The course of Stargardt’s disease is marked by central vision loss and development of eccentric fixation. In this study, we showed that perimetry can be a reliable method for assessment of fixation behavior. Its advantage is that it is a standardized method that it is available in most ophthalmology clinics.
However, it is not always possible to determine the FL in visual fields. In this cohort, 114 of 783 tests could not be analyzed due to absence or poor delineation of the blind spot. Moreover, if a patient develops a paracentral scotoma simulating a shifted central scotoma, in association with an ambiguous blind spot position, this could be erroneously interpreted as eccentric fixation. Nevertheless, if the blind spot is clearly delimited, visual fields can be used as an alternative, especially in retrospective evaluations or if fixation cannot be determined by direct fundus controlled devices (e.g., SLO). In doing so, the determined coordinates must be interpreted carefully and in association with other information about the fixation behavior, such as scotoma size and position, and funduscopic examination.
The PRL can be defined as the median of all fixation points recorded by SLO, whereas the FL is determined by the position of the blind spot. Despite discrepancies between methods (different stimulus contrast and pattern; as well as the fixation target) and uncertainties in the determination of the psychophysical blind spot, their good agreement shows that in cases with a well-established PRL (for details about SLO examination see Reinhard et al.
23 ) visual fields can allow a correct determination of the FL.
For the patient, knowledge about the FL is essential for rehabilitation, especially for reading, as one of the most important tasks in everyday life. We found that SMD patients often go through an intermediate phase before an absolute scotoma develops, showing a ring scotoma that can persist for up to 18.8 years. This phase can be critical for the patients in dealing with everyday tasks. The fovea maintains good visual acuity, but the central island may be too small for reading.
14 This phenomenon is evident in
Figure 5 , where a small cross is fixated centrally, but a 4° diameter diamond is fixated eccentrically.
In agreement with a previous report,
20 our data show that most of the SMD patients ultimately reached a visual acuity of 1 logMAR (20/200). Furthermore, visual acuity at the last test before eccentric fixation was adopted was not a predictive factor for the time needed for the development of eccentric fixation. This finding can be explained by the presence of the central ring scotoma, which can be associated with good or reduced visual acuity (e.g., the remaining fovea inside the ring scotoma may not be sensitive enough for recognizing an optotype), but can still be used to fixate the target in perimetry.
We estimated a median age for development of eccentric fixation of approximately 23 years. However, there was considerable variability concerning age and eccentric fixation, and this may reflect the different disease phenotypes, and the reported wide age ranges for the onset of symptoms.
2 4 Unaccountably, there was a statistically significant cohort effect showing that patients with early birth date developed eccentric fixation later in life compared with patients with late birth date. We believe that this is a bias in our data, provoked mainly by two factors: (1) once eccentric fixation has been adopted, visual field control testing was no longer performed on a regular basis and possibly therefore fewer older patients with early birth date who showed longstanding eccentric fixation were included in the study; and (2) the awareness and knowledge of eccentric fixation has been much higher in later decades, which may have provided the patients with more information about the benefits of using of an eccentric FL.
Most SMD patients in this study placed their eccentric FL below the scotoma (i.e., the PRL above the lesion on the retina;
Fig. 4 ). This is in accordance with our former study that also showed a clear predominance of the new FL to be placed on the lower visual field.
14 The preference for the upper retina (i.e., the lower visual field) cannot be explained by the spatial resolution in this area, since the cone and ganglion cells ratio distribution is radially asymmetrical, with the horizontal meridian having a higher density than the vertical.
25 However, placing the new FL between central scotoma and blind spot or in the temporal side of the scotoma would constrict lateral eye movements during reading, which requires a minimal horizontal visual field area.
15 26 27 28 29
In a review, Trauzettel-Klosinski
28 discussed the questions related to the preferential directions that patients with macular scotomas use to find the new FL in the visual field and concluded that placing the FL below the scotoma (upper retina) is considered favorable for horizontal reading, because it does not cover any part of the current line to be read. Accordingly, in reading tasks not involving eye movements, Petre et al.
30 showed that the fixation area plays an important role in reading performance.
Furthermore, it has been shown that a favorable FL can be trained to achieve better conditions for reading.
31 However, it has been shown in several studies that patients with macular scotomas and different diagnoses without goal-directed training establish the PRL in the upper retina much less frequently.
16 32
After an eccentric FL is developed, patients with SMD tend to keep the FL in the same area of the visual field in subsequent tests, which indicates a preference for a certain FL location that can be influenced by preexisting individual features. For instance, Altpeter et al.
33 presented evidence that focal visual attention mechanisms can influence the preference by demonstrating that areas with high attentional capabilities are candidates for a future FL.
In conclusion, patients with SMD experience development of a central scotoma, and they frequently choose one eccentric FL. However, this development is often delayed by the presence of a ring scotoma, which may persist for different periods. Once fixation is eccentric, SMD patients tend to keep the same FL over time, and there is a preference for placing the FL below the scotoma.
The authors thank Malte Weismann, MD, and Florian Gekeler, MD, from the Centre for Ophthalmology, University of Tübingen, for the assistance with the patients’ files.