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Homonymous hemianopia following stroke: visual field remediation.

This article considers the visual rehabilitation of patients presenting with homonymous hemianopia following stroke.

Optometrists [COMMUNICATION][OPTICAL APPLIANCES]

Dispensing opticians [COMMUNICATION][LOW VISION]

1 CET POINT

Introduction

The main goal of neuro-optometric rehabilitation in a patient with homonymous hemianopia (HH) is to take the visual stimulus that presents in the blind area of the field and transfer it for processing by the functioning area. Patients with HH should be seen by an eye care practitioner with knowledge in neuro-optometric rehabilitation at the earliest opportunity to avoid the development of unusual adaptive behaviours. optometric management of patients with HH includes: lenses, prisms, mirrors, and vision therapy, which will be considered in turn.

Lenses

The first step in managing patients with HH is to maximise the visual system input by correcting any underlying refractive error, or updating their habitual refractive prescription. In patients with acquired brain injury (ABI), binocular vision is often fragile, and in those with presbyopia, the ability to maintain accommodation and vergence may be affected. (1) Prescribing spectacles for a relatively small amount of refractive error in patients with ABI may stabilise both distance and near vision, including the ability to focus from one distance to another. (2) Due to the high incidence of vestibular dysfunction following AB1, which may result in these patients suffering from dizziness, (3) single vision spectacle lenses are often a good solution. (4) Multifocal spectacles can impair distance depth perception, contrast sensitivity, and exacerbate visual vertigo. (5,6) Bifocal lenses induce an apparent displacement of objects in the visual field due to prismatic jump at the top of the reading segment, altering both normal perception and increasing the risk of falls. (7,8) Following stroke, some patients can develop visual stress, a condition characterised by symptoms of eyestrain, headaches and glare, as well as illusions of colours, shape and motion, which are typically maximal when viewing high contrast stimuli at certain spatial frequencies, such as text.' The symptoms of visual stress in these patients can sometimes be alleviated using specific coloured filters, (10) or precisely tinted spectacle lenses. (11)

Prisms

Prisms work on the principle that the image of an object is displaced towards its apex. (12) For instance, in right hemianopia, the prism would be placed on the temporal side of the lens with its base to the right. In this way, the prism displaces the images of objects in the right hemianopic field towards its apex and into the seeing field of that eye. Prism in one eye only provides an expansion of the visual field but at the same time results in diplopia which patients are unlikely to tolerate. (13) As such, full field or partial field yoked prisms and compensatory prism systems might be considered as optical devices in patients with HH.

Full field or partial field yoked prisms

When yoked prisms are prescribed, both prisms are oriented with the base in the same direction. For instance, base left prisms refers to when the base is oriented towards the patient's left side; the prism base in the left eye is placed temporally while the prism base in the right eye is placed nasally. (14,15) In addition, yoked prisms allow a better posture, thus the prism counteracts the patient's tendency to tilt to one side. (16,17) Base right prisms, for example, shift the perception of space to the left; this shifts the head and body alignment towards the left side. Thus, the prisms change postural tone by influencing head /neck and body alignment in relation to the planes of space. (18) Yoked prisms are used to change the midline orientation of the body in patients with abnormal posture and /or balance disturbances. There are various ways to provide yoked prisms for hemianopic patients. Fresnel prisms can be easily applied to the front surface of the spectacle lenses. These prisms are inexpensive and are good for temporary intervention or as diagnostic devices. Once the amount of prism to prescribe is determined, Fresnel prisms are attached to the patient's glasses with instructions to use them first in sedentary situations and then around the home and during outdoor activities. Fresnel prisms unfortunately do compromise vision, and the greater the amount of the prism, the more marked this adverse effect becomes. However, full field yoked prisms ground into the patient's spectacle lens prescription can be used as a more permanent solution. Full field yoked prisms with about [20.sup.[DELTA]] oriented with the base towards the side of the visual field loss shifts the image about 10[degrees]. With the use of full diameter yoked prism there is unfortunately a corresponding optical field loss known as 'optical scotoma' in the far periphery on the seeing side even if the ocular movements negate the beneficial effect of the prism. (19)

Partial field yoked prisms ground into the patient's spectacle lens prescription, or as Fresnel prisms can be placed on the half of each lens corresponding to the affected hemifield space. When using the partial field approach it is important that the prisms do not interfere with the patient's central vision. Weiss suggested placing the prism by considering the patient's visual field; thus, it should be placed 15mm from the limit of the patient's visual field if it is 5[degrees] or less. (20) However, the prism's apex should be placed towards and close to the pupil, paying attention to ensure that it does not affect the patient's central vision. (21) Partial field yoked prisms provide only field of view relocation. The hemianopic patient looks into the prisms when information from the blind field is required. As soon as the patient directs fixation towards the prisms, the visual field is shifted, thereby reducing ocular movements necessary to locate targets in the blind visual area. The amount of prism is usually [12.sup.[DELTA]] to [18.sup.[DELTA]] and the shift of the field is approximately of 6[degrees] to 9[degrees]. (22) 'Jack-in-the-box scotoma' occurs with partial field prisms in the centre of the field of view. (23) However, head movements compensate for this visual disturbance, but unfortunately the potential benefit of the partial yoked prisms is reduced. (24) Yoked partial field prisms are more often used as a training device to improve the patient's visual scanning ability.

