Reading strategies in Stargardt's disease with foveal sparing
- Mira Goldschmidt†1Email author,
- Anouk Déruaz†1, 2,
- Erika N Lorincz†1,
- Andrew R Whatham1, 3,
- Christophe Mermoud1 and
- Avinoam B Safran1
© Goldschmidt et al; licensee BioMed Central Ltd. 2010
Received: 13 August 2009
Accepted: 22 January 2010
Published: 22 January 2010
Subjects with a ring scotoma can use two retinal loci, a foveal and a peripheral, for reading. Our aim was to investigate the relative use of both retinal loci as a function of the spared foveal area size and the spatial resolution at both retinal loci.
Two patients with Stargardt's disease and ring scotomas read through a scanning laser ophthalmoscope a series of letters and words at various character sizes. The number of fixations made using each retinal locus was quantified. The relative use of each retinal locus depended on character size of the stimulus. Both patients used exclusively the eccentric retinal locus to read words of large character sizes. At small character sizes, the central retinal locus was predominantly used. For reading letters or words, once foveal fixation was used, patients did not shift back to the eccentric retinal locus. When spatial resolution allowed deciphering at both the eccentric and the central areas, patients consistently fixated with the eccentric retinal locus.
Spatial resolution at the eccentric locus appears as a determinant factor to select the retinal area for reading. Reading strategies in patients with Stargardt's disease and a ring scotoma demonstrate a pattern of coordination of both eccentric and central retinal loci, reflecting a high degree of adaptation.
Central scotomas are frequently encountered in low vision populations and induce an impediment to efficient reading. Common causes of central scotomas include age-related macular degeneration (AMD) and Stargardt's disease. Patients with central scotoma use an extrafoveal locus for fixation in place of the fovea . Such a locus is commonly referred to as preferred retinal locus (PRL) .
Some patients affected by macular degeneration develop a ring scotoma. Such a condition is of particular interest with regard to its functional implications. As previously reported [3–5], subjects with a ring scotoma fixate small stimuli with the central locus, but wider stimuli with the eccentric locus. The alternation between central and eccentric fixations was found in 8 out of 40 eyes of patients with Stargart's disease , and it was suggested that this fixation pattern was a phase in the transition from foveal to eccentric fixation .
In the present study we investigated reading strategies in patients with Stargardt's disease and a ring scotoma. Our purpose was to assess the relative use of two PRLs, a foveal and a peripheral, as a function of the spared foveal area size and of the spatial resolution at both PRLs. The results showed that reading strategies strongly depended on the spatial resolution at the eccentric retinal locus.
Two female patients (CC and CN) with bilateral ring scotomas following Stargardt's disease were studied. They were aged 28 (CC) and 36 years (CN). The time since onset of the macular disease was 5 and 2 years respectively. None of the patients had received either low vision rehabilitation or optical magnifiers before data collection. Patients gave their informed consent to the testing procedures that were in accordance with the Declaration of Helsinki.
Experimental set-up and testing procedures
After locating the PRLs, the number of fixations was manually quantified, through frame-by-frame playback of the video recording. A fixation was defined as a stable retinal position of at least 3 successive frames, i.e., 120 ms. Reading strategies were assessed by the proportions of fixations performed with either the eccentric or the central PRL. The proportions of fixations were calculated for each subject and each letter or word. To determine the relative use of both PRLs as a function of character size and words length, the coefficient was computed from the number of fixations made with each PRL.
Patients' clinical characteristics
Patients' clinical characteristics
Visual Acuity (logMAR)
Spatial resolution at the eccentric PRL (logMAR)
Central PRL size
Spatial resolution at the central PRL (logMAR)
Better than 0.8*
Better than 0.8*
Spatial resolution at the eccentric PRL, as determined by the visumetry test, was 1.2 logMAR and 1.0 logMAR for patients CC and CN respectively. As could be expected from the EDTRS tests, the spatial resolution at the central PRL was better than 0.8 logMAR for both patients, which is the SLO's program spatial resolution limit.
