Visual field

In the assessment of the visual field we measure the total area of the binocular visual field by confrontation techniques and the quality of the central visual field either using proper tests, like the Damato Campimeter or by asking about distortion of the straight lines in the Amsler test and about disappearance of letters in the words looked at by the child. The usual Amsler test has such thin lines they may not be seen by the child. In such a case use a page of a regular college block for visually impaired students.

Peripheral visual field

Confrontation visual fields can be measured by using the tester's finger movements as the stimulus or a white or black ball on a thin stick. The object or the fingers are moved forwards from behind the person's head and the person tells when (s)he sees the object or the fingers. A young child turns the eyes as soon as the stimulus is seen in the peripheral visual field.

The approximate place of the stimulus detection is recorded, e.g." 90 degrees to the right, and left, 60 degrees up and down". In a person whose right eye is blind and left eye has slightly constricted visual field the finding could be e.g."30R, 60L, 30U and 45D". A more exact test for measurement of confrontation field by using a standard size white dot on a gray background is also available at VICE VERSA.

Visual field defects can be hemianopias, half field defects in one or both eyes, on the same side (homonymous hemianopias) or on both temporal sides (bitemporal hemianopias) or - rarely - on both nasal sides (binasal hemianopias). They can be quadrantanopias, quarter of the visual field defects, again in one or both eyes. The defect can also be a ring scotoma, loss of visual function in the "midperiphery" of the visual field, i.e. not in the very periphery but around the central visual field.

The functional importance of the child's visual field defect should be carefully assessed. There are two aspects that are not well covered in most medical reports. One is the case of bitemporal visual field loss. If both temporal halves are blind and the nasal halves of the visual fields function, the child has an approximately 120 degree visual field when looking straight ahead. However, when (s)he looks at something close by there is no visual field in a sector in front of that fixation point. That means that when the child looks on something on the desk, (s)he is unaware of a sector of the classroom in front of her/him and the same problem is present in physical education and during games.

Another feature that is not reported because it is not tested, is the presence of motion perception in some "blind" areas. This can be roughly tested with Goldmann visual field apparatus so that the tester moves the stimulus quickly back and forth within a scotoma found by using the normal testing technique. If the child responds to the quickly moving stimulus, the scotoma is not "absolute" even if the child does not respond to the maximum bright stimulus when it does not move. Most young children respond best if you ask them to shift fixation from the central fixation target to the stimulus as soon as it becomes visible somewhere.

Tunnel vision

The central visual field of only 10-20 degrees can be tested using plain white paper at a distance of 57 centimetres and a dark pen. Draw a cross in the middle of the paper and ask the person to look toward the centre of the cross all the time, independent of whether the actual crossing point is seen or not. Move the pen from the side of the paper toward the cross and ask the person to say when the pen becomes visible at the edge of the tubular field. Measure the extent from the right, left, up, down and diagonally. Then draw a line through these points and you have the size of the visual field depicted by that oval area. At 57cm each centimetre on the paper equals one degree of visual angle, so you can measure the size of the visual field simply by measuring the size of the area in centimetres. When the visual field is very limited, you may like to use a longer distance, 114cm or 228cm to make the measurement even more exact. At 114cm 2cm equals to one degree of visual angle and at 228cm 4cm equals to one degree.

Central visual field

The quality of the central visual field is important to assess for sustained near vision tasks, like reading and using picture materials. In these tasks small scotomas, areas of loss of sensitivity, and distortions of the image may greatly disturb perception of visual information. In clinical work, scotomas are mapped using perimeters and tangent screens, both are large and expensive. The campimeter developed by Dr. Bertil Damato is suitable for educational assessment. In it the stimulus appears in the middle of the test when the person is looking at the different numbers. If the child does not know numbers, a pointer can be placed at each number. When the child holds the pointer on the correct number, (s)he has to look in that direction. This way fixation becomes more stable. The cover on the eye, not being tested, is usually held in front of that eye by the parent or another adult, because otherwise there are too many demanding motor functions disturbing the response to the visual stimulus. The test comes with a complete manual.

The Damato campimetry is usually started with plotting of the blind spot. If the child has a central scotoma and the fixation is shifted to a new preferred retinal locus (PRL) then the blind spot is also shifted in the same direction and the child will respond when the stimulus is shown in the area of the normal blind spot. This should not confuse you to believe that the child's responses are unreliable. The same is true for all visual field testing: if the child does not fixate with the fovea the whole map of visual field is shifted in the direction opposite to the new fixation area, i.e. if the new fixation area is above the central lesion, the shadow of the lesion is located above the fixation area and the blind spot is shifted upwards.

If the person has nystagmus or he/she has difficulties in holding the fixation, testing the visual field becomes inaccurate and an approximation must suffice.

Amsler chart results should not be taken at face value. A child who says that the lines bend toward each other above the centre of the test picture may have:

  1. a scotoma in the upper part of the macular field, or

  2. a central scotoma that has lead to a change in fixation. There is a new preferred retinal locus (PRL) above the lesion and thus the scotoma is "pushed" upwards. The blind spot is also found to be "displaced" in the same direction. Note that the perceived scotoma usually is different from the small scotoma mapped with perimetry: like the scotoma of the normal blind spot, the non-functioning area in the visual field is filled by visual information from the nearby cells. Thus the scotoma is "filled in" and the child is not aware of the scotoma, except when looking on a grid or rows of text and knowingly analysing the image in the central visual field.

Extrafoveal Fixation Recorder was designed to help in measurement of shifts in fixation during reading by plotting the location of the blind spot. If there are several scotomas in the central visual field, islands of useful vision may be of different size and with varying visual acuity and contrast sensitivity. Thus fixation may vary from one PRL to another depending on the type of visual task.

Macular Mapping test developed by Manfred Mackeben covers the central 16 degrees. The stimuli are letters and thus depict recognition of forms in the central visual field. The size and contrast of the letters and the exposure time can be varied. The test is a computer program available from the designer (mm@ski.org).

Reading may also be disturbed by half field defects extending to the midline. A half field defect of the left side makes finding of the beginning of the next line difficult. In this situation a ruler or a card under the line to be read is helpful. If the right sided field loss is complete, with no macular sparing, reading toward the blind half field is difficult. In this situation there is a simple solution: turning the book upside down moves the blind half field behind the point of reading, which can be helped by the card or the ruler below the line while reading from right to left. If the child has learned to read before the accident the change in reading often occurs during the convalescence before the child comes back to school.

A. Right sided visual field loss without macular sparing and B. Right sided visual field loss with macular sparing. Macular sparing means that the nerve fibres from the macula have not been damaged although all other nerve fibres from the left side of both retinas leading to the right sided half field defect, have lost their function.

Scanning is an important strategy to compensate for visual field defects. The child can be taught to use regular scanning of the surrounding world, the hemianopic children "throwing" their gaze to the blind side and then moving it smoothly back to midline, the child with a tunnel field defects by moving the gaze from side to side like a long cane.

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