The development of the preferred retinal locus in macular disease 3063 Michael D' Crossland, Louise

1
The development of the preferred retinal locus in
macular disease (3063)Michael D. Crossland,
Louise E. Culham, Edmond S.W. Ng, Gary S. Rubin
Institute of Ophthalmology, University
College London, UK and Moorfields Eye Hospital
NHS Trust, London, UK(Except Medical
Statistics Unit, London School of Hygiene and
Tropical Medicine, UK)

m.crossland_at_ucl.ac.uk
THE DEVELOPMENT OF THE PRL Five elements of the
development of the preferred retinal locus have
been identified 1. The use of a repeatable
principal PRL (made by all patients) 2. A lack of
awareness of PRL use (experienced by 64 of
patients) 3. The use of a repeatable number of
PRLs under all conditions (76 of patients) 4.
The use of a single PRL (56 of patients) 5. An
improvement in fixation stability (44 of
patients) Stepwise statistical regression was
performed to investigate the relative importance
of these aspects of PRL development in terms of
determining change in reading speed over the
course of this study (table 1). TABLE 1
Result of statistical regression to determine the
factors affecting change in reading speed over
the course of this study. CONCLUSIONS All of the
patients examined in this study developed a
repeatable non-foveal preferred retinal locus for
fixation of a target. No patients developed a
PRL above the scotoma in visual field. Nearly
80 of the variance in change in reading speed
over the twelve months following the onset of a
scotoma is accounted for by changes in fixation
stability, a strategy of using the same number
of PRLs under all conditions and a
non-awareness of PRL use. PRL location does not
affect reading speed. This result reinforces the
findings of others.e.g.3 The development of a
strategy whereby the same number of PRLs are used
under all conditions is of more importance than
the number of PRLs used per se. These findings
have relevance for clinicians working in low
vision clinics and for the design of programmes
to train visual behaviour in patients with MD.
RESULTS I PRL location The location of the
principal PRL at baseline and at exit from the
study is shown in figures 3 and 4. PRL location
changed in 13 patients (52). In these 13
patients, the median time to adopt the final
PRL was 3 months. All of the patients used their
final PRL within six months. At the exit point
of the study, the distribution of PRL locations
is similar to those seen in previous
cross-sectional studies.e.g.2

• INTRODUCTION
• Patients with bilateral central field loss due
  to macular disease (MD) must use peripheral
  retina in place of their damaged fovea for visual
  tasks. Many patients develop one or more
  preferred retinal locus (PRL) or loci (PRLs).
  This poster presents the results of a
  longitudinal study investigating the development
  of the PRL over the first twelve months following
  scotoma onset in the better eye of patients with
  MD.
• METHODS
• Patients
• 25 patients with central scotomas due to MD
  were recruited. All patients had developed
  scotomas in their better eye within the previous
  month.
• Attendances
• Baseline, 1, 3, 6 and 12 months.
• Patients were excluded if VA dropped by gt0.2
  logMAR or scotoma size enlarged by gt0.5 disc
  diameters.
• PRL assessment
• The location of the principal PRL was found using
  a Rodenstock scanning laser ophthalmoscope (SLO,
  figure 1).
• Patients awareness of PRL use was assessed by
  comparing fixation position under these two
  instructions
• Please look at the red cross in the centre of
  the screen in front of you
• Please move your eyes so that you are looking
  straight at the cross, even if it disappears or
  becomes difficult to see
• Fixation stability was assessed when observing a
  point target using an infra-red gazetracker (SMI
  Eyelink I, figure 2). A bivariate contour
  ellipse area (BCEA) was calculated to encompass
  68 of the fixation locations.
• The number of PRLs was calculated using the
  gazetracker fixation data and an algorithm
  developed for this purpose.1 Variability in the
  number of PRLs used was assessed for five targets
  in different positions of gaze.

FIGURE 3 PRL location at baseline
FIGURE 4 PRL location at
exit
RESULTS II Awareness of PRL use Patients
awareness of using their PRL was assessed by
comparing the retinal area used under
instructions 1 and 2. By the end of the study,
16 patients had no awareness of their PRL use.
This adaptation did not occur before patients
adopted their final PRL location the median
time between using the final PRL and losing
awareness of PRL use was 5.5 months (IQR 0
11).
RESULTS III The number of PRLs used At the
baseline assessment, 16 patients (64) use more
than one PRL for some directions of gaze. This
number falls to 11 patients by the exit point.
The number of patients who use a repeatable
number of PRLs under all conditions rises from 12
at baseline to 19 at exit (figure 5).
RESULTS IV Fixation stability There were no
systematic changes in fixation stability over the
course of this study (mean BCEA at baseline
8,430 minarc2, at exit 8,100 minarc2). Fixation
stability improved in 11 patients, deteriorated
in 9 patients and remained constant in 5
patients.

• REFERENCES
• Crossland, M.D., et al. (2004). Evaluation of a
  new quantitative technique to assess the number
  and extent of preferred retinal loci in macular
  disease. Vision Research, In Press.
• Fletcher, D.C., Schuchard, R.A. (1997).
  Preferred retinal loci relationship to macular
  scotomas in a low-vision population.
  Ophthalmology, 104, 632-638.
• Sunness, J.S. et al (1996). Fixation patterns
  and reading rates in eyes with central scotomas
  from advanced atrophic age-related macular
  degeneration and Stargardt disease.
  Ophthalmology, 103, 1458-1466.

RESULTS V Reading speed Mean reading speed was
85 words/minute at baseline and 73 words/minute
at exit. Reading speed improved by gt25 in four
patients and deteriorated by gt25 in 7 patients.
There was no difference in reading speed in
patients who used each of the three PRL locations
at the exit point of the study (ANOVA,
F(df2,24)0.67, p0.52).
ACKNOWLEDGEMENTS The MN-read style sentences were
provided by Dr Elisabeth Fine of Schepens Eye
Institute, Harvard University, MA. MDC is
supported by Guide Dogs for the Blind Association
grant 2000-29a. GSR
is supported by European Commission grant
QLK6-CT-2002-00214. CR none.