Visual acuity

Visual acuity (VA) is one of many components of the visual perception sense and is defined as the eye's ability to resolve fine details. VA is a quantitative measure to see an in-focus image at a certain, standarized distance. VA is the most common measurement of visual function that is performed in a clinical setting.

History
In 1843 Kuechler, a German ophthalmologist, developed a set of three charts, but his work was almost completely forgotten.

In 1854 Jaeger published a set of reading samples to document functional vision. He published samples in German, French, English and other languages. He used fonts that were available in the State Printing House in Vienna in 1854 and labeled them with the numbers from that printing house catalog.

In 1861 Donders coined the term visual acuity to describe the “sharpness of vision” and defined it as the ratio between a subject's performance and a standard performance.

In 1862 Snellen published his famous letter chart. His most significant decision was not to use existing typefaces but to design special targets, which he called optotypes. This was crucial because it was a physical standard measure to reproduce the chart. Snellen defined “standard vision” as the ability to recognize one of his optotypes when it subtended 5 minutes of arc, thus the optotype can only be recognized if the person viewing it can discriminate a spatial pattern separated by a visual angle of 1 minute of arc. Since Snellen's days, few major improvements in visual acuity measurement have been made.

In 1875 Snellen changed from using feet to meters (from 20/20 to 6/6 respectively). Today, the 20 foot distance prevails in the United States and 6 meters prevails in Britain. Also in 1875 Monoye proposed to replace the fractional Snellen notation with its decimal equivalent (e.g., 20/40 = 0.5, 6/12 = 0.5, 5/10 = 0.5). Decimal notation makes it simple to compare visual acuity values, regardless of the original measurement distance.

In 1888 Landolt propose the Landolt C, a symbol that has only one element of detail and varies only in its orientation. The broken ring symbol is made with a "C" like figure in a 5 x 5 grid that, in the 20/20 optotype, subtends 5 minutes of arc and has an opening (oriented in the top, bottom, right or left) measuring 1 minute of arc. This proposal was based in the fact that not all of Snellen's optotypes were equally recognizable. This chart is actually the preferred visual acuity measurement symbol for laboratory experiments but gained only limited acceptance in clinical use.

In 1959 Sloan designed a new optotype set of 10 letters, all to be shown in each and every line tested, in order to avoid the problem that not all letters are equally recognizable. The larger letter sizes thus required more than one physical line. Louise Sloan also proposed a new letter size notation using the SI system stating that standard acuity (1.0, 20/20) represents the ability to recognize a standard letter size (1 M-bunit) at a standard distance (1 meter).

In 1976, Ian Bailey and Jan Lovie published a new chart featuring a new layout with five letters on each row and spacing between letters and rows equal to the letter size. This layout was created to standardized the crowding effect and the number of errors that could be made on each line, so letter size became the only variable between the acuity levels measured. This charts have the shape of an inverted triangle and are much wider at the top than traditional charts. Like Sloan's chart, they followed a geometric progression of letter sizes.

Also in 1976 Lea Hyvärinen created a chart using outlines of figures (an apple, a house, a circle and a square) to measure visual acuity in preschool children.

In this same year Taylor used these design principles for an illiterate Tumbling E Chart used to study the visual acuity of Australian Aborigines and it's actually used to screen visual acuity on illiterate people.

Physiology of visual acuity
Visual acuity is defined as the eye's ability to resolve fine details. To achieve this, the eye's optical system has to project a focused image on the clinical fovea, a region inside the clinical macula having the highest density of cone photoreceptors (the only kind of photoreceptors existing on the fovea), thus having the highest resolution and best color vision. Acuity and color vision, despite of being done by the same cells, are different physiologic functions that don't interrelate. Acuity and color vision can be affected independently without affecting the other function.

Visual cortex is part of the cerebral cortex in the posterior (occipital) part of the brain responsible for processing visual stimuli. The central 10° of field (approximately the extention of the macula) is represented by at least 60% of the visual cortex. Much of this neurons are believed to be involved directly into visual acuity processing.

