Vision and vision examination

2021-02-02 12:00 AM

Objects viewed to correspond to a certain visual angle at the eye's node, the smallest visual angle that the eye can distinguish between two distinct points.

Vision is an important part of visual function; it consists of many components of which the main ability is to distinguish light and the ability to distinguish space. Clinically, we often consider vision relative to the minimum resolving force, that is, the eye's ability to distinguish two separate points very close together.

A vision exam is a fundamental and important part of ophthalmology. Visual acuity allows evaluating the function of cones of the central retina, the central macular region. Vision assessment must always include both distant and near vision. Normally near and far vision is always equivalent, some conditions affecting eye regulation such as presbyopia, presbyopia without glasses, or central cataract, etc., may cause vision impairment. near while distant vision is not affected.

Visual angle

Objects are viewed corresponding to a certain visual angle at the node of the eye (this is immediately behind the lens). The smallest visual angle that the eye can even distinguish between two distinct points is called the minimum dissociation angle. In normal people, the dissociation angle is at least 1 arc minute (visual correspondence 10/10). In the vision panels, test letters are designed to be 5 arc minutes in size when the patient is 5 meters (or 6 meters depending on vision panel) from the vision panel and the test letter gap (approx. distance between 2 points) will correspond to 1 arc minute

Figure: Parts of the test letter corresponding to the visual angle

Figure: Test letters corresponding to different distances

Young people may have a minimum dissociation angle of less than 1 arc minute, even up to 30 arc seconds (visuals 20/10). In the elderly, eyesight is often impaired, so in some cases, normal eyesight may not reach the level of young people.

Vision panel

The visual acuity board consists of many lines of text, test letters are gradually smaller in size from above, all of which correspond to a 5-minute visual angle, but at different distances. Next to each test letter, there should be clearly stated the level of vision corresponding to that test word and the distance that the normal eye can read the word. For example, next to the top text (the largest letter) has 0.1 and 50 meters written, meaning that the eyesight is 1/10 when that line is read and the normal eye can read it at a distance of 50 meters. There are many types of vision test strips that are used clinically, the most common being:

Snellen board: contains many different letters, requires the patient to be able to read. The ability to distinguish letters can be different, for example the letter D is often confused with O, or the letter L is easy to distinguish from other letters.

Landolt table: only one test typeface is a circle with a gap in the top, bottom, right, or left directions. The patient needs to indicate the direction of the gap of the circle. This board can be used by children or people who are illiterate.

Letter E: patients also need to distinguish the direction of the letter E. It is easy for children to use because a hard plastic E-shape can be used for the patient to hold and compare with the letter on the vision board.

Picture board: test letters are different objects or animals. Usually used for young children.

Picture: Some types of a vision board

Near vision board: There are many types of boards, the most common is the Parinaud board (consisting of short sentences next to each line of vision) or the cardiogram (with letters, numbers, circles). open, or the letter E, next to the line with the fraction corresponding to visionary vision, or enter the number according to the Jaeger convention).

Conventional vision results

There are two types of vision recording conventions commonly used today. Snellen notation (common in English speaking countries) uses fractions where the numerator (always 6 or 20) is the test distance (i.e., 6 meters or 20 minutes) and the denominator indicates the distance that the normal eye can read the text (i.e., the distance for the test letter of that row corresponds to the 5-minute standard arc), for example 6/12 means the patient's eye can read at a distance of 6 meters normal eyes can read up to 12 meters. Decimal writing method (commonly used in Vietnam, France, etc.) in which visual acuity is recorded by decimal numbers from 1/10 to 15/10 or 20/10. Vision 6/6 (or 20/20) corresponds to 10/10, visual 6/60 (or 20/200) corresponds to 1/10, etc.

Factors affecting vision

There are many factors that can affect vision, the main of which are:

The brightness of the test room: low brightness stimulates the rod cell system, reducing vision. Strong luminosity stimulates the cone cell system, thereby increasing eyesight. Eye fatigue is much less likely when the test room brightness is about 30-40% lower than the vision panel brightness.

Brightness of the vision panel: Well-illuminated and evenly illuminated vision panel increases vision. The brightness of the vision panel should be between 1350 and 1700 lux. When reading black text on white paper, the best brightness is between 500-650 lux.

Test Text Contrast: The eye looks better when the test text has good contrast, the black test text on a white background is easier to read than the word on a blue background.

Pupil size: eyes with a refractive error often increase vision in bright environments because the light causes pupils to contract, thereby reducing the size of the blurred rings in the retina. This is also the reason why near-sighted people often squint when they need to see clearly. Clinically, when testing vision, people can use the pore glass as an artificial pupil to increase vision in people with refractive error.

Patient Age: The older the age, the greater the brightness requirement. Children can read easily in the middle of the morning, in the middle of the night, while adults can only read in well-lit places.

Eye Diseases: Certain eye conditions that affect the pupil, the clear environments of the eye (cornea, aqueous humour, vitreous, vitreous) or the retina can all cause vision loss.

Methods of vision measurement

Measure vision distance

The patient was placed in a dark room, 5 meters from the vision panel to avoid regulation. If a projector is used, the test distance may vary and the size of the test letter should be adjusted accordingly. The vision board should be bright enough, the test letters must be contrasting well and uniform. Cover the patient's left eye, taking care that the eye patch does not cover the patient's left eye and does not press the patient's eye during the test. Ask the patient to read each test letter (open circle or letter name, left to right or vice versa), taking turns from top to bottom until only more than half of the test letters are read. of a line. Cover the client's right eye and test the left eye as shown above. Have the client open both eyes and have both eyes vision tested simultaneously. Record the results of each eye test in the smallest text readable by the patient, for example:

Visual acuity: MP 6/10.

MP and MT: 10/10.

MT: 10/10.

If the patient cannot read less than half of that line then write the number of unreadable letters next to vision, eg 7 / 10-2 (cannot read 2 words of row 7/10).

If the patient's vision does not reach 1/10 (cannot read the largest line), bring the patient closer to the vision board, if the patient can read the top line 2.5 meters away, the vision is 1/20 if the patient reads this line at a distance of 1 meter then his vision is 1/50.

If the patient cannot read any words, have the patient count their fingers and write the results according to the finger count distance, eg DNT 2 m, DNT 50 cm.

If the patient cannot count fingers, then check the ability to distinguish light from the light direction. If the eye can still distinguish between light and light direction, write ST (+) and the light direction is good. If you cannot distinguish between light and dark, write ST (-).

Optometry with hole glasses

Eyeglass is an eyepiece with one or more holes, or it can be a black mask like a test lens in the centre with a small hole. The use of porthole allows to quickly distinguish vision loss caused by a refractive error from fundus or vitreous lesions. Here's how to do it:

Cover the patient's eye without testing. Place the hole in front of the eye to be tested, adjust the position of the pore glass so that the patient can see the test word clearly. Ask the client to read the words from top to bottom to the smallest visible word and record the results of their vision.

Photo: Vision table

Close vision measurement

The patient wears appropriate reading glasses, the near vision panel is positioned 33 cm to 35 cm away from the eye and is bright enough. Cover the patient's left eye and have the patient read the words on the smallest line of the test panel. Cover the client's right eye and measure left eye vision as above. Remove eye cover and have both eyes checked.

Record visual acuity in each eye and in both eyes. Example P2 (reading line 2 of the Parinaud table), J4 (reading line 4 of Jaeger), or 4/10 (near acuity equates to distant vision 4/10).