visual field expansion in patients with retinitis pigmentosa

Visual field expansion—Somani et al 27

Visual field expansion in patients with retinitis pigmentosa

Sohel Somani,*† MD, FRCSC; Michael H. Brent,† MD, FRCSC;

Samuel N. Markowitz,* MD, FRCSC

ABSTRACT • RÉSUMÉ

Background: To determine the effectiveness of using spectacle-mounted prisms forfield expansion in patients with retinitis pigmentosa (RP).

Methods: Vision-related activities of daily living (V-ADL) questionnaire scores andfunctional visual field score (FFS) measurements were conducted before and aftera one-month trial of spectacle-mounted prisms in those patients with RP who hadresidual central visual fields of less than 10 degrees.

Results: 16 patients were recruited who met study inclusion criteria. Mean V-ADLand FFS at baseline were 67.6 (73%) and 22.9 (46%), respectively.After a 1-monthtrial using spectacle-mounted prisms, V-ADL and FFS demonstrated significantimprovement to 73.4 (80%, p < 0.05) and 27.0 (54%, p < 0.001), respectively.

Interpretation: Spectacle-mounted prisms effectively create visual fieldexpansion and noticeable spatial orientation benefits in patients withRP. This may provide an adjunctive tool in low vision rehabilitation andshould be considered in all cases with RP with less than 10 degrees ofvisual field.

Contexte : Établir l’efficacité des prismes montés sur lunettes pour étendre le champvisuel des patients atteints de rétinite pigmentaire (RP).

Méthodes : On a mesuré les résultats d’un questionnaire sur les activités liée à la vuedans la vie quotidienne (V-ADL) et ceux d’un examen de la fonctionnalité du champvisuel (FFS) avant et après un mois d’utilisation de prismes montés sur lunetteschez des patients atteints de RP qui avaient un champ de vision résiduel de moinsde 10 degrés.

Résultats : L’on avait recruté 16 patients qui répondaient aux critères de l’étude.Audépart, les valeurs moyennes V-ADL et FFS étaient de 67,6 (73 %) et 22,9 (46 %)respectivement. Après un mois d’utilisation des prismes montés sur lunettes, lesdeux résultats s’étaient améliorés significativement, à 73,4 (80 %, p < 0,05) et 27,0(54 %, p < 0,001) respectivement.

Interprétation : Les prismes montés sur lunettes produisent effectivementune extension du champ visuel et des bienfaits remarquables pour

From *the Low Vision Service, University Health Network Hospitals,Department of Ophthalmology and Vision Sciences, University ofToronto, and †the Medical Retina Service, Mount Sinai Hospital,Department of Ophthalmology and Vision Sciences, University ofToronto, Toronto, Ont.

Presented at the Canadian Ophthalmological Society meeting inVancouver Jun. 16, 2004

Originally received Jan. 10, 2005Accepted for publication Sep. 9, 2005

Correspondence to: Dr. S.N. Markowitz, 1225 Davenport Rd., TorontoON M6H 2H1; fax (416) 531-6728; [email protected]

This article has been peer-reviewed.

Can J Ophthalmol 2006;41:27–33

l’orientation spatiale des patients atteints de RP. Cela peut ajouter unautre outil de rétablissement de la malvoyance et devrait être considérépour tous les cas de RP avec un champ visuel inférieur à 10 degrés.

Visual field expansion—Somani et al

28 CAN J OPHTHALMOL—VOL. 41, NO. 1, 2006

Retinitis pigmentosa (RP) is a variable retinal dys-trophy characterized by progressive and irre-

versible constriction of peripheral fields of vision.1 Asthis disease progresses, peripheral vision can becomealmost nonexistent, leaving only residual centralvision intact. Often referred to as “tunnel vision”,constricted visual fields can cause significant visualdisability with respect to orientation and mobility.Various optical devices have been introduced in anattempt to expand the residual visual field responsiblefor spatial orientation, all with various success. Thesedevices include minification of the targets observedwith reversed telescopes,2 negative lenses,3 and amor-phic lenses.4 Augmented-view devices,5 night visiondevices,6 and illumination devices7 are other aids usedto help with mobility in patients with RP. Commonproblems with these devices include degradation ofremaining central visual acuity, confusion, spatial ori-entation difficulties, and diplopia.

