Eccentric viewing

In the presence of a central scotoma, patients will need to use eccentric viewing (EV) and deliberately fixate to the side of the object of interest.
Patients use EV to move the scotoma so that it does not obstruct the target being viewed, hence these patients cannot use the fovea to fixate. It is important to assess which retinal region is best to use so as to offer the greatest chance of improvement.
This area of the retina, rather than the fovea, will be used for fixation. Since VA declines towards the periphery (away from the foveal region), it is the area closest to the fovea (parafoveal location) which will give best improvement.5
This is often the area closest to the edge of the scotoma and will best serve to replace the fovea. Hence, it is called the preferred retinal locus (PRL). The PRL should be large enough for the image of several magnified letters to be seen simultaneously. Patients may have more than one PRL, especially if the central scotoma is large (anything up to 10¡ from the fovea in size) and therefore patients may not be able to assign precedence to just one PRL. This may not be due to the poor adaptability of the patient, but just that each area may be better for a particular functional visual task.8
Reading also involves maintaining fixation on a word/group of words, for a period of time, before making an accurate saccade to the next word/group of words. Hence, as well as having the best possible PRL, it is important to have good fixation control. It must be remembered that parafoveal fixation is not as accurate as foveal fixation.1,8,9
Eccentric viewing can be time-consuming and difficult to teach, hence it is useful to have an indication of the likely success and which PRL is to be used. The PRL will need to resolve letters and, if used for reading, will need to image several letters simultaneously to achieve acceptable reading speed.1
Attempts have been made to determine the best part of the retina for PRL in terms of acuity, location and size. These methods are based on using measured central visual fields to determine the exact location and size of the (central) scotoma. This approach is successful if a suitable fixation target is used and working thresholds are determined for parafoveal, rather than foveal targets.
The Bjerrum screen is a possible method to determine the PRL. It is good for enlarging the apparent size of the scotoma by increasing viewing distance, so the patient is 2m or more from the screen. The screen is a detection test for single spot targets, and is not useful for letters, as discussed earlier. The patients are likely to have a central scotoma, hence the examiner may experience difficulty in encouraging the patient to maintain foveal fixation while initial measurements are taken. With a co-operative patient, single letter targets (the Sheridan-Gardiner test booklet) can be presented against the background of the Bjerrum screen at various locations in the visual field.1
Another method that may be attempted is the 'topography of residual vision', which is plotted by presenting letter targets to be recognised at 32 locations. The problem with these tests is that you cannot establish exactly how the patient is fixating while the test is being conducted.5
Examination of the retinal area used for fixation can be achieved using a scanning laser ophthalmoscope (SLO). Here the examiner projects the visual target onto the area of the retina to be tested, while simultaneously visualising both the retina and the visual target. The examiner, therefore, needs to know exactly which retinal location is under test. The SLO is the only method of measuring visual performance in a patient with a central scotoma. However, this method is currently only available in research settings.
In clinical practice, satisfactory results can be obtained using the Amsler grid to assess central field function. The Amsler grid can be very sensitive where VA is normal. On occasions, however, it shows an entirely spurious false negative response. For example, a patient may have severe macular pathology with poor acuity, yet report the grid as complete, clear and undistorted. This problem has been investigated in detail on many occasions, but attempts to rectify the errors have been unsuccessful.10
The best Amsler grid to use is the recording chart (black on white) with an additional fixation cross drawn on it (Figure 1). The test should be done monocularly with the non-viewing eye occluded. The patient holds the chart at a comfortable position to see it with the appropriate near correction worn. Even if vision is 1/60, most patients will be able to see the chart. However, if vision is particularly poor then the chart can be viewed via a CCTV, or a hand-drawn enlarged chart.1
The Amsler recording chart
Figure 2 illustrates a series of questions to establish if there is a central scotoma and which part of the visual field experiences clearest vision.
The patient must adopt a straight ahead viewing position initially and hold their head still. When using the grid to identify EV positions, the patient should not be instructed to 'look at the centre dot' unless the patient identifies the dot first.
In the presence of a central scotoma, the patient may be unable to view the centre dot without shifting gaze to an off-centre position. The purpose of this viewing technique is to determine where the scotoma(s) are located when the eyes are in the straight ahead position. The patient then describes how the grid looks when viewed from a straight ahead position and is encouraged not to shift to an off-centre position initially.
After any scotomas and distortion areas have been identified, the patient is asked to move their eyes only, superiorly, inferiorly, right and left, noting the central area of the grid. The patient may determine at what position the eye allows the least distorted or best view of the central grid. After a position has been selected, the patient is instructed to compare viewing positions of the two closest quadrants and select the better position. The best viewing position is most often in the direction of the densest scotoma or distortion area. If the viewing position of each eye is found to be different, the position found for the better eye is the one recommended for use.9
Binocular vision is very unlikely as optimally functional areas of the retina are not likely to coincide in the two eyes. However, if the patient develops EV in the better eye binocular vision may be possible.

determining direction of EV
Patient understanding of eccentric viewing is critical. Patients should be instructed that vision is better with their eyes in a certain position and what has to be done to move into that position.
It is important to stress to patients that they can get into a better viewing position by controlling their eye movements. Patients should be encouraged to shift their eyes to the position grossly identified as their best viewing position.
Patients can practise using the eccentric viewing positions with various objects in their own environment as well as specific reading exercises and visual training material. Patients with central scotomas who can identify an eccentric viewing position and have an understanding of the technique have a better chance of successful low vision device use.

References
References for all three articles in this series are available from optician features editor Rob Moss,
email rob.mossrbi.co.uk.

