Tonometry is a kind of eye examination where a part of the cornea is observed to measure the pressure of fluid inside the eye. Tonometry is also called intraocular pressure (IOP) measurement or even a glaucoma test because it is usually done to detect glaucoma and begin preventive treatment before it results to blindness. Glaucoma is the third leading cause of blindness globally. Often called the ‘sneak thief of sight’, this kind of visual defect does not have visible symptoms and can only be detected through ophthalmic eye examinations by the proper ophthalmic equipment. Caused by an increased pressure of the fluid in the eyes, glaucoma is an eye disease where optic nerves get damaged and result in permanent loss of vision. There are medications already available to lower intraocular pressure, but there is no cure for glaucoma as of yet. The early detection of this eye disease allows for careful preventive treatment to prevent actual loss of vision. Glaucoma may result from some systemic medications and even from other eye diseases like uveitis. Open angle glaucoma also runs in families. People who might have a high risk of this eye condition are recommended to have regular eye examination to be able to prevent or manage glaucoma in its early stage as soon as possible.
As mentioned above, one of the ways to test for glaucoma is through tonometry. The most common method is using an ophthalmic instrument called the applanation tonometer, which employs a probe to flatten one part of the cornea. Used with a slit lamp microscope, this method works by measuring how much force is needed to be exerted by the probe to flatten the cornea. Sometimes dyes are also applied onto the eye for the optometrist or ophthalmologist to observe the eye better. Anaesthetics may also be used. There is a small chance that the cornea might get scratched by the tonometer instrument probe if the applanation method is used. However, corneal abrasions heal in a few days, and the results from this type of tonometer equipment are quite accurate compared to the other methods.
Another method is the electronic indentation tonometry, which uses a pen-like ophthalmological tool as the instrument instead of a probe to touch the cornea. The results of the IOP measurement would then be shown in a small computer screen. Results from this method may vary from those taken by applanation. Electronic indentation tonometry is usually performed on young kids or people who could not reach the slit lamp microscope due to disability. Indentation or impression tonometry is seldom used except for emergencies where tonometry must be performed as soon as possible. This method uses a small plunger to measure the weight needed to flatten the cornea.
Dynamic contour tonometry (DCT) uses contour matching to measure IOP, taking into account factors that may affect the results of other methods like the thickness of the cornea, curvature, rigidity and elastic properties. Unlike the applanation method, DCT uses a non-variable force tonometer. Pneumatonometry uses an ophthalmic instrument with a pneumatic sensor to touch the cornea. IOP is measured here by calculating the balance between the force of the air coming from the pneumatic sensor and the pressure from inside the cornea. Rebound tonometry is a relatively new method that uses a probe that is light enough to trigger the most minimal corneal reflex, such as blinking. This probe is disposable and is carefully designed for the use of eye care professionals and nurses to make quite accurate measurements of IOP. Unlike the other methods of tonometry, this kind does not require the eyes to be anaesthetized.
Other methods of tonometry do not require any contact with the cornea at all. One of these methods is the transpalpebral tonometry, which uses a diaton tonometer ophthalmic instrument and is considered a relatively safe method for measuring IOP. Used mainly for routine eye examinations and even home-monitoring, the diaton tonometer measures IOP through the eyelid. This kind of tonometer is particularly helpful in the examination of children and those with corneal problems that prohibits the use of the other kinds of tonometry.
Another method is the non-contact or air-puff tonometry (this is different from pneumatonometry) which is like the applanation method, except that an air pulse is used instead of an actual probe. IOP is measured using this method by identifying the force of the air jet the moment it touches the cornea. The results for this kind of tonometry had not been considered to be as accurate as those from the other methods until recently. However, non-contact tonometry is particularly useful in taking IOP measurements more than once in the span of a day in order to observe IOP better. This type of ophthalmic equipment is frequently found in optometrist examining rooms.
The main reason for undergoing tonometry would be to monitor increase or decrease intraocular pressure that might mean a higher risk for glaucoma. Tonometry might be performed on a person already undergoing treatment for glaucoma in order to make sure that the medication approved by the optometrist. People who had already undergone corneal surgery might want to consider rebound tonometry or the non-contact methods. Many types of ophthalmic instruments are used for the field of tonometry. The accuracy of the equipment varies, as does the cost and the invasiveness of the procedure to the patient. The ophthalmologist needs to carefully review the types of equipment before making a selection for his examining room.
