Intraocular Pressure Measurement: Tonometry

Tonometry (synonym: intraocular pressure measurement) is a diagnostic procedure in ophthalmology for measuring intraocular pressure (intraocular pressure), which nowadays can be performed non-invasively (not penetrating the eyeball) using various techniques. In adults, normal intraocular pressure is between 10 and 21 mmHg. It is caused by a continuous flow of aqueous humor formed by the ciliary epithelium (epithelium of the cornea of the rays; a section of the medial eye membrane) and delivered to the posterior chamber. Here it washes around the lens of the eye and flows through the pupil into the anterior chamber at an average rate of about 2 µl/min. At the angle of the chamber, most of the aqueous humor leaves the eye and passes through the trabecular meshwork (tuft-like structure) into Schlemm’s canal and finally into the venous vasculature (trabecular outflow). A small portion of aqueous humor (approximately 15%) drains through the ciliary muscle and choroidal vessels (uveoscleral outflow). Maintaining the balance between aqueous humor production and outflow is critical to maintaining proper aqueous humor functions, which include maintaining a constant intraocular pressure. Intraocular pressure, in turn, is important for maintaining the shape of the eyeball, or the curvature of the cornea, so that the refraction (refraction of light for sharp vision) of the eye remains unchanged. Various diseases can lead to an increase in intraocular pressure, which in the long term leads to dangerous changes in the optic nerve and limitations in the field of vision (typical signs in glaucoma). Glaucoma is one of the most common causes of blindness worldwide. Therefore, control of intraocular pressure is of great importance.

Indications (areas of application)

Tonometry is performed when elevated intraocular pressure is suspected or as a screening test for early detection of glaucoma. The cause of increased intraocular pressure is an increase in aqueous humor, for which there are basically two possibilities:

  1. Overproduction of aqueous humor
  2. Obstruction of aqueous humor outflow (causative for glaucoma).

There are different forms of glaucoma, which are classified according to the cause of the outflow obstruction of the aqueous humor:

Primary glaucoma (spontaneous occurrence).

  • Primary open-angle glaucoma (POAG): slowly progressing ocular disease of the elderly, usually affecting both eyes and associated with typical visual field loss. Although the chamber angle remains open, the aqueous humor can not drain due to deposits of hyaline material (plaque deposits) in the trabecular meshwork, so that the intraocular pressure rises.
  • Primary angle-closure glaucoma (PWG): the cause is an occlusion of the chamber angle by the iris base (base of the iris), especially in the case of a congenitally narrow chamber angle or enlarged crystalline lens (age lens). Acute closure is an emergency situation (acute glaucoma attack) and must be treated immediately with medication and peripheral iridectomy (splitting of the iris by laser or surgically). Chronic angle-closure glaucoma is caused by goniosynechiae (adhesions of the chamber angle), which are usually the result of acute glaucoma cases not treated in time.
  • Primary congenital glaucoma (congenital glaucoma of the infant and toddler): congenital glaucoma arises from developmental abnormalities of the ventricular angle and usually manifests itself in the 1st year of life. The children are noticeable with excessively large cornea as well as photophobia, eyelid spasms and lacrimation.

Secondary glaucoma (consequence of other eye diseases).

  • Neovascularization glaucoma: diabetes mellitus or central retinal vein occlusion can lead to retinal ischemia (impaired blood flow to the retina). In response, the retina produces vascular endothelial growth factors (VEGF), which enter the anterior chamber via the aqueous humor. Here, these factors lead to neovascularization (formation of new vessels) on the iris or in the chamber angle, so that it is narrowed and displaced. Consequently, the aqueous humor can no longer drain and the intraocular pressure rises.
  • Pigment dispersion glaucoma: When the iris slackens, it rubs with its back against the zonular fibers (elastic fibers arranged in a circle around the lens of the eye), whereby pigment granules are exfoliated.These are transported with the aqueous humor into the anterior chamber and obstruct the chamber angle.
  • Pseudoexfoliative glaucoma: Fine fibrillar material (also called pseudoexfoliative material), which is predominantly formed by the ciliary epithelium, is deposited in the chamber angle. In this form of glaucoma, intraocular pressure values are often subject to high fluctuations. A measurement of the daily pressure curve can be helpful.
  • Cortisone glaucoma: administration of eye drops with corticosteroids can block the trabecular meshwork by accumulation of mucopolysaccharides. The chamber angle remains open. The prescription of eye drops containing corticosteroids always requires regular ophthalmological control.
  • Phacolytic glaucoma: proteins of the crystalline lens can penetrate through the lens capsule and block the trabecular meshwork in hypermature cataract (“overripe” cataract; lens opacity in old age).
  • Inflammatory glaucoma: inflammation may cause edema (swelling) of the trabecular cells, or inflammatory proteins may be produced, which in turn obstruct the trabecular meshwork.
  • Traumatic glaucoma: Injury may cause blood to obstruct the angle of the ventricle, and the vitreous may also press on the angle from the inside. Tears of the trabecular meshwork may cause compressive (constricting) scarring. Burns can lead to Schlemm’s canal obliteration.
  • Glaucoma in developmental disorders and malformations: Most often it is an increase in volume of the choroid or sclera (eg, hemangioma), so that ipsilateral (unilateral) glaucoma develops in childhood.