Light travels from the fixation object to the fovea thought an imaginary path called visual axis. The eye's tissues and structures that are in the visual axis (and also the tissues adjacent to it) affect the quality of the image. This structures are: Tear film, cornea, anterior chamber, pupil, lens, vitreous, and finally ending on the retina. The posterior part of the retina, called the Retinal Pigment Epithelium (RPE) is responsible, along with many other functions, to absorb light that crosses the retina so it cannot bounce to other parts of the retina.

Visual acuity expression
Visual acuity is often measured according to the size of letters viewed on a Snellen chart or the size of other symbols, such as Landolt Cs or Tumbling E.

In some countries, acuity is expressed as a vulgar fraction, and in some as a decimal number.

Using the foot as a unit of measurement, (fractional) visual acuity is expressed relative to 20/20. Otherwise, using the metre, visual acuity is expressed relative to 6/6. For all intents and purposes, 6/6 vision is equivalent to 20/20. In the decimal system, the acuity is defined as the reciprocal value of the size of the gap (measured in arc minutes) of the smallest Landolt C that can be reliably identified. A value of 1.0 is equal to 20/20.

LogMAR is another commonly used scale which is expressed as the logarithm of the minimum angle of resolution. LogMAR scale converts the geometric sequence of a traditional chart to a linear scale. It measures visual acuity loss; positive values indicate vision loss, while negative values denote normal or better visual acuity. This scale is rarely used clinically; it is more frequently used in statistical calculations because it provides a more scientific equivalent for the traditional clinical statement of “lines lost” or “lines gained,” which is valid only when all steps between lines are equal, which is not usually the case.

A visual acuity of 20/20 is frequently described as meaning that a person can see detail from 20 feet away the same as a person with normal eyesight would see from the same distance. If a person has a visual acuity of 20/40, that person is said to see detail from 20 feet away the same as a person with normal eyesight would see it from 40 feet away. It is possible to have vision superior to 20/20: the maximum acuity of the human eye without visual aids (such as binoculars) is generally thought to be around 20/10 (6/3). Recent developments in optometry have resulted in corrective lenses conferring upon the wearer a vision of up to 20/10. Some birds, such as hawks are believed to have an acuity of around 20/2, which is significantly better than human eyesight.

When visual acuity is below the largest optotype on the chart, either the chart is moved closer to the patient or the patient is moved closer to the chart until the patient can read it. Once the patient is able to read the chart, the letter size and test distance are noted. If the patient is unable to read the chart at any distance, he or she is tested as follows:

Many humans have one eye that has superior visual acuity over the other. If a person cannot achieve a visual acuity of 20/200 (6/60) or above in the better eye, even with the best possible glasses, then that person is considered legally blind in the United States. A person with a visual field narrower than 20 degrees in diameter also meets the definition of legally blind.

Visual acuity is registered documenting if the test was for distant or near vision, the eye evaluated and if corrective lenses (i.e. spectacles or contact lenses) were used:
 * Distance from the chart
 * D (distant) for the evaluation done at 20 feet (or 6 meters).
 * N (near) for the evaluation done at 14 inches (or 35 cm).
 * Eye evaluated
 * OD (Latin oculus dexter) for the right eye.
 * OS (Latin oculus sinister) for the left eye.
 * OU (Latin oculi uterque) for both eyes.
 * Usage of spectacles during the test
 * cc (Latin cum corrector) with correctors.
 * sc: (Latin sine corrector) without correctors.
 * Pinhole
 * PH abbreviation is used followed by the visual acuity measured with it.

So, distant visual acuity of 20/60 and 20/25 with pinhole in the right eye will be: DscOD 20/60 PH 20/25

Distant visual acuity of count fingers and 20/50 with pindole in the left eye will be: DscOS CF PH 20/50

Near visual acuity of 20/25 with pinhole remaining at 20/25 in both eyes with spectacles will be: NccOU 20/25 PH 20/25

Measurement
Visual acuity is typically measured monocularly rather than binocularly with the aid of an optotype chart for distant vision, an optotype chart for near vision, and an occluder to cover the eye not being tested. The examiner may also occlude an eye by sliding a tissue behind the patient's eyeglasses, or instructing the patient to use his or her hand. This latter method is typically avoided in professional settings as it may inadvertently allow the patient to peek through his or her fingers, or press the eye and alter the measurement when that eye is evaluated.


