3951力EI1四ue0l.a0.1L计.7w DFTONETIE ANDVIKIN ENCE ORIGINAL ARTICLE Is all Asthenopia the Same? JAMES E.SHEEDY,OD,PhD,FAAO,JOHN HAYES,PhD,and JON ENGLE,BS The Orio State Linieniy,CollegefOpryCaloiau,Diue ABSTRACT:Purpose.Any of several conditions can cause asthenopia.The purpose of this study was to determine whether the particular sersations or their location vary dependent on the symptom-inducing condition.Methods. Twenty subjects with good vision performed eight reading tasks in random order during different conditions.Each condition used different stimuli to induce asthenopia.The eisht conditions were mixed astigmatism,close viewing distance,upward gaze,dry eyes,lens flipper,small font,glare,and flickering light.Subjects were asked to read until attaining a level of discomfort self-defined as"barely tolerable."After each task,subjects rated the magnitude of several symptom descriptors (burning,ache,strain,irritation,tearing,blurred vision,double vision,dryness,and headache) and their location.Resulfs.Analysis of variance with repeated measures was used to determine that all of the individual symptom sensation measures (except blur)were significantly related (p values ranged from 0.003 to <0.0001)to the inducing condition.Principal factor analysis with orthogonal varimax rotation was used to test symptom by condition relationships and determined two latent factors,designated external and internal symptom factors (ESF and ISF),that related symptoms to inducing condition.The ESF pattern comprises burning,irritation,tearing,and dryness located in the front and bottom of the eye.ESF is caused by holding the eyelid open,glare,up gaze,small font,and flickering.ESF seems highly related to dry-eye symptoms.The ISF pattern comprises ache,strain,and headache located hehind the eyes.ISF is caused hy the close viewing distance,lens flipper,and mixed astigmatism conditions and is likely related to accommodative and vergence stress.Conclusions.Symptom descriptors and locations were able to distinguish discomfort on the basis of causative condition.Results support two different symptom constellations and,hence,at least two different afferent pathways for symptoms of asthenopia.(Optom Vis Sci 2003:00:732-739) Key Wordks:eyestrain,asthenopia,dry eye,accommodation,convergrnce,Bare,symptoms,eye sthenopia is the formal dugnostic term (ICD-9.368.13)caused by many factors.the treatments can be many and ofien that is nearly synonymous with the more familiar woed involve treatment of virl condtions as well as o the work eyestrainEyestrain is very commonfor exampl,it is environment.22.2 che most frequent sympcom reported by computer users.Secre Despice the fact thar numerous sympcom sensations have been taries,acoountants,bookkeepers,draftsmen,and others with de- associated with astbenopia and numcrous causative conditions manding near visual jobs have commonly experienced asthenopia have been identified,the condition of asthenopia is known as a The list of nonspecific symptom snstions mast commonly asso- single enrity,and the canerive factor of athenopi cannot cur- ciatod with asthcnopia includes cyotrain,cye frigue,disoomfort. rently he difforentiated on the badis of symptom sension or pcr- burning,iritatioo,pin,ach,o eyes and headachc.Other ccind location.The mcchanisms that crate the sympcom sensa- more specific symptoms include photophobia,blur,double vision. tions are also not lenown. itching,tearing,dryness,and foreign-body sensation. Collins ct al.sdied 79 camputer-using subjeets and catego- Asthenopia can be caused or induced by each of the following ried their symptoms as either oculr (asthenopia type symptoms) distinct conditions:glre from lighting termed "discomfort or visual (blur,double visicn),but did pot find any significant glare;anomalies of binocular vision sch as convergence insuffi- relationship to the visual examination findings that consisted of ciency and esophori accmmodative dysfunctions suchre refractive.binocular vision,and accommodative measurements. dneed amplitude and commodaive infacilir;uncorrected Likewisc,Neugrhuuer ct al.uded5 young adults with asthe- rfractive crror,3 including prsbyopias.compromiscd nopic sympeoms of headachec,ocr pain,forcign body snsation. yof thed imsuchspoor cn bilirydes photophobia,doublevisand difficulyhen chang less than optimal gaxe angles flckering stimuli suchs CRT ing fiation distance.They found no correlacion of specific symp- computer displays:and dry eye.Because asthenopia can be toms with the refrartion,hinoctlr vision,and accommeclation
ORIGINAL ARTICLE Is all Asthenopia the Same? JAMES E. SHEEDY, OD, PhD, FAAO, JOHN HAYES, PhD, and JON ENGLE, BS The Ohio State University, College of Optometry, Columbus, Ohio ABSTRACT: Purpose. Any of several conditions can cause asthenopia. The purpose of this study was to determine whether the particular sensations or their location vary dependent on the symptom-inducing condition. Methods. Twenty subjects with good vision performed eight reading tasks in random order during different conditions. Each condition used different stimuli to induce asthenopia. The eight conditions were mixed astigmatism, close viewing distance, upward gaze, dry eyes, lens flipper, small font, glare, and flickering light. Subjects were asked to read until attaining a level of discomfort self-defined as “barely tolerable.” After each task, subjects rated the magnitude of several symptom descriptors (burning, ache, strain, irritation, tearing, blurred vision, double vision, dryness, and headache) and their location. Results. Analysis of variance with repeated measures was used to determine that all of the individual symptom sensation measures (except blur) were significantly related (p values ranged from 0.003 to <0.0001) to the inducing condition. Principal factor analysis with orthogonal varimax rotation was used to test symptom by condition relationships and determined two latent factors, designated external and internal symptom factors (ESF and ISF), that related symptoms to inducing condition. The ESF pattern comprises burning, irritation, tearing, and dryness located in the front and bottom of the eye. ESF is caused by holding the eyelid open, glare, up gaze, small font, and flickering. ESF seems highly related to dry-eye symptoms. The ISF pattern comprises ache, strain, and headache located behind the eyes. ISF is caused by the close viewing distance, lens flipper, and mixed astigmatism conditions and is likely related to accommodative and vergence stress. Conclusions. Symptom descriptors and locations were able to distinguish discomfort on the basis of causative condition. Results support two different symptom constellations and, hence, at least two different afferent pathways for symptoms of asthenopia. (Optom Vis Sci 2003;80:732–739) Key Words: eyestrain, asthenopia, dry eye, accommodation, convergence, glare, symptoms, eye Asthenopia is the formal diagnostic term (ICD-9, 368.13)1 that is nearly synonymous with the more familiar word “eyestrain.”2 Eyestrain is very common—for example, it is the most frequent symptom reported by computer users.3, 4 Secretaries, accountants, bookkeepers, draftsmen, and others with demanding near visual jobs have commonly experienced asthenopia. The list of nonspecific symptom sensations