1040-5488/05/8211-0970/0 VOL. 82, NO. 11, PP. 970 975 OPTOMETRY AND VISION SCIENCE Copyright 2005 Americn Acdemy of Optometry ORIGINAL ARTICLE An Evlution of the Mrs Letter Contrst Sensitivity Test BRADLEY E. DOUGHERTY, BS, ROANNE E. FLOM, OD, FAAO, nd MARK A. BULLIMORE, MCOptom PhD, FAAO The Ohio Stte University College of Optometry, Columbus, Ohio ABSTRACT: Purpose. The Mrs Letter Contrst Sensitivity Test (initilly known s the Lighthouse Letter Contrst Sensitivity Test) is similr in design to the Pelli-Robson Test but my offer severl dvntges. This study evlutes the repetbility of the Mrs test nd its greement with the Pelli-Robson test in norml nd low-vision subjects. Methods. Fifty-four subjects were tested (ge 22 86 yers), including 20 normlly sighted young dults, 17 normlly sighted older dults, nd 17 dults with low vision (20/16 to 20/250). Subjects were tested with both contrst sensitivity tests nd with the ETDRS visul cuity chrt. After short brek, subjects were retested with n lternte form of ech contrst sensitivity test. The chrt forms used (two Pelli-Robson nd three Mrs) nd the order of testing were vried systemticlly. Testing ws monoculr with hbitul correction nd, for subjects over 40 yers of ge, included pproprite ner dd. Letter-by-letter scoring ws used for both tests. Repetbility nd greement were ssessed by determining the 95% limits of greement (LoA): 1.96 stndrd devitions of the differences between dministrtions or tests. Results. The Mrs test showed excellent greement with the Pelli-Robson test, with 95% LoA of 0.21 log units for ll subjects. The Mrs test ws similrly repetble (95% LoA 0.20 log units) to the Pelli-Robson test (95% LoA 0.20 log units) mong ll subjects. Conclusion. The new Mrs Letter Contrst Sensitivity Test shows excellent greement with the Pelli-Robson test nd hs similr repetbility. There re subtle differences in the ctul contrst levels on different forms of the Mrs test, nd djusting for these differences leds to superior repetbility of the Mrs test. Thus, the Mrs test my be useful lterntive to the Pelli-Robson test offering severl dvntges, including smller size, improved durbility, nd ese of use. (Optom Vis Sci 2005;82:970 975) Key Words: contrst sensitivity, low vision, Mrs test, Pelli-Robson test, repetbility Contrst sensitivity (CS) testing hs been suggested s useful mens of providing more complete ssessment of vision in vriety of clinicl situtions. 1 It hs been used s n indictor of mobility 2,3 nd reding speed 4 in low-vision ptients nd of qulity of life in ptients with glucom. 5 Other wys in which CS testing is used include quntifying vision loss resulting from ctrct 6 nd evluting vision fter refrctive surgery. 7,8 One commonly used test for the mesurement of contrst sensitivity is the Pelli-Robson test, 9 letter chrt with optotypes of constnt size nd vrying levels of contrst. The Pelli-Robson test hs been used to mesure CS in lrge-scle studies. 10,11 It is esy to dminister nd hs been demonstrted to be very repetble, 12 especilly when by-letter scoring method is used. 13 A new contrst test ws introduced by Mrs Perceptrix Corportion (Chppqu, NY) in 2004. The Mrs Letter Contrst Sensitivity Test 14 initilly mrketed s the Lighthouse Letter Contrst Sensitivity Test hs severl fetures tht might mke it desirble for use in clinicl prctice, including its smll size, durbility, nd portbility. It uses the sme Slon letter set s the Pelli- Robson test, but the mnner in which the contrst of the letters vries is slightly different. In the present study, we ssess the greement of the Mrs test with the Pelli-Robson test nd its repetbility in subjects with norml vision s well s subjects with low vision. We lso investigte potentil differences in the contrst levels of vrious forms of the Mrs test nd the effects of these differences on its repetbility. METHODS Subjects Fifty-four subjects prticipted in the study (ge rnge 22 86 yers). Subjects were from three groups: 20 younger norml-vision subjects (men [ stndrd devition] ge 24.4 2.0 yers), 17 older norml-vision subjects (men ge 58.9 11.7 yers), nd 17 low-vision subjects (men ge 56.9 24.3 yers). Norml
