^1,2Assistant Professor, Department of Ophthalmology, SMBT Institute of Medical Sciences and Research Centre, Dhamangoan, Nashik, INDIA.

Email: vidyabhojane1987@gmail.com  

Study conducted on 200 cases (400 eyes). Sociodemographic profile details are shown in table no.1. Mean age was 54.69+/- 14.86 years ranging from 18 years to 78 years of age. Percentage of cases belong to 18 to 25, 26 to 35, 36 to 45, 46 to 55 and more than 55 were 7%, 20%, 39%, 16% and 18% respectively. Highest cases were reported from 36 to 45 years of age. Female to male ratio was 0.87:1 as proportion of female and male were 46.5% and 53.5%. Mean values of intra-ocular pressure (IOP) measured by Goldmann Applanation Tonometer (GAT) and Non-contact Tonometer (NCT) and Central Corneal Thickness (CCT) are shown in table no. 2. Mean IOP measured by GAT was 19.18+/-6.89 mmHg ranging between 09 to 43 mmHg. Mean IOP measured by NCT was 22.42+/-9.17 mmHg ranging between 09 to 46 mmHg. Average CCT measured was 547.36+/-53.26 microm. ranging between 495 to 559 microm. The difference between the mean value of IOP measured by GAT and NCT was statistically significant (p<0.0001). Among different age groups, this difference was statistically significant in 26 to 35 years, and 46 to 55 years of age. In both, males as well as females, difference between IOP by two tonometers was not statistically significant (table no.1).  As shown in table no.3, IOP readings taken by GAT was divided into 3 groups viz. G1 (<12 mmHg), G2 (12 to 21 mmHg) and G3 (>21mmHg) to check difference in IOP measurements by two tonometers at different levels of IOP. Number of cases in group 1, 2 and 3 were 47, 210 and 143 respectively. Difference in IOP levels of group 1,2 and 3 were 1.44, 2.44 and 5.85 mmHG, respectively. Difference between IOP levels measured by GAT and NCT was statistically significant for IOP more than 12 mmHg i.e. group 2 and 3. Table no.4 depicts, IOP readings taken by GAT and NCT at different levels of CCT measured by Pachymeter (divided into 4 groups) to check difference in IOP measurements by two tonometers at different levels of CCT. Mean IOP by GAT and NCT at CCT (≤518µm) were 17.57and 20.9 mmHg, respectively. Mean IOP by GAT and NCT at CCT (519-551µm) were 18.96and 22 mmHg which increased to 20.13and23.51 mmHg at CCT (552-569 µm). The difference between mean levels of IOP measured by GAT and NCT at different levels of CCT were statistically significant (p<0.0001). Figure no.1 depicts the correlation between IOP measurements by GAT and NCT. The Pearson correlation coefficient(r) between the GAT and NCT was 0.90 (very strong positive correlation). Correlation was statistically significant (p<0.001). Linear regression analysis equation was [GAT=5.8+(0.597*NCT)] to predict IOP.

Table 1: Association between Intra-ocular pressure (IOP) by tonometers among sociodemographic groups of study participants (n=200)

*P<0.0001; GAT: Goldmann applanation tonometer, NCT: Non-contact tonometer; CCT: Central corneal thickness.

 

Table 2: Mean values of various clinical parameters

GAT: Goldmann applanation tonometer, NCT: Non-contacttonometer; CCT: Central corneal thickness

 

Table 3: Comparison of Mean Intraocular Pressure (IOP) measured by GAT and NCT

 *P<0.0001

 

Table 4: Comparison of Mean Intraocular Pressure (IOP) measured by GAT and NCT in different corneal thickness group

*P<0.0001; GAT: Goldmann applanation tonometer, NCT: Non-contact tonometer; CCT: Central corneal thickness.

