the pars plana vitrectomy ( ppv ) is a common procedure for the treatment of vitreoretinal diseases including vitreous hemorrhage , retinal detachment , epiretinal membrane , and macular hole . with the recent introduction of a sutureless vitrectomy technique , vitreoretinal surgeons are now performing vitrectomies more routinely and with fewer serious complications [ 1 , 2 ] . it has been well established that the intraocular pressure ( iop ) increases immediately after a vitrectomy [ 36 ] . the mechanism of this increased iop is unclear , although it responds well to conventional ocular hypotensive medications [ 3 , 4 ] . although several studies have reported that a vitrectomy may induce glaucoma long after surgery , the association remains unclear [ 3 , 7 , 8 ] . according to yu et al . , neither the incidence of late - onset open - angle glaucoma ( oag ) nor that of ocular hypertension ( oht ) increases in patients after a vitrectomy . diurnal iop change and peak iop are important factors in glaucoma development and they are essential for evaluating the precise state of the iop [ 1012 ] . the studies mentioned above did not investigate the diurnal iop pattern and its variation , and these patterns may provide a crucial link between the vitrectomy and development of glaucoma . in this study , we analyzed the effect of vitrectomy on the diurnal change of iop . from may to december 2007 , we recruited patients who were admitted for cataract surgery and who had undergone a previous vitrectomy . this exam included slit biomicroscopy , zeiss four - mirror indentation gonioscopy , dilated fundus indirect biomicroscopy , snellen best corrected visual acuity , specular biomicroscopy , and optical biometry ( iol - master , carl zeiss ) . patient history was evaluated , including other systemic and ocular diseases , current medications , and prior surgeries . an experienced ophthalmologist used a goldman applanation tonometer to measure the diurnal iop every two hours between 9 a.m. and 11 p.m .. we compared the diurnal iop data in patients who had one vitrectomized eye with a normal fellow eye and with age- , gender- , and axial length - matched controls . a vitrectomized eye was defined according to the following criteria : ( 1 ) the eye had undergone one successful ppv by a retinal specialist without any complications , ( 2 ) a phakic eye had an open angle without peripheral anterior synechiae ( pas ) , and ( 3 ) the eye was not exposed to antiglaucoma eye drops for at least four months prior to the study . to avoid the influence of tamponade foreign material on iop , we excluded eyes with a history of silicone injection or eyes that had undergone surgery within the last four months . a patient was defined to have a normal fellow eye by the following criteria : ( 1 ) the eye had an open angle without pas and had never been operated on , ( 2 ) the eye had not been exposed to antiglaucoma eye drops for at least four months prior to the study , and ( 3 ) the eye did not have any pathology or abnormality associated with iop on full ophthalmologic examination . the inclusion criteria for normal control eyes were the same as those for normal fellow eyes as described above . patients were excluded from analysis if they had a history of ocular surgery ( not including vitrectomy ) , use of antiglaucoma medication , pas > 90 degrees as measured with gonioscopy , inflammation , or anterior chamber reaction . we compared the diurnal iop and ocular biometries of vitrectomized eyes to their normal fellow eyes and to control eyes . to minimize selection bias , two separate ophthalmologists measured iop and the data independently and in parallel . paired t - tests were conducted using spss 18.0 ( chicago , il , usa ) to analyze the difference in iop between vitrectomized and normal eyes . this study included 21 individuals ( 12 males and 9 females ) with one vitrectomized eye and one normal eye and 21 normal control individuals . the mean patient age was 57.52  12.80 years ( range : 2178 years ) and 17 ( 81.0% ) of the vitrectomized eyes were right eyes . the preoperative indications for vitrectomy included vitreous hemorrhage ( 9 eyes , 42.9% ) , retinal detachment ( 5 eyes , 23.8% ) , epiretinal membrane ( 4 eyes , 19.0% ) , macular hole ( 2 eyes , 9.5% ) , and endophthalmitis ( 1 eye , 4.8% ) . after the vitrectomy , gas was injected into 11 eyes ( c3f8 in 8 eyes and sf6 in 3 eyes ) , and the other eyes were not injected with any tamponade material ( 10 eyes , 47.6% ) . mean iops were 13.33  2.90  mmhg in the eyes with c3f8 gas tamponade , 13.07  3.35  mmhg in the eyes with sf6 gas tamponade , and 13.61  2.88  mmhg in the eyes without tamponade . the mean interval time between surgery and iop measurement was 36.12  39.20 months ( range : 4140 months ) ( table 1 ) . there were no strong correlations between the iop of vitrectomized eyes and those of normal fellow eyes . also , the correlation coefficient between the iop of the vitrectomized eye and that of the normal fellow eye was low in the morning when the iop was relatively high . the mean correlation coefficients of the iops between bilateral eyes are summarized in table 2 . comparisons of iop and other ocular parameters between vitrectomized eyes and normal fellow eyes are described in table 3 . there were no significant differences between the diurnal measurements , the average iop , the minimum iop , the maximum iop , or the iop fluctuation ( calculated as [ maximum iop  minimum iop ] ) between vitrectomized and normal fellow eyes . other ocular parameters including endothelial cell count , corneal pachymetry , axial length , anterior chamber depth , and keratometry also were not significantly different between the two groups ( table 3 ) . the comparisons of iop and ocular parameters between vitrectomized eyes and age- , gender- , and axial length - matched control eyes are described in table 4 . there were no significant differences in the diurnal measurement , average iop values , minimum iop values , or maximum iop values between the two groups . however , there was significantly greater iop fluctuation in