Contraindications

Intraocular pressure measurements requiring direct corneal contact are contraindicated in infectious corneal disease because of the risk of germ spread.

Before the examination

Tonometry techniques requiring direct corneal contact require prior local anesthesia (numbing) of the cornea with eye drops.

The procedure

There are several methods to measure intraocular pressure, which differ in their technical implementation, accuracy, and applicability:

  • Palpation
    • By palpating (feeling) the bulb (eyeball), intraocular pressure can be estimated.
    • For the experienced ophthalmologist, this method can be a rough guide for the diagnosis of severely elevated pressure (e.g., acute glaucoma) in a side-by-side comparison.
    • The method is particularly indicated when device measurement is not possible (eg, in critically ill patients, infectious corneal ulcer).
    • When performed, the patient looks down with eyes closed and the physician palpates the eyeball with the tips of the index fingers. This should normally be fluctuantly depressible (tensio below 20 mmHg). However, if the bulb does not yield (rock hard eyeball), the pressure is about 60-70 mmHg.
  • Applanation tonometry
    • This method is the most accurate and is routinely performed on a sitting patient at the slit lamp using a Goldmann applanation tonometer.
    • A pressure corpuscle is pressed so far into the cornea that an area of about 3 mm in diameter is applanated (flattened). The force applied for this (contact pressure) can be read on a scale and corresponds to the intraocular pressure.
    • Hand-held applanation tonometers (e.g., Perkins tonometer) can be used for measurements on the supine patient.
  • Impression tonometry according to Schiötz
    • The principle of this method is based on a pen that sinks into the cornea to different depths depending on the intraocular pressure. The lower the pressure, the deeper the pen sinks and the greater the pointer deflection on the device.
    • However, this method is outdated and is only used in severely scarred corneas when applanation tonometry is not possible.
    • Especially in the myopic (nearsighted) eye, the error rate of this method is high. The measuring pin sinks due to the lowered compliance of the sclera (sclera) already due to this deeper than normal.
  • Air blast non-contact tonometry
    • The technique does not require direct corneal contact. An air blast is used to flatten the cornea and measure the altered reflex image.
    • Advantages: Since no direct contact is required, there is no need for a local anesthetic (topical anesthesia) and there is no risk of germ transmission.
    • Disadvantages: Accuracy is lower compared to applanation tonometry, especially at high pressures. The measurement is subjectively perceived as uncomfortable and the calibration of the device can be problematic.
  • Tono-Pen
    • This is a small, pen-shaped, battery-powered device that is held in the hand and contains a transducer (communication system) at the tip of the pen that measures the force. A microprocessor analyzes the readings and calculates the intraocular pressure. The main advantage of this method of measurement is the possibility of use even with irregular corneal surface, corneal edema and even (therapeutic) contact lenses.
  • Transpalpebral tonometer
    • These tonometers measure intraocular pressure through the eyelids and some are still in development. Similar to the tono-pen, they are pen-shaped and their small size also allows convenient home use by the patient.

Measurement of the daily pressure curve

A single measurement of intraocular pressure always represents only a “snapshot” and often can not capture pressure fluctuations. Also physiologically, intraocular pressure is subject to small fluctuations, but should not exceed 4-6 mmHg. The peak value is often at night or in the early morning. In patients with suspected glaucoma, measurement of the diurnal pressure curve may be indicated to detect larger fluctuations within 24 hours. This is nowadays also possible under home conditions by the patient himself or a partner.

  • Self-tonometry: a self-tonometer works according to the principle of applanation tonometry, whereby the patient fixes the tonometer to the forehead and brings it into the correct position by means of a light spot. A tonometer head automatically moves to the cornea and measures the pressure. The main advantage is that the patient can perform any number of measurements under his usual environmental and living conditions.
  • Partner tonometry: it is usually performed with a portable air blast tonometer. It can be held in the hand in front of the patient’s eye and allows an examiner-independent and therefore reliable measurement.

Possible complications

Minor corneal (corneal) injuries are possible with methods involving direct corneal contact. Germs may also spread from patient to patient with subsequent infectious conjunctivitis (conjunctivitis) or keratitis (corneal inflammation), e.g., keratoconjunctivitis epidemica (infectious conjunctivitis caused by adenoviruses).