Tonometry English: intraocular pressure measurement
Definition of intraocular pressure
By intraocular pressure measurement we understand different mechanisms to measure and determine the pressure present in the anterior segment of the eye.
The development of intraocular pressure
The eye, like almost every part of our body, depends on being supplied with sufficient fluid. On the one hand, so that there is no danger of dehydration, but also because the fluid and the substances dissolved in it ensure the supply of nutrients to some parts of the body that would otherwise not be adequately supplied by the blood. The anterior chamber of the eye is located in the front part of the eye between the cornea and the lens of the eye.
This chamber contains a liquid that is produced in certain quantities and drained off in corresponding quantities. This is the so-called aqueous humour, which supplies the cornea with sufficient nutrients and keeps it in shape by means of pressure. The aqueous humour is produced in the eye itself, in the ciliary body, a ring-shaped section of the middle eye skin (which is not only responsible for the production of the aqueous humour, but also for the fixation of the lens and for near accommodation).
From the ciliary body, the aqueous humor flows into the anterior chamber of the eye and from there is conducted through small channels into the bloodstream. In a healthy eye, as much aqueous humor is always produced as is released back into the blood, so there is a fine balance between production and outflow. In the case of diseases of the eye and disturbances of the aqueous humor circulation, this balance can be disturbed and a drop or increase in the pressure of the aqueous humor can occur, which is why it can be used as a good indicator for diseases affecting the eye.
The fluid also exerts a more or less strong pressure (intraocular pressure) on the entire eyeball and on the vitreous body, which in turn transmits the pressure to the back of the eye. The normal intraocular pressure is 15.5 mmHg. However, this intraocular pressure can fluctuate.
The normal values of intraocular pressure are fixed between 10 mmHg and 21 mmHg. The aqueous humor is formed by the ciliary epithelium in an amount of about 2.4 mm3 per minute and is released into the posterior chamber. It washes around the lens and finally flows into the anterior chamber.
The aqueous humor is then filtered off by the trabecular meshwork in the chamber angle and from there passes into the so-called Schlemm canal. From there, it finally flows through small channels into the veins of the conjunctiva and thus into the blood system. The production of aqueous humor is subject to a day-night rhythm and is reduced by about 40% at night.
The functions of the aqueous humour include feeding the lens and the cornea, maintaining the shape of the eyeball with the corresponding constant curvature of the front part of the eye (important for light refraction), and detoxification of the interior of the eye (interception of free radicals). Furthermore, the aqueous humor also serves as a lymphatic substitute, since the eye does not have its own lymphatic fluid. Reasons for an increased intraocular pressure are exclusively due to a disturbance of the outflow in the trabecular meshwork and never to an excessive production of the aqueous humor. The reason is usually pathological changes in the trabecular meshwork.