this retrospective study included 343 glaucoma subjects who were examined at the glaucoma clinic of the asan medical center . patients who met our inclusion criteria of at least three years of follow - up and seven or more reliable vf measurements were consecutively enrolled by medical record review . eyes that underwent intraocular surgery ( including cataract and glaucoma filtering procedures ) during the follow - up period were excluded . at initial diagnosis , each patient received a comprehensive ophthalmologic examination that included a review of medical history for the presence of systemic diseases like systemic hypertension , diabetes , cerebrovascular accident , dyslipidemia , and migraine , measurement of best - corrected visual acuity to confirm that visual acuity was adequate for automated perimetry performance , slit - lamp biomicroscopy , goldmann applanation tonometry , gonioscopy , dilated fundoscopic examination using a 90- or 78-diopter lens , stereoscopic optic disc and retinal nerve fiber layer ( rnfl ) photography , vf examination by standard automated perimetry ( humphrey swedish interactive threshold algorithm standard strategy 24 - 2 ; carl zeiss meditec , dublin , ca , usa ) , and measurement of central corneal thickness ( cct ; dgh-550 instrument , dgh technology inc . , all included patients met the following criteria at the baseline exam : best - corrected visual acuity of 20 / 30 or better , with a spherical equivalent ( se ) within 5 diopters and a cylinder correction within + 3 diopters ; vf mean deviation ( md ) higher than -20 db ; and the presence of a normal anterior chamber and open - angle on slit - lamp and gonioscopic examinations . diagnosis of glaucoma was based on both the presence of typical glaucomatous changes in the optic disc and glaucomatous vf defect . thus , eyes with a glaucomatous optic disc and a normal vf were not defined as having glaucoma . glaucomatous optic disc change included increased cupping ( vertical cup - disc ratio more than 0.7 ) , a difference in the vertical cup - disc ratio ( more than 0.2 between the eyes ) , diffuse or focal neural rim thinning , disc hemorrhage , or rnfl defects . eyes defined as having glaucomatous vf defects met two of the following three criteria ( as confirmed by more than two reliable consecutive tests ) : 1 ) a cluster of three points with a probability of less than 5% on a pattern deviation map in at least one hemifield , including at least one point with a probability of less than 1% or a cluster of two points with a probability of less than 1% ; 2 ) a glaucoma hemifield test result outside the normal limit ; and 3 ) a pattern standard deviation result of less than 5% . reliable vf assessment was defined as a vf test with a false - positive error of less than 15% , a false - negative error of less than 15% , and a fixation loss of less than 20% . if at least one eye showed glaucomatous changes , that patient was included for analysis . iop - lowering medication was prescribed if the eyes showed glaucomatous optic disc changes or had both a glaucomatous optic disc and vf changes . all procedures conformed to the declaration of helsinki , and the study was approved by the institutional review board of the asan medical center . since glaucomatous damage involves both structural and functional changes that may not appear at the same time , glaucoma progression was determined by either structural or functional measures . structural progression was assessed using a stereoscopic optic disc and red - free rnfl photographs . two glaucoma experts ( krs and jhn ) independently assessed all of the photographs in order to estimate the glaucoma progression between the patients ' first and last visits . both graders were blind to each other 's progression assessments and to all clinical and vf information . photographs were presented in chronological order and with masking of patient identification , age , and test date . each grader viewed all photographs of each eye before making an assessment and was asked to determine the possible presence of a glaucomatous optic disc or of rnfl progression , as revealed by an increase in the extent of neuroretinal rim thinning , enhancement of disc excavation , any widening or deepening of a rnfl defect , and/or the appearance of new disc hemorrhage . if the opinions of the two observers differed , a third examiner ( dj ) made the final decision . commercial software ( guided progression analysis , carl zeiss meditec ) was used to determine progressive functional loss of the vf . vf defect progression was defined as a significant deterioration from the baseline pattern of deviation at three or more of the same test points and was evaluated on three consecutive examinations . if only one eye showed progression by either structural or functional assessment during the follow - up period , the patient was included in the upg group . in contrast , if both eyes showed progression , the patient was included in the bpg group . age , gender , and the prevalence of systemic diseases ( systemic hypertension , diabetes , cerebrovascular accident , migraine , and dyslipidema ) were compared between the upg and bpg groups . in the upg group , pre - treatment iop , mean post - treatment iop , cct , se , baseline vf md , vf pattern standard deviation , and progression rates were compared between the pe and the npe within the same patient . the progression rates ( md slope ) were determined by linear regression analysis of md values in serial vf analyses . in the bpg group , the same parameters were compared between the fpe and the spe within the same patient . normally distributed data were compared between the upg and bpg groups using unpaired t - tests . non - normally distributed data were compared using either the mann - whitney or wilcoxon signed rank test . this retrospective study included 343 glaucoma subjects who were examined at the glaucoma clinic of the asan medical center . patients who met our inclusion criteria of at least three years of follow - up and seven or more reliable vf measurements were consecutively enrolled by medical record review . eyes that underwent intraocular surgery ( including cataract and glaucoma filtering procedures ) during the follow - up period were excluded . at initial diagnosis , each patient received a comprehensive ophthalmologic examination that included a review of medical history for the presence of systemic diseases like systemic hypertension , diabetes , cerebrovascular accident , dyslipidemia , and migraine , measurement of best - corrected visual acuity to confirm that visual acuity was adequate for automated perimetry performance , slit - lamp biomicroscopy , goldmann applanation tonometry , gonioscopy , dilated fundoscopic examination using a 90- or 78-diopter lens , stereoscopic optic disc and retinal nerve fiber layer ( rnfl ) photography , vf examination by standard automated perimetry ( humphrey swedish interactive threshold algorithm standard strategy 24 - 2 ; carl zeiss meditec , dublin , ca , usa ) , and measurement of central corneal thickness ( cct ; dgh-550 instrument , dgh technology inc . , all included patients met the following criteria at the baseline exam : best - corrected visual acuity of 20 / 30 or better , with a spherical equivalent ( se ) within 5 diopters and a cylinder correction within + 3 diopters ; vf mean deviation ( md ) higher than -20 db ; and the presence of a normal anterior chamber and open - angle on slit - lamp and gonioscopic examinations . diagnosis of glaucoma was based on both the presence of typical glaucomatous changes in the optic disc and glaucomatous vf defect . thus , eyes with a glaucomatous optic disc and a normal vf were not defined as having glaucoma . glaucomatous optic disc change included increased cupping ( vertical cup - disc ratio more than 0.7 ) , a difference in the vertical cup - disc ratio ( more than 0.2 between the eyes ) , diffuse or focal neural rim thinning , disc hemorrhage , or rnfl defects . eyes defined as having glaucomatous vf defects met two of the following three criteria ( as confirmed by more than two reliable consecutive tests ) : 1 ) a cluster of three points with a probability of less than 5% on a pattern deviation map in at least one hemifield , including at least one point with a probability of less than 1% or a cluster of two points with a probability of less than 1% ; 2 ) a glaucoma hemifield test result outside the normal limit ; and 3 ) a pattern standard deviation result of less than 5% . reliable vf assessment was defined as a vf test with a false - positive error of less than 15% , a false - negative error of less than 15% , and a fixation loss of less than 20% . if at least one eye showed glaucomatous changes , that patient was included for analysis . iop - lowering medication was prescribed if the eyes showed glaucomatous optic disc changes or had both a glaucomatous optic disc and vf changes . all procedures conformed to the declaration of helsinki , and the study was approved by the institutional review board of the asan medical center . since glaucomatous damage involves both structural and functional changes that may not appear at the same time , glaucoma progression was determined by either structural or functional measures . structural progression was assessed using a stereoscopic optic disc and red - free rnfl photographs . two glaucoma experts ( krs and jhn ) independently assessed all of the photographs in order to estimate the glaucoma progression between the patients ' first and last visits . both graders were blind to each other 's progression assessments and to all clinical and vf information . photographs were presented in