Patients with Stargardt's disease often show a significantly reduced ability to read despite fairly good visual acuity. We evaluated whether fundus perimetry with simultaneous observation of the point of fixation can help to explain these difficulties.
Methods: A total of 40 eyes in 21 patients with Stargardt's disease were examined by means of automatic fundus threshold perimetry and special fixation tasks using a scanning laser ophthalmoscope (SLO).
Results: During fundus perimetry 19 eyes showed a movement of the mean fixation point towards the top, and five to the right of the central scotoma. At the same time, the variation around the mean fixation point was orientated vertically. We observed an alternation between two different points of fixation in another eight eyes. While one fixation point was usually located in the center inside an area of reduced light sensitivity, the second new locus of fixation was at the top of the scotoma (six of eight eyes). Patients tried to fix small targets inside the scotomatous area.
Conclusion: Our results demonstrate that patients with Stargardt's disease exhibit a typical pattern of fixation. The development of eccentric fixation occurs in three steps: The initial phase is characterized by central fixation with decreased stability. In the intermediate phase, we observed alternation between central and eccentric fixation. Finally, patients develop a constant eccentric fixation. These results may explain the difficulties in reading which many Stargardt's patients experience and the problems in prescribing corrective devices for these patients.