Mrs Letter CS Test Dougherty et l. 971 vision subjects (visul cuity [VA] 20/25 or better) were recruited primrily from stff nd first yer students of The Ohio Stte University College of Optometry. Low-vision subjects (VA 20/16 to 20/250) were recruited from the Vision Rehbilittion Service t The Ohio Stte University College of Optometry. Informed consent ws obtined from ll prticipnts before testing nd fter n explntion of the procedure ws given. Study procedures were pproved by The Ohio Stte University Office of Responsible Reserch Prctices. Contrst Sensitivity Tests The Mrs Letter Contrst Sensitivity test mesures pproximtely 23 35.5 cm nd is printed on rigid plstic. It consists of 48 letters, 1.75 cm high, rrnged in eight rows of six letters ech. Stted contrst vries from 91% ( 0.04 log units) to 1.2% ( 1.92 log units) with the contrst of ech letter decresing by constnt fctor of 0.04 log units. Ech letter subtends 2 t the test distnce of 0.5 m (equivlent to 20/480). The Pelli-Robson test mesures pproximtely 59 84 cm nd is printed on rigid crdbord. It consists of 48 letters rrnged in eight rows of six letters ech. Ech line consists of two triplets of letters. Ech triplet contins letters of equl contrst, nd the contrst of ech triplet decreses by fctor of 0.15 log units. Stted contrst vries from 100% (0.00 log units) to 0.56% ( 2.25 log units). Ech letter subtends 2.8 t the test distnce of 1 m (equivlent to 20/672). Procedure All testing ws monoculr with hbitul correction nd the pproprite ner dd. All subjects over 40 yers of ge were provided with 2.00 D ner dd for the Mrs test nd with 0.75 D ner dd for the Pelli-Robson test, s specified by the mnufcturers instructions for ech test. Ech norml-vision subject used his or her right eye, but ech low-vision subject ws llowed to use his or her better eye. The Mrs test ws plced on reding stnd nd the Pelli- Robson test ws wll-mounted. Chrt luminnce rnged from 95 to 140 cd/m 2 on the Pelli-Robson test nd 95 to 120 cd/m 2 on the Mrs test. Both were illuminted by multiple ceiling-mounted fluorescent lmps. Contrst sensitivity ws mesured with both the Pelli-Robson nd Mrs tests, nd visul cuity ws tken with the ETDRS cuity chrt t 4 m. After short brek, contrst sensitivity ws mesured gin using lternte forms of both tests. The chrt forms used (two Pelli-Robson nd three Mrs) nd the order of testing were vried systemticlly. Hlf of the subjects were tested first with the Pelli- Robson test nd then with the Mrs test, wheres the other hlf were tested first with the Mrs test nd then with the Pelli-Robson test. For ll subjects, the order of testing ws reversed for the second dministrtion. The three forms of the Mrs test were used in blnced fshion cross subjects using ll six possible two-chrt permuttions. For both CS tests, subjects were instructed to red ll letters on ech CS test beginning with the highest contrst letters. Sidewys hed movements were lso llowed. On the Mrs test, subjects responding with letter not prt of the 10-letter set used on the chrt were informed of the set nd instructed to give nother response, s specified by the mnufcturer s instructions. Subjects were llowed up to 30 seconds per letter if needed nd forced to guess until the stopping rule ws reched. Testing ended on the Mrs test when the subject missed two consecutive letters. Contrst sensitivity ws scored s log contrst of the finl correct letter minus 0.04 for ech letter missed before tht. Becuse of the design of the chrt, this