vitrectomized eyes ( 3.38  3.54  mmhg ) compared to the control eyes ( 1.65  0.95  mmhg , p = 0.031 ) . no other ocular parameters were significantly different between the two groups ( table 4 ) . following a typical diurnal pattern , the iop in all subjects was the highest in the morning and gradually decreased over the course of the day ( figure 1 ) . in this study , we found that the iop profiles were not significantly different in vitrectomized eyes compared with that of normal eyes except for fluctuations in the diurnal iop . the iop diurnal fluctuation was larger in vitrectomized eyes than it was in age- , gender- , and axial length - matched controls ' normal eyes . furthermore , although it was not statistically significant , the iop diurnal fluctuation was larger in patients ' vitrectomized eyes compared to their normal fellow eyes . it is still controversial whether or not a vitrectomy affects the iop in the development of postoperative glaucoma . no prior studies have evaluated the diurnal iop . however , there are contradictory results from prior studies with regard to the association between vitrectomies and iop . yu et al .   demonstrated that a vitrectomy did not increase the risk of glaucoma or oht in 441 vitrectomized patients . they found that oht and oag occurred in 4.31% of vitrectomized eyes ; 2.49% of the control eyes were afflicted with oht and 2.95% of control eyes experienced oag . although a larger proportion of oht and oag occurred in vitrectomized eyes , there was no statistically significant difference between the two groups . in contrast , fujikawa et al . reported that the mean iop increased after a vitrectomy in patients with macular holes . similarly , ki - i et al . reported that although the immediate change in the iop after a phacovitrectomy was minimal , some eyes developed increased iop after several months . more recently , wu et al . reviewed the iop of 198 patients who underwent a vitrectomy and demonstrated that the incidence of high iop ( 24  mmhg ) or increased iop ( 5  mmhg ) was significantly greater in vitrectomized eyes than in control eyes . for example , iop can increase as a result of surgical complications including hemorrhage , inflammation , and gas tamponade [ 4 , 1618 ] . this temporary iop elevation can be easily controlled with antiglaucoma eye drops [ 3 , 4 ] . in other cases , late - onset glaucoma may develop [ 17 , 19 ] . given this risk , it is very important to identify any long - term harmful effects that vitrectomy has on the eye . there are many reports regarding iop and the prevalence of oht or oag after a vitrectomy . however , none of these studies reported the diurnal iop . therefore , the association between a vitrectomy and diurnal iop had not been previously characterized . it has been well established that there is a diurnal variation in iop and it may be an important factor in glaucoma development . iop is not a fixed value and it varies even over short - term periods . some studies have suggested that the diurnal fluctuation may be increased in glaucoma or oht patients [ 1012 , 20 ] . the circadian rhythm of ocular perfusion pressure could contribute to the mechanism of changing diurnal fluctuations , though the mechanism is not clearly understood . regardless , it is important to study diurnal iop patterns in order to understand the variation in iop and characterize pathological iop patterns . we investigated the diurnal iop to determine whether it is affected by a vitrectomy . in this study , we compared the diurnal iop of vitrectomized eyes with normal fellow eyes . there were no significant differences between vitrectomized eyes and normal fellow eyes in the diurnal iop , minimum iop , maximum iop , or fluctuations of iop . one retrospective study of oag patients suggested that daytime peak iop might be important in predicting the progression of glaucoma . konstas et al . reported that patients with a daytime peak iop of 18  mmhg or lower ( readings at 10 a.m. , 2 p.m. , and 6 p.m. ) generally did not experience worsening of their glaucoma . we measured iop with a shorter interval in between readings ( 2 hours ) , and there were no significant differences in the peak iop between vitrectomized eyes and normal eyes . we also compared the diurnal iop of the vitrectomized eye with age- , gender- , and axial length - matched control eyes to eliminate the effects of axial length on the iop . similarly , there were no significant differences in terms of diurnal iop , minimum iop , or maximum iop values between vitrectomized eyes and control eyes . however , the diurnal iop fluctuations were wider in the vitrectomized eyes than they were in the control eyes . we did not find a strong correlation in the iop between vitrectomized eyes and normal fellow eyes . in a previous study , the correlation between right and left eyes in normal subjects almost exceeded 0.8 . however in our study , there were relatively low correlations between vitrectomized eyes and normal fellow eyes . even if there are no major complications after the vitrectomy , the iop may change . chang reported that increased oxidative stress on the trabecular meshwork after a vitrectomy might increase the iop and the risk of glaucoma in the long term . moreover , fujikawa et al . suggested that inflammatory stress or the physical effects of the vitrectomy could be the cause of increased iop . although the mechanism is unclear , various effects of a vitrectomy can influence diurnal changes of the iop . we also used a small sample size and did not control for the preoperative diagnosis . in addition , because patients who had used antiglaucoma eye drops were excluded , the iop changes after the vitrectomy may have been underestimated . a well - designed , longitudinal , prospective study is necessary to further evaluate iop in vitrectomized eyes . in conclusion , a vitrectomy may change diurnal iop in the long term . however , the iop changes observed herein were within an acceptable range . considering the importance of the iop diurnal variation , these data help to describe the status of the eye after a vitrectomy .