most commonly associated with asthenopia includes eyestrain, eye fatigue, discomfort, burning, irritation, pain, ache, sore eyes, and headache. Other more specific symptoms include photophobia, blur, double vision, itching, tearing, dryness, and foreign-body sensation. Asthenopia can be caused or induced by each of the following distinct conditions: glare from lighting,5, 6 termed “discomfort glare”; anomalies of binocular vision such as convergence insufficiency and esophoria7–9; accommodative dysfunctions such as reduced amplitude and accommodative infacility10–12; uncorrected refractive error,13, 14 including presbyopia15, 16; compromised quality of the viewed image such as poor contrast or legibility17, 18; less than optimal gaze angles19; flickering stimuli such as CRT computer displays20; and dry eye.21 Because asthenopia can be caused by many factors, the treatments can be many and often involve treatment of visual conditions as well as of the work environment.22, 23 Despite the fact that numerous symptom sensations have been associated with asthenopia and numerous causative conditions have been identified, the condition of asthenopia is known as a single entity, and the causative factor of asthenopia cannot currently be differentiated on the basis of symptom sensation or perceived location. The mechanisms that create the symptom sensations are also not known.24 Collins et al.25 studied 79 computer-using subjects and categorized their symptoms as either ocular (asthenopia-type symptoms) or visual (blur, double vision), but did not find any significant relationship to the visual examination findings that consisted of refractive, binocular vision, and accommodative measurements. Likewise, Neugebauer et al.26 studied 50 young adults with asthenopic symptoms of headache, ocular pain, foreign body sensation, red eyes, photophobia, double vision, and difficulty when changing fixation distance. They found no correlation of specific symptoms with the refraction, binocular vision, and accommodation 1040-5488/03/8011-0732/0 VOL. 80, NO. 11, PP. 732–739 OPTOMETRY AND VISION SCIENCE Copyright © 2003 American Academy of Optometry Optometry and Vision Science, Vol. 80, No. 11, November 2003
Is all Asthenopia the Same?Shccdy ct al.733 finding.The lck of coerdations in thee studies supports the idea equalize them for difficulry.However,the absence of previous that the symptoms of asthcnopia arc nonspocific and not diffcrun- literature or work in induing asthenopia made such ouaication tiatod on the basis of the causitive factur. unatainable,and parameters were slected on the basis of clinical However,is it not likely that sensacion differences should be experience and iterative preiminary testing. distinguishable-if measured properly-because of the differ The following procedures were used for all conditions unless ences in the primary anatomical locarions of the action or stimu otherwise noced in Table 1:head and chin rest were used:subjects lation?The tisuc that is strosd or insaltod and penerates the sikncly ncad short storics for 15 min or until attaining a discomfurt feeings of discomfort is moat likely the same tissue of the primary kvel that was"barely tolerable stories were printed in 10-point action or stimulation.For exampl,the primary action of comver- font,were selected from the works of Edgar Allen Por,were placed gence imolves the medi reci,accommodation the cilry body. on a document holder 40 cm from the eyes,and were located so and dry eyes the anterior ocular surfaces.These are three discrete that the paragraph wis viewed with a range of 3.5 to 22 of tissus,and it is resonable that insults to thesc tisues could crcate downward gac.No story was ever prescnted more than oncetoa different snations.The Collins eta and Neugebaueral. given subject,and reading performance was not monitord.Sub- studies used retrospective anayses of clinical data.The reearch jects completed asymptom sensation questionnaire after ech con presented here isa prospective study of symptoms induced by dition and were given 5 min of rest berween tests.The sympcom prespecified controlled stimuli in a laboratory setting.We investi. sensation questionnaire contained nine identical analo scales oo gated whether the sympcom sstio or their perocived locations which the subject reoorded the magnitude of cach of the follkowing can be related to the causative factors.If the sensatioos of asthe- nine symptomo:burning.ache,strain.irritation.tearing,blurred popia can be differentiated.then it is likely that the anatomical vision,double vision,dryness,and headache.Each analog scale was location and'or the physiological mechanisms of ashenopia can a 100 mm line with descripcors at boch ends (0 none and 100= also be differentiaced and,in individual patienrs.tied to the caus severe)and at each qurtile location (mild,modest,and bad).The ative factor. subject indicaced magnirude with a vertical line along the scale, which was recorded as a valoe between 0 to 100.If the subject did METHODS not artain a discomfort level of barely tolerable within 15 min.the trial was stopped,the symptom sensation questionnaire adminis Twenty adult subjects (aged 18 to 35 years)sclected from stu- ered.and the subject【tpored《on a similar andlog scale）che dents and staff at The Chio Stare University were screened to have percentage of "barely comfortable"that had been arined.For 20/20 unaided (without glasses or contact lemses)distance visual each trial,the experimenter (E)watched the subject and estimaced acuity OU.no known ocular pathology,no dinically significant the percentage of time that the sbject noticeably squinced their asthenopia,and no asthenopia at the beginning of the tes session. evelids Eye cxamination data were not availahle for the subjects.Each consented to participate in the study according to a prooo ap- RESULTS proved by the Ohio State Universiry institutional review board Subjects read short stories during each of the eight discomfort. Analysis of variance with repeated measures was used to test che inducing coodicions listed in Table 1.Testing order was by Latin main effects of condition and testing order and interactions be squre doign.Condition parameters were sdected with intent to twcen the two o symptom snsations Each of the individual TABLE 1. Asthenopia induction conditions. Inducing Condition Induction Stimulus Lens flipper Subject read alternate senrences through t1.501 lenses using a hand-held hinocular lons flipper Close viewing distance Suhject wore glasses wirh +f.DO D5 QU;the distance between lens optical centers masched the subject interpupillary distance within 2.5 mm.Stories were placed an a document holder at 16.7 cm from the cyes unless a doser distance was required for clarity (o compensate proximal accummodatien)and were printed in 5-puint lont with recluced page width to componsate visual anglos for the clocer distance. Ury cye Subject manually held their eyolids open with thumbs and forefingers while reading. Flickering ligh城 A strobe light (1538-A Strobolac Ceneral Radio)run al 15 cycles'sec in a dark roorn was criented lo illuminale the reading maberial.A frooted plastic filter was placed in front of the strobe light to evenly distrihure the light anto the reading material. Glare Two desk lamps with 60 W tungsten bulbe,40 cm from and kvel with the eyes,were directed toward the subject's cyes. Srraall font The reading material was printed in 5-point font. Mixed astignalism Suject wore glasses with12.00 4.00 x 180 OU;the distarke between lers opticall centers matched the subpcct interpupellary distance within 2.5 mm. Upward gaze The sones wore placed on a document holder 40 cm from the cyes and located above cye level to require between 20 and 35 degrees of upward gaze. O小siwo向daa女w,Vd.0nIL,skhr28g
