Yes they are (provided you made a correct measurement and obtained a Q of 3 or better). In all corneas, but particularly in thicker than normal corneas, there is a wide variation in biomechanical properties (elasticity, rigidity, hydration, tissue type etc.). These peoperties cause the cornea to react differently to applanation, posing more or less resistance that an applanation tonometer has to overcome. In a soft or elastic cornea, there is little resistance to deformation, hence an applanation tonometer will produce a low reading, faking a lower than true IOP. A hard, stiff, cornea will resist applanation, causing a high reading. The PASCAL is not sensitive to these differences and will produce a reading that represents the true intra-ocular pressure.

OPA represents the mean difference between minimum and maximum points on the ocular pulse curve, i.e. the range of pressure change during the cardiac cycle. When the OPA value is added to the IOP value, the maximum intra-ocular pressure during the cardiac cycle, i.e. the "systolic IOP,” is obtained.

With any tonometer, the possibility of obtaining an erroneous reading always exists. Check the Q score of your PASCAL measurement, and discard the result if Q is higher than 3 (Q=1 is best). Make sure the SensorCap is properly mounted, without wrinkles or air bubbles. Repeat the measurement to confirm your result is reproducible. If you get a consistently different result, you probably have a correct reading. It is known that some corneas differ strongly in their biomechanical characteristics from a "standard cornea". If the cornea is soft and elastic, Goldmann may read low. If the cornea is stiff and thick, Goldmann may read high. The difference between Goldmann and PASCAL may in some cases exceed 5mmHg.

There is an obvious and a maybe less obvious answer: Obviously: the drug is expected to lower the IOP, and you will therefore excpect to measure a reduced IOP with PASCAL. On the less obvious side: Some drugs, when administered over an extended period of time, may have an effect on the corneal tissue, making it mechanically stiffer or softer. If that occurs, then an IOP reading obtained with an applanation tonometer (e.g. Goldmann, GAT) would be influenced not only by the IOP change but also by the change in the cornea's mechanical response. Hence, the apparent GAT IOP might be mieleading. PASCAL measures IOP only; corneal biomechanics has no effect on the measurement. Therefore, the IOP change seen by GAT and by PASCAL might appear to be different. Trust the PASCAL reading!

The short-term IOP changes you are observing with PASCAL may be real. It is well known that IOP may change within a few minutes. In some patients these changes are minimal (less than 1mmHg); in others the fluctuations may be surprisingly large (several mmHg). Possible reasons are: respiration, Valsalva maneuver, anxiety, vasospasms, symphaticotony etc. With the Goldmann tonometer these effects are often masked due to inferior sensitivity and due to oprator bias (operator inadvertently attempts to confirm previous reading rather than looking for differences when making manual force adjustments).

Applanation Tonometer measurements are subject to systematic errors if corneal thickness, radius, and rigidity deviate from values typical for a "standard cornea”. IOP estimates from Applanation Tonometers are therefore likely to be different from Contour Tonometer measurements in most cases.  Contour Tonometer readings are most likely a more accurate representation of true intra-ocular pressure.

Researchers have observed significant differences in the normative curves between DCT and GAT. Kaufmann et al (IOVS,  9/04) showed that DCT averaged 1.8 mm Hg higher than GAT over his study population. Varma, in the LALES Latino Eye Study showed a  difference that was somewhat higher.

The reason is that DCT's normative curve is about 2 mmHg higher than that of GAT. Therefore, a DCT of 20 mm is considered equal to GAT of 18 mm. Doctors often ask, "How, then, should I manage the case?" If you measure DCT = 20 mm, then manage the case as you would have if you had a GAT of 18 mm.

At some point, you will start to manage your individual cases in the new and more precise language of DCT.