Tonometry – Treatment, Effects & Risks

High intraocular pressure damages the optic nerve and can blind the patient. Before they notice the first symptoms, many patients with glaucoma have been suffering from elevated pressure levels for several years. However, tonometry is not only a measure for early detection, but is also used together with funduscopy in the therapy control of glaucoma.

Another area of application is drug therapies, which have been proven to have effects on intraocular pressure. An example of a therapy of this kind is cortisone therapy. Especially in the case of long-term treatments with cortisone, tonometry clarifies any pressure increases that may have arisen.

Basically, increased intraocular pressure can be associated with drainage disorders of the aqueous humor. Pathological changes in the trabecular meshwork, for example, can make it difficult to remove the aqueous humor. For this reason, tonometry also plays a diagnostically important role in connection with all damage to the trabecular meshwork. The same applies to diseases with increased aqueous humor production.

Methods and procedures

The best-known application methods of tonometry are applanation tonometry, dynamic contour tonometry and non-contact tonometry.

Applanation tonometry

Applanation tonometry is the most accurate measurement method and is routinely used. The Imbert-Fick principle forms the basis of the process. According to the principle, the pressure in a sphere is proportional to the force required to flatten the surface of the sphere. Applanation tonometry uses this principle to calculate the internal pressure of the eye.

Dynamic contour tonometry

Dynamic contour tonometry is also based on the applanation principle, but uses a modified tonometer prism with a pressure sensor and, unlike applanation tonometry, is relatively independent of the central corneal thickness.

Non-Contact Tonometry

Methods such as non-contact tonometry (NCT) do not use a measuring head. Instead, the applanation of the central cornea takes place with pulses of air. Other, although less routine, procedures include rebound or induction tonometry and transpalpebral tonometry.

Conduct of the Investigation

Applanation tonometry is the most common method of tonometry and originated from the Austrian-Swiss ophthalmologist Hans Goldmann. A small measuring body is attached to a slit lamp, the flat measuring head of which measures 3.06 millimeters. The examiner uses the measuring head to flatten the patient’s cornea. The force that the examiner applies for flattening is determined. The force applied generates a spring scale that hangs on a measuring drum. The pressure applied can be read directly from the measuring drum.

To ensure contact between the cornea and the measuring instrument, the ophthalmologist drips an aqueous solution containing the dye fluorescein into the conjunctival sac of the patient before the measurement. The application of these drops not only helps to visualize the contact with the cornea, but also has an anesthetic effect.

The tonometric measurement procedure differs from methods such as dynamic contour tonometry (DCT). During this measurement process, the examiner does not press the cornea with a pressure sensor, but stimulates a relaxed cornea. The pressure on both sides of the cornea is the same in the relaxed state. The measuring head on the outside of the cornea thus measures the same pressure as the inside.

In contrast to applanation tonometry, the different corneal properties have no influence on the measurement result in dynamic tonometry. Instead of a single pressure value, this method determines one hundred values per second, which are graphically displayed in a pulse pressure curve.

Self-service or health insurance – who bears the costs?

The measurement of the intraocular pressure is controversial as a glaucoma early detection measure. The reliability of the method remains unclear. For this reason, the statutory health insurance companies do not cover the costs for an early detection screening, or only if there is an increased risk or a solid suspicion of glaucoma. In all other cases, tonometry as an early detection measure represents an individual health service (IGeL) and costs the patient between 10 and 25 euros.

A tangible indication for the measurement method is an ongoing glaucoma therapy. Tonometry is medically necessary as part of therapy control. In addition, with long-term cortisone therapy, there is a medical necessity for the procedure.

In all cases of medical necessity, the statutory insurers cover the costs in terms of the treatment flat rate. In the case of privately insured persons, the assumption of costs depends on the contract conditions in detail. Some private insurance companies also contribute to the tonometry costs if the examination takes place as part of early detection.

Risks, dangers and complications

All tonometry procedures are associated with only a few risks and side effects. In extreme cases, the measuring instrument can transmit germs to the patient by touching the cornea, causing an infection.

However, such a scenario is rather unlikely due to the high standards of sterility in Western medicine. As long as the instruments are disinfected correctly, infections are virtually impossible. Tonometry procedures using air-puff instruments or interchangeable contact bodies carry the lowest risk of infection.If the tonometry is performed incorrectly, there is a risk of infection and mechanical damage to the cornea in individual cases. With professional execution, however, this risk can also be neglected. As a side effect of drop anesthesia, vision may remain blurred on the day of tonometry. As a consequence, phenomena such as Headaches or dizziness are also conceivable.