simplifies to 0.04 multiplied by the number of letters correct. Testing on the Pelli-Robson test ended when the subject missed two of three letters in triplet. Contrst sensitivity ws scored s 0.05 multiplied by the number of letters correct minus 0.15 becuse the first three letters re 100% contrst. All subject responses were recorded on stndrd score sheets. When n incorrect response ws mde, the incorrect letter ws recorded on the sheet so tht corrections could be mde lter. Specificlly, responses of O for C nd C for O were ccepted s correct 15 when scoring both tests nd when pplying the stopping rule. This represents smll devition from the published instructions. Correction ws mde for letters missed before the stopping point ws reched. Dt Anlysis Repetbility nd greement were ssessed by determining the 95% limits of greement (LoA): 1.96 stndrd devitions (SD) of the differences between tests. 16 This method hs been shown to be more pproprite in ssessing the repetbility nd vlidity of clinicl tests thn the correltion coefficient. 16 The difference between the CS scores for ech dministrtion or test ws clculted for ech subject. The distribution of these differences ws described by clculting the men, SD, nd the 95% LoA. The bredth of these LoA indictes the repetbility of the test. The nrrower these LoA, the more repetble the test. RESULTS Tble 1 shows men CS scores for ech test nd ech subject group. Men CS scores from the Pelli-Robson test nd the Mrs test did not differ significntly for the group s whole (t 0.11, p 0.90) or for ny subgroup (t 1.29, p 0.11). Men CS scores were not significntly different between the younger norml nd older norml-vision groups on either test (Pelli-Robson: t 1.47, p 0.15; Mrs: t 1.88, p 0.07). As expected, CS scores were reduced in the low-vision group (Pelli-Robson: t 6.52, p 0.001; Mrs: t 9.45, p 0.001). The greement between the Pelli-Robson nd Mrs tests is shown in Figure 1. The difference between CS scores from the first dministrtion of both tests is plotted s function of the men of these scores. Agreement ws good, with 95% LoA of 0.21 log units. There is some evidence tht in subjects with the poorest CS, scores re lower for the Pelli-Robson test thn for the Mrs test (Fig. 1). In the five low-vision subjects with the poorest CS, the men difference is 0.12 log units. A similr difference of 0.09 log units is observed if scores from the second dministrtion of ech test re compred. In Figure 2, the difference between CS scores from the two dministrtions of ech test is plotted s function of the men of the two CS scores to illustrte the repetbility of ech test. The repetbility of the Pelli-Robson test nd the Mrs test is summ-
972 Mrs Letter CS Test Dougherty et l. TABLE 1. Men ( stndrd devition) scores for ech dministrtion of the Pelli-Robson nd Mrs test Pelli-Robson Mrs Test Retest Test Retest Young norml vision 1.70 0.08 1.70 0.08 1.72 0.06 1.73 0.08 Older norml vision 1.74 0.09 1.70 0.08 1.76 0.05 1.74 0.07 Low vision 1.30 0.47 1.29 0.48 1.27 0.41 1.30 0.43 All subjects 1.59 0.29 1.57 0.30 1.59 0.27 1.60 0.27 Vlues re given for ll subjects nd for ech subgroup. FIGURE 1. Agreement between the Pelli-Robson nd Mrs tests. The difference between the scores for the first dministrtion of ech test is plotted ginst the men for the two tests. The shded re represents the 95% limits of greement. Overlpping points hve been dithered. rized in Tble 2. Overll, repetbility ws similr for both tests, with 95% LoA of 0.20 log units for the Pelli-Robson test nd 95% LoA of 0.20 log units for the Mrs test. Interestingly, in the low-vision subjects, repetbility ws slightly better for the Mrs test thn for the Pelli-Robson test (95% LoA 0.20 vs. 0.28). Conversely, in norml-vision subjects, repetbility ws slightly better for the Pelli-Robson