Besides non-contact tonometry, the other methods of measuring intraocular pressure have some risks that should be considered. First of all, the patient may have an allergic reaction to the anaesthetic drops previously placed in the eye. A burning sensation in the eyes after the administration of drops is normal and disappears after a few minutes.
However, an allergic reaction can include systemic reactions, such as shortness of breath up to anaphylactic shock. In addition, all methods of measuring intraocular pressure that involve direct contact with the cornea can also cause damage to the cornea and the surface of the eye. These include scratches and tears of the cornea caused by excessive pressure.
In extreme cases a corneal transplant may be necessary. Furthermore, there is a risk of germ transmission during intraocular pressure measurement, which can trigger an epidemic keratoconjunctivitis and makes antibiotic treatment necessary. The most important indication for intraocular pressure measurement is the diagnosis and monitoring of glaucoma.
The examination should be carried out from the age of 50 onwards in order to find out the corresponding new diseases. Depending on the results, the examination must be repeated at regular intervals. In the case of increased pressure values, the examination should be performed every six months. If glaucoma has already occurred in the patient’s family, an examination once a year is recommended.
The intraocular pressure measurement is a preventive examination and is usually not paid for by the health insurance company. It therefore falls into the category of so-called individual health services (IGeL), which have to be paid for by everyone. The costs amount to 20 EUR, which the patient has to pay by himself if no glaucoma is known (precaution). For all patients suspected of having glaucoma, the examination is carried out as a follow-up examination and is therefore covered by the health insurance company.
The standard values of intraocular pressure are usually in the range of approximately 10 to 22 mmHg. The mean value is approximately in the range of 15 mmHg. The value depends on the time of day and is subject to fluctuations.
The intraocular pressure is at its highest in the morning or after getting up. Fluctuations in daily pressure of up to 4 mmHg are considered normal and have no disease value. Values around 22 to 26 mmHg are suspected to be glaucoma, so that in case of doubt further intraocular pressure measurements have to be made.
All measurements that result in values above 26 mmHg are always considered pathological with regard to existing glaucoma. This requires clarification of the cause and treatment and reduction of the pressure to avoid or minimize consequential damage. The intraocular pressure is built up in the anterior chamber of the eye, which extends between the cornea and the lens in the front part of the eye.
The pressure is established by a balance of production and outflow of aqueous humor and is maintained in healthy patients. The aqueous humor is formed by the ciliary epithelium of the eye, then flows through the anterior eye region and finally reaches the venous blood system via the Schlemm canal. The built up intraocular pressure is necessary to maintain the shape of the eye and to ensure the refraction of light, among other things.
The intraocular pressure increases when the feet are moved to the blood system. The danger of increased intraocular pressure lies in the damage to the optic nerve, at the back of the eye, which can only tolerate a certain pressure range without damage. The normal eye pressure in humans is between 10 and 20 mmHg.
There is a wide range of norms, which depend on various factors. Therefore, in addition to regular intraocular pressure measurements, it is also necessary to check the field of vision to see whether correspondingly high pressure has already damaged the eye. There are various possibilities for measuring intraocular pressure.
Without apparatus, the doctor can determine a strongly increased intraocular pressure by pressing on the closed eye (e.g. in the case of a glaucoma attack=board hard eyeball). The so-called applanation tonometry is today the most accurate and most frequently performed examination for measuring intraocular pressure. A cylinder is placed on the cornea of a sitting patient and the pressure required to press in an area of the cornea of 0.3 mm is measured.
This pressure then corresponds to the intraocular pressure. Non-contact tonometry works according to a similar principle, except that the cornea is not pressed in by a cylinder but by a short blast of air. The resulting reflex is measured and a corresponding intraocular pressure is calculated.
An outdated method is impression tonometry, in which a pencil hits the cornea with its weight and determines how much force was necessary to push the cornea in. The intraocular pressure examination should be repeated regularly, especially if the intraocular pressure is elevated. It is paid by the health insurance company as a preventive examination but not as a follow-up examination and costs 20 EUR.
Risks and side effects may include allergy to the anaesthetic eye drops, which must be administered to the eye to be examined before the measurement, as well as injuries (scratching and tearing) to the cornea caused by the cylinder. Furthermore, an infection by pathogens introduced into the eye represents a rare danger.