Functional Effects of Peripheral Field Loss
Even people with very good visual acuity can be severely visually impaired if their visual field is reduced. When the visual field is constricted to less than 20° in total extent, or there is a hemianopia, the main difficulty for the patient is to gather sufficient information from the environment for effective orientation and mobility: where are they and how do they find a safe path to their destination, avoiding obstacles and hazards? To gain this information a patient must be able to systematically and quickly scan the visual scene in an ordered sequence of eye movements, have sufficient visual acuity to identify what they see, remember it, and coordinate all the information gathered into a coherent picture as quickly as possible: they are correctly and quickly assembling a jigsaw of the scene in their mind. An efficient scanner may only retain a 3° static visual field, but have a so-called dynamic field of perhaps 20° which can be quickly assessed. A poor scanner is one who uses erratic and inefficient head movements to slowly scan the scene, and whose dynamic field is no larger than their static field. For this patient, it is as if the jigsaw is lying broken up in its box, and a few pieces have been selected at random.
Many patients are very efficient scanners, especially if the field defect is less severe or long-standing, or if they have received training in visual search techniques (see Chapter 13 ). If the loss is monocular, or binocular but incongruous, intact areas in the fellow eye’s visual field will compensate for the missing regions sufficiently to maintain reasonable performance in mobility, navigation and orientation tasks. Formal tests like the Greene Hemianopsia test are used by some people to assess scanning, but it is possible to investigate how well a patient scans by observing their performance on a vision test (e.g. if they always miss letters on one side of the chart, or consistently lose their place on a reading test). Asking the patient to touch a pen held in different positions of the visual field and watching their eye movements can indicate how well they can make scanning movements to non-seeing regions of their visual field.
People are more likely to struggle with orientation and mobility when there is a binocular congruous field defect to within 10° of fixation and when the field loss is of recent onset, so efficient eye scanning technique has not yet developed.
Overall restriction of the visual field is most commonly caused by eye diseases like glaucoma, inherited retinal disease such as retinitis pigmentosa, retinal colobomas or bilateral retinal detachment. These field defects are not usually homonymous.
Homonymous field defects are usually caused by neurological disease or injury. Nearly half of those who survive a stroke and have suspected vision problems show some homonymous field loss, with about a third having a complete homonymous hemianopia ( ). As stroke is so common, a large proportion of the adult population have some field loss: 0.8% of those over 49 years old have a homonymous field defect of some kind ( ).
Clinical experience suggests that those with neurological field loss are usually easier to help with field expansion techniques than people with overall constriction of the visual field. Those with macular sparing and better visual acuity are more likely to succeed with field expansion than those with poor vision, and those without other diseases affecting their mobility also tend to do better. Some people with hemianopia also experience visuospatial neglect, where they are not aware of the existence of the part of the world corresponding to the region of visual field loss ( ). Signs of visuospatial neglect might include leaving half of a plate of food, or only applying makeup to one-half of the face. It is more difficult to use optical devices to rehabilitate those with neglect, although some successful cases are reported in the literature ( ). Tests to identify neglect are described in Chapter 3 .
Assessing Functional Adaptation
Under British law, people with significant field loss are not usually permitted to drive. However, they may apply to drive as an ‘exceptional case’ if their field loss is stable and has been present for at least 12 months; if they have no coexisting ocular condition which could cause visual loss; and if they have ‘clinical confirmation of full functional adaptation’ to their vision loss ( ). Patients with hemianopia are eligible for registration as sight impaired, but this would be incompatible with seeking to obtain a driving licence.
‘Functional adaptation’ is not defined in the regulations or in published research ( ) but, in the authors’ view, it may be considered if:
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the patient consistently makes scanning eye movements to a target presented in their blind field;
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they are quickly and accurately able to point towards a pentorch or similar target presented in their non-seeing field, when permitted to make eye movements;
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they do not report bumping into obstacles in their non-seeing side;
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they are aware of their hemianopia;
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they are able to scan effectively for other tasks, such as cycling, racket sports or reading; and/or
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they do not experience a positive scotoma, Charles Bonnet hallucinations, distortion, or other visual phenomena in their non-seeing region of field.