test (Tble 2). Further inspection of the dt for norml-vision subjects reveled tht men CS scores vried with different forms of the Mrs test. The men scores for ech of the three forms re shown in Tble 3. The men score for Form 3 is round 0.08 log units, or two letters, better thn the men for Forms 1 nd 2 (t 3.80, p 0.001 nd t 5.46, p 0.001, respectively), suggesting tht the contrst of the letters is slightly higher on Form 3. Pirwise comprison of norml-vision subjects tested with Forms 1 nd 3 (n 12, men difference 0.08 0.06 log units) nd with Forms 2 nd 3 (n 12, men difference 0.09 0.06 log units) led to similr conclusions. Frequency of seeing curves were plotted for the lst 12 letters of ech form of the Mrs test to further explore ny possible differences in contrst between forms of the Mrs test (Fig. 3). For ech letter, the percentge of ll norml- (young nd old) vision subjects who correctly identified tht letter ws plotted. Frequency of seeing curves for ll norml-vision subjects for ech of the three forms of the Mrs test re shown in Figure 3. From the figure, it gin ppers tht the contrst of Form 3 is slightly higher FIGURE 2. Repetbility of the Pelli-Robson nd Mrs tests. The difference between the scores for the second nd first dministrtion of ech test is plotted ginst the men of the two scores. The shded re represents the 95% limits of greement. Overlpping points hve been dithered. thn those of the other two forms nd tht there re some individul letters on certin forms whose contrst is different from the stted vlue. For exmple, the 44 th letter of Form 3 (V) ws identified correctly by 75% of norml-vision subjects, but the 44 th letter of Form 2 (Z), which hs the sme stted contrst, ws not identified correctly by ny norml-vision subjects. Given the pprent higher contrst of Form 3 of the Mrs test, correction fctor ws clculted by subtrcting the verge of the men CS scores of norml-vision subjects on Forms 1 nd 2 from the men CS score on Form 3. This fctor ws then pplied to ll CS scores of norml-vision subjects on Form 3, nd new men CS scores nd 95% LoA were clculted. This correction fctor im-
Mrs Letter CS Test Dougherty et l. 973 TABLE 2. Repetbility of the Pelli-Robson nd Mrs tests for ll subjects nd for ech subgroup proves the repetbility of the Mrs test from 95% LoA of 0.18 to 0.14 log units in the young norml vision subjects nd from 0.20 to 0.13 log units in the older norml-vision subjects. DISCUSSION Pelli-Robson Mrs Mrs With Correction Fctor Young norml vision 0.14 0.18 0.14 Older norml vision 0.17 0.20 0.13 Low vision 0.28 0.20 All subjects 0.20 0.20 All vlues represent the 95% LoA in log units. The finl column shows the 95% LoA when correction fctor of 0.08 log units is pplied to ll scores from Form 3 of the Mrs test. LoA, limits of greement. TABLE 3. Men scores for ech form of the Mrs test in norml-vision subjects Mrs Test Form n Men ( SD) Contrst Sensitivity Score 1 25 1.72 0.07 2 24 1.70 0.06 3 25 1.79 0.05 Number of subjects tested is round two thirds of the totl (37) s result of the experimentl design. SD, stndrd devition. FIGURE 3. Frequency of seeing curves derived from the responses of norml-vision subjects reding ech form of the Mrs test. Dt re shown for the lst 12 letters (two lines) on ech chrt corresponding to stted log contrsts of 1.48 to 1.92 (3.3 1.2%). Note tht not ll subjects hd the opportunity to red ll letters becuse testing stopped once two consecutive letters were missed. For the purposes of this nlysis, ll subsequent letters were ssumed to be missed. The new Mrs Letter Contrst Sensitivity Test shows good greement with the Pelli-Robson test nd similr test retest repetbility. These findings suggest tht it my be vible lterntive to the Pelli-Robson chrt for testing CS in clinicl prctice nd reserch. Tble 4 shows historicl men CS scores nd repetbility dt for the Pelli-Robson test, s well s those mesured in the present study. Men CS scores for the Pelli-Robson test in norml-vision subjects in the present study were slightly worse thn those of some other studies. 