The role of confocal scanning laser ophthalmoscopy in stereometric differentiation of eye papilla in ocular hypertension , normal tension glaucoma and primary open-angle glaucoma

Background/Aim. Primary open angle glaucoma (POAG) and normal tension glaucoma (NTG) demonstrate the same structural changes in the optic disc along with visual field defects but only POAG includes an abnormal elevation of intraocular pressure. Heidelberg retina tomograph based on confocal scanning laser ophthalmoscopy (HRT) and Moorfields regression analysis (MRA) have been employed to quantitatively assess the topography of eye papilla. We measured stereographic parameters of eye papilla in patients with POAG, NTG, and ocular hypertension (OH) using an HRT in order to determine whether HRT topographic parameters can be used to differentiate those conditions. Methods. The results of 145 eyes of 145 patients with OH, NTG and POAG were analyzed by age, refractive error, quality of HRT images, stereometric and MRA parameters. Results. Significant differences were found between NTG and other two groups for a majority of the HRT parameters, and also no differences between OH and POAG patients for a majority of the investigated parameters, except thickness of retinal nerve fiber layer. By reading the MRA no differences were found in the distribution of mostly damaged and mostly preserved neuroretinal rim sectors between NTG and POAG patients, and also all sectors of the neuroretinal rim in OH patients were preserved. Conclusion. HRT stereometric parameters are useful to differentiate patients with OH and NTG, and also for differentiation between NTG and POAG patients, but most of parameters showed no difference between OH and POAG patients. MRA may serve to confirm the diagnosis of OH, but not for precise distinction between NTG and POAG.

of the HRT parameters, and also no differences between OH and POAG patients for a majority of the investigated parameters, except thickness of retinal nerve fiber layer.By reading the MRA no differences were found in the distribution of mostly damaged and mostly preserved neuroretinal rim sectors between NTG and POAG patients, and also all sectors of the neuroretinal rim in OH patients were preserved.Conclusion.HRT stereometric parameters are useful to differentiate patients with OH and NTG, and also for differentiation between NTG and POAG patients, but most of parameters showed no difference between OH and POAG patients.MRA may serve to confirm the diagnosis of OH, but not for precise distinction between NTG and POAG.

Introduction
Primary open-angle glaucoma (POAG) is a disorder that demonstrates typical structural changes in the optic disc along with visual field defects related to an abnormal elevation of intraocular pressure (IOP), while normal-tension glaucoma (NTG) is a type of glaucoma that shares clinical features and mechanisms with POAG, except for the abnormal elevation of IOP.Ocular hypertension (OH) is intraocular pressure higher than normal in the absence of optic nerve damage or visual field loss.Assessment of the optic disc is included in the standard examination of patients with OH or suspected or manifest glaucoma.Such evaluation is performed not only by glaucoma experts, but also by general ophthalmologists, ophthalmology residents, and ophthalmologists with special skills in areas other than glaucoma.Glaucomatous optic nerve damage may manifest itself not only as a morphological change in the optic disc but also as a decrease in the thickness of the retinal nerve fiber layer (RNFL) 1 .Loss of axonal fibers results in the decreased thickness of the RNFL, and this structural change has been found to precede both any morphological changes of the optic disc and functional changes in the visual field 2-13 .Thus, measuring the RNFL thickness, along with morphological analysis of the optic disc lies at the cornerstone of early glaucoma detection.Various computerized quantitative imaging techniques have been developed to help doctors identify structural glaucomatous damage.Confocal scanning laser tomography using the Heidelberg retina tomograph II (HRT II); (Heidelberg Engineering, GmbH, Heidelberg, Germany) was introduced in the beginning of the 1990s and has been further developed since then.HRT II uses confocal scanning laser ophthalmoscopy to evaluate quantitatively the three-dimensional surface topography of the optic nerve head and the surrounding nerve fiber layer [14][15][16][17][18][19][20][21] .There fore, HRT II is considered to be a promising tool for the early detection of glaucoma.
The aim of this study was to investigate whether the HRT instrument can make distinguish between stereometrical characteristics of papilla in groups of patients with OH, NTG and POAG.We wanted to know whether the three examined identity may vary based on size of neuroretinal rim damage, as well as the distribution of damaged and preserved neuroretinal rim sectors.All that in order that the results obtained in our study help ophthalmologists in routine examinations of the HRT findings, to help them to avoid errors in diagnosis of the glaucomatous or OH damage to the optic disk and its surroundings.