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Driving regulations are updated frequently and it is essential that the practitioner is fully aware of the current guidance, and seeks professional advice where needed, before advising patients if they are able to drive.
Field Expanders
Reverse Telescopes
In order to present more information within the limited remaining visual field of the patient, the objects to be viewed can be minified—that is, they can be viewed with a magnification of less than 1.0. The patient can experience such a system by using a conventional telescope the wrong way round, although this limits the field-of-view because the lens which is now furthest from the eye, and is acting as the objective lens, has such a small diameter. Door viewers, of the kind used in hotel room doors or on some front doors, have long been suggested as field expansion devices, but the high amount of image compression with these systems limits their benefit as field expanders. Designs for purpose-made reverse telescopes have been suggested by several researchers ( ) but there are only limited options available commercially. The most common approach is to reverse a conventional handheld Galilean telescope or to use a commercially available bioptic minifying telescope.
Visual acuity will be reduced in proportion to the amount of minification: somebody with visual acuity of 0.0 logMAR (6/6) will have an acuity of 0.3 logMAR (6/12) through a reversed 2× telescope; for someone with acuity of 0.7 logMAR (6/30), the same telescope will reduce acuity to 1.0 logMAR (6/60). This poor acuity means that reversed telescopes can rarely be used as full-aperture spectacle-mounted systems, unless the amount of minification is relatively modest. In a case study of one older adult with retinitis pigmentosa, reported that a low amount of minification (1.3×) was used successfully on a constant basis in a spectacle-mounted telescope, with the wearer demonstrating a threefold increase in visual field diameter on perimetry.
The advantage of using a reversed telescope is that the increased field through the system allows the patient a better appreciation of objects and their relative locations, especially if the scene contains repetitive detail, such as a row of parked cars or a line of shop-fronts. It can also be used to identify obstacles in a corridor, for planning a route across a crowded area such as a railway station concourse, or for sightseeing ( Fig. 12.1 ).
When prescribing a reversed telescope, it is important to match the size of the patient’s field to that of the device. A 2×-minification telescope with a 5° field will compress 10° of the visual scene into that 5° image. If the patient’s remaining visual field is 5°, they will get the full benefit of this, but if the unaided field is 12°, they will in fact get a larger field without the device. A field-of-view mismatch of this type is particularly likely with a ‘normal’ telescope used the wrong way round as it was not designed for this purpose.
It is also important to consider the patient’s adaptations and their dynamic visual field. For example, if a patient with a 5° field is good at scanning, they might have an effective visual field of 15° by using head and eye movements, outweighing the benefit from a reversed 2× telescope. Sometimes a patient may react favourably in the consulting room, and peripheral field testing might confirm the expected increase in static field, yet the patient rejects the device once it is tried in the real world. The spatial distortion created by the minification will be more obvious under dynamic conditions, which may contribute to dissatisfaction. It is worth noting the very large depth-of-field obtained with any reversed telescope, due to the fact that the accommodative demand is divided by the magnification squared, rather than being multiplied by it as in the magnifying telescopes (see Chapter 9 ).
A reversed telescope could be constructed using a contact lens as the eyepiece lens and a spectacle lens as the objective ( ), although this approach has similar limitations to other contact lens telescopes (see Chapter 9 ). Reversed telescopes can also be made in bioptic form, so that the wearer drops their head in order to examine the overall scene.
Electronic vision enhancement systems can offer minification, particularly on head-mounted devices ( ). Electronic and augmented view techniques which simultaneously slow a minified and a ‘real’ view can overcome the problem of acuity reduction, as can an amorphic lens which expands the field horizontally without affecting the vertical image size ( ). Although amorphic lenses cause significant image distortion, they have only a small effect on visual acuity and, after training, can improve performance on various detection and mobility tasks ( ).
In practice, reversed telescope systems are most useful for specific tasks, such as a parent watching their child playing football or a commuter trying to navigate a crowded railway station. Very often a similar effect can be obtained by making efficient scanning eye movements or by using mobility aids (see Chapter 15 ). It is quite rare for patients to continue using a reversed telescope after the initial trial period.
