Tonometry: Treatment, Effect & Risks

Tonometry is a diagnostic measurement procedure in ophthalmology (eye care). Special equipment is used to determine the intraocular pressure. An increase in this value may indicate the presence of glaucoma, or glaucoma.

What is tonometry?

Tonometry is a diagnostic measurement procedure in ophthalmology (eye care). Intraocular pressure is considered an important diagnostic feature of glaucoma in ophthalmology. Intraocular pressure is caused by the aqueous humor, which supplies the cornea with nutrients. The aqueous humor flows into the anterior chamber of the eye and from there enters the bloodstream. The inflow and outflow of aqueous humor are balanced in a healthy eye. If there is an imbalance, the intraocular pressure increases. In healthy adults, the value of intraocular pressure is between 10 and 21 mmHg (millimeter mercury column). However, these values fluctuate depending on the time of day, age and body position of the patient. To obtain truly meaningful values, intraocular pressure is measured at different times. The results are summarized in a daily profile. Tonometry is an important tool to detect glaucoma in time, as this disease develops insidiously and does not cause any pain in the early stages. The diagnostic procedure is also used to monitor the progress of glaucoma. The high pressure values damage the optic nerve and in the worst case lead to blindness of the patient. Losses in the visual field are among the typical symptoms of glaucoma, such as a gray spot shifting into the field of vision. Or a narrowing of the visual field from the outside to the inside. Diabetics and people with age-related macular degeneration are among the risk groups for this disease, as are people with severe nearsightedness and farsightedness. Ophthalmologists therefore advise biennial tonometry from the age of 40. If there is a family history of glaucoma, an annual checkup is recommended.

Function, effect, and goals

Several measurement methods are available for tonometry, but not all with reliable results. The most commonly used is the Goldmann applanation tonometer. The examination proceeds with the patient lying or sitting. Local anesthesia of the cornea is required for the examination, which is performed by means of eye drops. Then the cornea is carefully pressed with a small, cylindrical measuring device, the tonometer. The pressure thus exerted is measured in mmHg and gives the current intraocular pressure value. The more force the ophthalmologist needs to press in the tonometer, the higher the intraocular pressure. The advantage of the Goldmann tonometer is that it can be attached to the slit lamp, the ophthalmologist‘s examination microscope. Non-contact tonometry does not require corneal contact. Anesthetic drops are not necessary. Here, the cornea is depressed by means of an air pulse. The deformation of the cornea is then measured. However, this method is rarely used because the measured values are not precise enough. This is also true for impression tonometry, an older method in which a metal pin is used for measurement. Here, anesthesia is again required. A pin is then sunk into the cornea and the doctor measures how deeply the pin indents the cornea. A relative novelty in ophthalmology is Dynamic Contour Tonometry. This provides the ophthalmologist with a very precise measuring instrument. Similar to an ECG, it is possible to display pulse curves of the eye pressure triggered by the heartbeat. A pressure sensor in the tonometer head can measure intraocular pressure regardless of corneal thickness, thinness, curvature or straightness. This method is being used more and more frequently because of its accuracy. In addition to the existing examination options, several measurement methods are still in the trial stage. One of these is the pressure-sensitive contact lens. The patient is supposed to wear this for several hours so that the intraocular pressure with its fluctuations can be measured over longer periods of time. Doctors hope that this will be a great step forward, and patients will find it easier to make a diagnosis. After all, once glaucoma has developed, damage to the optic nerve that has already occurred is usually irreversible. The most important measure now is to lower the intraocular pressure. A whole range of eye drops are available for this purpose.Whether the pressure could be lowered sustainably must be checked regularly with the tonometer. If the drops show no or insufficient effect, glaucoma surgery may be necessary to improve aqueous humor outflow.

Risks, side effects, and hazards

The risks of tonometry are low, and complications are rare. Only Goldmann applanation tonometry involves a risk of injury, albeit a small one. Therefore, it is recommended for patients to look for an ophthalmologist with appropriate experience in this diagnostic procedure. Since the tonometer is placed directly on the cornea, germ transmission is also conceivable. However, meticulous disinfection should prevent this. Even though tonometry is the method of choice for the diagnosis of glaucoma, it is not part of the screening catalog. Therefore the costs are not taken over by the legal health insurance companies. The tonometry belongs to the IGeL achievements. The patient must expect to pay around 20 euros. It is different, if urgent suspicious facts, or an increased risk for cataract exist. Then the health insurance companies also cover the costs of tonometry. Once glaucoma has been diagnosed, the insurance companies will cover all further treatment costs. Applanation tonometry can and may only be performed by ophthalmologists, as anesthetic eye drops must be administered. With non-contact tonometry, on the other hand, this is not necessary. For this reason, optometrists are now increasingly offering this examination procedure. But here, too, the following applies: no costs are covered by the health insurance funds.