Image relocation with spectacle-mounted prismshas been described earlier as a method intended toexpand visual field deficits in patients with homony-mous hemianopia,8,9 as well as in patients with tunnelvision, by using Fresnel prisms,10,11 Trifield glasses,12

and field-expanding channel lenses.13 Advantages ofspectacle-mounted prisms over other methodsinclude its inherent affordability, cosmetic appeal,and ready availability at optical or low-vision dispen-saries. Although the image relocation technique withprisms for cases with tunnel vision has been describedbefore, there have been no reports validating itsobjective or subjective success with a prospectiveinterventional case series.

We hypothesize that RP patients with tunnel visionmay have a more effective widening of their residualvisual fields using spectacle-mounted prisms. Thiscould be reflected in an improvement of activities ofdaily living, when tested subjectively, as well as betterperformance on visual function tests such as visualfield measurements, when tested objectively.

The purpose of our study was to formally validatethe technique of image relocation for expansion ofcentral visual fields in patients with retinitis pigmen-tosa using a subjective measure, the modified BelfastActivities of Daily Living questionnaire for low

vision,14 and an objective measure, functional fieldscore (FFS)15 obtained with the 10-2 program on theHumphrey visual fields instrument.

METHODS

Patients were identified prospectively and consecu-tively as they presented to either the Low Visionclinic (S.N.M.) or the Medical Retina clinic(M.H.B.). Inclusion criteria were patients with docu-mented RP, binocular central visual acuity better than20/80, central visual field less than 10°, and absenceof other major ocular comorbidity (e.g., cataract).Patients were excluded if they had a history of neuro-logical disease or cognitive impairment.

After we obtained research ethics approval andinformed consent, patients were enrolled in the study.Demographics, refraction, best-corrected visualacuity using the Early Treatment Diabetic RetinopathyStudy (ETDRS) charts, and contrast sensitivity usingthe Functional Acuity Contrast Test (FACT) chartswere measured and recorded. Baseline vision-relatedactivities of daily living (V-ADL) questionnaire and aHumphrey 10-2 visual field analysis were completed.Patients were then fitted and trained with Fresnelprisms. Carrier lenses included the compensation forrefractive errors measured during refraction. Fresnelsegments of 20 prism diopters were fitted around thevisual axis and applied to the carrier lens as in Fig. 1

Fig. 1—Fresnel prism illustrating spectacle-mounted placementused in the study.

with prism base towards the rim of the frame. After a1-month trial at home, patients returned to the officeto complete the same V-ADL questionnaire and the10-2 visual field analysis again.

Outcome measures selected were the V-ADL ques-tionnaire scores and FFS. The V-ADL questionnaire,based on the modified Belfast Activities of DailyLiving questionnaire for low vision,14 is composed ofdistance, intermediate, near, and peripheral tasks, asillustrated in Fig. 2. The maximum score for each

category is 32, 32, 16, and 12, respectively, for amaximum total of 92. The FFS is essentially a scoreof binocular functional visual field. One point isgiven for every 2° of intact field along 10 meridians,for both right and left fields. A superimposition ofthe 2 monocular field plots is also conducted, and themeasurement is repeated. The FFS then is a summa-tion of the right field score, left field score, and 3times the superimposition score, all divided by 5 togive a total score of 50 (i.e., FFS = [OD score + OS


CAN J OPHTHALMOL—VOL. 41, NO. 1, 2006 29

Difficulty Scale4 = No difficulty3 = Little difficulty2 = Lot of difficulty1 = Extreme difficulty/Unable

A. DISTANCECan You . . . Difficulty: 4 3 2 1

1. Read street signs?2. Recognize faces outside?3. Enjoy scenery when out for a drive?4. Recognize seasonal changes in the garden?5. Walk alone in your neighborhood?6. Walk alone outside your neighborhood?7. Adjust to dark coming from light?8. Adjust to light coming from dark?

