Here’s what you need to know about intraocular pressure and the instruments that are used to measure it.
To better understand intraocular pressure (IOP), picture the eye as a fluid-filled balloon or firm grape that is inflated by pressure. Pressure that is too high and left untreated can damage the delicate optic nerve, permanently affecting vision.
Aqueous humor is a fluid that is secreted by the ciliary body. It is in continuous circulation until it drains out of the trabecular meshwork. High eye pressure can be caused when too much fluid is produced by the ciliary body or if the aqueous drains too slowly from the trabecular meshwork. Eye trauma, certain medications and other eye conditions can also cause high eye pressure and glaucoma.
Fortunately, there are accurate methods to assess eye pressure by measuring the flexibility of the eye. In a sense, we measure how hard it is to poke the grape or balloon. With lower pressure, it is easier to poke, and with a high amount of pressure, it will be more difficult to poke. The “poking” is done with a variety of delicate contact and non-contact eye instruments.
Applanation tonometry is the process of flattening the cornea, and the Goldmann tonometer has long been considered the gold standard of tonometry. With this method, a topical anesthetic and fluorescein dye are instilled in the eye and a disinfected prism tip is gently placed against the cornea while the patient is seated at the slit lamp. The amount of force applied to the prism is measured when a tension spring within the device is adjusted.
The Tono-Pen AVIA and Tono-Pen XL from Reichert Technologies are used to measure IOP. These handheld devices are tapped gently on the anesthetized cornea, and the IOP value is displayed on the screen. A disposable tip cover, Ocu-Film, is used for each patient to reduce the risk of contamination and eliminate the need for instrument sterilization. Tono-Pen can be used in any position and is especially useful with children and patients who have difficulty getting in front of the slit lamp.
The Model 30 Pneumotonometer, also from Reichert, provides fast and accurate tonometry and optional tonography functions. The anesthetized cornea is gently touched by a probe tip that floats on an air bearing while a regulated flow of air applies force to the tip. Increasing pressure is continually applied to the cornea until the force being applied is equal to the pressure in the front of the eye. When these forces are in balance, a pneumatic sensor records the IOP.
Dynamic Contour Tonometry (DCT) measures true direct IOP highly independent of corneal properties. The PASCAL DCT from the Ziemer Group is mounted on a slit lamp. The sensor tip covered by a sterile single-use sensor cap contains a miniature pressure sensor and is placed on the central cornea. The concave sensor tip removes surface tension from the cornea and eliminates corneal influences on IOP measurement. PASCAL is ideally suited for measurements on eyes with scars or those that underwent surgical interventions, such as LASIK and PRK. Due to the high sampling rate, PASCAL measures precisely the ocular pulse amplitude (OPA).
The latest addition to the tonometry family is rebound tonometry. With rebound technology, a probe momentarily touches the central cornea and then rebounds at a certain rate. Icare USA has two handheld devices for eyecare professionals, the ic100 and the TA01l. There are two coils within the device, one for moving the probe toward the eye and one for measuring the probe speed. IOP is based on an algorithm at the point of contact and other motion parameters of the probe. A new probe is used for each patient, and no anesthetic drops are needed.
|The PASCAL DCT from the Ziemer Group
is ideally suited for measurements on eyes
with scars or those that underwent surgical interventions, such as LASIK and PRK.
|The ic100 from Icare USA utilizes
rebound technology, in which a probe
momentarily touches the central cornea and
then rebounds at a certain rate to determine IOP.
|The Diaton Tonometer from BiCOM, Inc.
is helpful for patients with corneal
abnormalities that would be difficult
to measure with contact tonometry.
|The non-contact Ocular Response Analyzer G3 from Reichert Technologies minimizes cross-contamination and does not require topical anesthesia or sterilization between patients.|
Non-contact tonometry (or air-puff tonometry) uses a pulse of air to applanate (flatten) the cornea while at the same time an infrared light beam is reflected by the flattened surface. The reflected light is compared to the amount of time it takes for the air puff to flatten the cornea and provide the IOP measurement. Non-contact tonometry is not the most accurate method of measuring IOP but has the advantage of very low risk of infection transmission since there is no contact with the patient.
The Ocular Response Analyzer G3 from Reichert uses dynamic bi-directional applanation to simultaneously measure the IOP and corneal biomechanical properties. This unique and patented method provides the unique corneal hysteresis measurement, which is a tissue property that is independently associated with glaucoma risk. In addition, this information is used to provide an IOP measurement that is compensated for corneal biomechanical properties (IOPcc), which has been shown to be more accurate than other methods of tonometry. The non-contact Ocular Response Analyzer G3 minimizes cross-contamination and does not require topical anesthesia or sterilization between patients.
Transpalpebral tonometry measures IOP through the eyelid. The Diaton Tonometer from BiCOM is a handheld device that uses a free-falling rod that rebounds against the outside of the upper eyelid at the tarsus and over the sclera. Corneal thickness does not influence the reading because it is measured against the sclera and not the cornea. It is effective for patients with corneal abnormalities that would be difficult to measure with contact tonometry.
There are some instances where we should not check an IOP measurement. If the patient has a red eye, foreign body or eye trauma, consult with the provider before any tonometry. OO
Kim Pickett, COMT, is a certified ophthalmic medical technologist and ophthalmic writer in Minneapolis, MN.
888.422.7313 | Icare-USA.com
716.686.4500 | Reichert.com