Compensatory prism systems

There are two types of prism system that can be prescribed for patients requiring compensation for peripheral visual field loss. The Peli field expansion system is a simultaneous perception device mounted on the spectacles above and below the line of sight with the base directed towards the patient's visual field impairment. (25) The amount of prism is around [40.sup.[DELTA]]. The prism system shifts the image of targets into the sighted field. The advantage of this device is that the use of higher-powered prisms creates wider field awareness while allowing simultaneous access to both the right and left visual field. The disadvantage is that the Peli system is hard to adapt to and may create visual confusion. (26) The Peli system works better in subjects suffering from hemianopia or unilateral neglect (UN) when applied only superiorly for about two weeks, and then inferior prism can be introduced. (27)

The Rekindle Visual Awareness System (VFAS) consists of a peripheral prism button fused into the lens. (28) The VFAS uses a combination of eye movements into the blind area and a relative high amount of prism, usually [18.5.sup.[DELTA]]to increase field awareness. Prism is ground into the patient's spectacle lens prescription with its base set towards the hemianopic field; this system does not usually disturb the patient's vision until they scan into the blind area. In addition, the system requires scanning into the defective visual field area, which may be crucial to visual field restitution.

Mirrors

Use of mirrors is an alternative approach that can be considered for patients who require compensation for peripheral visual impairment. Mirrors are more commonly prescribed in patients with right or left hemianopia after a cerebral stroke. (29) In hemianopic patients, the mirror can be fixed nasally, posteriorly, or anteriorly to the spectacle lens and angled towards the non-viewing area. By glancing into the mirror the patient can see objects from the blind area. Mirrors are also available as a clip-on or can be permanently attached to the frame.

Vision therapy

Some patients with HH with or without UN may not be fully aware of their visual field loss. The first step during vision therapy is to make the patient conscious of their visual field defect with awareness training to help them to appreciate that a part of the visual field is impaired; this can be achieved by using peripheral stimulation to establish where in the visual field objects are seen or not seen while the patient is looking straight ahead. Once the patient is able to recognise which side of the visual field is affected and the extent of scotoma, the goal of vision therapy is to improve visual scanning and reading ability. Patients with peripheral vision loss when scanning into the scotoma tend to lose information from the normally sighted side. Scanning the visual world with large sweeping eye movements would be the most obvious form of compensation in hemianopic patients. Some recommend always starting the visual scanning in the hemianopic area, and then performing it out in circular manner of the general environment. (30) However, during the therapy, hemianopic patients are trained to look toward the contralesional side, gather information, and move back to the other side as quickly as possible. (6) The patient is instructed to search for the target/stimulus into the blind hemifield, and then respond to it as quickly as possible. Games are useful functional activities for improving scanning ability. Berryman et al suggested using games such as solitaire or matching games. (31)

Another rehabilitative procedure, which may improve the patient's visual exploration ability, is to place coloured stickers or pictures on the doorframe (see Figure 1). Before the patient moves through the doorframe, they need to scan and look for all of the stickers. This procedure can also be performed by the patient while eating by placing four coloured stickers on each corner of a food tray; the patient must look for all stickers before eating (see Figure 2).

In addition, working on saccade and pursuit movements is helpful to maximise the performance of each eye in terms of latency and accuracy without body, neck and head movements. For this purpose, the Hart chart can be used (see Figure 3). (32) The Hart chart consists of 10 columns of letters. Each column contains 10 letters. The distance between the observer and the chart should be at least three metres. The patient is instructed to call out the first letter in column one and then the first letter in column 10, the second letter from the top in column one and the second letter from the top in column 10. The procedure is completed when the patient calls out all the letters from columns 1 and 10. The goal is to perform the task monocularly in 15 seconds. The practitioner should record errors made during the task. The level of difficulty can be raised by asking to the patient to call out the letters in the inner columns. The same procedure is then performed binocularly.

