Accuracy – True IOP!

The accuracy of tonometric readings is defined by how close they come to the true IOP as measured intracamerally. This accuracy is an important criterion for tonometry as IOP measurements are critical elements in making sound treatment decisions. The PASCAL was validated by comparing its measurements with manometric readings in a large series of cannulated eyes with patients undergoing cataract surgery. A mean difference of DCT to true IOP of only -0.2 mmHg was found, at the level of 20 mmHg.

Correcting applanation IOP for Corneal Thickness (CCT) does not work as it can be seen in the heavy scatter of GAT values. In many cases corrections would work in the wrong direction and even magnify the error. Other factors like corneal rigidity influence the applanation measurement to probably even a bigger extent.


Accuracy and Precision with unusual corneas

Many surgeries and treatments affect the biomechanical properties of the cornea and lead to an underestimation of IOP by applanation tonometry. PASCAL eliminates most false negative readings because its measurements are independent of corneal biomechanics.

PASCAL is accurate and precise with:

  • LASIK or PRK11
  • Lamellar and penetrating keratoplasty12
  • Trabeculectomy13
  • Prostaglandin therapy14
  • Keratoconus15
  • Corneal crosslinking therapy16



References:

11 E. M. Kirstein et al. Evaluation of the Orssengo-Pye IOP corrective algorithm in LASIK patients with thick corneas; (Optometry 2005;76:536–43.)

12
Papastergiou et al. Effect of recipient corneal pathology on DCT and GAT readings in eyes after PK; (Eur J Ophthalmol. 2010 Jan–Feb;20(1):29–34.)

13 E. Detorakis et al, Differences between Goldmann Applanation Tonometry and Dynamic Contour Tonometry following Trabeculectomy; (J of Ophthalmology doi: 10.1155/2010/357387)

14
E. Detorakis et al, GAT Versus DCT in Eyes Treated With Latanoprost; (J Glaucoma 2010;19:194–198)

15
Read SA, Collins MJ. IOP in keratoconus. (Acta Ophthalmol 2011 Jun;89(4):358–64)

16 Gkika MG;Tonometry in keratoconic eyes before and after riboflavin/UVA; corneal collagen crosslinking using three different tonometers; (Eur J Ophthalmol 2011 May 11)

Cannulation study by A. Böhm; 2009:

"DCT measurements come close to the true IOP"4 Correlation of in-vivo direct manometric measurements (black curve) with simultaneous PASCAL DCT measurements (red curve) performed during cataract surgery in the study of A. Böhm 2009.




References:

4 A. Böhm et al. DCT in Comparison to Intracameral IOP Measurements; (Invest Ophthalmol Vis Sci. 2008;49:2472–2477)

Distribution of IOP relative to CCT7

DCT, in contrast to GAT, is nearly independent of CCT. Correction of a single IOP value for CCT is not possible due to the heavy scatter.




References:

7
Schneider E, Grehn F; Intraocular pressure measurement – comparison of DCT and GAT; (J Glaucoma. 2006;15: 2–6)

IOP after LASIK

Surgery changes corneal biomechanics, not IOP. GAT and conventional Airpuffs give a false impression by underestimating IOP post-surgery – while PASCAL stays unaffected.


Δ IOP in different treatment groups

After trabeculectomy or with prostaglandin use, higher than normal Δ IOP = DCT – GAT was found. This finding may reflect structural changes in the ocular rigidity or hemodynamics which compromise the accuracy of GAT in a magnitude similar to LASIK.