findings. The lack of correlations in these studies supports the idea that the symptoms of asthenopia are nonspecific and not differentiated on the basis of the causative factor. However, is it not likely that sensation differences should be distinguishable—if measured properly—because of the differences in the primary anatomical locations of the action or stimulation? The tissue that is stressed or insulted and generates the feelings of discomfort is most likely the same tissue of the primary action or stimulation. For example, the primary action of convergence involves the medial recti, accommodation the ciliary body, and dry eyes the anterior ocular surfaces. These are three discrete tissues, and it is reasonable that insults to these tissues could create different sensations. The Collins et al.25 and Neugebauer et al.26 studies used retrospective analyses of clinical data. The research presented here is a prospective study of symptoms induced by prespecified controlled stimuli in a laboratory setting. We investigated whether the symptom sensations or their perceived locations can be related to the causative factors. If the sensations of asthenopia can be differentiated, then it is likely that the anatomical location and/or the physiological mechanisms of asthenopia can also be differentiated and, in individual patients, tied to the causative factor. METHODS Twenty adult subjects (aged 18 to 35 years) selected from students and staff at The Ohio State University were screened to have 20/20 unaided (without glasses or contact lenses) distance visual acuity OU, no known ocular pathology, no clinically significant asthenopia, and no asthenopia at the beginning of the test session. Eye examination data were not available for the subjects. Each consented to participate in the study according to a protocol approved by the Ohio State University institutional review board. Subjects read short stories during each of the eight discomfortinducing conditions listed in Table 1. Testing order was by Latin square design. Condition parameters were selected with intent to equalize them for difficulty. However, the absence of previous literature or work in inducing asthenopia made such equalization unattainable, and parameters were selected on the basis of clinical experience and iterative preliminary testing. The following procedures were used for all conditions unless otherwise noted in Table 1: head and chin rest were used; subjects silently read short stories for 15 min or until attaining a discomfort level that was “barely tolerable”; stories were printed in 10-point font, were selected from the works of Edgar Allen Poe, were placed on a document holder 40 cm from the eyes, and were located so that the paragraph was viewed with a range of 3.5° to 22° of downward gaze. No story was ever presented more than once to a given subject, and reading performance was not monitored. Subjects completed a symptom sensation questionnaire after each condition and were given 5 min of rest between tests. The symptom sensation questionnaire contained nine identical analog scales on which the subject recorded the magnitude of each of the following nine symptoms: burning, ache, strain, irritation, tearing, blurred vision, double vision, dryness, and headache. Each analog scale was a 100-mm line with descriptors at both ends (0 none and 100 severe) and at each quartile location (mild, modest, and bad). The subject indicated magnitude with a vertical line along the scale, which was recorded as a value between 0 to 100. If the subject did not attain a discomfort level of barely tolerable within 15 min, the trial was stopped, the symptom sensation questionnaire administered, and the subject reported (on a similar analog scale) the percentage of “barely comfortable” that had been attained. For each trial, the experimenter (JE) watched the subject and estimated the percentage of time that the subject noticeably squinted their eyelids. RESULTS Analysis of variance with repeated measures was used to test the main effects of condition and testing order and interactions between the two on symptom sensations. Each of the individual TABLE 1. Asthenopia induction conditions. Inducing Condition Induction Stimulus Lens flipper Subject read alternate sentences through 1.50 lenses using a hand-held binocular lens flipper. Close viewing distance Subject wore glasses with 6.00 DS OU; the distance between lens optical centers matched the subject interpupillary distance within 2.5 mm. Stories were placed on a document holder at 16.7 cm from the eyes unless a closer distance was required for clarity (to compensate proximal accommodation) and were printed in 5-point font with reduced page width to compensate visual angles for the closer distance. Dry eye Subject manually held their eyelids open with thumbs and forefingers while reading. Flickering light A strobe light (1538-A Strobotac General Radio) run at 15 cycles/sec in a dark room was oriented to illuminate the reading material. A frosted plastic filter was placed in front of the strobe light to evenly distribute the light onto the reading material. Glare Two desk lamps with 60 W tungsten bulbs, 40 cm from and level with the eyes, were directed toward the subject’s eyes. Small font The reading material was printed in 5-point font. Mixed astigmatism Subject wore glasses with 2.00 4.00 180 OU; the distance between lens optical centers matched the subject interpupillary distance within 2.5 mm. Upward gaze The stories were placed on a document holder 40 cm from the eyes and located above eye level to require between 20 and 35 degrees of upward gaze. Is all Asthenopia the Same?—Sheedy et al. 733 Optometry and Vision Science, Vol. 80, No. 11, November 2003
