Eye Conditions

Just as a camera has its lenses, diaphragm and film,  the eye has its cornea and lens to refract light, its pupil to optimally limit passage of light, and the retina to receive images.

The condition of your eye is determined by 2 factors: the refractive error (deviation from emmetropia) and the visual function.

Refractive Errors
The refractive power of the eye is predominantly determined by three variables: (i) the power of the cornea, (ii) the power of the lens, (iii) and the length of the eye. An emmteropic eye is an eye that has no refractive error when viewing a distant object. An eye that has refractive error is known as an ametropic eye.

Refractive errors are categorized into two: sphere and cylinder. A spherical aberration occurs when an image falls too short or too far from the retina (as in the case of myopia or hyperopia); a cylinderical aberration occurs when an image fails to focus on the retina (as in the case of astigmatism, where your cornea is shaped like a football).

Diopters are used as units of measurement to determine the refractive power. Diopters are determined by the inverse of the distance, in meters, needed to acquire a focused image . For example, if a patient is -2.00 Diopters then he/she would have a clear image at a distance equal to 0.50 meters. Autorefractors and phoropoters are equipment used to determine such errors. In addition to spehrical and cylinderical errors, patients may have higher order aberrations that can be measured using wavefront technology (see custom treatment).

Visual Function
The visual function of an eye is determined by visual acuity, contrast senstivity, and color vision tests.

Visual Acuity is a measure of how well a person can see objects, letters or figures, in different sizes at a fixed distance. A standardized tool used to measure visual acuity is a letter chart (Snellen) fixed at 20 feet. Therefore, the notation of visual acuity is written as a fraction.

20/20 is considered to be normal vision, 20/40 in at least one eye is required to pass the driving test, and 20/200 vision or worse is the legal definition for blindness. Best Spectacle Corrected Visual Acuity (BSCVA) is what a person can see with corrected vision based on the Snellen chart (with glasses or contact lenses), Uncorrected Visual Acuity (UCVA) is what a person can see without corrected vision based on the Snellen chart.

Normal Eye (emmetropia): The image is focused on the retina. A clear image is seen. Retina is the back part of the eye that corresponds to the film in a camera transmitting visual stimuli to the brain.

Nearsightedness (Myopia): The eye brings pencils of parallel rays of light to focus at points infront of the retina. The cornea, the front surface of the eye, in a myopic eye, has too much curvature, thus near objects can be seen clearly but distant objects do not fall into focus. Refractive surgical correction of myopia is achieved by flattening the cornea, using the excimer laser. When the myopic error is fully corrected, pencils of parallel rays are brought to focus on the retina and the patient can see without the aid of glasses or contact lenses.

Farsightedness (Hyperopia): The hyperopic eye brings parallel rays of light to focus at points behind the retina. This is the opposite case of a myopic eye; the cornea has too little curvature, thus far objects can be seen clearly, but near objects do not fall into focus. Refractive surgical correction of hyperopia is achieved by steepening the cornea using the excimer laser. When the myopic error is fully corrected, pencils of parallel rays are brought to focus on the retina and patients can see without the aid of glasses or contact lenses.

Astigmatism: The parallel rays of light, in this condition, do not focus properly on the retina. This may be caused by asymmetry-irregularity of the cornea, or less frequently, of the lens. Objects are not seen clearly at all distances. Refractive surgical correction of astigmatism varies, either by flattening or steepening (or both) the cornea to retrieve its regular shape.

Presbyopia: is the age-related loss of the ability to sustain comfortably the accommodation necessary for clear vision as objects are moved closer. Age of onset of presbyopia will vary with the refractive error and its method of correction. A promising treatment for presbyopia, known as monovision, can be refractive surgical correction of one eye for distance and one eye for near.

References:

1. Poothullil AM, Azar DT. Terminology, classification, and history of refractive surgery. In: Azar DT (editor), Gatinel D, Hoang-Xuan T (Associate editors). Refractive Surgery 2nd edition. Mosby Elsevier: 2007, P:1-18.