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MattiSeppänen

Assessment of Vision

Essentials

  • Basic functions of vision can be accurately assessed in primary health care.
  • Basic visual workup
    • Visual acuity
    • Binocularity
    • Visual field
    • Colour vision
  • Further examinations
    • Accommodation (particularly in presbyopic persons)
    • Night vision
    • Contrast sensitivity
    • Glare sensitivity
    • Perception of movement

Visual acuity

Principles

  • Visual acuity is measured as the size of the smallest optotypes that can be correctly recognized.
  • Another variable significantly associated with visual acuity is the crowding phenomenon.
    • The crowding phenomenon means that the recognition of targets, such as letters, is impaired as the space between them gets smaller.
    • The crowding phenomenon can be detected in a test if the person can see individual small letters well if they are placed wide apart on the line but is not capable of seeing letters of the same size if they are placed close to each other.
    • Tolerance to crowding improves with age and is usually at the adult level at the beginning of school age, but it may also be considerably lower.
    • Crowding is not routinely tested for, although the test may be available at child health clinics, for example, on the reverse side of the near vision test.
    • If a child has reading problems despite good visual acuity, the crowding phenomenon can be tested for by an ophthalmologist. If crowding is detected, it can be addressed by vision training.

Choice of optotype charts and optotypes

  • Visual acuity can be tested either by optotype tests or by grating tests (see e.g. http://www.lea-test.fi/en/vistests/instruct/instruct.html and http://www.lea-test.fi/en/vistests/instruct/2503/index.html).
    • Visual acuity is defined by the line of smallest optotypes that can be correctly recognized.
    • Visual acuity should be measured with a chart where the space between the optotypes in a row is at least as wide as the optotypes in that row (WHO/PBL/03.91).
    • The tumbling E test is not used for screening purposes any more because
      • in small children the understanding of directional concepts develops late
      • the tumbling E test is not a symbol test because the E shape always remains the same.
    • In addition to LEA symbol charts, charts containing letters (Sloan) or numbers are used in the assessment of functional vision.
  • Visual acuity should be measured with both near and distance vision tests.

Testing distance

Distance vision

  • A testing distance of 4 metres is used in adults, with a test chart positioned or projected at this distance.
  • In children, the testing distance is 3 metres.
    • Symbol charts for smaller children
    • Number charts for school children

Near vision

  • In adults, near vision is assessed at a distance of 40 cm or, if the patient uses reading glasses, at the distance of most accurate vision with the glasses.
  • If the testing distance differs from 40 cm, the result should be written down as, for example, "reads line 0.8 at a distance of 52 cm".
    • If the acuity of near vision was examined at a distance of 52 cm, visual acuity can be calculated by dividing the measured value by 40 and multiplying it by the line read (52/40) × 0.8 = 1.0.
  • In vision screening of children, the near vision test is also done at a distance of 40 cm. If the child wants to hold the card very close to the eyes or at an angle, the vision is abnormal and the child should be referred to an ophthalmologist for further investigations.

Illumination

  • Good illumination of the room, with no reflection, is essential.
  • The use of lightbox tests removes the problem of non-standardized illumination. Lightbox tests can be used both in occupational health services and for screening of children and elderly persons.

Naming of optotypes

  • Ask the patient to name one optotype on each line (the first or the last one).
  • When the patient starts to hesitate, ask them to read all the optotypes on the line.
  • If the patient cannot read the line correctly, ask them to read the line above.

Recording visual acuity

  • Visual acuity is the value of the smallest line where at least 3 out of 5 optotypes were correctly identified.
  • The most accurate way of documenting the result is to write, for example, 0.8(-2) if the subject had two wrong answers on line 0.8.
  • The test-specific instructions should be carefully read.

Examination of visual acuity in special circumstances

Suspected brain damage

  • If brain damage is suspected, visual acuity should be measured as a part of the functional vision assessment.
  • Difficulty identifying abstract shapes may apply to only one of the optotypes used. Therefore visual acuity should be examined using letter, number and symbol charts.
  • When examining elderly people with brain damage, a near vision test combining in parallel all three tests in one should be used.
  • As recognition functions decline with age, first letters, then numbers and lastly LEA symbols are usually affected.