Computer-based therapy such as the VisionBuilder program may improve ocular motion in normal and in neurological patients and is available in both a home and consulting room version. (33)

Computer Orthoptics (VHT3), developed and produced by HTS Inc, contains diagnostic and therapeutic programs. The diagnostic program includes tests to assess heterophorias, fixation disparity, fusional vergence, suppression, motor fields, pursuits, and saccades. The therapeutic program includes procedures designed to improve ocular motility, vergence, and accommodative function. Scanning visual therapy can be conducted with or without partial field yoked prisms. Most of patients with UN have difficulty when instructed to move their eyes towards the contralesional side. Proprioceptive procedures such as the Margolis eye throwing technique--where the patient is taught to throw their eyes as far as physically possible into the direction of the scotoma area--can be very helpful in maximising use of neglected visual cues. (34) Once the patient is able to perform this procedure, they can begin scanning activity therapy in the consulting room, which can then be introduced into real world situations.

Pambakian et al found that 76% of patients who were enrolled in saccadic visual search therapy reached faster search times, 14% showed no improvement, and 10% of subjects had slower search times. (35) Kim et al studied 10 patients with left visual neglect due to right hemisphere stroke. (36) The patients took part in virtual environment training which included crossing the road by detecting cars approaching either from the left or right, with findings showing that asymmetry for visual neglect decreased during the rehabilitation; this effect remained at three month follow up.

In addition to scanning visual therapy, there are several therapies designed to ameliorate concepts of body awareness and visual spatial awareness, even visual spatial relationship. Body awareness is the ability to effectively and accurately position and move the body in space, while visual spatial awareness is the ability to accurately and effectively orient in space using visual representations and spatial relationship. Patients after visual scanning therapy still require more saccades to view their full field. Therefore, patients with HH spend a lot of time and energy scanning in the environment and must have good visual perceptual speed and visual spatial awareness to process information.

Simple techniques can be very effective in improving reading ability. For instance, instructing the patient with right HH to read by looking at the end of words rather than the start of words can improve the patient's reading ability. In this case last letter cancellation therapy can be helpful. Thus, using a newspaper and a red pen, the patient is trained to locate and cancel out the last letter of each word (see Figure 4).

Hemi-reading cards are helpful to mark the boundaries on the column of text during reading. Using sticky notes along the side of text to mark the beginning or the end of a column is another useful technique for these patients (see Figure 5).

Conclusion

Neuro-optometric therapy in patients with HH begins with educating the patient so that they are fully aware of their visual field impairment. There is no standard treatment approach for hemianopic patients but use of optical devices such as lenses, prisms, in particular yoked prisms, and compensatory prism systems associated with vision therapy, are the mainstay options. Lenses are used to maximise the visual system input by correcting underlying refractive error. Prisms and mirrors shift incoming information from the affected field to the sighted area. The goal of vision therapy is to improve ocular motion, in particular the patient's visual scanning ability reducing head and/or body movements.

Exam questions

Under the enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk. Please complete online by midnight on 9 June 2017. You will be unable to submit exams after this date. Please note that when taking an exam, the MCQs may require practitioners to apply additional knowledge that has not been covered in the related CET article.

CET points will be uploaded to the GOC within 10 working days. You will then need to log into your CET portfolio by clicking on 'MyGOC' on the GOC website (www.optical.org) to confirm your points.

References

Visit www.optometry.co.uk, and click on the 'Related CET article' title to view the article and accompanying 'references' in full.

Course code: C-55727 Deadline: 9 June 2017

Learning objectives

* Be able to explain options for visual rehabilitation to stroke patients (Group 1.2.4)

* Understand use of optical appliances for visual rehabilitation in stroke patients (Group 4.2.2)

* Be able to explain options for visual rehabilitation to stroke patients (Group 1.2.4)

* Understand use of optical appliances for visual rehabilitation in stroke patients (Group 6.3.1)

Fabrizio Bonci Dip Optom, MCOptom

* Fabrizio Bonci is the clinical optometry lead at Ocular-Optikus Clinic in Kecskemet, Hungary. He has mostly worked in eye clinics in Italy and was also a clinical research fellow at the Department of Clinical Neuroscience and Mental Health, Imperial College London, Charing Cross Hospital.

Caption: Figure 1 Six coloured notes are placed on the doorframe. The patient has to scan and look for all coloured notes before they move through the door

Caption: Figure 2 Four stickers are placed on each corner of the food tray. The patient has to look for all four stickers before eating

Caption: Figure 3 Hart chart

Caption: Figure 4 The last letter cancellation therapy

Caption: Figure 5 Coloured notes alongside of a column of text are helpful to mark the beginning of each row
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Author:Bonci, Fabrizio
Publication:Optometry Today
Date:May 1, 2017
Words:2810
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