For stationary tasks, such as watching television, or other display screens, increasing the viewing distance (if practicable) will produce minification, and so effectively increase the linear field of view.
Handheld Minus Lenses
A handheld minus lens positioned at about 20 to 30 cm from the eye allows viewing of an image with reduced size but expanded field. This device is in fact also a reverse Galilean telescope, with the handheld minus lens being the objective, and the user’s accommodative power providing the positive eyepiece component. The higher the power of the minus lens, and the closer it is held to the eye, then the more accommodation is required. Typically, however, the accommodative demand is modest (<2.50 DS). If the patient’s amplitude of accommodation is insufficient, the field expansion is still achieved, but the image is slightly blurred. This positive power can be provided by a small Fresnel stick-on portion in the superior part of the spectacle lens, or the use of the positive segment addition if the patient wears multifocal lenses. Obviously, in either case, the patient will need to tilt their head vertically in order to use the appropriate zone of the lens. This does not normally pose a problem as the device is only intended to be used for short spotting tasks.
Lens powers of up to −50.00 DS have been suggested by some authors ( ), but relatively low-power lenses are more often used.
Kozlowski and colleagues devised a systematic method of prescribing such minus lenses, which overcomes the rather arbitrary way in which these devices are usually presented ( ). Taking a high-minus lens and holding it at a distance from the eye, it quickly becomes obvious that the greater this distance, the smaller the magnification (or greater the minification) and the larger the field-of-view through the lens becomes. This field expansion occurs at the expense of acuity, so it is important that the field-of-view created by the lens exactly matches that of the patient. This is illustrated in Fig. 12.2 , which shows an eye which has considerable constriction of the visual field to a remaining diameter of α degrees. A high-minus lens of diameter d is placed at a distance s from the nodal point (N) of the eye. Position (2) is the optimum position in which to place the lens, because the expanded field exactly fills the patient’s own usable field—the edge of the lens subtends an angle at the nodal point of the eye which is equal to the size of the patient’s visual field. In Position (1), the lens is held too far away and the view is excessively minified: whilst a very large area of the visual field will be ‘sampled’ by the high-minus lens, the user will need to move the hand holding the lens around in order to actually use the full extent of his remaining peripheral field. By contrast, in Position (3), the edge of the field-of-view of the high-minus lens is falling onto scotomatous retina, and the user would need to use eye movements in order to scan across the whole of the field-of-view provided by the device.
The optimum distance of the lens from the eye (eye-to-lens distance, s ) can be found from simple geometry:
tan(α/2)=(d/2)/s
so
s=(d/2)/(tanα/2)
where α is the remaining visual field diameter of the patient and d is the diameter of the high-minus lens.
Magnification values of between 0.33 and 0.1 are likely to be most useful. If M = 0.33, for example, a field expansion of 3× (from perhaps 5° to 15° in diameter) will be accompanied by a corresponding loss of acuity (e.g. from 0.3 logMAR to 0.9 logMAR). If a device with M = 0.1× was used by the same patient, the visual field would expand to 50° but the acuity would only be 1.3 logMAR within that field. The smaller the patient’s field, and the better the acuity, the lower the value of M which could reasonably be selected.
‘Standard’ telescope formulae (see section ‘The Optical Principles of Telescopes’ in Chapter 9 ) can be used to determine the appropriate objective and eyepiece powers:
M=−FE/FO
and
t=fO’+fE’
where F O and f O ′ are the power and focal length respectively of the objective lens; F E and f E ‘ are the power and focal length respectively of the eyepiece lens; and t is the length of the telescope, the separation of F O and F E .
These equations are just as applicable to reverse telescopes, although now 0 < M <1.00. F O in this case is the handheld minus lens ( F HHM ), which is positioned at a remote distance from the eye. The power F E is provided by the user’s accommodation ( F ACC ) and t is now the eye-to-lens distance s . Therefore:
M=(−FACC)/FHHM
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