched out or blurry. Other symptoms may include double vision, headaches, and eye strain.

Fig.2. Common Complaints of a Person having Refractive error

DIAGNOSIS:

A complete eye examination can easily diagnose refractive error. Blurry vision may result from any number of conditions not necessarily related to refractive errors. Vision defects caused by refractive error can be distinguished from other problems using a pinhole occluder, which will improve vision only in the case of refractive error.

A doctor uses a trial frame and trial lenses to measure the person's refractive error. The diagnosis of a refractive error is usually confirmed by an eye care professional during an eye examination using a large number of lenses of different optical powers, and often a retinoscope or an automated refractor to objectively estimate a person's refractive error.Following that "objective refraction" the clinician typically shows the person lenses of progressively higher or weaker powers in a process known as subjective refraction. Cycloplegic agents are frequently used to more accurately determine the amount of refractive error, particularly in children.

      Fig.3. Subjective Refraction                                   Fig.4. Objective Refraction 

COMPLICATIONS:

• Recurrent eyelid infections (styes/ blepharitis/chalazion).
• Development of Squint (cross eyes)
• Amblyopia (Lazy Eye)
• Predisposition to other eye diseases (eg. Glaucoma, Retinal holes and Retinal Detachment, Keratoconus etc.).

MANAGEMENT:

How refractive errors are treated or managed depends upon the amount and severity of the condition and any associated complications if present. Those who possess mild amounts of refractive error may elect to leave the condition uncorrected, particularly if the person is asymptomatic. For those who are symptomatic, glasses, contact lenses, refractive surgery, or a combination of the three are typically used. Complications are actively looked for and managed accordingly.

                   Fig.5. Principles of management of Refractive errors

Please comment and give your suggestions for topics of your interest.

Thank You.

* The author is a Consultant Ophthalmologist,Eye Surgeon and Medical Director of Prem Jyoti Eye Care and Hospital, Bhilad(India)