B. INTERMEDIATECan You . . . Difficulty: 4 3 2 1

1. Watching television?2. Distinguish a person’s feature in the room?3. Distinguish objects in the room?4. Notice steps and use them?5. Prepare food in the kitchen?6. Handle food on your plate?7. Pour yourself a drink?8. Cut your fingernails?

C. NEARCan You . . . Difficulty: 4 3 2 1

1. Read newspaper headlines?2. Read regular print material?3. Write and sign documents?4. Can you identify money?

D. PERIPHERALCan You . . . Difficulty: 4 3 2 1

1. Notice stationary or moving targets with your side vision?2. Navigate your way among stationary or moving targets?3. Locate a target from among other targets crowding the field?

Fig. 2—Vision-related activities of daily living (V-ADL) questionnaire.

score + (3 × OU score)]/5). A statistical analysis wasperformed on both outcome measures using a 2-tailed paired Student t test.

RESULTS

A total of 16 consecutive patients who met theinclusion and exclusion criteria were identified. Theaverage age was 46 (range 30–65) years and 10 (63%)patients were female. In all patients, RP was diag-nosed and documented elsewhere before the visit tothe low vision clinic.

All patients had best-corrected binocular visualacuities better than 20/80: 9 (56%) patients hadbetter than 20/25 vision, 4 (25%) patients had 20/30to 20/40 vision, and 3 (19%) patients had between20/50 and 20/70 vision (Fig. 3). The average spheri-cal equivalent refraction in both eyes was –0.5 (range+2.00 to –7.00) dioptres. Average contrast sensitivi-ties at 1.5, 3, 6, 12, and 18 cycles per degree are illus-trated in Fig. 4, and the plotted values were within thenormal range.

Fig. 5 illustrates visual field scores (VFS) and FFS atbaseline and following a 1-month trial with prisms.In summary, the baseline FFS was 22.9 (45.8%),which significantly improved following a 1-monthtrial with prisms to 27.0 (54%, p < 0.001). BaselineV-ADL questionnaire scores and the scores after the1-month trial are illustrated in Fig. 6. Baseline V-ADL scores for distance tasks, intermediate tasks,near tasks, peripheral tasks, and the total score were23.7 (74%), 25.5 (80%), 12.6 (79%), 5.75 (48%),

and 67.6 (73%), respectively. Following the 1-monthtrial, V-ADL scores for distance tasks (25.1, 78%),intermediate tasks (27.4, 86%), and near tasks (13.4,83%) showed a very modest improvement that wasnot statistically significant. However, peripheral tasks(7.4, 62%, p = 0.006) and the total score (73.4, 80%,p < 0.05) also demonstrated a modest, yet significant,improvement.

INTERPRETATION

As persons with low vision are a growing part of ourophthalmic patient population, we are faced with theongoing challenges of vision rehabilitation needs.The RP population is a specific group with very chal-lenging needs in terms of vision rehabilitation. Thereis little published clinical experience regarding themanagement of these cases. One of the most impor-tant aspects in low vision assessment in the RP popu-lation is the need to identify and manage constrictedvisual fields. When peripheral vision loss is severe(leaving central visual fields of less than 20°, mobilitycan be reduced.16 In addition to navigational prob-lems, social interactions may also be affected.5

Clinical rehabilitation options in dealing withtunnel vision using nonprismatic methods are limitedand have had variable success.2–7 The most commonproblem among with these devices is degradation ofremaining central visual field. Fresnel prisms may notcause the same central visual field degradation andhave added advantages of cosmetic appeal, relativeavailability, and ease of fitting. The rationale for using



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Fig. 3—Distribution of baseline ETDRS (Early Treatment Diabetic Retinopathy Study) binocu-lar visual acuities (VA) of 16 patients with retinitis pigmentosa enrolled in the study.

prisms for field expansion involves increasing scan-ning effectiveness for patients, resulting in improvedperipheral awareness. We constantly scan our envi-ronment using low spatial-frequency visual channelsas we also intermittently spot and view points ofinterest in visual fields scanned, using various highspatial-frequency visual channels. Cortical temporalmultiplexing processes create visual perception as weknow it by using the information obtained fromscanning and spotting. In the presence of tunnelvision, prisms project peripheral fields informationotherwise unavailable, thereby enhancing the scan-ning abilities of the eye. Enhanced scanning abilitywill produce new spotting eye movements andtogether both visual skills in fact expand peripheralfield awareness.