chronological order and with masking of patient identification , age , and test date . each grader viewed all photographs of each eye before making an assessment and was asked to determine the possible presence of a glaucomatous optic disc or of rnfl progression , as revealed by an increase in the extent of neuroretinal rim thinning , enhancement of disc excavation , any widening or deepening of a rnfl defect , and/or the appearance of new disc hemorrhage . if the opinions of the two observers differed , a third examiner ( dj ) made the final decision . commercial software ( guided progression analysis , carl zeiss meditec ) was used to determine progressive functional loss of the vf . vf defect progression was defined as a significant deterioration from the baseline pattern of deviation at three or more of the same test points and was evaluated on three consecutive examinations . if only one eye showed progression by either structural or functional assessment during the follow - up period , the patient was included in the upg group . in contrast , if both eyes showed progression , the patient was included in the bpg group . age , gender , and the prevalence of systemic diseases ( systemic hypertension , diabetes , cerebrovascular accident , migraine , and dyslipidema ) were compared between the upg and bpg groups . in the upg group , pre - treatment iop , mean post - treatment iop , cct , se , baseline vf md , vf pattern standard deviation , and progression rates were compared between the pe and the npe within the same patient . the progression rates ( md slope ) were determined by linear regression analysis of md values in serial vf analyses . in the bpg group , the same parameters were compared between the fpe and the spe within the same patient . normally distributed data were compared between the upg and bpg groups using unpaired t - tests . non - normally distributed data were compared using either the mann - whitney or wilcoxon signed rank test . a total of 343 korean patients were included in the final analysis , of which 182 were men and 161 were women . the mean follow - up period was 4.2 years . at baseline , 134 patients had unilateral glaucoma . among these 134 patients , 102 showed glaucomatous optic disc changes but a normal vf in the fellow eye , while the remaining 32 patients had a normal appearing optic disc and a normal vf in the fellow eye . among the 134 unilateral glaucoma patients , 25 patients showed unilateral progression during follow - up while 6 patients showed bilateral progression ; thus , 31 patients had either unilateral or bilateral progression while 103 patients were stable . among the 32 patients with unilateral glaucoma and a normal optic disc and vf in the fellow eye , none of the fellow eyes showed progression . at baseline , 209 patients had bilateral glaucoma , of which 43 showed progression with 25 showing bilateral progression . 1 . hence , the bpg group consisted of 31 patients ( 6 from the unilateral glaucoma at baseline group and 25 from the bilateral glaucoma at baseline group ) , while the upg group consisted of 43 patients ( 25 with unilateral glaucoma at baseline and 18 with bilateral glaucoma at baseline ) . the mean age was significantly higher in the upg group than in the bpg group ( 58.7  10.0 vs. 52.6  3.9 years , p = 0.035 ) . however , the prevalence of all analyzed systemic diseases was not different between the two groups ( table 1 ) . all pes in the upg group showed more severe clinical characteristics in terms of baseline vf parameters . however , baseline iop , mean follow - up iop , cct , and se were not significantly different between the pe and the npe ( table 2 ) . the progression rate determined by the md slope was compared between the fpe and the spe within the same patient in the bpg group . as expected , the progression rate was higher in the fpe , but other parameters did not differ significantly ( table 3 ) . when the pe in the upg group was compared to the fpe eye in the bpg group , the progression rate was significantly higher in the fpe in the bpg group , but no other parameters differed ( table 4 ) . according to the data from 343 patients analyzed in this study , 134 ( 39.1% ) presented with unilateral glaucoma at baseline . among these unilateral glaucoma patients , 76.1% showed glaucomatous optic disc changes in the fellow eye , while only 32 patients out of all glaucomatous patients ( 9.3% ) had completely unilateral glaucoma with both a normal optic disc and a normal vf in the fellow eye . among the 134 unilateral glaucoma patients , 23.1% showed progression during the follow - up period . the fellow eye of the 32 patients with unilateral glaucoma did not show progression during follow - up , although iop - lowering treatment was not performed in those eyes . among the 209 bilateral glaucoma patients , 20.6% showed