Central Field Loss
When someone with bilateral central field loss looks straight at an object, it will fall into the scotoma and not be seen. This effect has been known for centuries: for example, in the late 19th century, friends of the impressionist artist Degas reported that he found it ‘torment to draw, when he could only see around the spot at which he was looking, and never the spot itself’ ( ). This method of using peripheral retina to observe something is known as eccentric viewing (EV) .
Eccentric Viewing and the Preferred Retinal Locus
Most people with central field loss will use a single region of peripheral retina in place of the damaged fovea to look at a scene ( ). When this part of the retina is used repeatedly for observation, it is known as the preferred retinal locus (PRL).
The PRL has been shown to develop within 6 months in most people with bilateral macular disease ( ), although training may be needed for the patient to use the most effective region for every task. In studies, microperimeters are usually used to identify whether a patient is using a PRL, but careful observation of the patient whilst they fixate a target can also show their fixation behaviour. For example, if the patient is asked to observe a budgie stick in front of the clinician’s face, their eye position can often be determined. Asking the patient to switch their gaze between a distance acuity test and the budgie stick can indicate how well established the PRL is: if the patient always quickly adopts the same eye position, they are likely to have a more stable PRL than if they make several scanning movements or use a different position each time.
Another method of determining the PRL location is to use an Amsler grid. The patient is asked to look towards the centre of the chart, and then to report any missing regions. If the top of the chart is missing, for example, it is likely that they are using a PRL beneath the scotoma; if the left hand side is distorted then their PRL is to the right. If the central dot is not visible, or the centre is missing, they are probably not using EV. A similar approach, using a clock, is often used for training EV (see Section ‘PRL Training’ later).
The best Amsler Grid to use is the recording chart (black-on-white) with an additional diagonal fixation cross drawn on it. The testing must be carried out monocularly, and the nonviewing eye occluded. An appropriate reading addition is used as required, and the patient is allowed to hold the chart at any comfortable distance in order to see it. Even with an acuity of 1.60 logMAR, the chart can usually be seen under these circumstances, but if vision is extremely poor, the chart can be presented on an electronic vision enhancement system, or a hand-drawn enlarged chart can be used. The California Central Visual Field Test (see Chapter 3 ) can be used if it is necessary to plot the position and depth of the scotoma more precisely.
Systematic errors on a vision test can also indicate the PRL position. For example, if the last letter on a distance acuity chart is always missed, or the patient skips the last word on each line of a reading task, it is likely that the PRL is to the left of the scotoma in visual field space.
Patients are often not aware of using EV, even when they make very obvious head or eye movements to see, or when they move the object they are looking at around. When asked, for example, why they move a reading test around to read it, they often say that it is to get the lighting right or to bring the print into focus.
Someone who does not use EV strategies effectively may report being able to read only a few words before ‘all the words run together’ or ‘it all runs into one’. This person may not perform any better if magnification is used: in fact, they sometimes do worse as fewer magnified letters can be visualised simultaneously. When asked about their reading, they might report that they have just as much difficulty with newspaper headlines as with the small text underneath. A careful history can establish whether a patient has adapted to using a PRL effectively ( Table 13.1 ).
| More Likely to Have an Established Preferred Retinal Locus | Less Likely to Have an Established Preferred Retinal Locus |
|---|---|
| Can see large print, but difficulties increase as print gets smaller | Finds headlines just as difficult to read as smaller print |
| Accurately ranks magnifiers in terms of power and utility | Has a variety of magnifiers of different strengths but cannot tell any difference between them |
| Progressive increases in magnification produce predictable improvements in performance | Acuity improvement with magnification is less than expected |
| Reads single letters, short words, and long words (of appropriate size) equally easily | Can read single letters, or part of a line on a sight chart, but not full words |
| Can read a whole sentence over multiple lines | Words ‘run into one another’ |
| Head and text are held still, and eyes move ‘normally’ when reading | Moves head, eye and text around in an irregular pattern when attempting to read |
Location of the PRL
Considered in terms of resolution ability, which decreases dramatically with distance from the foveal centre, it makes sense for the patient to ‘choose’ a PRL on the edge of the scotoma nearest to the fovea. Even with the best area of remaining retina selected, the patient would also be expected to require magnification of the image to compensate for the poorer resolution.
People may use different PRLs for different tasks ( ). For example, a smaller, better resolution area of retina may be used for reading single letters on an acuity chart, but a larger PRL with worse acuity may be better for reading with a magnifier, as more letters would be seen at once.
A PRL to the left or right of the scotoma is unlikely to be optimal for reading, as the scotoma would obscure either the start or the end of the line: the patient would be reading ‘into’ or ‘out of’ the scotoma ( Fig. 13.1 ). Given the choice between placing the PRL above or below the scotoma, it is slightly better to use a PRL under the scotoma (i.e. to move the eye up to see), as there is a slightly higher photoreceptor density ( ) and better attentional deployment ( ) in that part of the retina, and the upper eyelid is less likely to obstruct the vision. Despite this theoretical advantage, there does not seem to be a systematic difference in reading speed between people who use PRLs in different retinal regions ( ). Other factors such as eye movements and fixation stability may be more important in determining the success of EV.
