Diabetic retinopathy is responsible for many retinal changes observed in long-term diabetes. The permanent increase in the morbidity and the prolonged life time of the diabetics result in an increased occurrence of ophthalmic complications leading to irreversible blindness in developed countries.

The diabetic changes in blood vessels consist in thickening the basement membrane of capillaries, damage and proliferation of the capillary endothelial cells and loss of pericytes. The abnormalities contribute to the occlusion of small retinal vessels and to the reduction of the perfusion in the capillaries. The resulting retinal ischaemia and hypoxia lead to the formation of arteriovenous shunts, intraretinal microvascular abnormalities and leakage through the damaged vessel walls as well as localized and diffused retinal edema. The development of diabetic retinopathy results in formation of hemorrhages, microaneurysms, hard exudates, multiple cotton wool spots, neovascularization of retinal vessels, vascular-fibrous proliferations to the vitreous corpus and retinal detachment.

The most frequent reason for the blindness in diabetic patients is maculopathy consisting in retinal edema or hard exudates observed in the foveal region. In the case of clinically significant retinal edema, the treatment consists in photocoagulation of leakage sites with argon laser and resorption of the exudate. The good visual acuity can be retained after the treatment of the patient (1, 2).

Evaluation of retina by using the contact lens biomicroscopy at the dilated pupil is the basic clinical method of detecting of the macular edema. The efficiency of the evaluation depends, however, on the experience of the physician as well as on the macular edema extension. The blood vessel status and the extent of the leakages in the macula region can be evaluated by using fluorescein angiography (3). This method also allows to qualify diabetic patients to the laser coagulation treatment. The optical coherent tomography is also useful in evidencing the morphological changes of retina in diabetes.

The retinal thickness analyzer (RTA) seems also to be a good tool for the screening investigations. It enables a non-invasive examination of the retinal thickness in macula and it makes possible to identify the macular edema in the early stage of the disease (4, 5).

The aim of the study was to evaluate the retina thickness in the macular region in diabetic patients on various stages of retinopathy by using the RTA analyzer and an assessment of applicability of the RTA method in diagnosis of diabetic macular edema.

Sixty-eight eyes of 40 diabetic patients treated in the CMKP Department of Ophthalmology in 2005-2008 were included into the studied population. All patients underwent a detailed investigation, consisting of determination of visual acuity, intraocular pressure (applanation Goldmann tonometer) and lens transparency as well as evaluation of the fundus with Volk contact lens after pupil dilatation with tropicamide (1% solution). Patients with advanced hypertension, thyroid hyperactivity, pseudophakia and other eye surgery, retinal vein occlusion, advanced glaucoma and retina degeneration were excluded from the study. The eyes with lens and cornea opacity, low visual acuity and difficulties with the pupil dilatation were also excluded. The studied population was presented in the table 1.

The diabetic retinopathy was classified according to the recommendations of the Research Group of Examination and Early Treatment of Diabetic Retinopathy (6). Both patients without any changes in the retina, patients with nonproliferative minimum and mild retinopathy (without any visible macular edema) and patients with visible macular edema were included in the studied population. The patients with visible macular edema underwent fluorescein angiography with Heidelberg Retina Angiograph HRA (Heidelberg Engineering, Germany).

Eighty healthy subjects (155 eyes) free of systemic and ocular diseases were in the control group. They appeared in the CMKP outpatient clinic for prophylaxis or getting glasses. The group was described in other paper (7) in detail. In the group, the retinal thickness did depended neither on patients´ sex nor on age.

The retinal thickness analysis was carried out with a non-invasive RTA analyzer (Talia Technology Ltd., Israel). The principle of the RTA is based on projecting a thin helium-neon laser slit through the dilated pupil obliquely on the retina and viewing it at an angle in a manner similar to slit lamp biomicroscopy. The separation between the reflections and scatter from the vitreoretinal interface and the chorioretinal interface is a measure of the retinal thickness. As the result, colored maps of the retinal thickness, graphs and respective numerical data were obtained (4). In particular, foveal average thickness FAV (area of 600 μm radius from fixation point) and posterior pole average thickness PPAV (area 6x6 mm excluding fovea) were determined both in the studied population and in the control group. The data were statistically evaluated and compared with the data of the control group (8, 9). The significance of differences between the average FAV and PFAV values in both groups was checked by Student t test.

A permission of the CMKP Bioethical Commission, Warsaw, was received (resolutions of 2^nd March 2005 and 30^th January 2008). Although no invasive methods were used in the study, the patients were fully informed of the aim and course of the study and they agreed.

The results of the study were collected in the table 2. Both FAV and PPAV values in the studied population of diabetic patients were significantly thicker than those in the control group.

An early detection of the macular edema is very important for taking the decision on fast treatment with laser coagulation and decreasing the risk of visual loss. The clinical evaluation of macular edema with biomicroscopy is supported with fluorescein angiography. This method can evidence the leakage from abnormal retinal blood vessels, however, does bring neither any information about degree of the fluid accumulation on the retina nor on the edema formation rate. At the first stage, no fluid accumulation in retina can take place at the normal absorption of the exudate. During the progress of the diabetic retinopathy, the exudate fluid is, however, accumulated in the retina what results in edema and in an increase of retinal thickness in macular region (3, 5, 10, 11). The fluorescein angiography has not any quantitative character and does not allow estimating the exudate fluid accumulation in the retina. Moreover, the angiography is an invasive method (fluorescein injection). Use of the RTA for an objective determination of the retinal thickness gives numerical data on the edema size and localization and its distance from the fovea (11, 12). Retinal thickness maps of healthy patients show a specific shape of the macula. It has C-shaped thickening above, below and nasally to the fovea. The analog maps for diabetic patients do not show any characteristic configuration in the foveal and perifoveal regions but evidence a thickening of retina in the studied area (fig. 1, fig. 2).

An increased thickness of retina in macular region in diabetic patients was described in many papers based on RTA and OCT methods (11, 13, 14, 15, 16). Our RTA studies showed a significant difference in the retinal thickness between the healthy and diabetic subjects and confirm our former observations (17). The observed increase in retinal thickness in diabetic patients can be assumed as subclinic and clinic symptoms of the macular edema and the measurement of the retinal thickness may be used for the diagnostic purposes.

The eyes with minimum and mild nonproliferative diabetic retinopathy without any visible macular edema should be examined by some independent objective methods, like RTA, to establish the increased retinal thickness in its initial stage. The method allows improving the accuracy of diabetic edema diagnosis, more frequent monitoring and an early start of the treatment in reduced area. The method is not widely used in Poland yet, but it can be recommended especially for screening the macular edema in diabetic patients because of its non-invasiveness, simplicity of the examination and further data evaluation.