Methods
This retrospective study included 145 eyes of 145 patients from the Institute of Ophtalmological of Medical of Medicine Faculty of the Belgrade University.The research followed the tenets of the Declaration of Helsinki and was approved by the Regional Ethical Review Board.Stereographic parameters of 61 eyes in 61 patients with POAG, and 45 eyes in 45 patients with NTG, and 39 eyes in 39 patients with were investigated OH (Table 1).The restriction of the study considering the one eye of each patient for each group was made in order to facilitate statistical analysis.Those eyes with excessive refractive error (of more than +6 diopters or less than -6 diopters), cataracts, diabetic retinopathy or with any history of surgical treatment or eye trauma were excluded.The NTG eyes included in this study were defined as those showing both glaucomatous optic disc changes and IOP never exceeded 21 mmHg on repeated measurements.Subsequently, the eyes enrolled as POAG group included those whose IOP exceeded 21 mmHg prior to or after initiation of therapy.The OH eyes included in this study were defined as those showing no glaucomatous optic disc changes and IOP exceeded 21 mmHg on repeated measurements.We used HRT II in our study for the collection of all necesary data.By using HRT II we can get a series of photographies of the cross section of the optical nerve head of different deepness and after 3D reconstruction it produces topographical photographies of the papilla and peripapillar retina 22 .To quantify morphometric rim and cup parameters in optic disc topography, a reference plane is defined.The reference plane is parallel to the retinal surface.It needs to be stable so that the parameters change only when true structural changes in the optic disc occur.Within the disc margin, the retinal surface located above the reference plane is defined as rim and below the reference level as cup.
In order to verify the quality of topographic images we used images with standard deviation less than 40 μm.Eight stereometric parametres were taken into consideration in this study: disc area (mm 2 ), cup area (mm 2 ), rim area (mm²), cup-to-disc area ratio (C/D ratio), cup volume (mm³), rim volume (mm³), cup shape measure (mm) and mean retinal nerve fiber layer (mRNFL) thickness (mm).Moorfields regression analysis (MRA), a program contained in the basic package of HRT device was used for comparison of the examined six sectors of neuroretinal rim (1.temporal, 2. supero-temporal, 3. infero-temporal, 4. nasal, 5. supero-nasal, and 6. infero-nasal) with a normative.It defines these sectors as damaged, borderline and normal based on the 95% and 99.9% confidence intervals.
The aim of this study was to find which group has larger damage (in percent) of the of the neuroretinal rim and which segment of the neuroretinal rim is the most frequently and the least frequently often represented as the damaged for each group separately.Also, the aim of this study was to find if we can confirm by reading the MRA findings that the neuroretinal rim of OH patients is preserved.
By using SPSS version 20 we analysed the basic demographic characteristics (age, gender), also a refractive error and standard deviation of HRT images and examined eight morphometrical parametres of the optical disc in all three groups, with the aim to establish the existance of statisticaly significant difference between the same parametres in the groups (statisticaly significant difference is when p < 0.05).We used methods of descriptive statistics, and an analytical statistical methods (t-test).

Results
Basic statistics relating to sex, age, size of refractive error in patients eyes and standard deviation of topographic HRT images were shown in table 1. Between all patients who met the entry criteria there were no significant differences in refractive error and standard deviation of topographic HRT images among the three groups.There was statistically significant differences in age between patients wih OH and NTG, and patients with OH and POAG.Patients with OH were significantly younger than patients with NTG and POAG.A basic statistical summary of the results of the HRT parameter measurements was shown in Table 2. Examining the significance of differences among parameters between groups we found different values.The mean values of disc area (mm²) were significantly larger in NTG than in the other two groups.The NTG group also showed significantly the largest cup area (mm²), followed in order by POAG and OH group.On the other hand, the mean values of rim area (mm²) were significantly higher in OH than in NTG group.Rim area were not significantly different between OH and POAG patients, and also between NTG and POAG patients.Cup volume (mm³) were significantly higher in HTG than in other groups, and the same parameter were not significantly different between OH and POAG group.The mean values of rim volume (mm³) were significantly larger in OH than in NTG group, but there was no statistically significant differences between other groups in relation to this parameter.Further, the mean values of cup to disc area ratio were significantly larger in NTG eyes than in OH and POAG eyes, though at was not significantly different between OH and POAG group.The NTG patients showed the significantly larger values for cup shape measure (mm) compare to OH and POAG patients, and the same parameter were not significantly different between OH and POAG group.Last investigated parameter of optic disc was mean RNFL thickness (mm), and it was significantly larger in OH than in other two groups of patients.Mean RNFL thickness were not significantly different between NTG and POAG group (Table 2).

