Ocular Tonometry

Tonometry is the procedure eye care experts perform to figure out the intraocular pressure (IOP), the fluid pressure inside the eye. It is an important test in the examination of patients at risk from glaucoma. Most tonometers are calibrated to determine pressure in millimeters of mercury (mmHg).

Ocular Tonometry Types and Techniques

All adults attending an eye unit must have their intraocular pressure (IOP) measured, unless there is a contraindication (e.g. injury or corneal ulcer). Lots of people with glaucoma have no symptoms and do not know they have the condition. All children who have actually had cataract surgery need to also have their IOP determined at every follow-up see, if possible. Discovering glaucoma early permits treatment to be provided which will maintain sight. Although elevated IOP is not the only sign of glaucoma, measuring it is basic and fast to do. It ought to therefore be done routinely on all adults attending eye care facilities. Applanation tonometry, explained in this article, is the preferred method (the ‘gold standard’) together with other types of tonometry used in ophthalmology.

Applanation tonometry

In applanation tonometry the intraocular pressure (IOP) is presumed from the force needed to flatten (applanate) a constant area of the cornea, for the Imbert-Fick law. The Maklakoff tonometer was an early example of this approach, while the Goldmann tonometer is the most commonly used version in present practice. [3] Due to the fact that the probe reaches the cornea, a topical anesthetic, such as proxymetacaine, is introduced on to the surface area of the eye in the form of an eye drop.

tonometry
tonometry (tuh-NAH-mih-tree). Measurement of intraocular pressure.

Goldmann tonometry

Goldmann tonometry is thought about to be the gold basic IOP test and is the most widely accepted technique. An unique disinfected prism is mounted on the tonometer head then placed against the cornea. The examiner then uses a cobalt blue filter to view two green semi circles. The force used to the tonometer head is then changed using a dial connected to a variable stress spring till the inner edges of the green semicircles in the viewfinder meet. When an area of 3.06 mm (0.120 in) has been flattened, the opposing forces of corneal rigidity and the tear film are roughly approximate and cancel each other out enabling the pressure in the eye to be figured out from the force used. Like all non-invasive approaches, it is inherently inaccurate and might have to be adjusted.

Perkins tonometer

The Perkins tonometer is a kind of portable applanation tonometer, which might work in children, anesthetised patients who have to lie flat, or patients not able to co-operate with a sitting slit light examination, that yields scientific outcomes comparable to the Goldmann.

Dynamic contour tonometry

Dynamic shape tonometry (DCT) uses the concept of contour matching rather of applanation. The pointer consists of a hollow the same shape as the cornea with a mini pressure sensor in its centre. In contrast to applanation tonometry it is designed to avoid warping the cornea during measurement and is therefore believed to be less influenced by corneal density and other biomechanical properties of the cornea than other techniques but since the suggestion shape is created for the shape of a normal cornea, it is more affected by corneal curvature.

The probe is placed on the pre-corneal tear movie on the main cornea and the integrated piezoresistive pressure sensor instantly begins to acquire data, measuring IOP 100 times per second. The tonometer tip rests on the cornea with a constant appositional force of one gram. When the sensor undergoes a change in pressure, the electrical resistance is altered and the tonometer’s computer computes a change in pressure according to the change in resistance. A complete measurement cycle needs about 8 seconds of contact time. The device likewise measures the variation in pressure that occurs with the cardiac cycle.

Electronic indentation tonometry

Electronic imprint tonometers are customized Mackay-Marg tonometers that use a complimentary drifting transducer to find the transmitted pressure. The transducer is surrounded by an outer ring that flattens the nearby cornea minimizing its influence on measurement. Since the device touches the cornea, topical anesthetic eye drops are used to numb the eye but just like non-contact tonometry, these devices are typically used in children and non-cooperative patients since of their mobility and ease of use. Portable electronic tonometers also play a significant function in veterinary tonometry

Rebound tonometry

Rebound tonometers figure out intraocular pressure by bouncing a small plastic tipped metal probe versus the cornea. The device uses an induction coil to magnetise the probe and fire it versus the cornea. As the probe bounces versus the cornea and back into the device, it produces an induction existing from which the intraocular pressure is determined. The device is simple and simple to use and self-use versions are offered. It is portable, does not require making use of eye drops and is particularly ideal for children and non-cooperative patients.