13,17,18 Repetbility of CS scores from the Pelli- Robson test in the present study ws quite comprble to tht of previous studies. 18 20 Men CS scores for different forms of the Mrs test (Tble 3) suggest tht the contrst vlues of the lower letters on Form 3 of the test re slightly higher thn those of the other two forms. Frequency of seeing curves plotted for the lst 12 letters (the lst two lines) on ech of the three forms (Fig. 3) lso suggest tht Form 3 hs slightly higher level of contrst. Some of the vritions between nd within chrts my lso be the result of differences in legibility of letters. 15 Nonetheless, this cnnot explin ll of the differences between forms, especilly given tht the 46th letter of ll three forms of the test is D. Given tht it is in the sme position on ll three forms, its contrst should theoreticlly be the sme, s would be the frequency with which norml-vision subjects identify the letter correctly. Nonetheless, 60% of norml-vision subjects correctly identified the D on Form 3, wheres only 5% correctly identified it on Form 1 nd 10.5% on Form 2. The men CS score in norml-vision subjects ws 1.79 for Form 3 compred with 1.72 nd 1.69 for Forms 1 nd 2, respectively. This represents bout two-letter discrepncy nd dversely ffects the repetbility of the test, so scores from Form 3 were djusted by correction fctor of 0.08 log units for ll norml-vision subjects (Tble 2). This correction fctor improved the repetbility of the test, nrrowing the 95% LoA to 0.14 log units in ll normlvision subjects (n 37), which is ctully better thn the repetbility of the Pelli-Robson test (95% LoA 0.18 log units). This grees with other recently reported vlues. 21 It should be noted tht these contrst discrepncies did not ffect the repetbility of the Mrs test in low-vision subjects, becuse they were unlikely to rech the lower contrst portions of the chrts. Until the mnufcturers of the Mrs test cn resolve these between-chrt discrepncies, clinicins nd reserchers my need to pply similr correction fctors or void using different forms of the test. We mde some photometric mesurements of the contrst of the chrts used in the present study using Spectr Pritchrd Photometer Model 1980 A (Kollmorgen Corp., Burbnk, CA). As expected, ccurtely mesuring the contrst of letters with stted contrst vlues of 1% to 2% ws very difficult. In most cses, the letters could not be seen through the eyepiece of the photometer nd estimtes of contrst were highly vrible. Given tht subjects with poorer contrst sensitivity hd slightly higher scores on the Mrs test thn on the Pelli-Robson test (Fig. 1), we lso mesured the contrst of letters in the 0.75- to 1.00-log unit rnge on both tests. We found tht the contrst of the letters on the Mrs test ws on verge 0.07 log units higher thn the stted vlue, wheres the vlues on the Pelli-Robson test were within 0.02 log units of the stted vlue. This is consistent with the low-vision subjects scoring higher on the Mrs test. Reserchers hve shown tht the repetbility of visul cuity or CS test is relted to the number of letters per unit chnge in size
