In ophthalmology, tonometry is the procedure eye care professionals perform to determine the intraocular pressure (IOP), the fluid pressure inside the eye. It is an important test in the evaluation of ocular conditions such as glaucoma as well as conditions such as phthisis bulbi, and iritis. Most tonometers are calibrated to measure pressure in mmHg.
In Applanation tonometry, a special calibrated disinfected probe attached to a slit lamp biomicroscope is used to flatten the central cornea a fixed amount. Because the probe makes contact with the cornea, a topical anesthetic, such as oxybuprocaine, tetracaine, alcaine, proxymetacaine or proparacaine is introduced onto the surface of the eye in the form of one or a few eye drops. A yellow fluorescein dye is used in conjunction with a cobalt blue filter to aid the examiner in determining the IOP.
Dynamic contour tonometry (DCT) is a novel measuring technique using the principle of contour matching instead of applanation to eliminate the systematic errors inherent in previous tonometers. These factors include the influence of corneal thickness, rigidity, curvature, and elastic properties. It is not influenced by mechanical changes, such as those seen in refractive surgery that would otherwise cause error in applanation tonometers. The PASCAL Dynamic Contour Tonometer (Ziemer Ophthalmics) is a relatively new device that uses DCT to measure IOP. Although this device is similar in appearance to a Goldmann, the PASCAL it is unlike Goldmann applanation in that it is not a variable force tonometer.
PASCAL uses a miniature pressure sensor embedded within a tonometer tip contour-matched to the shape of the cornea. The tonometer tip rests on the cornea with a constant appositional force of one gram. This is an important difference from all forms of applanation tonometry in which the probe force is variable. When the sensor is subjected to a change in pressure, the electrical resistance is altered and the PASCAL's computer calculates a change in pressure in concordance with the change in resistance.
The contour matched tip has a Concave surface of radius 10.5 mm, which approximates the cornea’s shape when the pressures on both sides of it are equal. This is the key to the PASCAL’s ability to neutralize the effect of intra-individual variation in corneal properties. Once a portion of the central cornea has taken up the shape of the tip, the integrated piezoresistive pressure sensor begins to acquire data, measuring IOP 100 times per second. A complete measurement cycle requires about 8 seconds of contact time. During the measurement cycle, audio feedback is generated, which helps the clinician insure proper contact with the cornea. Literature references:
Diaton tonometer measures intraocular pressure through the Eyelid. It is regarded as a simple and safe method of ophthalmotonometry. Transpalpebral tonometry requires no contact with the cornea, therefore sterilization of the device and topical anesthetic drops are not required and there is very little risk of infection. Compared with the current standard of Goldmann applanation tonometry and Tono-Pen, recent data suggests that transpalpebral tonometry via the Diaton tonometer is clinically useful device for measuring IOP in routine eye exams. Although requiring practice of the technique , transpalpebral tonometery has been described as sufficient for clinical use and it may have applications in the home-monitoring of intraocular pressures. Use of transpalpebral tonometry may be indicated in those for whom Goldmann tonometry is not indicated, such as in children, those with corneal pathology, or those who have had corneal surgery.
Thus, corneal thickness is an important factor of IOP evaluation and monitoring, it requires the necessity of including corneal pachymetry in the program of examination the patients with suspicion of glaucoma and hypertension, especially after various keratorefractive surgeries while using the traditional corneal methods of ophthalmotonometry. At the same time clinical application of trans-palpebral and trans-scleral Diaton tonometer makes it possible to evaluate IOP using only one device, the procedure being efficient, economical, simple in performance and requiring no additional instrumental examination.In practice the device appears to be difficult to use in pediatric cases. Positioning the patient is key and can at times be difficult. When the eye lid is lifted off the globe, false low pressures are obtained. More studies are needed to understand the true utility of this device.
The images to the right and below show painlessness of the Diaton tonometry procedure and its proper positioning on the Eyelid, where the edge of the lid coincides with the edge of the limbus. Since the screening occurs above the cornea, contact lenses are not being taken out.
Rebound tonometry (such as iCare Tonometer) is a new method for measuring intra-ocular pressure. Unique and carefully studied and tested measurement technique which requires no local anaesthesia. An ultralight, sensitive probe touches the eye for a moment, and does not even always cause, for example, the corneal reflex (blinking). The disposable probe is also safe for the eye in microbiological terms. The accurate measurement device is primarily intended for professional use by general practitioners, opticians, nurses.
It is the only portable device of its kind that does not require anaesthesia. Compact and easy to use, it is suited to the investigation of patients such as children. Made of durable plastic and anodized aluminium, the device is light and small. Comfort and ease of use in different positions have been taken into account. The device employs an electromagnetic field, and the disposable probe is easy to change. It is also sturdy and the possible movement of the patient during investigation will not affect it.