Low IOP is measured with great accuracy using non-contact applanation tonometer

August 1, 2009

A non-contact applanation tonometer (Ocular Response Analyzer, Reichert) appears to be more effective than the Goldmann applanation tonometer (GAT) in assessing very low and negative IOP values using the non-contact applanation tonometer's corneal-compensated IOP (IOPcc) parameter. The GAT may be the long-accepted gold standard, but the more accurate assessment of IOP using the non-contact applanation tonometer challenges this gold standard and raises questions as to which of the two techniques is optimal in measuring IOP.

Key Points

Fort Lauderdale, FL-A recent study found a non-contact applanation tonometer (Ocular Response Analyzer [ORA], Reichert) to be more accurate in the assessment of IOP when compared with the Goldmann applanation tonometer (GAT), particularly when measuring for extremely low and negative IOP, according David A. Luce, PhD, speaking at the annual meeting of the Association for Research in Vision in Ophthalmology.

"Corneal stiffness forces vary over a wide range, causing both over- and under-estimation of the true IOP," said Dr. Luce, chief scientist, Reichert. "The significant impact of the corneal biomechanical effect on the measurement of extremely low and negative IOP has become more apparent to ophthalmologists, and this realization has led to the dawn of other IOP assessment devices such as [this non-contact applanation tonometer], which can achieve more accurate IOP values by determining the negative Goldmann-equivalent IOP (IOPg) and more accurately compensate for corneal resistance."

In a recent study, Dr. Luce and colleagues evaluated the effectiveness of the non-contact applanation tonometer in measuring extremely low IOP in eyes with "soft" corneas. Twelve eyes (nine with primary open-angle glaucoma, one with dry eye, and two normal eyes) with negative IOPg (defined as less than 1 mm Hg) were selected for analysis in order to determine the effect of corneal biomechanical compensation using this non-contact applanation tonometer's IOPg and corneal-compensated IOP (IOPcc) parameters. Also, the progression over time of the IOPg and IOPcc was evaluated for two of the participants.

Results for the 12 eyes showed a mean IOPg of –1.40 mm Hg with a standard deviation of 1.29 mm Hg, ranging from –4.98 to –0.07 mm Hg. The mean IOPcc was +3.02 with a standard deviation 1.66 mm Hg, ranging from +0.2 to +6.3 mm Hg. The IOPcc was greater than zero in all of the eyes, and for the two patients who underwent multiple measurement sessions following glaucoma surgery, the IOPcc showed a regular increasing "recovery" IOP.

The IOP measurements taken by ophthalmologists are generally accepted to be the true values; however, because very low and negative IOP values can occur when assessing IOP, why they occur is of particular interest. According to Dr. Luce, very low and even negative IOP readings are not at all uncommon following a trabeculectomy. Whatever the cause, the biomechanical properties of the cornea, such as tear film and corneal resistance, can significantly affect the true IOP value, and using a device that appropriately and accurately compensates for this is clearly the optimal IOP measurement technique.

"Even though extremely low and negative IOP values are an unusual situation, the fact remains that they do occur and can be due to these properties of the cornea," Dr. Luce said. "After we understand why this happens, it would be advantageous to use a device that can accurately compensate for the corneal effect and subsequently produce a true IOP value."