Variations of vision come in many flavors. Some are dramatic and lend themselves to fairly accurate descriptions and timing, e g, transitory retinal strokes (which usually evolve over a few minutes) and classical migraine auras (of the order of 20 minutes). At the other end of the spectrum, variations may be much more subtle, more difficult to describe, and repetitive on short time-scales (minutes, seconds, fractions of seconds). A physiological variant is the Troxler phenomenon, the fading-from-view that normally occurs with a steady fixation, with immediate return of vision upon blinking or shifting fixation. For a striking demonstration of the Troxler effect, try the
Lilac Chaser
illusion. A familiar example of abnormal variation of vision is the obscurations frequently associated with papilledema from raised intracranial pressure. There are many other variants of short-term, repetitive variations of vision which might fit under a heading of unstable vision. This is the topic of this presentation.
Unstable vision is a frequent complaint but many patients find it difficult to provide detailed descriptions and very few are prepared to illustrate their observations graphically. Indeed, graphic libraries of unstable visual perceptions seem to be lacking. Part of the difficulty of coming to grips with symptoms like these is the lack of a colloquial terminology and the very instability of the percept, i e, its variation over time. Fortunately, variation over time is easily arranged in computer graphics, so allowing the generation of a symptomimetic reference library.
By modifying the fixed reference image shown to the right, the display aims to present several variants of unstable vision on an adjustable time scale. The idea is to try to find an approximate match to the individual patient's perceptions and to obtain a rough quantitative measure. Select a type of image modification under the Feature heading and click on the display to start variation. Click again to stop. The Mode heading allows selection of regular or irregular variations. Adjust the rate of change by means of the slider. Older browsers cannot display sliders but will instead present a text input field. To vary the rate of change, enter a value in the 50 to 95 range and tap the Enter key.
The types of variations included in the display aim to match real patients' descriptions of unstable vision with physical image properties as follows:
External light - mimics variations in the mains supply to a single light source
Fog - mimics true environmental fog, i e, dispersion of light from a single light source by water droplets
Brightness - variation of intensity values
Contrast - variation of the range of intensity values
Brightness & Contrast - combination of the two preceding procedures
Saturation - variation of gray content
Blur - gaussian combination of neighboring pixels
Noise - laplacian addition of blank pixels
Flicker - interspersing blank frames
Jitter - random movements of image
Waves - structured deformations
The display deliberately has been made very simple. Readers familiar with modern drawing software will recognize the potential for much finer graduations and also for mixing various types of unstable percepts. However, such refinements would require a much more complex interface, with a much steeper learning curve, and also would cause slower renditions.
New 2016: a refined version of the above display has been included with a host of other symptomimetic displays in a new dynamic visual library. The library has the format of an iPhone/iPad app named
Visual Disorder Atlas.
Interpretation
Broadly speaking, unstable vision generally seems attributable to optical causes, neural causes, or both. Optical causes like variations in corneal shape, contact lens movements, and swirling anterior chamber or vitreous hemorrhages will be left aside here, where neural causes are of primary interest. Unfortunately, neural causes of unstable vision are much more poorly understood and the following comments must be regarded as tentative only. Side effects of drugs always need to be considered. Much remains to be explored and documented about unstable vision...
Mains faults - probably equivalent to perceived variations of brightness, or contrast, or both
Fog - this term is probably often used as a synonym of blur
Brightness ?
Contrast ?
Brightness & Contrast ?
Saturation ?
Blur - probably most often optical in origin
Noise - consider inflammatory and paraneoplastic retinopathies
By modifying the fixed reference image shown to the right, the display aims to present several variants of unstable vision on an adjustable time scale. The idea is to try to find an approximate match to the individual patient's perceptions and to obtain a rough quantitative measure. Select a type of image modification under the Feature heading and click on the display to start variation. Click again to stop. The Mode heading allows selection of regular or irregular variations. Adjust the rate of change by means of the slider. Older browsers cannot display sliders but will instead present a text input field. To vary the rate of change, enter a value in the 50 to 95 range and tap the Enter key.