Anecdotal evidence exists that demonstrates somebenefit of using prisms in RP patients with tunnelvision.10,11,17 To date, there have been no prospectivedata on the efficacy of prism-mounted spectacles

using both objective and subjective measures.Furthermore, while lack of consensus exists as to thetechnique of fitting,11 there is some suggestion thatthose patients with less than 10° of central visionbenefit the most.18 Therefore, we limited our study toRP patients with less than 10° of residual centralfield.

Our results indicate that with spectacle-mountedprisms, central visual field measurements on theHumphrey 10-2 visual field improve by 8 percentagepoints (46% to 54%, p < 0.001). This improvementin visual field is supported by a modest improvementof 7 percentage points (73% to 80%, p < 0.05) inoverall V-ADL tasks. The most significant improve-ment was in the peripheral-related tasks, whichimproved by 14 percentage points (48% to 62%, p =0.006) when the spectacle-mounted prisms wereused. While there was not a significant improvementin distance, intermediate, or near tasks, there was amodest trend to some tolerance in performing these



Fig. 4—Average baseline contrast sensitivity illustrated by solid single black line, n = 16. Standarddeviation = dotted line; shaded area = normal population range; cpd = cycles per degree.

Spatial frequency (cpd)

Percent contrastC

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tasks with the spectacle-mounted prisms. This isimportant, given that field expansion with otherdevices has been shown to compromise central visionin exchange for the benefit of increased peripheralvision.2 Perhaps the reason nonperipheral-relatedtasks were tolerated in our study is a result of the waythe prisms were fitted. While there is variability in thetechnique of prism fitting,11,12,18 the method chosenin this study was the traditional method advocated bymost clinical practitioners. However, some have sug-

gested that overall satisfaction with prisms may beindependent of prism power; distance from theoptical axis; binocular versus monocular fit; andunderlying pathology.10 This may hold some promisefor other disorders of reduced visual field, includingglaucoma and neurological causes, conditions thatwere not included in this study. Also emphasized inour experience and those before us is the importanceof initial and ongoing training for these patients,both with respect to the device and its benefits.



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Fig. 5—Field Scores.Visual field scores (VFS) for right (OD) and left (OS) eyes, and functional fieldscores (FFS), comparing baseline scores to scores measured after 1-month trial with spectacle-mounted prisms. FFS improved from 22.9 (45.8%) at baseline to 27.0 (54%) after 1-month withprisms, *significant at p < 0.001.

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Fig. 6—Performance on vision-related activities of daily living (V-ADL) questionnaire comparingbaseline scores to scores measured after 1-month trial with spectacle-mounted prism. Peripheraltasks (48% to 62%, *p = 0.006) and the total score (73% to 80%, **p < 0.05) demonstrated signifi-cant improvement.

There are certain limitations to consider in thisstudy. Although tolerance was reported by virtue of amodest improvement in V-ADL scores, this measureis subjective and prone to recall bias. Furthermore, itwas not ascertained what proportion of the time thespectacle-mounted prisms where worn, and perhapspatients chose to wear them for certain tasks only,and thus reported favorably on the V-ADL question-naire. Finally, patients who chose to be enrolled inour study may reflect a motivated segment of our RPpopulation.

In conclusion, while visual field loss is an unfortu-nate and irreversible consequence of RP, spectacle-mounted prisms may prove to be a beneficial tool inhelping rehabilitate these patients, particularly thosewith residual central fields of less than 10°. Spectacle-mounted prisms seem to effectively create visual fieldexpansion and noticeable spatial orientation benefitsin patients with RP. Further research is encouragedwith other technical designs, training methods, andevaluation tools to better understand this aspect oflow vision rehabilitation.

We acknowledge support for this study by the Saul WagmanFund for Medical Research.

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Key words: low vision, field expansion, retinitis pigmentosa,prism



visual field expansion in patients with retinitis pigmentosa

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