Current IOP measurement techniques have limited accuracy and do not allow for 24-hour monitoring. A system based on a small, implantable passive transducer and external probe is being developed to address these limitations.
Miami-Progress continues to be made in a project to develop a continuous, non-contact IOP monitor, said Ronald E. Frenkel, MD.
The system is based on the implantation of a small, passive transducer that consists of a parallel plate capacitor and a discrete inductor connected in series to form an L-C circuit. In response to a change in IOP, displacement of the deforming plates changes the circuit capacitance and, in turn, the resonance frequency of the circuit. Non-contact pressure readings are made by measuring the resonance frequency of the coil using radiofrequency energy from an external probe.
So far, a prototype device measuring 3.3 mm in diameter and 0.8 mm in height has been fabricated using silicon micromachining techniques. In initial performance testing, it has been shown to measure changes in pressure with an accuracy of ±2 mm Hg.
The research represents a collaborative effort among ophthalmology, bioengineering, and biophysics, and it is supported by a grant from the National Eye Institute.
Discussing the background for developing the continuous IOP monitor, Dr. Frenkel reviewed the importance of obtaining round-the-clock measurements and the limitations of existing IOP measurement techniques.
"Numerous studies have shown IOP fluctuation is harmful, and based on its review of existing evidence, a consensus panel at the Association for Research in Vision and Ophthalmology concluded it is helpful to measure IOP fluctuation both in the short term and the long term," said Dr. Frenkel.
Because glaucoma progression continues to occur despite the use of IOP-lowering therapy, clinicians are left to wonder whether the worsening is due to medication non-adherence, undetected IOP spikes, or factors unrelated to IOP.
"However, IOP is the only known treatable factor, and so clinicians assume the IOP has been too high and intervene to lower it further. This approach essentially determines the adequacy of therapy in a retrospective fashion after irreversible damage has occurred. Until we have a more accurate method of measuring circadian IOP, this problem is likely to endure," Dr. Frenkel said.
Recognition that there is circadian variation in IOP as well as variability in diurnal curves measured on separate occasions further underlines the need for continuous IOP monitoring techniques, he said, adding that measuring 24-hour IOP using pneumotonometry is a cumbersome process with multiple drawbacks.
"Nocturnal IOP varies during different stages of sleep, differs depending on whether it is measured with the patient supine or sitting, and may decrease substantially after awakening. These factors confound the accuracy of IOP measurement with pneumotonometry since it only allows a limited number of readings and requires that patients be disturbed during sleep," said Dr. Frenkel.
Moreover, current techniques for measuring IOP suffer from many limitations relating to both insufficient accuracy and inability to meet the need for continuous measurements. The drawbacks of applanation tonometry are well recognized, and although various correction algorithms have been developed to overcome the effects of corneal thickness on IOP, they have not been validated.
New technologies for measuring IOP (Pascal Dynamic Contour Tonometer, Swiss Microtechnology; Ocular Response Analyzer, Reichert; Icare, Tiolat Oy) have been developed to avoid confounding related to corneal thickness and elasticity, Dr. Frenkel said. Accumulating research, however, reveals problems with the accuracy of these methods as well and also points to non-cornea factors that affect IOP, he added.