Continuous IOP Monitoring

Applanation tonometry determines IOP based on the Imbert-Fick principle, which states that the pressure inside a dry, thin-walled sphere equals the counterpressure necessary to flatten its surface divided by the area of flattening (P = F/A, where P = pressure, F = force, and A = area). This principle works on the condition that the membrane of the sphere is endlessly thin and without stiffness of its own, with no other forces affecting it. The cornea, however, is not infinitely thin or perfectly elastic, and it does not have a dry surface. Although Goldmann applanation tonometry (GAT) is still considered the gold standard for IOP measurement, this method is not without limitations.

Today, it is understood that a source of error with GAT stems from the interindividual differences in the biomechanical properties of the cornea. These properties are influenced by central corneal thickness (CCT), corneal curvature, and the material properties of the cornea, including the amount of collagen and structure of the collagen fibers, other parts of the extracellular matrix, and the hydration of the cornea.

When Goldmann invented his tonometer, he estimated an average CCT of 0.5 mm (or around 500 µm), as it was not possible then to measure in micrometers.¹ The value used in GAT today is an interpolation based on that estimation. In reality, CCT measurements vary from eye to eye, with an average of 537 µm and a range of 427 to 620 µm.²

In an effort to increase the specificity of these calculations, other methods and devices have since been developed for IOP measurement.

POINTWISE IOP MEASUREMENTS VERSUS DIURNAL CURVES

Most spikes in IOP occur outside of office hours.³ In 20% of cases of normal-tension glaucoma, IOP measures more than 21 mm Hg at night.⁴ One possible method of imitating nocturnal IOP values in the clinic is to compare measurements with the patient in a supine position and a seated position. This can help to predict fluctuations throughout a 24-hour period.^5,6 With this approach in my clinic, we have seen a difference of 3 or 4 mm Hg between IOP measured in a supine versus a seated position.

IOP FLUCTUATIONS

Fluctuations in IOP include short-term fluctuations over 24 hours and long-term fluctuations, sometimes referred to as variability. Discussion is ongoing about the importance of IOP fluctuation and whether it is an independent risk factor in patients with glaucoma; some studies show that it is, whereas others show no correlation.^7-9

CONTINUOUS IOP-MEASURING DEVICES

The history of implantable devices for continuous IOP measurement dates back more than 30 years, as the first IOL with a biomedical sensor for measuring IOP was invented in 1992.¹⁰ Currently, both noninvasive and invasive devices are available for continuous IOP monitoring. Potential issues to consider with such devices are the delivery of energy (internal with batteries or solar cells or external with wire or electromagnetic waves) and the transmission of data (wire or electromagnetic waves). Two types of intraocular technologies in this category are sensors and microfluids.

Sensors

A sensor is a transducer that measures a physical quantity and converts it into a signal that can be read by an observer or an electronic instrument. Intraocular sensors can be placed on the cornea, on the sclera, in the anterior chamber, or in the vitreous. It is important that the sensor is safe, accurate, and stable over a long period of time. The frequency must be compatible with eye pressure dynamics, or between 0 and 30 Hz.¹¹

Microfluids

In microfluidic systems, intraocular fluids are collected in microshaped channels, which can be inserted in the eye as an artificial drainage system. These work as manometric devices and thus measure pressure not with external power but using the IOP itself. The combination of microfluids and microelectromechanical systems (MEMS) will lead to a new generation of small implantable sensors. These sensors could be not only of diagnostic value but also of therapeutic value.^11,12

NONINVASIVE DEVICES

The first investigation of contact lens tonometry was conducted in 1998,¹³ and a contact lens for IOP monitoring was developed in 2003.¹⁴ Dynamic contour tonometry was the basis for a prototype of an IOP-sensitive contact lens introduced in 2009.¹⁵ This contact lens was reused in 2010 and showed concordance with dynamic contour tonometry and GAT handheld devices.¹⁶

A newer version of this approach is an implantable sensor that uses a miniaturization of the MEMS technology.¹¹ These include systems in which a sensor is embedded in a contact lens, such as the Triggerfish (Sensimed), a soft contact lens system that transfers data to an external device. The primary issue with this type of technology is that it does not measure eye pressure, but instead measures the ocular dimension changes in electrical units. It therefore does not provide a direct measurement of IOP.

A newer promising technology being developed by Sensimed is the Pressure-Measuring Contact Lens, or PMCL, which features a centrally embedded MEMS pressure sensor.¹⁷

INVASIVE DEVICES

The Eyemate-IO (Implandata Ophthalmic Products; not available in the United States) is a telemetric microsensor designed for implantation in the ciliary sulcus during cataract surgery. Powered by an external transmitter, the Eyemate-IO transmits IOP readings through a handheld device.¹⁸ Ten-year results showed that IOP measurements produced by the Eyemate-IO were reliable.¹⁹ Another study looking at IOP measurements with the Eyemate-IO before and after implantation of the Preserflo Microshunt (Santen; not available in the United States) showed reliability as well.²⁰

The Eyemate-SC (Implandata Ophthalmic Products; not available in the United States) is a telemetric microsensor designed for implantation in the suprachoroidal space after nonpenetrating glaucoma surgery. A recent study showed that IOP measurements recorded with the external handheld device had good concordance with GAT. In addition, the device was reportedly easy to implant and well tolerated by patients.²¹

HOME TONOMETRY

Home-based applanation tonometry is a viable option, but it may not be effective in all cases. Whereas some patients like to be in charge of their care, others may perceive this as a burden. Part of the challenge, therefore, is identifying the right patient for use of this technology.

The best type of device for home tonometry is a rebound tonometer such as the iCare Home (Icare USA). Most values produced by devices like this are within 5 mm Hg of GAT measurements, although they are consistently lower than with GAT.²²

CONCLUSION

GAT is the gold standard for IOP measurement, but this method provides only snapshots of a patient’s disease. A range of continuous IOP-monitoring devices are available or in development to help provide a more complete diagnostic picture. Implantable devices such as the Eyemate system may provide precise IOP measurements, but they are more invasive. Some promising noninvasive options exist in the form of smart contact lenses, but questions remain about whether it is possible to effectively determine IOP through a contact lens. Home tonometry has potential, but this approach requires effort from both patients and physicians. Continued exploration and investigation will help elucidate the use of these tools in glaucoma care.