| Reichert’s Tono-Pen can be used for
screening patients who can’t put their
chins on the slit lamp.
ECPs have many different ways of measuring IOP and these methods have varying degrees of accuracy.
The eye is an enclosed system with a drainage structure similar to that of a kitchen sink. Aqueous humor is secreted by the ciliary body, which is like a sink faucet. The aqueous travels from the posterior chamber into the anterior chamber and exits the eye through the trabecular meshwork, which is like a sink drain. If there is an inconsistency between the amount of fluid being secreted and the amount exiting the eye, there is an increase in intraocular pressure (IOP). This pressure builds up and causes tension on the walls of the eye and the optic nerve. If left untreated, increased IOP will ultimately result in loss of vision.
IOP can be determined by measuring the flexibility of the eye. If the cornea is more flexible, the eye will have an IOP that is higher than measured. A cornea that is less flexible will have an actual IOP that is lower than measured. The two most common methods of measuring IOP are contact and non-contact.
Applanation Tonometry Applanation tonometry is the process of flattening the cornea, and the Goldmann tonometer is the most widely accepted method of measuring IOP. With this method, a special disinfected prism is gently placed against the cornea after topical anesthetic is instilled. The amount of force that is applied to the tonometer prism head is measured when a tension spring within the device is adjusted. The corneal thickness can influence the accuracy of the IOP reading. For patients with thinner corneas, the reading may be underestimated, and for patients with thicker corneas, the readings may be overestimated.
Tono-Pen: Reichert’s Tono-Pen‘ is a handheld device (available in two models””AVIA‘ and XL) using the applanation method to check IOP. The device is tapped gently on the cornea after a drop of topical anesthetic to get the IOP measurement. A new sterile cover is used for each patient, so the risk of transferring germs from patient to patient is low. The Tono-Pen is often used as a screening method and also for children or patients who can’t put their chins on the slit lamp.
Pneumotonometer The pneumotonometer uses a pneumatic sensor which when placed against the cornea, pumps air through a piston. The air pumped into the piston travels through a small membrane placed against the cornea, and the balance between the airflow and corneal resistance is calculated. Topical anesthetic is needed to ensure patient comfort.
Tonometry This type of tonometer uses the principle of contour matching. The instrument is mounted on a slit lamp and the disposable hollow tip contains a miniature pressure sensor. The probe tip is placed on the central corneal pre-corneal tear film and the pressure sensor located tip and slightly forces the cornea into its curvature. The pressure measurement is obtained when equilibrium is achieved on both sides of the cornea. The pressure sensor in the instrument measures the IOP on the external surface of the cornea. Because the tip of the probe is designed to avoid changing the shape of the cornea, the corneal thickness does not influence the accuracy of the reading. The device requires approximately eight seconds of contact with the cornea, during which it measures the IOP 100 times per second.
Indentation Tonometry The ShiØtz tonometer is the most commonly used device for this method of tonometry. It measures the depth of corneal indentation made by a small plunger carrying a known weight. The reading is obtained by placing the weight in the instrument and measuring how much the plunger sinks into the cornea. High IOP will require a heavier weight to indent the cornea. After the measurement is obtained, a conversion table converts the reading into mmHg. Myopic patients with thicker corneas may have inaccurate false low reading because of scleral rigidity.
Non-contact tonometry (or air-puff tonometer) 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 provides the IOP measurement. Non-contact tonometry is not the most accurate method of estimating intraocular pressure as it can overestimate low pressures and underestimate high pressures.
Ocular Response Analyzer: The ocular response analyzer uses bi-directional corneal applanation to measure the intraocular pressure. This instrument measures the difference between the pressures when the cornea is pushed inward and then outward during applanation. This difference is known as corneal hysteresis. Based on this information, the device is able to compensate for corneal thickness and provide an accurate intraocular pressure. The ocular response analyzer minimizes patient germ cross-contamination as it does not require topical anesthesia.
Non-Corneal and Transpalbebral Tonometry This tonometer measures the intraocular pressure through the eyelid. A free-falling rod rebounds against the outside of the upper lid at the tarsus and over the sclera. Because the measurement is taken by rebounding against the sclera and not the cornea, central corneal thickness does not influence the accuracy of the pressure reading.
Rebound Tonometry: The most recent addition to the tonometry family, rebound tonometry measures the eye pressure by bouncing a solenoid coil against the cornea. The coil propels a small, magnetized probe that’s monitored by a specialized sensor. The bouncing probe creates a voltage current within the system, which is calculated, and the IOP is determined. The force applied by the probe is so minimal in duration that it does not elicit the blink reflex. As a result, no anesthetic drops are needed. It is particularly useful with children and non-cooperative patients.
Technique for Measuring IOP
The slit-lamp mounted Goldmann tonometer is the most common way to measure IOP. Prior to measurement, the examiner should explain the procedure to the patient, and disinfect the tonometer prism. The prism can be cleaned with 70% isopropyl alcohol, 3% hydrogen peroxide, and allowed to completely dry prior to measurement. A prism that is not completely dry may cause a caustic burn to the cornea. Here are the specific steps to follow.
•Instill a drop of topical anesthetic into the patient’s eye along with fluorescein. Care should be taken to not instill too much fluorescein since this may cause an inaccurate measurement.
•Adjust the slit lamp illumination light to the widest beam possible and direct the beam to a 45Â° to 60Â° angle so that it completely illuminates the prism tip.
•Advise the patient to look straight ahead, keep still, and breathe normally.
•Bring the tonometer prism tip slowly forward toward the center of the cornea of the patient’s eye, stopping when it gently rest on the cornea.
•Upon touching the cornea, two semicircular mires will appear when viewed through the slit lamp oculars.
•Turn the measuring drum wheel, while observing
the mires, until the inner edges of the semicircles
•Note the measurement on the dial and remove prism from the cornea.
•Wipe the prism dry and repeat the procedure for the other eye. OO
Janet Hunter, president of Eye Source, LLC, specializes in ophthalmic technician training.