primary open - angle glaucoma ( poag ) is a group of chronic , slowly progressive neuropathies of the optic nerve , with distinctive defects in the visual field and morphological changes of the optic disc and of the retinal nerve fibre layer . according to weinreb et al . the first stage , which is the beginning of the apoptosis of ganglion cells and of the retinal nerve fibre atrophy , is undetectable with available diagnostic methods . in the second stage , changes in the retinal nerve fibre layer thickness and initial changes in the visual field can be ascertained . diagnosing poag and initiating its treatment at this stage increases the chance of stopping the progression of the disease . in the third stage the glaucomatous changes in the optic nerve and nerve fibre layer are advanced and although this makes the detection easy , the chance for stopping the progress of the disease is low . the aforementioned reasons make it clear that early diagnosis and initiation of treatment are crucial . no objective symptoms of poag are notable until slowly growing defects in the visual field encompass central vision and the patient suddenly loses useful visual acuity . therefore , an active quest for early features of optic disc glaucomatous damage during an ophthalmological examination is necessary . the recent decade saw a tremendous growth in the development of auxiliary examination techniques that facilitate precise evaluation of the optic disc , the nerve fibre layer and the retinal sensitivity . currently , methods such as heidelberg retinal tomography ( hrt ) , laser scanning ophthalmoscopy , gdxvcc laser scanning polarimetry and fdt ii perimetry are among the more commonly used . the use of the hrt method allows obtaining a 3d morphological image and determining detailed diagnostic parameters of the optic disc , such as the cup / disc ratio ( c / d ) . the gdx laser scanning polarimetry allows the evaluation of retinal nerve fibre layers thickness in the peripapillary region , expressed as the nerve fibre index ( nfi ) . the fdt enables the possibility of assessing the visual field and determining the retinal sensitivity mean deviation ( md ) . the aim of this study was to evaluate diagnostic parameters ( c / d , nfi , md ) and analyze their advantages and limitations in the early diagnostics of poag . during the period 20072008 , 194 patients ( 371 eyes ) 3065 years of age were recruited from the outpatient department of the clinic of ophthalmology , medical center of postgraduate education . group a consisted of 50 patients ( 95 eyes , average age 43.79.0 years ) with diagnosed poag , confirmed by additional examinations . group b had 67 poag suspects ( 128 eyes , average age 43.211.0 years ) without glaucomatous visual field defects . control group c included healthy subjects with normal results on examination for poag ( 77 subjects , 148 eyes , average age 46.310.9 years ) . inclusion criteria for a group were specified in accordance with the guidelines of the european glaucoma society   and described in detail in previous publications : age over 35 years , regular anterior chamber angle , glaucomatous damage of the optic nerve head , as well as abnormal results of gdx and visual field examinations . for inclusion to group b , glaucoma - suspicious appearance of optic head nerve and gdx results , but normal results of visual field examination , were adopted . exclusion criteria were : eye refraction error greater than 2 dsph and media opacity , which lowers the quality of hrt and gdx results . subjects with very small or very large optic disc area ( less than 1.69 mm or more than 2.82 mm ) evaluated in hrt examination were also excluded . patients underwent detailed ophthalmological examinations and hrt ( hrt ii heidelberg engineering , germany ) , gdx vcc ( laser diagnostic technologies , usa ) and fdt ( hamphrey matrix , zeiss , germany ) examinations , to determine standard diagnostic parameters . c / d ratio was determined as square root of surface ratio of cup - to - disc . nfi was determined as the probability of glaucomatous damage of the retinal nerve fibre layer . md value was determined as the difference between retinal sensitivity in a healthy eye ( taking age into consideration ) and retinal sensitivity values measured in the examined subject in all examined points . all the basic ophthalmological examinations were performed at the clinic of ophthalmology , medical center of postgraduate education in warsaw while gdx , hrt and fdt examinations were performed at the institute of glaucoma and eye diseases in warsaw . the study was approved by the bioethics commission of the medical center of postgraduate education in warsaw ( resolution of 2 march 2005 and of 30 january 2008 ) . the significance of differences between the particular group pairs was tested by using student s t test . histograms showing the frequency of occurrence of those parameters in individual groups were also drawn . results of the measurements of the nerve fibre layer thickness index ( nfi ) , cup - to - disc ratio ( c / d ) and mean deviation of retinal sensitivity ( md ) in poag patients ( group a ) , poag suspects ( group b ) and in healthy subjects ( group c ) are shown in table 2 . results of the evaluation of the significance of differences between the group pairs of examined subjects are shown in table 3 . histograms showing the frequency of occurrence of selected parameters ( c / d , nfi and md ) in individual groups are shown in figures 13 . the juxtaposition of the results of c / d ratio measurements shows , as expected , a great difference of the parameter s mean value ( p=0.001 ) between healthy subjects ( group c ) and groups b ( glaucoma suspects ) and a ( glaucoma patients ) . the difference is smallest between the mean values of the parameter in groups a and b , which makes evident the difficulties in differentiation between glaucoma patients and glaucoma suspects based on the evaluation of the optic disc . the results of our study are in agreement in particular with jonas et al . observations and confirm that