734 Is all Astheropia the Same-shecdy et al symptom sensation measurcs (except blur)was significantly related to determine the underlying causes of epestrain based on the re- (pvalues ranged from 0.003 to<00001)to inducing condition.A ported symptom sensations. summary of the mean scores by symptom is provided in Table 2. Although the individual symptom snsaion mesures(exept The symptom headache was the only one to be significantly related blur)were significantly related to inducing condition.a much (p -0.0151)to testing order-there was a cumulative effect of stronger relarionship was obrained by calculating the lacent factocs testing time on headache magnitude.This is n indication that with principal factor anlysis.Two factoes sccounted for Giof beadache is in some way different from the other symptoms or that the ttal varianoe.Eigenvalues were<10 for the remaining factocs the subjects,in unison.scaled headacbe differently across the and,thus,were dropped from consideration.For each sympcom course of the experimental session.Alo,the fact that all of the measure,the analysis determined a coefficient by which that symp other symptoms were not relaced to testing oder heips to validate tom measure contributed to each calulated latent factor.Fi.I is the mechodology and the relatively short resting time (5 min) a graphical representation of the latent faco lodings foe the nine allotted berween conditions.Although some symptom carryover sensation eares used in this study.Two ditinct groupingsof seems likely,ocher than headache,such carryover was not stacisti the symptom meres were evident.One group comprises burn cally significant,and the Latin order testing design helped tomit ing,irritation,dryness,and cearing:and each beavily conmributes to igate rentaining order effects. fictor I and noc to factor 2.The other group comprises strain, There were significant simple comrelations among the symptom headache,ache.double vision,and (to a leser extent)blur:and scores (Table 3).The simple cocreations ugeed the existence of each heaviy contributes to factor 2 and not factor 1.Because the underlying primary latent variables that could reduce the dimen first group.which contributes heavily to factor 1,was comprised of sicoality of the sympom space.Principal factor analysis withor symptoms that seemed related to the ocular surface.we labeled thogonl varimax rotation was used to take advantige of the inter- fictor I as "external symptom factor"(ESF),and with similar correlatioes and reduce the dimenanaliry of the symptom space. resoning.we labeled factor 2 as"interal symptom factoe"(TSF) The number of underlying ltent varables was deermined by se- After rotation,the ESP accounted for 33.7%nf the total variance, lecting fictors with eigenvalues1.An eigenvue is the amount of and ISF accounted for 30.4%. variance accounted for by the latent variable.An eigenvalue of one Fig.2shows the mean value (wich standard error of the mean)of is equal to the amount of variance accounted for by any one of the the calculated ESF and ISF for each of the eight inducing condi- symptom scores.An underlying latent vartble reduces dimension- tions ued in this sdy.The no facrors were significantly different aliry only if it explains more varianoe than a single variahle.The from ane another for a given condition if the ermoe bars do not fctor analysis revedled fundamental latent factors that were used overlap.Very clearly.the dry-eye condition shows the grearest TABLE 2. Summary oi mean SD symptom scores (scale o 0 to 100)by conditioxn. sd山erd Symptim Coed tion Buming Ache 口n Imitvion Teuriag Douhle Viins Dryness Ibyhe Luo leper 158±193 J59±31.3 522±27.1 191±19.8 97±15.9 15.5=272 202±324 183±22.1 523±312 Cloe viewin 267±297 441±31.3 0B±166 27.4±292 126±16 201±312 301±33.6 23.7±269 6.1±34 Dry ere 633±357 198±296 238±25.1 507±352 588±36 13.9±29.4 65±17.2 532±428 112±199 Flickoring lidnt 264±31 137±31.5 434±297 30±31.2 7±264 163=24.9 12.9±229 29.7±322 35.4±40 CLev 352±299 351±32.3 454±291 318±31 129±204 196±29.2 109±23 31.3±258 319±321 Small iont 155士25 109士27.5 52丁生2K1 1%5生10 156士24 2h1=121 21.1±1.5 可2±161 24.5±1写7 Mxrd adgrtats 123±21.4 4±11.5 ±25.4 109士2k1 145士1队5 171±274 2士4用 ±66 55.5±13 Upwrad gaae 10±24 4儿7±27.指 553±25.9 277±22226±24.7 17=2.1 15.3±244 17±144 44±18 TABLE 3. Pearson correlation coellicients between symptom scores. Ache Strain lcadache Double Vision Blur Tcaring Burning Irntation Dryness Ache 0.66 059- 0.46 0.56 0270 0.38 0.41- 0.26 Strain 0602 0,434 0.37e D04 0210 D23b 01a I Icadache 0.32 02B9 0.13 0.17 017 0.13 Douhle vision 0.h2 0.17 0.25 D.29h 0.25 Blur 03 0.39- 044+ 0.43+ Tearing 0.66- 056 0.43 Burning D79 0.65 lrra对on 64 Dryness 'p<0.0001. p<0.01 Oproetry aad Viiow .Sirwer,Vol.30,No 11.Neveether 2003
symptom sensation measures (except blur) was significantly related (p values ranged from 0.003 to 0.0001) to inducing condition. A summary of the mean scores by symptom is provided in Table 2. The symptom headache was the only one to be significantly related (p 0.0454) to testing order—there was a cumulative effect of testing time on headache magnitude. This is an indication that headache is in some way different from the other symptoms or that the subjects, in unison, scaled headache differently across the course of the experimental session. Also, the fact that all of the other symptoms were not related to testing order helps to validate the methodology and the relatively short resting time (5 min) allotted between conditions. Although some symptom carryover seems likely, other than headache, such carryover was not statistically significant, and the Latin order testing design helped to mitigate remaining order effects. There were significant simple correlations among the symptom scores (Table 3). The simple correlations suggested the existence of underlying primary latent variables that could reduce the dimensionality of the symptom space. Principal factor analysis with orthogonal varimax rotation was used to take advantage of the intercorrelations and reduce the dimensionality of the symptom space. The number of underlying latent variables was determined by selecting factors with eigenvalues 1. An eigenvalue is the amount of variance accounted for by the latent variable. An eigenvalue of one is equal to the amount of variance accounted for by any one of the symptom scores. An underlying latent variable reduces dimensionality only if it explains more variance than a single variable. The factor analysis revealed fundamental latent factors that were used to determine the underlying causes of eyestrain based on the reported symptom sensations. Although the individual symptom sensation measures (except blur) were significantly related to inducing condition, a much stronger relationship was obtained by calculating the latent factors with principal factor analysis. Two factors accounted for 64% of the total variance. Eigenvalues were 1.0 for the remaining factors and, thus, were dropped from consideration. For each symptom measure, the analysis determined a coefficient by which that symptom measure contributed to each calculated latent factor. Fig. 1 is a graphical representation of the latent factor loadings for the nine sensation measures used in this study. Two distinct groupings of the symptom measures were evident. One group comprises burning, irritation, dryness, and tearing; and each heavily contributes to factor 1 and not to factor 2. The other group comprises strain, headache, ache, double vision, and (to a lesser extent) blur; and each heavily contributes to factor 2 and not factor 1. Because the first group, which contributes heavily to factor 1, was comprised of symptoms that seemed related to the ocular surface, we labeled factor 1 as “external symptom factor” (ESF), and with similar reasoning, we labeled factor 2 as “internal symptom factor” (ISF). After rotation, the ESF accounted for 33.7% of the total variance, and ISF accounted for 30.3%. Fig. 2 shows the mean value (with standard error of the mean) of the calculated ESF and ISF for each of the eight inducing conditions used in this study. The two factors were significantly different from one another for a given condition if the error bars do not overlap. Very clearly, the dry-eye condition shows the greatest TABLE 2. Summary of mean SD symptom scores (scale of 0 to 100) by condition. Inducing