Fixation Stability
It is less easy to hold the eye steady with peripheral retina than with the fovea. People with central vision loss who are better able to hold the eye still when observing a target—in other words, those with better fixation stability—have better visual acuity ( ) and tend to be better at reading ( ) than those who have less precise fixation. Fixation stability tends to decrease as the scotoma increases in size and the PRL becomes more eccentric, although it is often acceptable for scotomas of less than 20 degrees in diameter ( ).
Eye Movements and Steady Eye Strategy
If someone is fully adapted to using EV, they will make an eye movement straight to their PRL rather than to their fovea: that is, they will make the PRL the centre of their oculomotor system. This is relatively rare: in one study, only one-third of patients had made this adaptation ( ). In a longitudinal study, one of the authors found that all of their subjects with early-onset inherited macular disease were unaware of using EV, which implies some degree of rereferencing their oculomotor system, but only about half of those with age-related macular degeneration (AMD) had made this adaptation within 1 year of losing central vision ( ).
Someone who does not automatically make an eye movement to their PRL will always be aware of using EV. When they read a vision chart and get close to their threshold acuity, they will often start moving their eyes to identify the letters and may spontaneously report ‘trying to catch the letter out of the corner of the eye’ or ‘moving the blind spot out of the way’.
When a patient is unable to make eye movements to their PRL, they may adopt a technique known as steady eye strategy (SES), where the eye is held steady, but the print is moved right to left so that it scrolls through their PRL ( ). The patient fixates the first letter on the line of print and is instructed to obtain the clearest possible view of it. This should mean that they are using EV to place the letter on the PRL. As they keep the eye still and move the print, each succeeding letter will be imaged in turn on the PRL and the words are read accurately, letter by letter.
At first, this technique can make reading slow and frustrating: patients often report that it is ‘like learning to read again’, but when SES is mastered it can support a reading speed which is fast enough for enjoyable leisure reading, of up to about 120 words per minute ( ).
An alternative to asking the patient to move the text themselves is to present scrolled text on a screen. This can be performed using software such as Zoomtext ( ), through an app such as MD_evReader ( ), Fig. 13.2 , or on the EV internet news system managed by Royal Holloway, University of London ( http://www.mdevreader.rhul.ac.uk/ev-news/ ).
It should be noted that even though the patient is being asked to keep their eyes still, and believes this is what they are doing, successful SES does not actually result in eyes which are stationary. The moving text elicits an optokinetic-like or sawtooth eye movement, with alternating fast (saccadic) and slow (pursuit) eye movements. As the text is moved to the left, the eyes fixate (eccentrically) on the letter of interest and match their speed to that of the text, making a smooth pursuit movement. After analysis of that letter is complete, the eyes reset to the correct position relative to the next letter/word (i.e. with the correct EV angle) with a saccade. This appears as the fast phase of the nystagmoid sawtooth movement.
SES can be used without EV, and it has proved to be a fast and efficient method of reading with a magnifier. It has the additional advantage that the task can be aligned so that rays of light from the object of interest always pass through the optical centre of the lens, and image quality is optimised: as the task is moved under the magnifier, the patient is always viewing the object through the optimal portion of the lens. In binocular viewing, an additional advantage is that as the eyes consistently use the same portion of the lens, they experience a constant prismatic effect. This means that there is no need for the patient to alter the degree of convergence, which would be necessary if they were to move the eyes laterally and use different zones on the lens.
Eccentric Viewing Training
As many people with central vision loss do not use EV efficiently, an obvious question is whether better EV techniques can be trained. Many different training programmes have been suggested, including maximising the use of the existing PRL; encouraging people to adopt a new, trained retinal locus (TRL); improving eye movements to the PRL; and improving fixation stability. Some training programmes use sophisticated technology such as microperimeters or scanning laser ophthalmoscopes, whilst others use only basic equipment.
EV training is most commonly provided by rehabilitation officers, but it is sometimes offered by optometrists, orthoptists or by volunteers with macular disease ( ). Home-based training has also been offered, using an audio CD or computer programme ( ).
There is limited evidence for the benefit of low vision training on improving reading ability ( ). Despite the absence of high-quality evidence for its benefit, there are many anecdotal reports of success with EV training, and up to half of low-vision clinics in some countries offer this training ( ). It is known that some individual patients do well from training, but this improvement is not universal enough for studies to determine a systematic benefit for everyone with central field loss ( ). In one study, almost three-quarters of participants found EV training helpful, despite not demonstrating faster reading or improvements in visual task performance ( ). This might indicate that this training confers benefits which are not easily measured in clinical trials.
EV training is usually provided as part of an integrated rehabilitation programme ( Fig. 13.3 , ).There is considerable variation between professionals on the content of an EV programme, even within the same organisation ( ). The methods described here are based on techniques used by some rehabilitation workers in the United Kingdom and do not require specialist equipment.
PRL Training
To train EV, the patient is made aware of the location of their PRL and encouraged to make eye movements to this location. A clockface is often used to identify the location of their PRL, and the patient is shown how to move their eyes so that the PRL falls on other items of interest ( Fig. 13.4 ). Training is performed monocularly on the better eye, unless both eyes are similar in which case the dominant eye is used ( ).