Table 2 Heidelberg retina tomograph (HRT) parameters in ocular hypertension (OH), primary open angle glaucoma (POAG) and normal tension glaucoma (NTG)
OH By the reading of MRA findings of all three groups, we found that the size of the damage of neuroretinal rim is higher in the NTG group (12.4 %), than in the group of the patients with POAG (6.5 %).We found that in group of the patients with POAG the segment which is most often classified as damaged was nasal, and the least often temporal, also in the group of the patients with NTG the segment the most often classified as damaged was nasal, and the least often temporal (Table 3).All six sectors of neroretinal rim of each eye in the OH group were classified as normal.

Discussion
Since the development of the HRT, many reports have demonstrated its advantages for quantitative assessments of optic disc topography during diagnosis and follow-up of glaucoma patients [23][24][25][26][27][28][29][30][31][32][33][34] .Several authors have made comparisons between the topographic parameters of optic discs among patients with glaucoma, individuals with OH, and normal controls 26,28,32,34,35 .However, there are few reports of studies that have compared those parameters among OH, NTG, and POAG patients 26,27 .In our study we did not find any HRT parameter witch is significantly different among all three groups.In previous reports 26,35,36 , disc area showed no significant difference among these disorders; however in our study the NTG patients have significantly largest disc area.In our study the NTG group also showed significantly the largest cup area, cup volume and cup to disc area ratio, followed in order by POAG and OH group.Same results can be found in study of Kiriyama et al. 36 .The mean values of rim area and volume were significantly higher in OH than in NTG group, and the same results we can find in the study of Kiriyama et al. 36 .Our study demonstrated no difference in the some parameters between NTG and POAG had shown group, as in a study of Iester et al. 37 .Further on, our study showed that the NTG patients had larger values of cup shape measure compared to OH and POAG patients, and the same parameter was not significantly different between OH and POAG group.Kiriyama et al. 36 reported that when comparing the same parameters cup shape was significantly different between POAG and OH, and NTG and OH eyes, but not between POAG and NTG eyes.Previous mentioned study 36 also showed that mRNFL thickness was significantly different between POAG and NTG eyes, and NTG and OH eyes, however, did not between POAG and OH eyes.Our study showed that mRNFL thickness was significantly larger in OH than in other two groups of patients.In sum, we found significant differences between NTG and other two groups for a majority of the parameters, and also no differences bettwen OH and POAG patients for a majority of the investigated parameters, except mRNFL.By reading the MRA findings we found no differences in the distribution of mostly damaged and mostly preserved neuroretinal rim sector between NTG and POAG patients.MRA findings also showed that all sectors of the neuroretinal rim in OH patients were preserved.Another study showed that by using HRT and following throw time parameters of the neuroretinal rim may prove the diagnosis of ocular hypertension 38 .

Conclusion
Patients with NTG tend to have larger disc, larger cupping, smaller rims, and thinner RNFL as compared to POAG and OH patients.Patients with NTG also had a larger area of damage of the neuroretinal rim, compared to POAG.On the other hand, patients with OH were younger, and had largest rim area and largest cup shape measure.Looking at the size of the all tested parameters, the patients with POAG were positioned in the middle, with respect to all three groups.Thus, HRT topographic parameters are useful to differentiate patients with OH and NTG, and also for differentiation between NTG and POAG patients, but in most of them showed no difference between OH and POAG patients.Also, we can conclude that MRA may serve to confirm the diagnosis of OH, but not for precise distinction between NTG and POAG.

R E F E R E N C E S
1. Tuulonen A, Airaksinen PJ.Initial glaucomatous optic disk and retinal nerve fiber layer abnormalities and their progression.Am J Ophthalmol 1991; 111(4): 485 90. 2. Quigley HA, Katz

Table 1 Basic data of patients with ocular hypertension (OH), normal tension glaucoma (NTG), and primary open angle glaucoma (POAG)
*p <

Table 3 Distribution of damaged sectors of neuroretinal rim in patients with normal tension flaucoma (NTG) and primary open angle glaucoma (POAG) based on Moorfield's regression analysis (MRA) findings
J, Derick RJ, Gilbert D, Sommer A. An evaluation of optic disc and nerve fiber layer examinations in monitoring progression of early glaucoma damage.Ophthalmology 1992; 99(1): 19 28. 3. Quigley HA, Addicks EM, Green WR.Optic nerve damage in human glaucoma.III.Quantitative correlation of nerve fiber loss and visual field defect in glaucoma, ischemic neuropathy, papilledema, and toxic neuropathy.Arch Ophthalmol 1982; 100(1): 135 46. 4. Tuulonen A, Lehtola J, Airaksinen PJ.Nerve fiber layer defects with normal visual fields.Do normal optic disc and normal