| Background Glaucoma is the second common cause of blindness worldwide and seems to be a multifactorial disease. Elevated intraocular pressure (IOP) is the most important risk factor and the only risk factor directly affected by treatment in the etiology of glaucoma. Therefore, accurate tonometry is crucial for diagnosis and control of treatment success in glaucoma. Nocontact tonometer(NCT), a kind of applanation tonometer, is used widely in rabbits. Studies showed evidences that central corneal thickness(CCT) affects the estimation of IOP with NCT. The rabbit's eye is more different with human eye. The accuracy and feasibility measuring IOP with NCT remains unkown in rabbits.Objective To investigate the accuracy of NCT value compared with actual IOP using intracameral manometry, and the relationship among the actual IOP, NCT value, NCT error, CCT in New Zealand White rabbits, and attempt to establish a mathematic model of correlation between the actual IOP, NCT value, and CCT.Methods The CCT was measured five times using a ultrasonic pachymeter in each subject after topical anesthesia, and the average of five measurements was calculated. This measurement was performed for three days, and then the results of three days were averaged as CCT reading. The rabbits were divided into two groups with thin and thick CCTs. Rabbits were examined under general anesthesia. The actual IOP was measured with a 24G needle inserted the anterior chamber and connected to a pressure transducer, and NCT values was taken simultaneously with noncontact tonometer(Topcon-60). NCT measurements were taken three consecutive times until the difference of them was less than 3 mmHg. The readings of three times were averaged. Readings of the two techniques were compared with a paired t-test. TheNCT error(actual IOP minus NCT value) was calculated. NCT errors between thin CCT and thick CCT were compared with an independent Mest. Linear regression analysis was performed in these variables. Differences at a level of P=0.05 were accepted as statistically significant changes.Results No relationship was found between the CCT and actual IOP, NCT value or NCT error (R=0.148, P=0.436;R=0.135, P=0.477;R=0.154, />=0.416, a Pearson correlation), respectively. Reliability analysis showed the intraclass correlation coefficient was 0.9786(Alpha) between actual IOP and NCT values. NCT showed lower readings than intracameral manometry in the same individuals. The mean value was 8.97+5.14mmHg(mean±SD) for NCT tonomatry and 22.22±6.40mmHg for manometry. The readings of two methods were of statistically significant differences (t=43.188, F<0.001, a paired Mest) in the entire group. The mean NCT error was 13.24±1.68mmHg. In the most extreme case, NCT error of 16.49 mmHg was obtained. Mean CCT was 360.3±10.0//m(342 to 374/*m), 393.8±13.5/?n(375 to 424/?n) in thin and thick CCT groups. NCT error in two groups (13.08±1.68mmHg, 13.43±1.72mmHg, respectively) showed no significant difference (f=-0.570, P=0.573, a independent Mest). A high positive correlation with statistical significance between readings of two methods was also found (R=0.981, P< 0.001). Linear regression analysis showed there is a regression equation for direct intracameral IOP, Y=11.278+1.220X (F=713.521, P < 0.001), namely each 0.820 mmHg increase in actual IOP(Y) caused 1 mmHg increase in NCT readings(X).Conclusion In this in vivo study NCT error was found no significant differences in thin and thick corneal. It is necessary to calibrate the readings of IOP measured with NCT to get a more accurate ones, because NCT showed lower readings than direct manometry as a whole in rabbits. |
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