As optometrists, we may encounter patients who have restricted mobility, as well as reading difficulty, resulting from some form of brain injury. Often a stroke results in the sequelae of a left or right sided homonymous hemianopia. Should these patients be fortunate enough to have retained the ability to walk, they may find themselves bumping into objects or tripping on obstacles to the side of the vision loss. Reading ability is often considerably reduced. Peli Prisms, also called Vision Multiplexing Prisms, can offer profound relief to these patients suffering from hemianopia.
Traditionally, we have heard of prism relocation therapy using sector prisms, and some of us have tried them. I personally have never had much success with prism relocation and my rehabilitation was more based on improving scanning eye movements into the area of vision loss. Similarly, reading skills were improved by advising how to locate the next line of print using typoscopes and typical reading skills therapy, spotting, scanning and saccades.
The breakthrough in prism relocation and mobility rehabilitation, with which I have recently had success, comes in what the inventor Professor Eli Peli calls “Multiplexing Prisms”. I have had the privilege of attending Professor Peli’s lectures and workshops at two International Low Vision Conferences.
Homonymous hemianopia is usually caused by post-chiasmal brain injury, such as stroke, trauma, tumours, infection or following brain surgery. The field defect or impairment is on the contralateral side to the injury and always respects the vertical midline. The extent of the defect is dependent on the severity and positioning of the lesion. Patients with a full congruous homonymous hemianopia are most disabled. Those with a temporal lobe lesion may have a one- sided superior quadrantanopia “pie in the sky” defect and will be least disabled, while the parietal lobe lesion will result in an inferior quadrantanopia “pie on the floor”, resulting in a tripping disability by objects not seen in the lower field on the side of the defect. Those with a right sided hemianopia have more trouble with reading.
See figure 1. In the example below, our patient was shot in the head during a hijacking. The CT scan in figure 1 shows bone fragments in the right parietal lobe, with gliosis seen in the temporal and parietal lobes. The resulting visual field was a full left homonymous hemianopia with macula splitting.
Knowing the cause of the defect is just the beginning of the rehabilitation process. Understanding the disability is more relevant and if the handicap is not fully volunteered by the patient, it should be obtained through detailed questioning. My most recent patient with a left hemianopia described his vison to be poor, but was actually unaware that his entire left field was non-existent!
Questioning patients with hemianopias revolves around their ability to interact with their environment. For example, finding objects in a room, daily living skills, loss of independence, driving, risk of tripping, bumping into overhead objects and reading difficulty. The term hemianopic dyslexia describes the pattern of associated reading problems. These disabilities have a significant impact on social and emotional functioning.
Since reading is probably the most often mentioned goal during the low vision interview, we need to first understand how the different hemianopias affect reading ability. Reading performance is related to the side of the visual field defect, the size of macular sparing and the time since onset. Figure 2 is my rehash of a slide I copied during a presentation by S. Trauzettel-Klozinsky at a past Low Vision Conference. A scanning laser ophthalmoscope was used in three scenarios. Subjects were timed while reading an identical line of print. The first subject has a normal visual field. The second has a right hemianopia, who takes considerably longer with many small saccades to the right (into the blind area). Note, this subject has no problem finding the following line of print. The third subject has a left hemianopia and reads slower than the normal subject but significantly faster than the right hemianopia. Note, the return to the next line of print has a fast saccade, which slows and becomes jerky while trying to find the beginning of the next line.
It has been noted, that with time, most hemianopes read faster. This is partly due to expected adaptation over time. Some full homonymous hemianopes without macula sparing develop a spontaneous adaptive strategy, where they will over time adopt an eccentric fixation of up to 5 degrees into the blind side. This will have a positive effect on reading ability.
There are three rehabilitation strategies for those with hemianopia.
Vision restitution therapy
This is the most ambitious and controversial and assumes some plasticity of the brain. Blind areas are stimulated in an attempt to awaken new pathways. Studies have shown that visual restoration therapy in adults did not lead to increases in visual field size, but consistently yielded subjective improvements. The basis of this is unclear. This therapy requires an excess of effort and time and is costly.
Compensatory training
This teaches patients more effective ways of using their eyes. Ideally, it should be done for one hour a day over four weeks. It can be done at home and does not require a therapist. Successful patients have demonstrated up to thirty degrees increase in visual search field, increased reading speed and/or decreased errors. Moreover, there is good transfer of training related improvements into skills of daily living. Research scientists at the university of Tubingen Germany, have developed a compensatory training program, which the patient can download for a fee. This can be found at www.visiocoach.de. Research shows that compensatory rehabilitation training can improve the visual searching skills and reading performance of those with homonymous field defects1.