 

Figure 1: Correlation between between IOP Value GAT and NCT

DISCUSSION

IOP measurement largely influenced by the technique of measurement and central corneal thickness (CCT). Goldmann Applanation Tonometer (GAT) and Non-contact Tonometer (NCT) are widely used methods. But IOP measurements through these methods influenced by corneal properties. In present study, most of cases were of age between 36 to 45 years with average age of 54.69 years and male to female ratio was 0.87:1. IOP increased with age measured through GAT and NCT. Average IOP among males (19.58) was more when compared with females (18.78) by both method. Study done by Sood et al.⁹ reported 33% cases from 33 to 45 years age group, 42.06 years as mean age and 52.7% males in their study. In present study, difference in mean IOP levels of GAT and NCT was statistically significant in 26 to 35 years, and 46 to 55 years of age but same was not true for males as well as females as difference between IOP by two tonometers was not statistically significant. Sood et al.⁹ reported comparable findings. Study done by Chakrabarty et al.¹⁰ reported 57% females and 44% were males with mean age of 52.48 years (range: 25 to 83 years). In current study, mean levels of IOP measured with NCT (22.42) was higher than that of GAT (19.18), so the difference was 3.236 mmHg and this difference was statistically significant (p<0.0001). Study done by Sood et al.⁹ reported, statistically significant difference (3.99 mmHg) between mean levels of IOP measured with GAT (17.35) and NCT (22.02). Ahmad et al.¹¹ and Pande et al.¹² reported similar findings in their studies. They reported statistically insignificant effect of age, gender or laterality of eyes on IOP measurements through GAT or NCT. Studies done by Gunvant et al.¹³, Foster et al.¹⁴ and Christopher et al.¹⁵ reported mean IOP between 12.6 to 16 mmHG and mean CCT between 495 and 559 micrometer. In present study, difference between mean levels of IOP measured through GAT and NCT increased as the IOP increased. These differences were statistically significant in IOP more than 12 mmHg and not significant for IOP levels lower than 12 mmHG. So use of NCT over GAT for higher IOP levels should be cautiously done. Study done by Sood et al.⁹ reported, significant difference in IOP levels measured through both techniques at all levels of IOP. Studies done by Qasim et al.¹⁶ and Rochtchina et al.¹⁷ reported that, in cases where IOP (measured by GAT) levels is in the lower range (<12 mmHg), in such cases IOP measurement by NCT are more reliable than in IOP (measured by GAT) levels is in the higher range (>12 mmHg). This was in comparison with present study findings. Highest number of cases had IOP levels between 12 to 21 mmHg in study done by Chakrabarty et al.¹⁰ Present study reported similar finding. Different findings were reported by some studies which reported, NCT overestimates IOP at lower values and underestimates at higher values when compared with GAT.^18-21 In present study, IOP levels measured with GAT and NCT significantly differed at all levels of CCT. CCT affects IOP values measured with both techniques but effect on NCT was more pronounced than GAT. Study done by Sood et al.⁹ reported, positive correlation between IOP measurement of both methods and CCT. They reported coefficient of correlation for NCT-CCT was 0.318 and for GAT-CCT was 0.325. So these findings were similar to findings reported by current study. Hiroki et al.²² found in their study that, for every 10 μm change in the CCT, the IOP changed by 0.23 mmHg with the GAT and 0.29 mmHg with the NCT. Nair et al.²³ and Gupta et al.²⁴ also reported comparable findings. Chakrabarty et al.¹⁰ reported good positive correlation between GAT and NCT IOP measurement s at all levels of CCT. Most studies done by many authors have also reported NCT to be affected more by CCT.^2,4,25 Very strong positive correlation found between IOP levels measured by GAT and NCT with correlation coefficient (r) of 0.90 which was statistically significant (p<0.0001). Sood et al.⁹ also found statistically significant positive correlation between GAT and NCT (r=0.882; p<0.0001). Chakrabarty et al.¹⁰ reported a significant correlation between NCT and GAT (r=0.95, p<0.001) and positive correlation with CCT with a regression of 0.35mmHg per 10μm for both GAT and NCT which is comparable with current study. Previous studies reported good correlation between GAT and NCT [r: 0.27 to 0.9 (p=0.03 to p<0.001)].^18-21

 

CONCLUSION

Out of two techniques for measurement of IOP GAT is the gold standard but being an invasive procedure which requires contact with cornea, it increases risk of cross infection and corneal abrasions. lso it takes long time to conduct so less useful in places where case load is high. NCT had shown statistically significant moderate to strong positive correlation with GAT in current as well as previous studies. Considering this and being a non-invasive and non-contact technique, NCT can be used routinely as a screening procedure for IOP measurements in out-patient department (OPD) and mass screening camps. However, in certain cases like patients with CCT, adjustment of IOP readings measured with NCT should be done to avoid underestimation or overestimation.

 

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