Visual Adaption

VISUAL adaptation is the most often altered visual function in retinal diseases. The change may affect either the time course of adaptation, the range of adaptation or both. Depending whether cones or rods are more affected the problems of delayed or restricted adaptation occur predominantly in photopic or scotopic conditions. Quite often patients who are light sensitive and easily dazzled due to changes in retinal function, also have small subcapsular lens opacities which cause increased scatter of light and worsen the condition further.

Usher's Syndrome is the most common cause for adaptation problems in the deafblind population. It is often described as night blindness but naturally occurs during daytime as well if there is a marked change in luminance. This is often forgotten when assessing school children. Some of the Usher youngsters have such slow adaptation that it may take them 20-30 minutes before they start seeing in the classroom if they have been in bright sunlight. Finding objects from cupboards and closets may be impossible in the home economics class. If these problems are not noticed and dealt with we force the child to function tactually without having taught the use of tactile information. The same problem is common in many work places where some observations or tasks must be done in mesopic range of vision and the change of luminance is rapid.

Our present tests give rather little information for assessment of delays in visual adaptation. The LEA test system has the CONE Adaptation test, a set of plastic chips, blue, red, and white (Thornton 1977) for screening and also for preliminary assessment of delayed and/or reduced cone adaptation of deaf children. This test is useful in the schools and homes of deaf children informing the teachers and parents how rapidly the child starts seeing colours when entering a dimly fit area.

Goldmann-Weeker's adapation test gives us information on both the delay and the final threshold of visual adaptation. The usual execution of the test can be modified when examining young deaf children by covering one eye for half an hour and measuring only the final threshold. If the child does not seem to, understand the test when in the dark, we can cover the dark-adapted eye in the dark, then turn on the lights and explain again to the child who now watches with his less adapted eye and then continues the test in the dark.

During the measurement of visual fields at different luminance levels we also learn something about visual adaptation. In most cases the size of the visual field reaches its maximum within half a minute. We may observe some increases at the points measured as the very first ones. In other cases there is a slow increase in the size of the visual field over 3-4 minutes and fluctuation in the thresholds which would make the patient "unreliable" in automated measurements.

The tests for glare sensitivity and glare recovery suitable for examination of low vision patients are not commercially available. The test target should be designed so that it can be seen by patients who have lost some of their central contrast sensitivity.

Absorption glasses

If our clinical tests for visual adaptation need improvement, our testing for absorption glasses needs to be built starting from its foundation. Today the prescription of absorptive lenses is based on the patient's subjective experience when the lenses are demonstrated. Often this is done in room illuminance or while the patient is looking through a window. Usually we have no proof that we have given the best possible prescription for absorption lenses.

Absorptive lenses are either filter lenses that cut out a part of the spectrum nearly totally or totally, or lenses that reduce transmission of light at all wavelengths almost equally. Some of the lenses are phototropic. We thus have four basically different types of lenses: filter lenses and non-filter lenses both of which can be phototropic or have only one transmission level.

Ever since Corning introduced the CPF- lenses there has been much discussion concerning which patients should have these more expensive photochromatic filter lenses. When we try to answer this question we should know what happens in the retina when the filter lens is placed in front of the eye. The total amount of light is reduced so that the eye's adaptive state is changed slightly toward lower photopic range. This same change can be achieved with any absorptive lens. However, because of the filtering off of the blue light simultaneously there occure another change that we know very little about: blue cones and rods are selectively brought to mesopic luminance levels while green and red cones function in photopic conditions. Depending on which filter we choose, the effect on illuminance available for blue cones and rods will vary. It may be the reason why different patients experience different filters better than other almost similar filters.

The change in the quality of the image that many RP-patients describe, when looking through CPF- or similar lenses, sounds as if the filter would stop the rods making noise. Now there is a possibility of assessing the filter effect more carefully when we can separate the effect on luminance from the filter effect.

Since we have no tests for selecting absorption lenses, the best way to make a sensible choice is to have as many absorptive lenses as possible for comparison during a camp or weekend gathering for the deafblind. We have used the skiing camps in March and the family learning vacations during the summer in Finland for this purpose. When the different absorptive glasses are compared at different times of the day in full sunlight, in shaded areas and when moving in and out of buildings, individuals with Usher Syndrome learn the typical features of each lens and make very well founded decisions on what to use.

Photochromatic or nonphotochromatic lenses

The photochromatic filter lenses are more expensive than the non-photochromatic and therefore there has to be a good reason for prescribing them. When the type of filter has been decided upon, I lend the corresponding commercially available photocromatic clip-on pair and the corresponding tinted plastic lenses for a week or two weeks after which time the patient is asked to describe what is good and what is problematic in both lenses. If the photochromatic feature is considered important enough, the patient gets the photocromatic lense. If it is not appreciated, the patient usually prefers having the lighter weight plastic lenses.