Optical aids
Optical aids used for homonymous hemianopia are mirrors, monocular sector prisms, binocular sector prisms and vision multiplexing prisms. In the past, innovative opticians would solder small mirrors onto a patient’s spectacles, which would be used much like a car rear view mirror in bringing items from the blind field into view. Sector prisms are fitted either monocularly or binocularly. They are aligned on the largest pupil margin with the base pointing in the direction of the field loss. During straight ahead gaze, they are therefore not seen by the patient. A small flick of the eye into the prism apex will allow objects in the blind field to appear into the sighted side. My argument against sector prisms is, that if one is going to teach the patient to flick the eye into the prism, why not just teach them to continuously flick the eye into the blind field without any prism! If fitted monocularly, it will result in double vision and if fitted binocularly, the placement needs to be precise in order to avoid diplopia as well. Sector prisms have been manufactured commercially and go by the name of Visual Field Enhancement Systems (VFAS) and Inwave. VFA is a 22mm button prism of 9° and Inwave is a single vision lens, cut vertically along the outer pupil edge with a 6° prism base out laminated to this (Fig. 3). The best option for sector prisms in my opinion are Fresnel prisms, which are inexpensive, easy to fit, to adjust and can be made up to forty prism dioptres, which is a much larger prism value than both the Inwave and the VFAS.
This patient from figure 1 was fitted with binocular thirty prism dioptre Fresnel sector prisms and extensively counselled by a low vision rehabilitation therapist. It may be anecdotal, but according to his post-traumatic stress counsellor, the sector prism coincided with a turning point in his post-traumatic stress disorder therapy. This is the one and only sector prism success I have had in almost forty years of practicing Low Vision Rehabilitation.
Peli Prisms, also called Vision Multiplexing Prisms, are in my opinion one of the most significant advances in field expansion prism therapy I have ever encountered. Two rectangular segments measuring 8 x 22mm are presented monocularly, base out, to a single vision lens on the eye, on the side of the visual field loss. This can be done using Fresnel prisms, or can be custom manufactured from CR39 prisms, which look like Fresnel prisms fitted into the patient’s single vision prescription2. The patient’s pupil centre is dotted in forward gaze position and the prisms (40PD Fresnel’s) are fitted 6mm above and below the centre. While walking, an obstacle within the blind zone will appear above and below in the patient’s sighted peripheral vision area. It will not necessarily be clear, but the patient will however be aware of the obstacle and be able to take steps to avoid it. Unlike sector prisms or mirrors, these prisms do not require eye movements into the prism zone. On the contrary, the patient is instructed never to look into the prisms! For demonstration purposes, I fit these Fresnel prisms to the patient’s own spectacles. Where not available, I will choose a frame off the rack or I have a clear fit-over onto which the trial prisms can be attached. We go walking around the practice and then across the road to the walking lane with lots of A-frame advertising boards. I will ask the patient to follow me, while I deliberately walk close to these boards. The most likely successes are the patients who immediately see the benefit. If this demonstration is a success, we will then make up single vision spectacles and fit the Fresnel prisms.
The following three photos demonstrate firstly, what the normal sighted sees, what the hemianope sees, and what a hemianope with forty prism dioptre Multiplexing Prism sees.
I order a forty dioptre Fresnel prism and cut it up into about ten 8 x 22 segments, which get stored by the patient as replacements when required. After a short training session and frame fitting, the patient is discharged. Chadwick Optical can also supply you with trial fitting Fresnel prisms, ready cut to dimension. The intention is that the patient has a one-month trial with the Fresnel prisms. All the fine adjustments get made to the Fresnel prisms and then the final Peli Prism is ordered from Chadwick optical. Although the Fresnel prisms work very well, and some patients are happy to continue with them, the Optical Peli lenses have the advantage of being available in 40 PD, 57 PD and 57 PD oblique prisms. These do not only bring objects into the sighted field laterally, but the oblique base also brings the upper image down and the lower image up, to provide an almost simultaneous vision scenario. They are also optically clearer and more permanent. Clinical trials of the Peli prism were done on forty-three subjects in 8 different locations. The purpose of the study was to evaluate the prisms as a mobility aid for patients with homonymous hemianopia. Forty-seven per cent of the participants were still wearing the prism glasses after twelve months, typically for 8 hours per day. They reported significant benefits for obstacle avoidance in a variety of mobility situations3.
Fitting procedure
- Properly adjust spectacles
- Fit prisms on side of field loss – base out
- Measure pupil centre in normal head posture
- Apply prism 6mm above and/or below
- Discuss looking through central lens (non-prism)
Train and explain
- Walk around and observe head posture
- Minor placement adjustments.
- Four-week trial.