For both patients the relative use of the central PRL and the eccentric PRL strongly depended on the character size (Figure 3). The analysis performed on the PC coefficient revealed that the proportion of fixations between the eccentric and the central PRLs varied significantly across the 8 character sizes (Kruskal-Wallis test, CC: χ2 = 24.73, df = 7, p = 0.001; CN: χ2 = 22.44, df = 7, p = 0.002), although the total number of fixations did not differ significantly between character sizes (non parametric Kruskal-Wallis test, CC: χ2 = 3.44, df = 7, p = 0.841; CN: χ2 = 7.30, df = 7, p = 0.398). Patients used exclusively the eccentric PRL to read words of large character sizes (CC: 1.3 logMar; CN: 1.2 logMar). They mainly used eccentric fixations until 1.2 logMAR (CC) and 1 logMAR (CN). At the smallest character size they employed predominantly the central PRL.
In contrast to character size, word's length did not influence the relative use of each PRL (Figure 4). The proportion of eccentric or central fixations was similar across word length for each patient (Kruskal-Wallis test, CC: χ2 = 0.82, df = 3, p = 0.85; CN: χ2 = 0.06, df = 3, p = 0.996). As expected, patients made more fixations for reading longer words (Kruskal-Wallis test, CC: χ2 = 19.69, df = 3, p < 0.0005; CN: χ2 = 8.60, df = 3, p = 0.035).
In both patients with Stargardt's disease and a ring scotoma reading strategies involved the combined use of both the eccentric PRL and the central PRL. The relative use of each PRL depended on character size. For reading words with large character sizes, patients used exclusively the eccentric PRL. For smaller character sizes, patients combined benefits of the eccentric PRL, i.e. a wider visual span and presumably a better spatial word localisation, with those of the preserved foveal zone, i.e. a higher spatial resolution.
The use of multiple PRLs has been attributed to several factors. Whittaker  noticed that multiple PRLs were more likely to occur with large central scotoma above 20°, while Crossland  reported higher occurrence in patients with recent vision loss (<12 weeks).
The selection and the location of the PRLs also depend on the task requirements, including visual acuity, visual span, stimulus and background luminance, correspondence in binocular vision, attention load [12–16]. Alternation between multiple PRLs with complementary functional advantages might improve performance of the visual task.
Our findings emphasised the importance of the spatial resolution at the eccentric PRL as one of the determinant factor in selecting a fixation locus for reading. Interestingly, when characters could be deciphered in both PRLs, patients consistently used the eccentric locus. Although patients had a better spatial resolution at the central than at the eccentric PRL, they did not shift fixation to the central PRL as soon as one letter could fit within it, but only when character size was below the resolution of the eccentric PRL.
In contrast to character size, word length did not appear to influence the relative use of the central or the eccentric PRL. Although the central area could accommodate more than 1 or 2 letters, patients selected the eccentric PRL as long as its spatial resolution enabled reading.
Another interesting finding was that once foveal fixation was used, patients did not shift back to the eccentric PRL. This phenomenon can be attributed to the fact that using the eccentric PRL implies the development of new oculomotor skills, involving shift of the oculomotor reference from the fovea to a non foveal locus . Subsequently, eye movements need to be recoded in another coordinate system as soon as changes in PRLs occur, which is a constraining task, requiring a major plastic reorganisation in the oculomotor control.
Reading strategies in patients with Stargardt's disease and a ring scotoma demonstrate a pattern of coordination of both eccentric and central PRLs. The spatial resolution at the eccentric PRL was a determinant factor in selecting a fixation locus. When spatial resolution allowed deciphering in both eccentric and central areas, patients consistently fixated with the eccentric PRL. More studies with greater patients' samples are needed to confirm these preliminary results.
This work was supported by a grant from the "Pro Visu" Foundation, and from the "Fondation pour les Aveugles".
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