Effect of Central Corneal Thickness On IOP Using Various Tonometers

Accurate estimation of intraocular pressure is an essential aspect in the diagnosis and management of glaucomatous eye disease. Glaucoma is a progressive optic neuropathy and a leading cause of blindness in the United States. In glaucoma, vision is lost through apoptosis (programmed cell death) of retinal ganglion cells, a type of cell in the retina that transmits visual information to the brain. Diagnosis of glaucoma is usually based on a combination of progressive, characteristic vision loss (measured using visual field testing) and progressive optic nerve head damage (as detected through dilated fundus examinations or disc photography). While a high pressure inside the eye (ocular hypertension, OHT) is not sufficient for a diagnosis of glaucoma, it is the greatest single risk factor for disease onset.

For decades, the gold standard for intraocular pressure measurement has been the Goldmann applanation tonometer. The Goldmann tonometer is calibrated based on a central corneal thickness of 500 micrometers. But, it is known that CCT varies between individuals and even between individual eyes. The various central corneal thicknesses (CCT)s are thought to influence the accuracy of IOP measurements and thus the diagnosis, screening, and treatment of patients with glaucoma. A study performed by Copt (1999) showed that ocular hypertensive patients have relatively thick corneas and normal tension glaucoma patients have relatively thin corneas. Thus it is thought that a thicker cornea creates falsely high IOP readings and a thinner cornea creates falsely low IOP readings. This was followed by a study performed by Lleo et al (2003) which showed that applanation tonometry and CCT positively correlated in normal eyes. More recently the following studies have also found a positive correlation between CCT and tonometry: Lui et al (2005), Tonnu et all (2005), Pourjavan et al (2005), Doyle et al (2005), Gunvant et al (2005), Brunsini et al (2005) and Rask et al (2005).

To further evaluate the effect of CCT on IOP measurement, the method and instruments used to measure IOP were compared. In a clinical setting IOP's are measured by four instruments, Goldmann applanation tonometer, ocular blood flow pneumatonometer, pneumatonometer or Tono-Pen. Singh et al (2001) concluded that the effect of central corneal thickness on Goldmann applanation tonometry accuracy appears not to be clinically relevant. However, Bhan et al (2002) showed that in eyes with normal corneas, the ocular blood flow pneumatonometer appears to be more affected by variation in CCT than the Goldmann tonometer, and the Tono-Pen is least affected by CCT. A limitation of this study is that corneal curvature (a measure of astigmatism) was not performed prior to measuring IOP. High amounts of astigmatism can impact IOP readings obtained from the Goldmann tonometer.

The PASCAL Dynamic Contour Tonometer is a new digital tonometer that provides a direct trans-corneal measurement of IOP and is sensitive enough to detect the ocular pulse amplitude (OPA) due to the patient's heartbeat. Supposedly, when using the principle of contour matching instead of applanation, the PASCAL eliminates the systematic errors inherent in all previous tonometers, such as the influence of corneal thickness and rigidity. Ku et al (2005) demonstrated that the Pascal tonometer was effective in reducing corneal thickness bias during IOP measurements.

It has long been thought that there is a significant positive correlation of scleral rigidity to IOP, thus with increasing pressure in the eye, a resulting increase in rigidity is noted. (1960). A study completed by Pallikaris et al. (2005) investigated a possible correlation of rigidity to other ocular found no correlation between the two.

The effect of CCT on IOP measurement is a clinical concern as erroneous IOP measurements may lead to misdiagnosis. The purpose of this study is to evaluate whether IOP measurements by Goldmann applanation, pneumotonometryocular blood flow pneumotonometry, y, Tono-Pen and the PASCAL Dynamic Contour Tonometer are affected by central corneal thickness, and if so, to what extent.