 * 1) Place the chart at 20 feet (or 6 meters).
 * 2) If the patient uses glasses, then the test is performed using them.
 * 3) Place the occluder in front of the eye that is not being evaluated. The first evaluated eye is the one that is believed to see less or the one the patient says that is seeing less.
 * 4) Start first with the big optotypes and proceed to the smaller ones. The patient has to identify every one on the line being presented and communicate it to the physician.
 * 5) If the measurement is reduced (below 20/20) then the test using a pinhole should be done and register the visual acuity using the pinhole. Both measures should be registered, with and without using pinhole.
 * 6) Change the occluder to the other eye and proceed again from the 4th step.
 * 7) After both eyes have been evaluated in distant visual acuity, proceed to evaluate near visual acuity placing a modified snellen chart for near vision (such as the Rosembaum chart) at 14 inches (or 35 centimeters). Then repeat the test from the 2nd step.

In some cases, binocular visual acuity will be measured, because usually binocular visual acuity is slightly better than monocular visual acuity.

Measurement considerations
Visual acuity measurement involves more than being able to see the optotypes. The patient should be cooperative, understand the optotypes, be able to communicate with the physician, and many more. If any of these factors is missing, then the measurement will not represent the patient's real visual acuity.

Visual acuity is a subjective test meaning that if the patient is unwilling or unable to cooperate, the test cannot be done. A patient being sleepy, intoxicated, or having any disease that can alter the patient's consciousness or his mental status can make the measured visual acuity worse than it actually is.

Illiterate patients who cannot read letters and/or numbers will be registered as having very low visual acuity if this is not known. Some of the patients will not tell the physician that they don't know the optotypes unless asked directly about it. A patient having a brain damage can make him not recognize printed letters, or being unable to spell them.

Any motor inability can make a person not respond correctly to the optotype shown and affect negatively the visual acuity measurement done.

Variables as pupil size, background adaptation luminance, duration of presentation, type of optotype used, interaction effects from adjacent visual contours (or “crowding") can all affect visual acuity measurement.

The measurement of visual acuity in special populations (e.g., young children and handicapped individuals) is not always possible with a letter chart. For these populations, “preferential looking” techniques such as registering if a visual stimuli can be fixed, centered and followed, oculomotor responses such as optokinetic nystagmus, and electrophysiologic measures such as the visual evoked potential can be used to moderately estimate visual acuity.

"Normal" vision
Visual acuity depends upon how accurately light is focused on the retina (mostly the macular region), the integrity of the eye's neural elements, and the interpretative faculty of the brain. "Normal" visual acuity is frequently considered to be what was defined by Snellen as the ability to recognize an optotype when it subtended 5 minutes of arc, that is Snellen's chart 20/20 feet, 6/6 meter, 1.00 decimal or 0.0 logMAR. In humans, the maximum acuity of a healthy, emmetropic eye (and even ammetropic eyes with correctors) is approximately 20/16 to 20/12, so it is inaccurate to refer to 20/20 visual acuity as “perfect” vision. The 20/20 standard is the visual acuity needed to discriminate two points separated by 1 arc minute, that's all. The significance of the 20/20 standard can best be thought of as the lower limit of normal or as a screening cutoff. When used as a screening test subjects that reach this level need no further investigation, even though the average visual acuity of healthy eyes is 20/16 or 20/12.

Some people may suffer from other visual problems, such as color blindness, reduced contrast, or inability to track fast-moving objects and still have normal visual acuity. Thus, normal visual acuity does not mean normal vision. The reason visual acuity is very widely used is that it is a test that corresponds very well with the normal daily activities a person can handle, and evaluate their impairment to do them.