*4.2. Proportion of the Examined Visual Field Area in the Paracentral Visual Field*

Another important difference between the different VF testing methods is the portion of the visual field actually examined. Testing the central VF using either Octopus, G1 program (17 points), or HFA3, 10-2 program (68 points) and employing Goldmann point size III, i.e., 4 mm<sup>2</sup> [4], the area examined only covers 3.1% and <1.0% of the central visual field, respectively. Here, at the examination distance of 30 cm, the 10◦ area of the central visual field is 87.58 cm<sup>2</sup> with a radius of 5.28 cm with minimal error variability due to the spherical surface deviation. These values explain why the arcuate scotoma was not found in F.H. with the Octopus perimeter. More accurate results can be expected with rapid campimetry where 6.75% of the paracentral visual field is tested.

#### *4.3. Accuracy of Rapid Campimetry*

To further test the accuracy of rapid campimetry vs. other standard perimetry, i.e., HFA3, we assessed whether glaucomatous VF defects were comparable in both techniques. Here, the examination of glaucomatous VF defects of five participants demonstrated agreement in the findings. In addition, rapid campimetry appeared to detect scotomata areas that were missed in the standard HFA3 test. The findings of subjects 1, 3, and 5, as well as F.H. suggest a superiority of the rapid campimetry vs. HFA3: For example, for F.H., the HFA found a relative scotoma in the upper visual field, whereas the campimetry instead found an absolute narrow arcuate scotoma at the same location (Figure 1e). As shown in Table 2, the angular diameter of the test point at the edge of the central 10◦ field of view is 0.31◦ compared to the conventional perimeter test mark III with a diameter of 0.43◦ at any point in the VF [18]. This latter large test mark cannot totally disappear in the narrow, approximately 0.35◦-wide scotoma of Figure 1e, and cannot be perceived as an absolute scotoma, but only as a relative one. Finding these narrow scotomas appears to be facilitated by rapid campimetry's technique of a continuous vertical light line of ≈1400 closely spaced test points that overlap during motion and intersect all nerve fibres running to the blind spot.

#### *4.4. Detection of Arc Scotomas*

The method of rapid campimetry is similar to the campimetry described by Rönne and developed by his teacher Bjerrum, where a 1 cm-sized test point with angular diameter of 0.29◦ moves slowly on a black rod at 4 m2 square black wall at 1–2 m distance [20]. In rapid campimetry, the dimensions are reduced and tailored to today's technology, as well as having the crucial feature of rapid movement. According to Rönne, the first early defects in glaucoma usually present in the Bjerrum area as small paramacular scotomas, which may be arcuately connected to the blind spot [20]. An explanation for arcuate scotomas is easily given by comparing the nerve fibre course in the retina with the shape and location of the arcuate scotomas where glaucoma damages individual optic nerve bundles and leads to interruption of the input from the corresponding retinal sites, leaving other bundles intact [21]. With today's standard examination methods, arcuate scotomas are hardly detected as such early stages, although their presence is theoretically probable [21].

Recently, finer patterns than the standard 24-2 VF tests, e.g., a 6 × 6◦ grid, have been applied, and studies have confirmed that multiple macular VF defects can occur in glaucoma, of which arcuate scotoma is the most common [22,23]. These VF defects could also correspond to structural damage [24]. More recently, a new testing paradigm, the 24-2C, has been developed, in which 10 asymmetrically distributed test points from the 10-2 grid are integrated into the 24-2 grid so that both the central and peripheral visual fields can be tested. Nevertheless, testing the central 10 degrees supports higher resolution in terms of a detailed description of VF defects and better structure–function agreement [25].

#### *4.5. Automation of Test Point Movement in the Paramacular Visual Field*

Glaucoma is a group of progressive optic neuropathies characterized by degeneration of retinal ganglion cells [26]. The probable consequence of such ganglion cell degeneration is absolute rather than relative scotoma. Aulhorn, in reviewing 961 visual fields of glaucoma subjects, found that very early scotomas, despite their small extent, are usually absolute and very rarely relative [17]. In principle, the shape of a scotoma can be described well with kinetic perimetry, but small paramacular scotomas can be easily missed [17]. The requirement for slow test point movement can be met by the instrument only if large movement distances on the examiner's side correspond to a small visual angle on the subject's side. This is only possible, however, if very large-area examination screens are used for direct test point guidance, as for example with the Bjerrum wall, or if a translation mechanism is used for indirect test point movement [17].

The combination of the two demands may seem absurd, to increase the running speed of the test point for a safer scotoma detection on the one hand, and to move the test point as slowly as possible for an accurate definition of the scotoma margins on the other hand. However, both demands belong together, and only together do they fulfill their task perfectly. With the automatic test point movement, which can be slowed down by a factor of 4 or 8, the rapid campimetry meets Aulhorn's demand of translation mechanics in the paramacular range. In this way, it is possible to translate the advantages of Bjerrum's and Rönne's campimetry [20] into a novel technique and to combine it with the attentional enhancement of the fast movement.

#### *4.6. Limitations of the Study*

Our case-series study has a number of limitations which need to be addressed in a follow-up study on a larger participant cohort. The study was designed to provide proof-of-concept of rapid campimetry and was not designed to assess the sensitivity and specificity of the approach. For the latter purpose, a systematic investigation with a greater sample size is essential, including patients with different disease states and healthy controls. Further, potentially confounding effects of visual pathologies, e.g., optic media opacities, deserve attention in future studies. Finally, the quality of the fixation and its relation to the campimetry outcome has not been addressed in the present study, where patients were instructed to fixate the central target during testing and repeatedly reminded of the importance of central fixation. Online tracking of eye movements and fixation monitoring would help to assess whether maintaining central fixation is an issue during rapid campimetry testing.
