Visual acuity (VA)

Visual acuity is how well one can see details. It is one of several factors in vision, including color vision, depth perception, and peripheral vision and part of a comprehensive eye exam. Visual acuity tests are used to determine how well one sees details of a symbol or letter from a specific distance. The tests may be performed by optometrists, ophthalmologists, opticians, technicians, family doctors, or nurses, depending on the type and purpose of test.

Visual acuity testing usually relies on the Snellen chart or Snellen eye chart. This consists of lines of progressively smaller type on a chart viewed at a distance of 20 feet (6 meters). The smallest line that the patient can read is the individual’s visual acuity. The size and shape of the type are standardized, although the spacing of letters and lines is not. This inconsistency has led to some criticism of the Snellen chart and the development of alternative eye charts.

Background

Herman Snellen was born in Zeist, The Netherlands, in 1834. He studied medicine in nearby Utrecht. He earned his medical doctorate in 1858 and was hired as assistant physician for an ophthalmological clinic founded by his mentor, Franz Cornelius Donders. Four years later, he became a primary physician and in 1884 was named director. He also became a professor of ophthalmology at Utrecht University in 1877.

Snellen was well known for making many advances in his field. His work included diseases of the retina and connective tissue, defective color vision, and glaucoma treatment. He is best known for improving eyelid operations and the development of visual acuity testing. While Snellen was not the first to develop a visual acuity chart, he developed special characters to measure visual acuity and calibrated them, so they could be reproduced and calibrated by others.

He began publishing his ideas about visual acuity tests in 1862. He suggested using a series of letters, called optotypes. Snellen designed his own block type using geometry. The thickness of the lines is equal to the thickness of the white spaces between the lines of optotypes. He made the height of the letter five times the thickness of the optotypes’ lines.

He placed a number above each letter series. The number indicates the distance at which the letters subtend five minutes of arc. An arcminute is a unit of measure for angles and is equal to 1/16 of one degree. He used the arcminute because rays of light form angles when they come together. According to Snellen, standard vision is the ability to recognize an optotype when it subtends five minutes of arc. This means that standard vision is when a person can distinguish a space pattern separated by one minute of arc. He expressed his concept as V=d/D, in which V represents the degree of acuteness of vision. It is equal to the relation of the distance at which the letter is seen (d) to the distance at which the letter is apparent at an angle of five minutes (D).

Snellen chose 20 feet (6 meters) as the standard distance for viewing the optotypes, hence, 20/20 vision means an individual can discriminate space of one minute of arc when it subtends five minutes of arc at 20 feet. (Countries that use the metric system express normal vision as 6/6.) Numbers other than 20 indicate the distance from which a person with normal vision could clearly see the chart; for example, an individual with 20/40 vision must be 20 feet from something to see it as clearly as someone with normal vision could see it at 40 feet (12 meters). Some individuals may have better than normal vision, for example, 20/15.

Overview

Light is reflected off objects. This light enters the eye through the front, which consists of the cornea, iris, pupil, and lens. The image of the object is focused on the retina, which is a light-sensitive membrane at the back of the eye. The retina comprises millions of nerve cells, which form the optic nerve. A small area of the retina at the center in back of the eye is known as the macula. The macula is responsible for discerning fine detail in the center of the field of vision. The light that enters the pupil should be focused on this spot.

A test result that is not 20/20 means that the patient has a refractive error. This means that light rays pass through the lens of the eye but do not bend the way they should, causing distortion of the image. It does not reach the macula correctly. Refractive errors are the most common form of vision problem.

Visual acuity is most often evaluated using the Snellen chart. The chart may be on a wall or video monitor. Individuals who cannot read, such as children, are tested using alternative charts showing numbers, shapes, figures, or a tumbling E. The E is read based on which way its “arms” are pointing.

The traditional Snellen chart consists of eleven lines of optotypes, with the largest line, containing a single letter E, at the top. The chart uses nine letters: C, D, E, F, L, O, P, T, and Z. Some criticisms of the chart include that visual crowding on lines that contain more letters makes differentiating letters difficult and the spacing between rows and letters varies, which could affect results.

Each eye is tested individually. The other eye is covered with a small paddle, a card, or one’s hand. Practitioners often use a refractor or phoropter, which is a device that has a mask-like component that contains many lenses. The patient looks at the chart through a variety of lenses switched by the practitioner. The patient will be asked to read specific lines and may be instructed to read the smallest line he or she can read clearly. The test may then be repeated with glasses or contact lenses.

The results of the test may indicate that an individual needs corrective lenses, either glasses or contact lenses. Testing may also indicate the need for surgery or the presense of an eye condition, such as an infection or injury. Test results may show that patients who wear corrective lenses need a new prescription because the individual’s eyesight has changed. In some cases, patients opt for surgical vision correction.

Bibliography

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