Children with communication difficulty

Contrast sensitivity

  • Contrast sensitivity is the ability to perceive small differences in luminance.
  • The fainter a shadow a person can see the better the contrast sensitivity.
  • Contrast sensitivity can be measured by using either an optotype test or a grating test.
  • Testing is useful in, for instance,
    • follow-up of vision in persons with diabetes
      • Low-contrast function is often affected before high-contrast vision.
    • occupational health care
      • Some nerve toxins affect contrast sensitivity.
    • investigation of unclear visual symptoms
      • Incipient optic neuritis
      • Some intoxications

Visual fields

  • The visual field is the area that a person can see without moving his/her eyes.
  • Because of the continuous movement of the eyes, the functional visual field is wider than the tested visual field.
  • Central visual field is important for precise vision, the peripheral parts of the visual field are important for moving around and for observing the surroundings.
  • Visual fields can be measured by finger perimetry.
    • Restriction of visual fields, particularly hemianopia, can be detected by the test.
    • Small scotomas can often not be detected.
  • Automated perimeters (such as Humphrey and Octopus) can be used for accurate measurement of visual fields.
    • These are used for diagnosing and monitoring ophthalmological (particularly glaucoma) and neurological diseases, for instance.

Examination procedure

  • Visual fields can be measured by finger perimetry or by using a small ball on a thin rod.
  • The person examined sits looking straight ahead and responds when and in what direction he/she detects the movement of the fingers or the ball.
  • The person examining moves his/her fingers or the ball on a thin rod first in the periphery of the examined person's visual field, gradually moving it towards the centre of the visual field.
    • The person examining must not follow the movement of the fingers or the ball with his/her eyes but watch that the eyes of the person examined stay fixed in the midline.
    • The test should be repeated at least obliquely in each visual field quadrant.
  • Clinical measurements do not differentiate between total (absolute) visual field loss and partial loss where the seeing of movement is preserved. In the latter case, rehabilitation may substantially improve the functional ability.

Accommodation and presbyopia

  • With age, the lens will become stiffer and the ability of the eye to adjust to seeing at various distances (accommodation) becomes impaired. Near vision is gradually affected.
  • Most people over 40 can read small text at a close distance better with the help of reading glasses.
  • This is not a disease but a phenomenon associated with normal ageing that is called presbyopia.
  • Presbyopia essentially affects the performance of various tasks in many occupations. Presbyopia requires special attention at workplaces. See also Work with Display Screen Equipment, and Special Work Glasses.

Accommodation problems in special groups

  • Remember the occurrence of impaired accommodation in people with Down's syndrome and when examining infants and children with locomotor handicaps.
  • In special groups, examination by an ophthalmologist is warranted during the first 3 months of life, or even earlier if the infant does not make adequate eye contact at the age of 8 weeks at the latest.

Colour vision

  • Colour vision can be screened in primary care with Ishihara Methods for Colour Vision Screening, Velhagen or HRR tests, if the examinee has normal contrast sensitivity (these tests are low contrast).
  • Typical red-green colour defects can be detected with these tests, but some persons with normal vision are also caught in the screen.
  • Abnormal results therefore need to be confirmed with sorting tests performed by a nurse, doctor or optometrist with adequate expertise.
    • Farnsworth Panel D-15 type test
    • The Good-Lite Panel 16 test can be used for the examination of colour vision in persons with impaired vision as well.
  • In sorting tests, the type and severity of the colour vision disorder can be determined when the results are entered on the recording form.
  • Colour vision must be measured using a blue "daylight" lamp (colour temperature > 6000 K) or in indirect sunlight (i.e. at a window facing north).

Night vision

  • Cone cells are active in daylight (photopic vision), rod cells are active in very dim light (scotopic vision) and both cell types are active in the intermediate area (mesopic vision).
  • The speed of adaptation and how dim the light can be for the person to still see are significant for working capacity and daily activities.
  • The cone cells' speed of adaptation can be measured with, for example, Cone Adaptation Test.
    • In the test, the patient's ability to detect the colours of the test chips is compared to that of the normal-sighted examiner.

References