Condition Symptom Burning Ache Strain Irritation Tearing Blur Double Vision Dryness Headache Lens flipper 15.8 19.3 35.9 31.3 52.2 27.1 19.2 19.8 9.7 15.9 15.5 27.2 20.2 32.8 18.3 22.1 32.3 31.2 Close viewing 26.7 29.7 44.1 31.3 70.8 16.8 27.4 29.2 12.6 18 20.1 31.2 30.1 39.6 23.7 26.9 36.1 36.4 Dry eye 63.3 35.7 19.8 29.6 23.8 25.1 50.7 35.2 58.8 36 13.9 29.4 6.5 17.7 53.2 42.8 11.2 19.9 Flickering light 26.4 31.2 33.7 31.5 43.4 29.7 30 31.2 17 26.4 16.3 24.9 12.9 22.9 29.7 32.2 35.4 40 Glare 35.2 29.9 35.1 32.3 45.4 29.1 31.8 31 12.9 20.4 19.8 29.2 10.9 23 31.7 25.8 31.9 32.1 Small font 33.5 29.5 30.9 27.5 52.7 28.3 28.5 30 15.8 24 26.3 32.3 23.1 31.5 23.2 26.1 28.5 33.7 Mixed astigmatis 22.3 23.4 49 31.5 63.8 25.4 30.9 28.1 14.5 19.5 17.3 27.8 38.2 43 23 26.6 35.5 31.3 Upward gaze 39 26.4 49.7 27.8 55.3 25.9 27.7 26.2 22.6 24.7 17 28.1 15.5 24.8 31.7 24.4 34.9 31.8 TABLE 3. Pearson correlation coefficients between symptom scores. Ache Strain Headache Double Vision Blur Tearing Burning Irritation Dryness Ache — 0.66a 0.59a 0.46a 0.56a 0.27b 0.38a 0.41a 0.26b Strain — — 0.60a 0.43a 0.37a 0.04 0.21b 0.23b 0.10 Headache —— — 0.32a 0.28b 0.13 0.17 0.17 0.13 Double vision —— — — 0.62a 0.17 0.25b 0.29b 0.25b Blur —— — — — 0.31 0.39a 0.44a 0.43a Tearing —— — — — — 0.66a 0.56a 0.43a Burning —— — — — — — 0.79a 0.65a Irritation —— — — — — — — 0.64a Dryness —— — — — — — — — a p 0.0001. b p 0.01. 734 Is all Asthenopia the Same?—Sheedy et al. Optometry and Vision Science, Vol. 80, No. 11, November 2003
Is all Asthenopia the Same?-Sheedy e al.735 090 eye data remowved.Analysis of variance with repeated measres determined that burning (p =0.0005),ache (p=0.0148).strain (p 080 eran 垂A中到 0.0111),and heachche (p =0.03)were significantly related to 0.70 inducing condition.Principal factor analysis without the dry-ey ◆Handache cundition revealed the same two latent factors for which the load- ◆Do.ble ing factors of the component sympom sensations are graphed in ◆E卧山 Vision Fig 3 and for which the mean values for cach condition are shemn 050 in Fig 4.Without the dry-cyc condition,the overall fctor model 040 acoounted for63%of the variance with 34%bcing accounted for by ESF and 29%acoounted for by ISF.Again,factors with cigen- 020 values1 were not considered. 020 ◆lTii2n Figs.3 and 4 are the same as Figs 1 and 2,respectively,cxopt ◆uTin that data from the dry have bcen remaved from the ◆Drynes 0.10 analysis.The ESF and ISF clusters of symptoms in Fig 3 are ncarly ◆T8sna the same is in Fig.1.Buming.irritation,and dryness were still 0004 associated with ESF,and strain.headache,and ache were still as c.00 Q29 040 0.60 080 1.00 sociatod with ISF.The symptoms of tcaring,blur,and double Extomal Symplom Factor (ESF) vision were not as well asociatod with the latent factors with re- moval of the dry-eye d由ta. FIGURE 1. Likewise,the reationships between the two latent factocs and Ceaphical repnmntation of the lucfing:Lactors for the exemal symplum the inducing conditions weresimilar in Figs.2 and 4.Lens flippers. factor and the internal sympoom factor for each of the nine symptom wensalian masures uwetl in this stuly. close viewing dstance,and mixed astigmatism continued to have signifiantly greater ISF than ESF weightings dospite absnce of data from the dry-eye condiion.However,after the strong effecrs 1.50 of the dry-eye data were removed,the data in Fig ishow that other conditions arereaed to ESF.Glare had significantly greaer ESF 1,0 compared with ISF,and fickering,small foot.and up gaze abso showed higher ESF than ISF.These ctfocts were soemingly over- whelmed by the very strong ESF effects of the dry-tye condition shown in Fig.2.The remarkable similarities becween the factor C.CO analysis results wich (Figs.I and 2)and without (Figs.3 and che influential dry-eye dar senthens the validiry of the rwo latent 0.50 factors [ESF and ISF).Table 4 contains the same raw data mcans of ▣ESF 1.00 08C ■ISF 0 ◆Ston ◆%中阳 1.50 0e dry eye 重eC ◆H4lG4 ◆E山 050 ◆co左5 Visian Symptom Inducing Condition 0.4c FIGLRE 2. Tearrg Mun+SEM c the exmal sympiom facter (FSF)and inkemal sympinm 030 factor IISF)for each inducin condition. ◆Bumring Mmtaion 02元 sepuration herween the ro symptom factoes with a high ESF and *prynees low ISF.Therefore,the ESF component sensations (burning.irri 0.1G tation.drynes.and tearing)were relaed to dry eyes.The dry-eye condition was also the only condition with a significantly high 000+ ESF.ISF (strain,headcbe,ache,double vision,and blur)was 0c0 0.29 0.0 Q80 080 100 significantly higher than FSF for the accommodatnve strexs,con- External Sympiom Factor (ESF) vergno,and mixed astigmmarism condirions FIGURE 3. The lrge contribution of the dry-eye condition to ISF and to the separation berween ESF and ISF led to a concern that the Graphical representation of the loading fackoes tor the exemal symptom symptoms elicited by this one condition overwhelmed the analysis sersation me2su5ucd箱hisu,but with data1om童edse Far this reason,statigical analysis was reperfoemed with the dry- cnnditinn mmnved tm anlysis
separation between the two symptom factors with a high ESF and low ISF. Therefore, the ESF component sensations (burning, irritation, dryness, and tearing) were related to dry eyes. The dry-eye condition was also the only condition with a significantly high ESF. ISF (strain, headache, ache, double vision, and blur) was significantly higher than ESF for the accommodative stress, convergence, and mixed astigmatism conditions. The large contribution of the dry-eye condition to ESF and to the separation between ESF and ISF led to a concern that the symptoms elicited by this one condition overwhelmed the analysis. For this reason, statistical analysis was reperformed with the dryeye data removed. Analysis of variance with repeated measures determined that burning (p 0.0005), ache (p 0.048), strain (p 0.0111), and headache (p 0.03) were significantly related to inducing condition. Principal factor analysis without the dry-eye condition revealed the same two latent factors for which the loading factors of the component symptom sensations are graphed in Fig. 3 and for which the mean values for each condition are shown in Fig. 4. Without the dry-eye condition, the overall factor model accounted for 63% of the variance with 34% being accounted for by ESF and 29% accounted for by ISF. Again, factors with eigenvalues 1 were not considered. Figs. 3 and 4 are the same as Figs. 1 and 2, respectively, except that data from the dry-eye condition have been removed from the analysis. The ESF and ISF clusters of symptoms in Fig. 3 are nearly the same as in Fig. 1. Burning, irritation, and dryness were still associated with ESF, and strain, headache, and ache were still associated with ISF. The symptoms of tearing, blur, and double vision were not as well associated with the latent factors with removal of the dry-eye data. Likewise, the relationships between the two latent factors and the inducing conditions were similar in Figs. 2 and 4. Lens flippers, close viewing distance, and mixed astigmatism continued to have significantly greater ISF than ESF weightings despite absence of data from the dry-eye condition. However, after the strong effects of the dry-eye data were removed, the data in Fig. 4 show that other conditions are