How to Define Clusters of Test Locations
Many approaches to the detection and quantitative assessment of local visual defects are described in literature relating to this subject. There is evidence that in normal as well as in pathological visual fields, test locations are relative to one another.1 Since the threshold in single test locations is highly influenced by fluctuation,2 grouping of test locations (so-called clustering) is a means of reducing the magnitude of fluctuation. In the assessment of visual fields, judging neighbouring test locations with slightly reduced sensitivity as clusters is considered a valid procedure for the recognition of abnormality, while single test locations at a defect level of eight decibels (dB) are considered to result from normal fluctuation.3 Local changes are thus identified more easily if they are all part of the investigated cluster of test locations.
Correlations Between Test Locations
In 1982, Wirtschafter et al.4 gave a first analysis of correlation between test locations. They projected the test locations of visual field examinations onto fundus photographs of primate eyes and grouped them into 15 clusters. Asman and Heijl5 proved that evaluating clusters following the nerve fibre bundles improved glaucoma detection in humans. Their studies resulted in the Glaucoma Hemifield Test (GHT)6 using 10 clusters. Weber and Ulrich7 pursued a different approach, analysing the scotoma borders of 159 glaucoma fields and building 21 clusters according to theaffected location’s course. Other teams8,9 applied statistical analysis of glaucomatous visual fields for the definition of the clusters. Garway-Heath et al.10 followed a different path by measuring the angles of visible retinal nerve fibre layer (RNFL) wedges at the optic disc in 1° steps and correlating them with the corresponding visual field defects. Newly available diagnostic methods have been included in the hunt for improved structure–function correlation. Anton et al.,11 like Gardiner et al.,12 compared focal visual field defects with the topographical measurements in 36 sectors of the Heidelberg retina tomograph (HRT) result. Buerki et al.13 decided to choose a procedure similar to the polar representation at the optic disc. He superimposed the Octopus program G test locations onto a nerve fibre image and followed the corresponding nerve fibres back to the optic disc. Test locations with an exitangle difference of no more than 5° were grouped to build 22 small clusters. Practical evaluation of the 22 clusters(see Figure 1a), which consisted of two test locations, showed that the reduction of fluctuation was not satisfactory. As a consequence, the 22 small clusters were joined to form 10 larger clusters (see Figure 1b). These clusters served as starting points for all further computations. Due to the course of the nerve fibres, the clusters do not mirror one another along the horizontal axis.