974 Mrs Letter CS Test Dougherty et l. TABLE 4. Previous studies of the repetbility of the Pelli-Robson test Author Method Subjects Men SD 95% LoA Elliot et l. 13 Elliot et l. 19 Lovie-Kitchin nd Brown 20 Elliot nd Bullimore 18 Hymes nd Chen 17 Present study Monoculr, dominnt eye, best correction Monoculr, dominnt eye, best correction Monoculr, better eye, hbitul correction, test distnce 3m Monoculr, best correction Monoculr, better eye, best correction in low vision subjects; right eye with hbitul correction in soft contct lens subjects Monoculr, right (or better) eye with hbitul correction Vlues re in log units. Test distnce 1 m, unless specified. SD, stndrd devition; LoA, limits of greement; VA, visul cuity. Age rnge 22 79, N 40, 1.83 0.14 0.20 VA 6/9 Young norml, N 30 1.88 0.08 0.16 Older norml, N 42 1.75 0.12 0.23 Young nd older norml, N 79 1.74 0.17 Young norml, N 24 1.86 0.09 0.18 (41 subjects from ll 3 groups) Older norml, N 23 1.80 0.11 Ctrct group: VA 6/21, N 33 1.48 0.25 Low-vision group: men ge 74, 1.11 0.38 0.25 N 22; VA rnge 0.36 1.50 logmar, soft contct lens group: men ge 22, N 20, VA rnge 0.12 to 0.16 1.88 0.08 0.18 Young norml (N 20) 1.70 0.08 0.14 Older norml (N 17) 1.74 0.09 0.18 Low vision (N 17) 1.30 0.47 0.20 or contrst. 22,23 In essence, CS tests with finer scles, in which ech letter represents smller chnge in contrst, should hve better repetbility. On the Mrs test, ech letter corresponds to 0.04 log units nd thus the test should hve better repetbility, nd nrrower 95% LoA, thn the Pelli-Robson test when, lthough the contrst decreses in 0.15-log unit steps, ech letter cn be tken to correspond to 0.05 log units. 14 This theory is supported by our results once the correction fctor is pplied to Form 3 of the Mrs test (Tble 2). Previous studies hve reported tht vision tests in low-vision subjects re likely to be less repetble thn in people with norml vision. 24,25 Given tht CS testing is frequently indicted in lowvision ptients, it is crucil tht new test of CS hs comprble repetbility to those tests tht re commonly used. The fct tht the Mrs test uses letters of smller size nd lesser ngulr subtense thn the Pelli-Robson test (2 vs. 2.8 deg) my rise some concerns bout its use in subjects with poorer visul cuity. Nonetheless, the Mrs test performed well in the low-vision subjects tested in this study. The Mrs test hd better repetbility (95% LoA 0.20 log units) thn the Pelli-Robson test (95% LoA 0.28 log units) in low-vision subjects. Agreement between the two tests in low-vision subjects (95% LoA 0.30 log units) ws poorer thn in ll subjects (95% LoA 0.21 log units), nd we did not test subjects with visul cuity worse thn 20/250. Overll, our results suggest tht the Mrs test is n pproprite test for use in low-vision popultion. Using the test t distnce of 40 cm insted of the recommended 50 cm would result in the letters hving similr ngulr subtense s the letters on the Pelli-Robson test t 1 m. Elliot et l. demonstrted the vlue of ccepting response of C for O nd O for C on the Pelli-Robson test. 15 The dt presented for our min nlyses in the current study re bsed on ccepting these common misclls on both the Pelli-Robson test nd the Mrs test. Nonetheless, we recorded such misclls nd nlyzed repetbility with nd without their cceptnce. We found tht CS scores from the Mrs test demonstrted improved repetbility when C nd O misclls were ccepted with 95% limits of greement of 0.20 log units when the misclls were ccepted nd 0.22 log units when they were not ccepted. These results suggest tht there is vlue in ccepting C for O nd O for C responses on the Mrs test. There re severl physicl fetures of the Mrs test tht mke it desirble for use in clinicl setting. The chrts re smll, mesuring 23 cm by 35.5 cm. This mkes them much esier to trnsport nd store thn the Pelli-Robson test, which is considerbly lrger. A plstic folder is provided for storing the Mrs chrts, protecting them from dmge. The Mrs test is lso printed on durble plstic, which my mke it less susceptible to dmge thn the Pelli-Robson test, which is printed on thin crdbord. Finlly, the smller dimensions of the Mrs test mke it esier to illuminte evenly. All of these fetures mke the Mrs test firly verstile nd
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