Training with the prisms involves; stressing the importance of only looking through the 12 mm window between the two prisms. When an object is located (which will be shifted from its true position), then the patient is encouraged to shift the eyes within the clear zone to see that object in true space. At first, this will require deliberate attention, but with time this will become natural. We need to then demonstrate the visual field expansion effect of the prism. Next, we practice turning the head and eyes to directly view the object on the blind side. We then demonstrate undesirable central double vision, should the patient look through the upper or lower prism. Reach and touch training is done next, where the patient learns to accurately locate objects in the blind field (your waving hand) and touch them while still looking straight ahead. Next, a training-walk first through an uncluttered environment and then advancing to a cluttered place. It is important to observe the patient’s head posture and to make small adjustments to the prism positioning for better head posture if required. One or two millimeters is usually all that will be needed to rectify the posture. Chadwick Optical produces a comprehensive fitting guide, which they supply with a starting kit. This has detailed information with full fitting and training instructions. This also includes an order form for the true Peli prism product.
Comparison Fresnel and Peli Prism
| Fresnel prism | Peli Prism |
|---|---|
| Poorer optics | Clearer optics |
| Placed on back curve of spectacle lens | Laminated into a spectacle lens |
| Can slide or fall off | Permanent |
| Plastic denatures over time | CR39 wear and tear only |
| Return office trips to re-align prisms | |
| Limited to 40 prism diopters | 40, 57 and 57 oblique |
| Cost effective – can cut approximately 10 stick-on pieces from one Fresnel | |
| Lacks Rx power in the prism section | |
| Any Rx can accept a Fresnel lens | Not suggested for Rx greater than +/-5.00D |
Case Examples. (only relevant data shown)
Case 1. GMcQ. Measureable visual fields improvement
An eighteen-year old female consulted me with an enquiry regarding driving. At age twelve she had surgery for a right parietal astrocytoma (type of cancer of the brain). She was studying and complained of seeing only half of the whiteboard. She would also get a fright walking in public when things would jump at her from the left side. She had never worn spectacles and her refraction was OD -1.00DS 6/6 and OS -0.75DS 6/6. A 40PD Multiplexing Fresnel was fitted base left in the upper and lower left lens. Visual field in figure 4 and 5 below, without and with prism below shows the effect of a 40 PD base out left eye Fresnel Multiplexing Prism. On a follow up phone call, she reported success with the prisms while walking in public and in class. She was however financially unable to complete the fitting of Peli Lenses.
Comment: My experience is that in children, after six years of adaptation to any disability results in relatively little loss of ability. I do not fit Peli prisms for driving purposes and my observation is that patients with hemianopia, who have consulted me with the primary goal to drive again, often do not have strong enough motivation to use Peli prisms for other mobility purposes.
Case 2. AMacC. Full Peli prism
A sixty-two-year old male suffered a medical misadventure during bypass surgery. He was left with a left sided hemianopia since 1999. He was bumping into objects on his left side, especially while walking in shopping centres and crowds. He has adopted head scanning techniques. He also smashes his spectacles, spoils his clothes with food, can’t see his fork and has become socially withdrawn. He had adapted well to reading on his PC, on which he spent about five hours a day.
The field plot showed a left inferior homonymous quadrantanopia, extending one eighth into the superior field. See figure 6. Amsler grid showed macula sparing.
40 PD Peli prisms were ordered on single vision spectacles after a successful trial period with equivalent Fresnel’s. On a follow up phone call the patient reported benefits in many aspects of daily living. He found the lower prism more beneficial. He was bumping his head less often. He was much more confident in crowds at his retired serviceman association meetings, where children were running around him. He would recommend the Peli lens to those in the same situation.
Comment: As far as reading and PC are concerned, a left homonymous hemianopia with macula sparing is the least disabling.
Summary
I can confidently say that the principle of applying Multiplexing Prisms in patients with homonymous hemianopia can be a rewarding experience for patient and optometrist alike. Like all rehabilitation, this should be goal orientated and based on the patient’s needs. Of critical importance is the patient’s desire to improve their mobility skills. It may also help to link the therapy to the duration of the illness. Longstanding hemianopes may have adapted satisfactorily in their own way, whilst on the other hand, presenting this therapy too soon after the field loss occurred may fail. This may be due to the patient not yet having realised how the brain damage has affected them.
References:
- Bouwmeester et. al. Analysis of 14 studies
- chadwickoptical.com
- Community-Based Trial of a Peripheral Prism Visual Field Expansion Device for Hemianopia. Eli Peli et al. Arch ophthalmol/vol126 (no5) May 2008
Useful links:
www.visiocoach.de PC based compensatory training.
www.hemianopia.org Webpage of Chadwick optical.
www.eri.harvard.edu/faculty/peli/papers/Bowers%20et%20al%202008.pdf Peli lens study.