related to ESF. Glare had significantly greater ESF compared with ISF, and flickering, small font, and up gaze also showed higher ESF than ISF. These effects were seemingly overwhelmed by the very strong ESF effects of the dry-eye condition shown in Fig. 2. The remarkable similarities between the factor analysis results with (Figs. 1 and 2) and without (Figs. 3 and 4) the influential dry-eye data strengthens the validity of the two latent factors (ESF and ISF). Table 4 contains the same raw data means of FIGURE 1. Graphical representation of the loading factors for the external symptom factor and the internal symptom factor for each of the nine symptom sensation measures used in this study. FIGURE 2. Mean SEM of the external symptom factor (ESF) and internal symptom factor (ISF) for each inducing condition. FIGURE 3. Graphical representation of the loading factors for the external symptom factor and the internal symptom factor for each of the nine symptom sensation measures used in this study, but with data from the dry-eye condition removed from analysis. Is all Asthenopia the Same?—Sheedy et al. 735 Optometry and Vision Science, Vol. 80, No. 11, November 2003
736 Is all Astheropia the SameShocdy ct al 0.o with ISF and a nogative rcltionship with ESF."Between"the cyes has a negative rclationship with ESF. 0.40 The peroentage cstimate of squinting time was significantly- lated to condition (p0.0001),values are displayed in Tablk 5. 0.20 The glare,refractive error,and comvergence stress conditions had the grearest amount of squinsing Squinting provides a visual ben 0.00 cfit by roducing blur for uncorrectod refractive errur and reducing retinal illumination for ghre,and.bence,the high kvel of squint during these conditions is reascoable.The visual benefit of squinting during the clse viewing coodition is less clear,though 口E3F -0.40 it s posible that blur occurred due to inppropriae accommoda ■好F tive response to the doe vicwing condition. 0.0 The mean amount of time required to reach bardy tolerabk and the mean percentige o bardly tolerable atained are presenced in Table 6.The latter mean was determined by using 100%when the subject artained barely tolerable within 15 min and using,their peroentage otimate on the anabg scak when they didn't reach that Symplom Inducing Conditfon kevel.The dry-eye condition was much faster to threshold than the other cooditions The values in Table 6 calibrate the symptom FIGLRF 4. inducing conditioes used in this study as reported in Table I and Mean SEM of the extemal sympiom factor I[SIl and intemal symptnm can help to equalize the difficulry of discomfon-inducing condi fackr (5F)for tskch intucing cunftiun,buld山ue山y-t tions for furure study. condition removed from anahsis DISCUSSION symptom ratings as shawn in Table2,hww年，in Tahle4.he inducing conditions and symptoms are srgrgtd int ESF and The symptoen ntions were significantly rted to the induc ISF caeorics.Inspection of data in Table 4 shows the catrica ing conditions.The rdationships wer deermincd to be strongost (ISF and ESF)differenoes in the magnitudes of spucific sympeoms by cakcubting two factor groups hercin named cxternal sympcom based on the category(ISF or ESF)of the inducing condition. factor(ESF)and intemal symptom facror (ISF).ESF continues as ESF (p-0.0013)and ISF (p -0.0033)were also significantly a separate identifiable factor even when data from the dry-eye reated to perecived location with all of the data and still have gnod inducing condition are remaved from the analysis This strongly rcationships with the dry-cye data removed (ESF,p-0.0016s suppores ESF and ISF as fundamentally identifiabk and diffenent ISF,p -0.0549).Fig.5 A and B shoms the mcan factor scores with factors.The reltionships of these two facturs to the sympcom and without the dry-eye data.respectively."Bottom"of the eyes sensations,inducing conditions,and perceived locations are sum bears a strong positive relationship with ESF and a negaive rela marized in Table 7. tiunship with ISF in both Fis.5 A and B-"frunt"of the cyes has The data amlysis demonstrates at least two constellations of the same relationships but only with indlusion of the dryeye data sbjecthe symptoms tht ar differentiated by ni type,n Bthind"the eyes shows the oppoaite-i.ea positive relationship sation location,and induing condition.The fact that the subject TABLE 4. Mean symptom ratings for symptomns and inducing conditions calegorized as ESF or ISF. Symptoms Inducing Conditions S ESF Neither Ache Scrain Headache Burning Imitation Dryness Tearing Blur Double Vision ISF Lens flipper 36 52 1 19 18 10 16 2D Close viewing 44 71 36 27 24 13 20 30 Mixed astigmatim 49 64 6 22 1 17 38 FS Dry e 20 24 ) 51 53 59 14 Glare 35 45 2 3 32 32 13 20 11 Small font 1 53 34 2 23 16 3 Upward gaze 50 55 35 39 28 32 23 17 16 Flickering light 34 43 35 26 30 30 17 16 13 E5F,extemnal symplom Lactor ISF,intemal symplom factor
symptom ratings as shown in Table 2, however, in Table 4, the inducing conditions and symptoms are segregated into ESF and ISF categories. Inspection of data in Table 4 shows the categorical (ISF and ESF) differences in the magnitudes of specific symptoms based on the category (ISF or ESF) of the inducing condition. ESF (p 0.0013) and ISF (p 0.0033) were also significantly related to perceived location with all of the data and still have good relationships with the dry-eye data removed (ESF, p 0.0016; ISF, p 0.0549). Fig. 5 A and B shows the mean factor scores with and without the dry-eye data, respectively. “Bottom” of the eyes bears a strong positive relationship with ESF and a negative relationship with ISF in both Fig. 5 A and B—“front” of the eyes has the same relationships but only with inclusion of the dry-eye data. “Behind” the eyes shows the opposite—i.e., a positive relationship with ISF and a negative relationship with ESF. “Between” the eyes has a negative relationship with ESF. The percentage estimate of squinting time was significantly related to condition (p 0.0001), values are displayed in Table 5. The glare, refractive error, and convergence stress conditions had the greatest amount of squinting. Squinting provides a visual benefit by reducing blur for uncorrected refractive error and reducing retinal illumination for glare, and, hence, the high level of squint during these conditions is reasonable.27 The visual benefit of squinting during the close viewing condition is less clear, although it is possible that blur occurred due to inappropriate accommodative response to the close viewing condition. The mean amount of time required to reach barely tolerable and the mean percentage of barely tolerable attained are presented in Table 6. The latter mean was determined by using 100% when the subject attained barely tolerable within 15 min and using their percentage estimate on the analog scale when they didn’t reach that level. The dry-eye condition was much faster to threshold than the other conditions. The values in Table 6 calibrate the symptominducing conditions used in this study as reported in Table 1 and can help to equalize the difficulty of discomfort-inducing conditions for future study. DISCUSSION The symptom sensations were significantly related to the inducing conditions. The relationships were determined to be strongest