Application Of OCT In Early Diagnosis Of Chronic Angle-closure Glaucoma

Chronic angle-closure glaucoma (CACG) is asymptomatic until there is serious damage of visual function. It is a serious irreversible disease which could end up in blindness. Therefore early diagnosis in glaucoma prevention and treatment is particularly important, as it is the key to prevent visual loss in glaucoma patients. In the past, many scholars believe that the characteristic visual field defect is the gold standard for early diagnosis of glaucoma. But Quigley and his colleagues believed that when the retinal ganglion cell death is more than 50%, the visual field begins to change. When the decline in retinal light sensitivity reaches 5dB, more than 20% of the RGC would die. In most cases, the optic disc and retinal nerve fiber layer (RNFL) thickness change earlier than the glaucoma visual field defect. The previous inspection was done based on the ophthalmologists'direct observation of fundus changes, therefore the results were considered as too subjective. Currently available technologies such as Optical coherence tomography (OCT), the qualitative and quantitative detection RNFL thickness or the relative thickness, have become more useful in evaluating the degree of RNFL defects in early glaucoma diagnosis, provision of an objective basis for the early diagnosis of glaucoma and the observation of changes in condition. OCT is a principle design using high-resolution optical coherence cross-sectional CT imaging methods. The resolution could be precised up to 10μm and could provide the histology of RNFL thickness measurements with good correlation. OCT can objectively and quantitatively measure RNFL thickness and optic disc structural parameters. It is an optical imaging technique which can produce high resolution reproducible images of the RNFL that discriminates glaucomatous from healthy subjects. It provides a new means for diagnosis in the early glaucoma.In this study, a total of 91 patients were examined. 51 eyes with early glaucoma, 51 eyes suspected glaucoma and 37 healthy eyes subjects were included. All patients underwent optic disc and RNFL thickness with OCT during the same visit. Comparisons were made with regard to superior, inferior, temple, nasal quadrant and average RNFL thickness. Comparison of Cup/Disc horizontal ratio, vertical ratio and C/D area ratio among the groups were made, as well as the comparison of MD between the normal vision and the SG group. Analysis of the correlation between the average RNFL thickness and visual field was also carried out.Clinical data showed that the Superior, inferior, nasal, temporal and average RNFL thickness +/-standard deviation in the normal group were 139.43±11.444, 124.95±14.600, 78.43±10.062, 79.03±15.323, 105.7365±7.78173, respectively. In SG group RNFL thickness were 120.78±17.607, 113.08±16.479, 73.65±13.868, 74.20±9.660, 95.3596±9.37894, respectively. In CACG group RNFL thickness were 98.90±23.993, 93.29±18.801, 70.04±12.983, 56.90±11.848, 78.3922±12.75257, respectively. There was significant difference of RNFL thickness in various quadrants and means among the three groups (P<0.01). Normal and SACG group: RNFL thickness in superior, inferior and the average had significant difference (P<0.01), while in the nasal and temporal, there was no difference. Normal and CACG group: every quadrant and average RNFL thickness had significant difference (P<0.01). CACG and SACG Group: the superior, inferior, nasal and the average RNFL thickness was significantly different (P<0.01), temporal RNFL thickness showed no significant difference. Among the normal, SG, early CACG group, C/D horizontal, vertical radio and C/D AR were significantly different (P<0.01). The average RNFL thickness and visual field mean defect (MD) showed a strong and positive linear relationship (P<0.01). Correlation coefficient was 0.703 with linear regression equation as y = 107.834 +4.754 x.The purpose of this study was to compare the ability of OCT RNFL thickness and optic disc parameters to differentiate between healthy, SG and early CACG eyes. The study also investigated if there could be improvement in the accuracy of OCT in early diagnosis of glaucoma when used in combination with visual field MD change. OCT results detected showed that RNFL normal Form is "double humped", that is, the RNFL thickness of the disc above and below performance of the two "peaks", the nasal and temporal side of the two "valleys" is the same as the histological features of normal RNFL thickness. Results from this study show that OCT, can at an early stage, reflect the changes of RNFL thickness; we found SG RNFL damage before the vision field change, smaller RNFL damage range, and confined to the optic nerve at the superior and / or inferior. As the disease progressed, the early CACG of RNFL damage began to extend when compared with SG, above superior and inferior RNFL thickness was further thinned, nasal RNFL thinning also occurred, but the temple was not damage. Gradual diffusion led to gradual development of defects and at this time, vision had begun to change. These RNFL changes were consistent with morphological changes. First there was pressure on the RNFL at the top below the optic disc, and then RNFL started thinning. The double peak structure disappeared or double peak structure asymmetric, as disease progressed showing diffusion/flattening as change. We also found that using OCT in the normal, SG, early CACG group of C/D horizontal, vertical ratio and C/D AR, proved that with the progression of disease, the optic disc C/D of early CACG and the SG had different degrees of change, as reflected in the bigger cup, narrower plate and so on. With the RNFL thinning, damage viewed in the C/D and C/D AR also became bigger. This trend is consistent with the glaucoma's optic disc damage and synchronous with RNFL change and visual field damage. At the same time, OCT detected average RNFL thickness and visual field MD correlation analysis was carried out to prove its relevance. Results showed that the two had strong and positive linear relationship. OCT detected RNFL thickness changes and visual field defect have good correspondence. With the progression of glaucoma RNFL thickness to thinness, diffused light sensitivity decreased vision and localized visual field defect also increased.In summary, OCT can reflect early the changes in glaucoma by measuring the RNFL thickness and optic disc parameters, find the RNFL thickness damage before the damage of visual field occurs, and having a better correlation with the vision, it can serve as a method of early diagnosis in glaucoma.