Early diagnosis of visual disorders


Dr. K. Spang
Dipl. Ing. M. Repnow
Prof. M. Fahle

Quite a number of diseases can influence visual perception, part of them especially central vision while others start by impairing visual resolution in the periphery of the visual field. The latter disorders are generally even more dangerous than the former, since patients become well aware of a decrease of central acuity while they usually miss the symptoms of degradation of vision in the peripheral visual field. Therefore, disorders such as glaucoma (increased intraocular pressure), or brain tumors affecting parts of the visual system concerned with peripheral rather than central vision may exist and progress without the patient noticing, and irreversible damage is done that could have been prevented if the disorder would have been detected and diagnosed earlier, leading to an adequate therapy. Moreover, defects of so-called ‘higher’ cortical visual areas, which constitute about 25% of the human cortex, may lead to perceptual disorders not apparent in clinical tests of visual function such as tests of visual acuity and visual fields by (conventional) perimetry, but may only affect some specialized subfunctions of vision, such as colour vision, motion perception, or perception of stereoscopic depth in part of the visual field.

Project aims: To develop and improve screening methods allowing to detect visual field defects in patients at an early stage. These screening methods should detect not only defects in visual resolution or contrast detection simultaneously over the visual field, but also be able to elucidate defects of other subfunctions or components of visual perception such as isolated defects of colour- or motion perception in parts of the visual field.

Methods: Test stimuli are presented simultaneously over a part of the visual field as large as possible, either on a large monitor or by means of a video-projector under control of a computer. For presentations on the monitor, the patient’s head is supported by means of a head-rest and brow bar, and distance to the monitor is as small as possible to allow to test as large a part of the visual field as possible. On the monitor, one of thirteen types of test stimuli is displayed at a time. Examples of stimuli are shown in Fig. 1. However, this figure can only display one frame of each of the stimuli, while the main characteristic of the stimuli is that they are continuously changing, i.e. dynamic. The simplest of the stimuli is a type of visual white noise, similar to the appearance of a TV set if the antenna is disconnected. This stimulus was originally proposed by Aulhorn and Köst (1988). We have modified this stimulus in several ways, a) by increasing grain size of the elements towards the periphery of the visual field in order to compensate for the decreasing resolution there; b) by colouring both the elements and the background, for example red and green, and with (almost) no luminance contrast, hence the elements differ only regarding colour and cannot be detected by a colour-blind system. Moreover, we designed additional stimuli that test patient’s ability to simultaneously process motion information, colour, and line orientation in the entire visual field (cf. Fig. 1). Patients were asked to fixate the middle of the monitor as steady as possible, to let their mind’s eye (rather than their real eyes) wander over the entire stimulus, and to pay attention whether the stimulus appeared homogeneous over the entire extend of the monitor or whether there were any parts where it appeared to move slower, not at all, where the colours seemed paler, or where depth was not as clear, etc. If such regions were present, the patients encircled them by means of a felt-tip directly on an acetate sheet that covered the monitor, while keeping their eyes on the center of the monitor. The size and position of these areas were afterwards calculated by feeding the lines from the acetate into a computer by means of a digitizing tablet.

Figure 1

Figure 1: Typical stimuli of component perimetry, testing the ability of the patient to detect differences in line orientation, colour flicker appearance of coloured dots, rotation of Landolt Cs, rotation of line elements, or interruptions in straight lines. To start an animated example stimulus click here

Results: The results are encouraging: all patients with suprageniculate lesions subjec-tively experienced their visual field defects in component-perimetry. Sizes of visual field defects obtained with both methods correspond well with each other, the correlation is highly significant (Fig. 2). The specificity of component perimetry is higher than that of the original noise-field campimetry (Bachmann & Fahle, 1999).

Figure 2

Figure 2: Correlation factors between results of all 13 patterns of component perimetry with the results of conventional perimetry.

Conclusions: This pilot study indicates that component perimetry is a subjective but relatively reliable method to detect dis-orders of visual perception caused by lesions at different stages along the visual pathway, permitting a fast screening of the visual field. In addition, this method seems to allow examining the visual field not just regarding defects in contrast sensitivity – as conventional light perimetry does – but regarding other components of vision such as color or motion perception, too. Further evaluation with larger patient cohorts will be required to allow exact assessment of the clinical usefulness of the method.

References:

  1. Aulhorn E, Köst G. Rauschfeldkampmetrie: Eine neuartige perimetrische Untersuchungsweise. Klinische Monatsblätter für Augenheilkunde 1988; 192: 284-288
  2. Bachmann G, Fahle M. Component perimetry: A fast method to detect visual field defects caused by brain lesions. Investigative Ophthalmology & Visual Science 1999; ( in Revision)

 

 

 

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