by calculating two factor groups herein named external symptom factor (ESF) and internal symptom factor (ISF). ESF continues as a separate identifiable factor even when data from the dry-eyeinducing condition are removed from the analysis. This strongly supports ESF and ISF as fundamentally identifiable and different factors. The relationships of these two factors to the symptom sensations, inducing conditions, and perceived locations are summarized in Table 7. The data analysis demonstrates at least two constellations of subjective symptoms that are differentiated by sensation type, sensation location, and inducing condition. The fact that the subject FIGURE 4. Mean SEM of the external symptom factor (ESF) and internal symptom factor (ISF) for each inducing condition, but with data from the dry-eye condition removed from analysis. TABLE 4. Mean symptom ratings for symptoms and inducing conditions categorized as ESF or ISF.a Inducing Conditions Symptoms ISF ESF Neither Ache Strain Headache Burning Irritation Dryness Tearing Blur Double Vision ISF Lens flipper 36 52 32 16 19 18 10 16 20 Close viewing 44 71 36 27 27 24 13 20 30 Mixed astigmatism 49 64 36 22 31 23 15 17 38 ESF Dry eye 20 24 11 63 51 53 59 14 7 Glare 35 45 32 35 32 32 13 20 11 Small font 31 53 29 34 29 23 16 26 23 Upward gaze 50 55 35 39 28 32 23 17 16 Flickering light 34 43 35 26 30 30 17 16 13 a ESF, external symptom factor; ISF, internal symptom factor. 736 Is all Asthenopia the Same?—Sheedy et al. Optometry and Vision Science, Vol. 80, No. 11, November 2003
Is all Asthencpia the Same-sheody ct al.737 TABLE 5. Mean estimate,by condition,of the percentage time sub- 1000 jects squinted during testing. Squint Cundilion 0500 Behird Glan 56.5 Mied astigmatism 53.1 。Batasen 0000 -Center Clse viewing distance 51.8 lens llipper 22.8 Small font 18.9 Upward gaze 5.5 000 Butlom FTixckering gh 5.0 TABLE 6. -1c0 The mean amount of time for symptoms to become "barely -1.000 -0.500 0.000 0800 1.000 tolerable"and the mean magnitude attained relative to External Symptom Factor [ESF] barely tolerable. B. Conditicn Mean Time Barely Tolerable X 1.030 Mixocd astigmatism 336.6 100 Dry cyes 2.5 99.6 53三 Ckse viewing dislance 409.6 90.8 0530 Bchind Upward paze 571.0 83.6 Oside Small font 04.6 79.6 Lene flipper 399.8 77.6 -Center Flickering light 605.3 76.0 0.020 Glare 672.D 733 Around TABLE 7. 0500 Botlon Summary of the ISF and ESF symptom/condition/location relationships. -1020 Factor Symptoms Inducing Conditions Location -1.D03 0.500 0000 0500 1.C00 S Huming Lry eye Hottom of ewes Exteral Symmptom Facter (ESF) Iritation Clare Front of eyes Dryness Up gaze FIGURE 5. Small font Mun scnnm of the twn facton (trmal vymptom facter and imemal flicker S列mp2m白ctor or each location.Me因n scores were calculated for IA) Strain Lens flipper Bchind the cyes amhses o all drta and [B)anahysi withmt data fmm dry-ne cnncitinn Ache Clse viewing dislance Hesdache Mixed asigmalism popularion could differentiate asthenopu demoetrates at least ISF,intemal sympom factor;FSF,extemal symptom factor. rwo different afferent pathways that are involvod in asthenopia The composition of the two constellations is rational basod on Manually holding open the cye lids will dcarly cnate a dry-eye current understanding of anatomy and physiology,ie..ESF sms cundition and ESF symptoms.The upge cnditionaoms clearly related to dry eye and TSF is mast likely related to other reasanbly related to dry eyes and sympoms ofESP because supe- visual fmnetions such as accommodation and comergence. rior gixe res in a greater esposed ocur srce.Blinking is The sensations of ESP (huming,irritation,and drynos)are retarded in up game because of fusion of the fibrous sheaths of the similar to the mot oommun symptums cnsdend rdated tu dry supcrior rctus and levator palpcbrac supcrioris musdes3 The eye(gritty,buming.red,and dry)in a population-based study of other ESF-inducing conditions (glare,small font.and Hicker)secm elderly subjects and is aso coosisent with dinical reports of not as directly related to dry eyes.However,each of these condi- sympioms acd with drysAlo,one of the of tions commonly cmuter workstation,and work dry eye is staining in the lower cornca and comjunciva.which is computr has hoen hown to gnifcantlyduce blink rate esentially the same location as the perccived lucatiun of ESF (front These conditions could in some manner roduce blink rate and and bottom of the eye). result in dry-eye condionsIt isa posibk that they operate by 小w时ad Viiow Sciruer,Vd.30Nn1I,eeber20G
population could differentiate asthenopia demonstrates at least two different afferent pathways that are involved in asthenopia. The composition of the two constellations is rational based on current understanding of anatomy and physiology, i.e., ESF seems clearly related to dry eye and ISF is most likely related to other visual functions such as accommodation and convergence. The sensations of ESF (burning, irritation, and dryness) are similar to the most common symptoms considered related to dry eye (gritty, burning, red, and dry) in a population-based study of elderly subjects28 and is also consistent with clinical reports of symptoms associated with dry eyes.29, 30 Also, one of the signs of dry eye is staining in the lower cornea and conjunctiva,30 which is essentially the same location as the perceived location of ESF (front and bottom of the eye). Manually holding open the eye lids will clearly create a dry-eye condition and ESF symptoms. The up-gaze condition also seems reasonably related to dry eyes and symptoms of ESF because superior gaze results in a greater exposed ocular surface.31 Blinking is retarded in up gaze because of fusion of the fibrous sheaths of the superior rectus and levator palpebrae superioris muscles.32 The other ESF-inducing conditions (glare, small font, and flicker) seem not as directly related to dry eyes. However, each of these conditions commonly exist at computer workstations, and work at a computer has been shown to significantly reduce blink rate.33, 34 These conditions could in some manner reduce blink rate and result in dry-eye conditions. It is also possible that they operate by FIGURE 5. Mean scores of the two factors (external symptom factor and internal symptom factor) for each location. Mean scores were calculated for (A) analyses of all data and (B) analysis without data from dry-eye condition. TABLE 5. Mean estimate, by condition, of the percentage time subjects squinted during testing. Condition Squint (%) Glare 56.5 Mixed astigmatism 53.1 Close viewing distance 51.8 Lens flipper 22.8 Small font 18.9 Upward gaze 5.5 Flickering light 5.0 TABLE 6. The mean amount of time for symptoms to become “barely tolerable” and the mean magnitude attained relative to barely tolerable. Condition Mean Time (sec) Barely Tolerable (%) Mixed astigmatism 336.6 100 Dry eyes 26.5 99.6 Close viewing distance 409.6 90.8 Upward gaze 571.0 83.6 Small font 604.6 79.6 Lens flipper 599.8 77.6 Flickering light 605.3 76.0 Glare 672.0 73.3 TABLE 7. Summary of the ISF and ESF symptom/condition/location relationships.a Factor Symptoms Inducing Conditions Location ESF Burning Dry eye Bottom of eyes Irritation Glare Front of eyes Dryness Up gaze Small font Flicker ISF Strain Lens flipper Behind the eyes Ache Close viewing distance Headache Mixed astigmatism a ISF, internal symptom factor; ESF, external symptom factor. Is all Asthenopia the Same?—Sheedy et al. 737 Optometry and Vision Science, Vol. 80, No. 11, November 2003