Pneumatonometry

A pneumatonometer uses a pneumatic sensing unit (including a piston drifting on an air bearing). Filtered air is pumped into the piston and travels through a little (5 mm (0.20 in) diameter) fenestrated membrane at one end. This membrane is put against the cornea. The balance in between the flow of air from the maker and the resistance to flow from the cornea affect the motion of the piston and this movement is used to determine the intra-ocular pressure.

Impression tonometry

Impression tonometry (likewise known as indentation tonometry) measures the depth of corneal indentation made by a small plunger carrying a recognized weight. The greater the intraocular pressure, the more difficult it is to press against and indent the cornea. For extremely high levels of IOP, extra weights can be contributed to make the plunger push harder. The movement of the plunger is measured using an adjusted scale. The Schiøtz tonometer is the most typical device to use this concept.

Non-corneal and transpalpebral tonometry

Transpalpebral tonometry refers to techniques of measuring intraocular pressure through the eyelid. The Diaton non-corneal tonometer calculates pressure by determining the action of a free-falling rod, as it rebounds versus the tarsal plate of the eyelid through the sclera. The patient is positioned so that the tip of the device and cover are overlying sclera. Non-corneal and transpalpebral tonometry does not include contact with the cornea and does not need topical anesthetic during regular use. Transpalpebral tonometry might work for measuring postsurgery IOP after myopic LASIK ablation because this strategy is not affected by the treatment. The Diaton tonometer still requires additional assessment and is not a substitute or alternative for more recognized approaches. The Diaton tonometer has a large margin of error compared to commonly used tonometers (e.g., GAT) in most patients (including those with ocular hypertension, glaucoma, and glaucoma tube shunts).

Non-contact tonometry

Non-contact tonometry (or air-puff tonometry) is different from pneumatonometry and was invented by Bernard Grolman of Reichert, Inc (previously American Optical). It uses a quick air pulse to applanate (flatten) the cornea. Corneal applanation is found by means of an electro-optical system. Intraocular pressure is estimated by discovering the force of the air jet at the instance of applanation. Historically, non-contact tonometers were ruled out to be an accurate way to determine IOP but instead a quick and simple way to screen for high IOP. However, modern-day non-contact tonometers have been shown to associate well with Goldmann tonometry measurements and are particularly helpful for determining IOP in children and other non-compliant patient groups.

Ocular response analyzer

The ocular response analyser (ORA) is a non-contact (air puff) tonometer that does not require topical anaesthesia and provides additional information on the biomechanical properties of the cornea. It uses an air pulse to deform the cornea into a minor concavity. The difference between the pressures at which the cornea flattens inward and outside is measured by the device and described corneal hysteresis (CH). The machine uses this worth to correct for the effects of the cornea on measurement.

Palpation

Palpation (also known as digital tonometry) is the approach of estimating intraocular pressure by carefully pushing the forefinger against the cornea of a closed eye. This method is infamously unreliable.

Affecting elements

Central corneal thickness (CCT): The thickness of the cornea impacts most non-invasive approaches by varying resistance to the tonometer probe. A thick cornea generates a greater probability of an IOP being overestimated (and a thin cornea of an IOP being ignored), however the extent of measurement mistake in private patients can not be determined from the CCT alone. The Ocular Response Analyzer and Pascal DCT Tonometers are less impacted by CCT than the Goldmann tonometer. On the other hand, non-contact and rebound tonometers are more impacted. Corneal density differs among individuals in addition to with age and race. It is reduced in certain disease and following LASIK surgery.

Reyus Mammadli (Eyexan Team Leader) / author of the article
Bachelor in biomedical and electrical apparatus and systems. For more than 20 years he has been studying methods to improve health using affordable and safe methods. Collaborates with eye care charity organization of the CCP. Specialization is a vision correction by laser surgery, including LASIK.
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