In a recent study on patients undergoing phacoemulsification, the anterior chamber was cannulated in a closed system with IOP set to 15, 20, and 35 mmHg by a manometric water column, and IOP measurements were taken by DCT. Measurements with the DCT showed a good concordance with the intracameral IOP. CCT showed a statistically significant but clinically irrelevant effect on measurements with the DCT. All other parameters including corneal curvature, astigmatism, axial length, and age had no statistically significant effect on the difference of DCT and intracameral IOP [18].

It has also been suggested that the ORA measures IOP independently of CCT and CH. However, there is conflicting data regarding this topic. In one study of 153 eyes (78 patients) without glaucoma, not taking ocular medications, no association was found between CCT and ORA IOP measurements [19]. In another study of 48 eyes of 48 glaucoma patients on topical glaucoma medications, a statistically significant association was found between IOP and CCT [20]. More studies are needed before firm conclusions can be drawn as to whether ORA can measure IOP independently of CCT.

Summary for the Clinician

››The dynamic contour tonometer (DCT) is least affected by corneal biomechanics of all instruments used to estimate IOP.

››The DCT may be particularly useful in corneas thinned by LASIK.

››The ocular response analyzer data shows mixed results in terms of the effects of corneal properties on its IOP estimations.

References

 1. Laiquzzaman M, Bhojwani R, Cunliffe I, et al. Diurnal variation of ocular hysteresis in normal subjects: relevance in clinical context. Clin Exp Ophthalmol 2006;34:114–118.

 2. http://www.ocularresponseanalyzer.com/how.html.

 3. Liu J, Roberts CJ. Influence of corneal biomechanical properties on intraocular pressure measurement quantitative analysis. J Cataract Refract Surg 2005;31:146–155.

 4. Roberts C. Biomechanics of the cornea and wavefront-guided laser refractive surgery. J Refract Surg 2002;18:S589–S592.

 5. Touboul D, Roberts C, Kerautret J, et al. Correlations between corneal hysteresis, intraocular pressure, and corneal central pachymetry. J Cataract Refract Surg 2008;34:616–622.

 6. Bochmann F, Ang GS, Azuara-Blanco A. Lower corneal hysteresis in glaucoma patients with acquired pit of the optic nerve (APON). Graefes Arch Clin Exp Ophthalmol 2008; 246:735–738.

 7. Congdon NG, Broman AT, Bandeen-Roche K, et al. Central corneal thickness and corneal hysteresis associated with glaucoma damage. Am J Ophthalmol 2006;141:495–505.

 8. Kirwan C, O’keefe M, Lanigan B. Corneal hysteresis and intraocular pressure measurement in children using the Reichert ocular response analyzer. Am J Ophthalmol 2006; 142:990–992.

 9. Goldmann H, Schmidt T. Applanation tonometry [German]. Ophthalmologica 1957;134:221–242.

10.Ehlers N, Hansen FK, Aasved H. Biometric correlations of corneal thickness. Acta Ophthalmol (Copenh) 1975;53:652–659.

11.Ehlers N, Hansen FK. Corneal thickness in low-tension glaucoma. Acta Ophthalmol (Copenh) 1974;52:740–746.

12.Ehelers N, Bramsen T, Sperling S. Applanation tonometry and central corneal thickness. Acta Ophthalmol (Copenh) 1975; 53:34–43.

13.Orssengo G, Pye D. Determination of true intraocular pressure and modulus of elasticity of the human cornea in vivo. Bull Math Biol 1999;61:551–572.

14.Kotecha A. What biomechanical properties of the cornea are relevant for the clinician? Surv Ophthalmol 2007;52(Suppl 2): S109–S1014.

15.Kaufmann C, Bachmann LM, Robert YC, et al. Ocular pulse amplitude in healthy subjects as measured by dynamic contour tonometry. Arch Ophthalmol 2006;124:1104–1108.

16.Kaufmann C, Bachmann LM, Thiel MA. Intraocular pressure measurements using dynamic contour tonometry after laser in situ keratomileusis. Invest Ophthalmol Vis Sci 2003; 44:3790–3794.

17.Kniestedt C, Nee M, Stamper RL. Accuracy of dynamic contour tonometry compared with applanation tonometry in human cadaver eyes of different hydration states. Graefs Arch Exp Ophthalmol 2005;243:359–366.

18.Boehm AG, Weber A, Spoerl E, Koch R, Pillunat LE. Dynamic contour tonometry in comparison to intracameral IOP measurements. IOVS 2008;49:2472–2477.

19.Medeiros FA, Weinreb RN. Evaluation of corneal biomechanical properties on intraocular pressure measurements using the ocular response analyzer. J Glaucoma 2006;15:364–370.

20.Martinez-de-la-Casa JM, Garcia-Feijoo J, Fernandez-Vidal A, et al. Ocular response analyzer versus goldmann applana-