linear c / d ratio is of great use in diagnosing primary open - angle glaucoma . histograms showing the distribution of particular c / d values frequency occurrence in individual groups ( figure 1 ) demonstrate partial overlapping of curves . however , their shift between the control group of healthy patients and the glaucoma patients group is clearly visible , which is evidenced by a high significance of mean values differences in these groups ( table 3 ) . the results of measurements of the retinal nerve fibre layer thickness in the peripapillary region , shown in table 2 , expressed as a probability index of glaucomatous damage nfi , differ statistically significant between individual groups of tested subjects . , who have shown that the nfi is lower than 18 in healthy subjects and does not exceed 31 in glaucoma patients . the diagnostic value of the nfi has also been confirmed by reus and lemij , zheng et al .   and medeiros et al . . according to the data collected in the gdx corporate materials , the glaucomatous damage probability index nfi below 30 defines this probability as low , nfi of 3050 as suspicion of glaucoma , and nfi over 50 indicates an increased probability of glaucomatous damage to the retinal nerve fibre layer in the peripapillary region . histograms showing the distribution of particular nfi values frequency of occurrence in individual groups ( figure 2 ) demonstrate overlapping of curves to a larger degree than in the case of c / d parameter ; however , their shift between the control group of healthy patients and the glaucoma patients group is also visible , which can be confirmed by the high significance of differences of the nfi mean values in these groups ( table 3 ) . the high dispersion of the nfi indicator observed in primary open - angle glaucoma patients ( from 11 to 98 ) limits its diagnostic usability , which is confirmed by results of other studies . therefore , the suggestion of medeiros et al .   to improve the method of analysis and interpretation of the gdx examination due to a very high dispersion of measurement results , both in healthy subjects as well as in glaucoma patients , is understandable . results of the measurements of the mean defect of retinal sensitivity ( md ) , compiled in table 2 , show , according to the accepted criteria of primary open - angle glaucoma diagnosis , a statistically significant difference between the control group of healthy subjects ( group c ) and the glaucoma patients ( group a ) , as well as between the glaucoma suspects ( group b ) and glaucoma patients ( group a ) . no statistically significant difference has been observed between the healthy patients ( group c ) and glaucoma suspects ( group b ) . this is connected with the criteria for selection of patients for group b , in whom no significant change in the visual field has been observed . the mean value of md parameter was 0.29 db in the group of healthy patients and was close to the value given by bowd et al . , who determined an md value of 0.16 db for an analogous group of healthy subjects . the mean value of the md parameter in the group of glaucoma patients was 3.00 db , which is also close to the values found in the literature . for an analogous group of glaucoma patients , bowd et al . an md value of 2.9 db , and racette   an md value of 3.69 db . the cause of these slight discrepancies is probably a difference in the glaucoma stage in the examined subjects . according to the esg guidelines , the deterioration of the retinal sensitivity is connected with lowering of the md value until reaching maximum values of the so - called perimetrically blind field ( from 27 db to 34 db ) . histograms showing the distribution of particular md values frequency of occurrence in individual groups ( figure 3 ) demonstrate clear overlapping of curves , proof of which can be seen in the high significance of differences of the md mean values in these groups ( table 3 ) . the high dispersion of the md index in primary open - angle glaucoma patients ( from 20 to + 4 ) observed in the study indicates that it is necessary to use additional diagnostic parameters . mean values of the basic diagnostic parameters ( c / d nfi and md ) in primary open - angle glaucoma patients , primary open - angle glaucoma suspects , and healthy subjects differed significantly . what limits the diagnostic value of these parameters is their wide span and overlapping in populations of primary open - angle glaucoma patients and healthy patients .
summarybackgroundthe aim of this study was to evaluate diagnostic parameters and analysis of their advantages and limitations in early diagnostics of primary open - angle glaucoma ( poag ) . cup / disc ratio ( c / d ) , nerve fiber index ( nfi ) and mean deviation of retinal sensitivity ( md ) were considered.material/methodsfifty primary open - angle glaucoma patients ( 95 eyes ) , 67 primary open - angle glaucoma suspects ( 128 eyes ) , and 77 healthy subjects ( 148 eyes ) underwent full ophthalmologic examination and also hrt , gdx and fdt examinations for determination of c / d , nfi , and md parameters . 
 student s t test was used to confirm the statistical significance of the differences between the particular group pairs . 
 histograms of distribution of the occurrence frequency of the parameter values in the groups were plotted.resultsthe mean values of c / d were 0.650.11 , 0.580.11 and 0.430.11 , nfi 37.022.7 , 18.55.6 and 15.14.8 and md 3.005.07 , 0772.49 and 0.291.94 , respectively . 
 statistically significant differences between the particular groups were found . there was a partial overlapping of the histograms of distribution of the occurrence frequency of the parameter values.conclusionsthe basic diagnostic c / d nfi and md parameters in primary open - angle glaucoma patients , primary open - angle glaucoma suspects and healthy subjects differed significantly . 
 these parameters are important diagnostic tools in glaucoma diagnosis . 
 a limitation of their applicability is related to a high scatter of the results and their overlapping in particular groups .