Correlation between central corneal thickness, applanation tonometry, and direct intracameral IOP readings

Abstract

Background: Several authors reported incorrect high intraocular pressure (IOP) values in eyes with a thick cornea using applanation tonometry. This hypothesis was checked by comparing applanation tonometry with direct intracameral manometry.

Methods: 73 patients, scheduled for intraocular surgery, were enrolled. Immediately before surgery, the following were registered: (i) central corneal thickness (CCT), (ii) applanatory IOP (Perkins/Tonopen), and (iii) intracameral IOP.

Results: The difference between applanatory and intraocular measurements was completely independent of CCT (y= -3.43 + 3.8x; where y is the difference between applanatory and intracamerally measured IOP (mm Hg) and x is CCT (mm); r(2) = 0.002; p = 0.72).

Conclusions: There is no systematic error of applanation tonometry with increasing CCT. Therefore it is inadequate to recalculate IOP based on regression formula of applanatory IOP versus CCT.

Figure 1

Cannula placed in the anterior chamber.

Figure 2

Difference between IOPappl and intracameral cannula correlated with CCT. Readings differing by more than 10 mm Hg are marked as circles. y = −3.43+3.8*CCT, r²=0.002; p=0.72, n=73.