An increasing number of new technologies have become available to measure and monitor intraocular pressure over the past decade, and more are on the horizon.
The answer, according Dr. Gaddie, is a simple one: Because our growing knowledge and understanding of glaucoma enables and demands it. He explained the limitations of Goldmann applanation tonometry, and reviewed several newer IOP measurement technologies.
"Goldman tonometry is based on the principle that pressure equals force divided by area. Therefore, the internal pressure of a fluid-filled sphere is proportionate to the force required to applanate the external area of the sphere," said Dr. Gaddie, Gaddie Eye Centers, Louisville, KY, and adjunct assistant professor of optometry, Northeastern State University College of Optometry, Tahlequah, OK.
Many common visco-elastic materials, such as foam mattresses, automotive struts, viscous fluids like honey and oil, and door closers exhibit hysteresis: they have properties that do not instantly follow forces applied to them, but rather react slowly to such forces, and do not return completely to their original state after the force is ceased.
The cornea is similarly visco-elastic, and corneal hysteresis (CH) is a characterization of the cornea's biomechanical tissue properties. It is the result of visco-elastic damping (energy absorption) as the cornea moves rapidly in or out.
Taking this hysteresis into account enables newer IOP measurement devices to quantify and minimize the cornea's influence on IOP measurement.
"Central corneal thickness-based IOP correction is flawed because corneal resistance to bending is not dependent on thickness, but on material properties. Thick and thin is wrong; think weak and strong instead," Dr. Gaddie said.
Examples of IOP measurement technologies cited by Dr. Gaddie include:
It measures CH and corneal resistance factor, both indicators of the overall resistance of the cornea, including both the viscous and elastic properties. This measurement, referred to as corneal compensated IOP (IOPcc) is minimally affected by corneal visco-elastic properties, thickness, or previous surgical procedures such as LASIK.
A proprietary ocular response analyzer (Reichert 7CR Auto Tomometer) also provides Goldmann-correlated pressure measurement (IOPg), which is similar to an actual Goldmann measurement. Seeing the IOPcc and IOPg values simultaneously can provide clinicians with a better understanding of a patient's tonometry values, according to Dr. Gaddie, and can help optometrists assess glaucoma patients or suspects as progressing or stable. For example, in a progressing glaucoma patient IOPcc may be higher than IOPg, while in a stable glaucoma patient IOPcc and IOPg are often equal and low.
"The idea behind this instrument is that if the eye were enclosed by a tight, contour fitting shell, the pulsatile forces generated by IOP would act on the shell wall," Dr. Gaddie said. "It replaces part of that shell wall with a pressure sensor that enables measurement of pulsatile blood flow and IOP."
The dynamic contour tonometer has a contoured tip surface designed to match contour of cornea, and, as its name indicates, employs dynamic rather than static IOP measurement: the instrument continuously reads the diastolic and systolic values of the ocular pulse pressure curve.
The instrument uses an induction-based rebound principle. It is held 4mm to 8 mm from the cornea, with the patient in a sitting position. The device shoots toward the cornea a small, fine plastic probe with a round bulb 1 mm wide. IOP is determined by the speed with which the bulb hits the cornea and bounces back or rebounds.
Anesthetic isn't necessary because the disposable probe only touches the cornea for milliseconds. This instrument provides readings that correlate well with Goldmann tonometry, Dr. Gaddie said, and is useful when physical limitations preclude the use of Goldmann tonometry, such as patients who are wheelchair-bound, or uncooperative.
Looking at the future, a constant IOP monitor that could provide 24-hour and supine IOP monitoring still represents the "holy grail" of IOP monitoring. "Check back later this year on that one," Dr. Gaddie concluded.
I. Ben Gaddie, OD, FAAO
Dr. Gaddie is a member of the advisory boards of Allergan, Bausch + Lomb, Inspire Pharmaceuticals, Pfizer, and Zeiss Humphrey. He is a consultant for Allergan, Bausch + Lomb, Inspire Pharmaceuticals, ISTA, Ocusoft, and Optovue; and has received research support from Alcon, Allergan, Inspire Pharmaceuticals, Ocusoft, Tearlab, and Zeimer Ophthalmics.