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Recent studies evaluating intraocular pressure (IOP) measurements by dynamic contour tonometry (DCT) have shown that they are more comparable to true manometric IOP than Goldmann applanation tonometry (GAT),1 and less affected by changes in naturally occurring central corneal thickness (CCT)2,3 as well as surgically induced thinning of the cornea following laser in situ keratomileusis.4 Following penetrating keratoplasty, further errors in the measurement of IOP by GAT may also occur due to irregular astigmatism, and technical difficulties in aligning distorted mires on the tonometer head.
We prospectively studied 10 patients with unilateral keratoplasty to detect a significant difference between the measurement of IOP by GAT and by DCT. Contralateral eyes served as paired controls. six horizontal and six vertical GAT readings were averaged for each study eye and each control eye. Six DCT readings were also taken. Keratometry and corneal topography was performed by the Baush & Lomb Orbscan IIz, and CCT was measured ultrasonically with the Tomey Corporation Pachymeter SP‐3000.
Demographic data of the study population are summarised in table 11.
IOP measurement in both grafts and controls were found to be significantly higher using DCT than GAT (p=0.004 and p<0.001 respectively). The mean difference in IOP measured by DCT against GAT was +2.67 mm Hg (95% C.I. 0.86–4.47) in grafted eyes, compared to a mean difference of +3.26 mm Hg (95% C.I. 2.16–4.35) in controls. An under‐reading of true IOP by GAT has also been shown against manometry in cadaver eyes,1 as well as in vivo.5
In grafted eyes, there appeared to be no correlation between CCT and DCT‐GAT difference (Pearson's correlation coefficient −0.202, p=0.122), but in controls there was a significant correlation (Pearson's correlation coefficient −0.262, p=0.043). The effect of CCT on IOP measurement by GAT in 56 post‐ keratoplasty eyes has been previously studied, and again no statistically significant relationship was found.6 This unexpected finding could possibly be explained by the alteration in biomechanical forces following keratoplasty such as compliance forces present in the host eye, as well as variable graft‐host interface mechanics.
In grafted eyes, there was a significant correlation between amount of astigmatism and DCT‐GAT difference (Pearson's correlation coefficient −0.398, p=0.002), but not in controls (Pearson's correlation coefficient 0.142, p=0.278). Corneal curvature has previously been shown to have no significant effect on DCT or GAT measurements in non‐PK eyes.7 The correlation found in keratoplasty eyes in this study may possibly be explained by the fact that these eyes had higher degrees of astigmatism than the control eyes (means = 4.38D vs 2.33D).
Previous studies have assessed other tonometers (Tono‐pen, Pro‐Ton and ocular blood flow tonometer) in the measurement of IOP following keratoplasty,8,9,10 but none have shown an advantage over GAT. This study suggests that DCT may be closer to providing the true IOP in these eyes, and may allow it to challenge GAT as the new gold standard in IOP measurement in this setting.
Competing interests: None.