Rabiah Narband is a pre-reg optometrist at SpecsaversIn the presence of a central scotoma, patients will need to use eccentric viewing (EV) and deliberately fixate to the side of the object of interest.
Patients use EV to move the scotoma so that it does not obstruct the target being viewed, hence these patients cannot use the fovea to fixate. It is important to assess which retinal region is best to use so as to offer the greatest chance of improvement.
This area of the retina, rather than the fovea, will be used for fixation. Since VA declines towards the periphery (away from the foveal region), it is the area closest to the fovea (parafoveal location) which will give best improvement.5
This is often the area closest to the edge of the scotoma and will best serve to replace the fovea. Hence, it is called the preferred retinal locus (PRL). The PRL should be large enough for the image of several magnified letters to be seen simultaneously. Patients may have more than one PRL, especially if the central scotoma is large (anything up to 10¡ from the fovea in size) and therefore patients may not be able to assign precedence to just one PRL. This may not be due to the poor adaptability of the patient, but just that each area may be better for a particular functional visual task.8
Reading also involves maintaining fixation on a word/group of words, for a period of time, before making an accurate saccade to the next word/group of words. Hence, as well as having the best possible PRL, it is important to have good fixation control. It must be remembered that parafoveal fixation is not as accurate as foveal fixation.1,8,9
Eccentric viewing can be time-consuming and difficult to teach, hence it is useful to have an indication of the likely success and which PRL is to be used. The PRL will need to resolve letters and, if used for reading, will need to image several letters simultaneously to achieve acceptable reading speed.1
Attempts have been made to determine the best part of the retina for PRL in terms of acuity, location and size. These methods are based on using measured central visual fields to determine the exact location and size of the (central) scotoma. This approach is successful if a suitable fixation target is used and working thresholds are determined for parafoveal, rather than foveal targets.
The Bjerrum screen is a possible method to determine the PRL. It is good for enlarging the apparent size of the scotoma by increasing viewing distance, so the patient is 2m or more from the screen. The screen is a detection test for single spot targets, and is not useful for letters, as discussed earlier. The patients are likely to have a central scotoma, hence the examiner may experience difficulty in encouraging the patient to maintain foveal fixation while initial measurements are taken. With a co-operative patient, single letter targets (the Sheridan-Gardiner test booklet) can be presented against the background of the Bjerrum screen at various locations in the visual field.1
Another method that may be attempted is the 'topography of residual vision', which is plotted by presenting letter targets to be recognised at 32 locations. The problem with these tests is that you cannot establish exactly how the patient is fixating while the test is being conducted.5
Examination of the retinal area used for fixation can be achieved using a scanning laser ophthalmoscope (SLO). Here the examiner projects the visual target onto the area of the retina to be tested, while simultaneously visualising both the retina and the visual target. The examiner, therefore, needs to know exactly which retinal location is under test. The SLO is the only method of measuring visual performance in a patient with a central scotoma. However, this method is currently only available in research settings.
In clinical practice, satisfactory results can be obtained using the Amsler grid to assess central field function. The Amsler grid can be very sensitive where VA is normal. On occasions, however, it shows an entirely spurious false negative response. For example, a patient may have severe macular pathology with poor acuity, yet report the grid as complete, clear and undistorted. This problem has been investigated in detail on many occasions, but attempts to rectify the errors have been unsuccessful.10
The best Amsler grid to use is the recording chart (black on white) with an additional fixation cross drawn on it (Figure 1). The test should be done monocularly with the non-viewing eye occluded. The patient holds the chart at a comfortable position to see it with the appropriate near correction worn. Even if vision is 1/60, most patients will be able to see the chart. However, if vision is particularly poor then the chart can be viewed via a CCTV, or a hand-drawn enlarged chart.1
The Amsler recording chart
Figure 2 illustrates a series of questions to establish if there is a central scotoma and which part of the visual field experiences clearest vision.
The patient must adopt a straight ahead viewing position initially and hold their head still. When using the grid to identify EV positions, the patient should not be instructed to 'look at the centre dot' unless the patient identifies the dot first.
In the presence of a central scotoma, the patient may be unable to view the centre dot without shifting gaze to an off-centre position. The purpose of this viewing technique is to determine where the scotoma(s) are located when the eyes are in the straight ahead position. The patient then describes how the grid looks when viewed from a straight ahead position and is encouraged not to shift to an off-centre position initially.
After any scotomas and distortion areas have been identified, the patient is asked to move their eyes only, superiorly, inferiorly, right and left, noting the central area of the grid. The patient may determine at what position the eye allows the least distorted or best view of the central grid. After a position has been selected, the patient is instructed to compare viewing positions of the two closest quadrants and select the better position. The best viewing position is most often in the direction of the densest scotoma or distortion area. If the viewing position of each eye is found to be different, the position found for the better eye is the one recommended for use.9
Binocular vision is very unlikely as optimally functional areas of the retina are not likely to coincide in the two eyes. However, if the patient develops EV in the better eye binocular vision may be possible.

determining direction of EV
Patient understanding of eccentric viewing is critical. Patients should be instructed that vision is better with their eyes in a certain position and what has to be done to move into that position.
It is important to stress to patients that they can get into a better viewing position by controlling their eye movements. Patients should be encouraged to shift their eyes to the position grossly identified as their best viewing position.
Patients can practise using the eccentric viewing positions with various objects in their own environment as well as specific reading exercises and visual training material. Patients with central scotomas who can identify an eccentric viewing position and have an understanding of the technique have a better chance of successful low vision device use.

References
References for all three articles in this series are available from optician features editor Rob Moss,
email rob.mossrbi.co.uk.

Rabiah Narband is a pre-reg optometrist at Specsavers