738 k all Asthenopia the Samef-Shocdy ct al. different mechnism (e.g..glare could stress the retim,irs.or 4.Nilun E,SalbelloC.Survey of LS upiume:riesaearding prevaence visual pathway)that were no distinguble by the testi in ad emnvssmps In:Slvendy G.Smith MJ. this study. Knubeck RH.d.Deign of Compuring Sye:Precerdingsthe The ISF sympcoms of strain,ache,and headache seem caegor 7th Incemacioesl Conference on Human-Compurer Interaction ically distinct from those associated with dry eyeand more re- (HCI Iauematiund 97).Sn Fraiwo.CA Augt 24-29.1997. New York:Ebever.1997:26-10. btod to thaa:commanly describod aocin with accummoda- tive or binoculr vision disorders.Hedaches,eyestrain,and eye S.Giuth SK.Prentice memoeial lecture:the scirnceofarring-ascarch fur criteris Am Opcom Physiol Opr 1981:5:8/0-85 fatigue are reported as sympoms commonly relaed to conver- fi.American Narional Srandard Inrirare.American Narinnal Standand cendmino disurdsof Practice for Office Lehting,ANSI/IESNA RP.1-1993.New York Bhr,headache,and asthcnogia wen th:most common symptoms Amcrican Natiored Standards lutitue,1993. nated in a retrospeetive snady of pitient recnrds containing a dig- 7.Sheody IE.Saladin l.Phocia.vergenoe.ad fcio disparity in nosis of accommodative inauficiency In a paper on vergence acem国probem.Am Optum Piyainl○pt197:54s474-8. Tyrell and Leibuwire stat:"A common complaint associared 8.Sheo动JE5lh水Asociaronof乎ps with of with prolangrd n work i of a vage discomfort,which may oculommor deficiencin.Am J Opmm Phydnl Ope 1978556570-6. seem to be localied in cither the head or the eyes."ISF symptoms 9.Grisham D.Visual therapy results fo comergence insufficiency: are similar to those previously repored for accommodative and literalure peview.Am Opiu田Plrpaiul Opt19图：65a448-54 binoculr vision pruhlems. 10.Henneey D.looue RA.Rouse MW.Rebrion of symptom co ac Twn of the [SF-inducing ooditians,the lens flipper and the cummodative infciEy of sthoal-apodichldrem m ]Optum Phyaiol 0pr1:61t77-83. clase viewing conditons,primarily sresed the accommodtie 11.Levine 5.Ciuffreda KJ.Seimnow A,Flax N.Clinical auromere of and vergence mechanisms.It is possible that the miedimatism accommodanve ficiliry in sympoomaric and asymptomatic individu. conditica Loewise stressed accomnodation and convergence if the 山.Jm0 pium Ao1985:56286-90. large astigmatic range (4.00 D)resuleed in accommodative uncer 12.Juchins-Kru W.Sclweflinghs W.Rdacns betwern dark ac. tainty and fluctution.The location of ISF (hehind the eyes)is cummodation and pyth ympm.Opchlmic Phyidl consistent with the descripcion by Tyrell and Leibowitz"and is 0pr12:115-5. also clse to the locations of the ocmoor and ciliary musdes 13.Wiggins NP,Daum KM.Val dicmnmfoet and aeigmaric refractiv that are responsible for binocular alignment and accommodation errors in VDT use.J Am Opoom Assoc 19912:680-4 respectieh. 1d.Wagin NP,Daum KM.Sayder CA.上lla山ofroichal igmonom in contact lero wear on vbual dsoomfur ia VDT uie.J Am Opoom Aa3c1992:6k177-81. CONCLUSIONS 15.Bachman G.Compurer-pecific pecrade lers design preference of presbynpic npermrs.J Oeeup Med 1992-34:1023-7. Sympcom descriproes and locatices were able to distinguish dis- 16.Booon SP.Sheody JE.Nilsen E The efficacy of computer glisses in comfort on the hasis of cwasative condition,indicating at least rwo raluciin ul uepu wuekerxyupom Opiomnriry 002:71:271-30 different afferen:pachways for the symptoms of asthenopia Re 17.Sheody IE.Reading performaace and visusl comhigh eio. ssupport two diferent sympcom constellations herein labeled lutiun mo umaredaViA moaimr.Flectroni:Iming estemal sympeum factor (ESF)and internal symprom factor (SF) 1y2h145-1 The ESF patern comprises burning irritaion.tearing,and dry 18.Shendy JF,MeCarhy M.Rrading perfoeromer and viual enmfoet ness located in the front and boctom of the eye.ESF is caused by with s o gray comped with black and white scaned print. holding the eyelid open,glare,upga,small font,and flickering Diphy319941527-0 FSF ams highly ratod to dryy ympeom.The ISF pm 19.Sheody IE.Kang [M.O WT.Verical eye gue posicon:effout on comprises ache.strain,and headche locatd hehind the ryes.ISP tauk perfoemance and vival rumloet.Ia:Sheedy JE,Sharw-McMinn is caused by the close viewing digae lens flipper.and mixed P.eds Diapnosing ad Treating Compurer-Relared Vision Proh- astigmatism conditions and is cly rdatod to actummodtive Iems Bostor Buterworth-Hringmanss 2000:190-1. 20.Wlkins AJ.Nimmo-Smith I.Slarer Al.Bedocs L.Fuorescene lighe and/or hingular visinn streo. iwha山hh3ad5siL1 hing Ro Te山198211l1-8. 21.Toda 1,Fujbhima H.Tsubou K.Ocubr fatigue is the mojor symp ACKNOWLEDGMENTS tom of dry eye.Aoa Ophthalmul (Copenh)1993:71-347-52. 22.Sheedy JE.Parsont SD.The Video Dsplay Termical Eye Chnic Sappored.in part y Nanival Eyr frurimar Neoal furiar of Heab dinical mpeet.Omom Vis Sci 96:6-6. w235-707)5i. 23.Aars A Hocgen G.Borset HH.Ro O.Thooesen M.Muscdloskekeul Reerited OrTolr 19,22 rvirian reened Ivae 25.NXB viual poycbcial in VDU eperoo bdur: dL年lif写ergveomic atervetioas.4plBe19952335-51. 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different mechanism (e.g., glare could stress the retina, iris, or visual pathway35) that were not distinguishable by the testing in this study. The ISF symptoms of strain, ache, and headache seem categorically distinct from those associated with dry eyes—and more related to those commonly described as occurring with accommodative or binocular vision disorders. Headaches, eyestrain, and eye fatigue are reported as symptoms commonly related to convergence insufficiency36 and to minor disorders of ocular vergence.37 Blur, headache, and asthenopia were the most common symptoms noted in a retrospective study of patient records containing a diagnosis of accommodative insufficiency.38 In a paper on vergence, Tyrell and Leibowitz39 state: “A common complaint associated with prolonged near work is of a vague discomfort, which may seem to be localized in either the head or the eyes.” ISF symptoms are similar to those previously reported for accommodative and binocular vision problems. Two of the ISF-inducing conditions, the lens flipper and the close viewing conditions, primarily stressed the accommodative and vergence mechanisms. It is possible that the mixed astigmatism condition likewise stressed accommodation and convergence if the large astigmatic range (4.00 D) resulted in accommodative uncertainty and fluctuation. The location of ISF (behind the eyes) is consistent with the description by Tyrell and Leibowitz39 and is also close to the locations of the oculomotor and ciliary muscles that are responsible for binocular alignment and accommodation, respectively. CONCLUSIONS Symptom descriptors and locations were able to distinguish discomfort on the basis of causative condition, indicating at least two different afferent pathways for the symptoms of asthenopia. Results support two different symptom constellations herein labeled external symptom factor (ESF) and internal symptom factor (ISF). 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