progression . thus , the overall prevalence of progression was similar between patients with unilateral glaucoma and bilateral glaucoma . interestingly , 80.6% of patients with progressing disease in the unilateral glaucoma group demonstrated unilateral progression , while 58.1% of patients with progressing disease in the bilateral glaucoma group showed bilateral progression . chen and park   showed that 6.2% of fellow eyes progressed over a period of 8.7 years . among 134 unilateral perimetric glaucoma patients , 6 ( 4.5% ) showed progression in the fellow eye in the present study . the results from the current study indicated no significant differences in the prevalence of any analyzed systemic disease between the upg and bpg groups . overall , the prevalence of systemic disease was very low in both groups , which may affect the statistical analysis and contribute to the lack of significance of the data . in contrast to the present data , a study by kim and kim   showed that the prevalence of systemic disease was different between patients with unilateral and bilateral glaucoma . the discrepancy is likely due to the fact that systemic disease can affect the development of bilateral glaucoma , but the progression of disease is modulated by various factors thus reducing the effect of systemic disease on glaucoma progression . for instance , myopia is known to be a risk factor for the development of glaucoma but not for progression of the disease , although this remains under debate . when we compared the pe and the npe in the upg group , the pe showed a more advanced disease status at baseline . however , there was no difference in iop , cct , and se between the fpe and spe in the bpg group . in the comparison between the pe and npe in the upg group , iop similarly , iop did not differ between the fpe and the spe in the bpg group . as shown by mean baseline iop measurements , most of the patients in the current study had low iop and can be categorized as having normal - tension or low - tension glaucoma . whether a correlation exists between iop asymmetry and glaucoma severity in cases of normal - tension glaucoma has been under debate in previous studies . some studies have shown that higher iop is related to more advanced stages of glaucoma . however , the low - pressure glaucoma treatment study reported that iop asymmetry is unrelated to vf asymmetry . the present study revealed no differences in iop between the pe and the npe in the upg group or between the fpe and the spe in the bpg group . while iop reduction substantially decreased the progression rate in the collaborative normal - tension glaucoma study , some treated eyes also showed progression in that randomized clinical trial . thus , taken together , our results and those of previous studies suggest that factors other than iop , which may affect the progression of glaucoma with lower iop readings , should be investigated in future studies . furthermore , no differences were noted in baseline vf defect severity and iop values between the pe in the upg group and the fpe in the bpg group ; however , the progression rate was significantly higher in the fpe in the bpg group . the progression rate of the pe in the upg group was -0.70 db / year , which is approximately two times higher than the usual progression rates reported in previous randomized clinical trials ( -0.36 to -0.41 db / yr ) . the progression rate of the fpe in the bpg group was approximately five times higher ( -3.43 vs. -0.70 db / yr ) than that of the pe in the upg group . thus , we speculate that in simultaneously bilaterally progressing patients , both eyes progress faster than the pe in upg . in contrast , baseline vf defect severity was not different between the pe in the upg group and the fpe in the bpg group . first , the prevalence of systemic disease was determined by self - reported histories from the patients . second , the follow - up period ( mean 4.2 years ) was relatively short . considering the slow - progressing nature of glaucoma , this period may not be sufficient to observe the natural progression of the disease . some cases of unilateral glaucoma may eventually progress to bilateral glaucoma and some cases with a unilaterally progressing eye may show progression in the fellow eye over time . however , during the four years of observation in this study , unilateral glaucoma tended to progress only in the glaucomatous eye , while simultaneously bilaterally progressing patients showed much faster progression rates than those with a unilaterally progressing eye . no systemic factors analyzed in this study significantly differed in prevalence between the upg and bpg groups . therefore , glaucoma patients with simultaneous bilateral progression should be treated with caution in clinical practice because these cases can show relatively rapid progression .