ATul Jain, M.D.
Contributing Author
Tonometry, the measurement of intraocular pressure (IOP), is a routine part of the ophthalmic examination and often regarded as one of the ophthalmic “vital signs” in addition to visual acuity. Technology has come a long way since 1863 when Albrecht von Grafe devised the first tonometer. But without belaboring the historical aspect, is it most convenient to describe modern tonometers in one of two ways: non-contact (in which direct contact is not made with the globe) and contact (direct contact with the globe).
NON-CONTACT TONOMETERS
The benefits of non-contact tonometry are that there is virtually no risk of infection and that IOP can be measured several times a day (as no anesthetic is used in addition to any contact). The downside is non-contact tonometry is more variable and less reliable than contact tonometry.
There are three major non-contact tonometers: 1) air-puff; 2) transpalpebral (Diaton); and 3) Ocular Response Analyzer (ORA). Most are familiar with the myriad of air-puff tonometers that are generally used for shopping mall screenings in which a puff of air applanates the cornea and is measured by an electro-optical system which estimates an IOP.
The diaton tonometer (BiCOM, about $2500) is something new that measures IOP through the eyelid, thus, no anesthetic or sterilization measures are necessary. It is placed on the upper eyelid, corresponding to a location on the globe a few millimeters posterior to the superior limbus, when the eye is open. A small metal rod then indents the eye and several measurements are recorded (electronically) and averaged. The results are in mm Hg. It does not require the removal of contact lenses. The claim to fame is that this device can be used to safely measure IOP at home (once users are properly trained). It is portable and similar in size to a Tono-Pen.
The ORA (Reichart, about $28,500) is an advanced air puff tonometer which shoots a puff of air onto the cornea and takes two measurements, one as the cornea is indented and one as the cornea rebounds back, the difference in these two measurements reveals information about the biomechanical properties of the cornea, or corneal hysteresis. Because the ORA takes corneal biomechanics and ridigity into account in determining the IOP, the resultant value is not significantly affected by refractive procedures or corneal thickness, but further studies to verify this are required.
CONTACT TONOMETERS
The most commonly used contact applanation tonometer is the Goldmann which is affixed to most slit-lamp systems, and the portable Tono-Pen. However, they have their limitations, especially in post-refractive surgery patients, keratoconus, edematous corneas, and several others conditions in which the cornea is not smooth and regular. There are several new contact tonometers that seek to rival Goldmann tonometry as the new “gold standard,” some of the popular ones include: 1) pneumatonometry; 2) rebound tonometry; and 3) dynamic contour tonometry (DCT).
Pneumatonometers have been around for some time now and measure IOP using a floating pneumatic sensor (5 mm in diameter) that touches the surface of the anesthetized cornea with the exact amount of applanation force (produced by filtered air) required to take the measurement. Several measurements are taken each second. This method of IOP measurement is less affected by corneal thickness and irregularities as the sensor “molds” to the shape of the cornea.
Rebound tonometry (iCare, about $2000) is a relatively new method of IOP measurement in which a very small disposable probe indents a NON-anesthesized cornea and measures the pressure based on the force required for indentation of a very small area. This is a combination between non-contact and contact tonometry, in which there is corneal contact, but it is with a disposable small probe and anesthesia is not required. The unit is portable and similar in size to a conventional Tono-Pen. Only a single measurement is taken per indentation.
The Pascal Dynamic Contour Tonometer (Ziemer Ophthalmics, about $6500) seeks to compensate for differences in central corneal thickness and corneal rigidity to provide a more accurate IOP measurement. While mounted on a slit lamp and similar in appearance to a Goldmann, DCT is not a force tonometer, it uses contour matching in which a miniature pressure sensor embedded within a tonometer tip is contour-matched to the shape of the cornea. The 10.5 mm tonometer tip rests on the cornea with a constant appositional force of one gram (herein lies the main difference from other tonometers in which the probe force is variable). Once a portion of the central cornea has taken up the shape of the tip, the pressure sensor begins to acquire data, measuring IOP about 100 times per second. A complete measurement cycle requires about 8 seconds of contact time. IOP measurements using DCT have been shown to be unchanged by refractive surgeries.
In conclusion, there are a lot of tonometers available each with significant differences, however, only the Goldmann and Pascal DCT remain affixed to a standard diagnostic slit lamp. Air puff and pneumatonometers remain stand alone devices, while rebound and diaton tonometers offer portability and non or only minimal contact.