In this article we summarise the key investigations used in the diagnosis and monitoring of glaucoma.
Tonometry is a technique used to measure intraocular pressure (IOP).
The most widely used method is Goldmann applanation tonometry, which is based on the Imbert-Fick law.
<aside> 💡 The Imbert-Fick principle states: for a dry thin-walled sphere, the pressure (P) inside the sphere = the force (F) necessary to flatten its surface, divided by the area (A) of flattening. Therefore, P = F/A.
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The Imbert-Fick law is used to calculate the amount of force required to flatten an area of cornea that is 3.06mm in diameter, assuming central corneal thickness (CCT) is 520 micrometers.
Technique:
Topical anaesthetic and fluorescein are instilled into the conjunctival sac
The prism of the tonometer is advanced until it touches the apex of the cornea
A pattern of two green semi circles is seen, one above and one below the horizontal midline, which represent the flourescein stained tear film touching the upper and lower outer halves of the prism
As the dial on the tonometer is rotated, the force applied varies, and the inner margins of the semi circles align with a circular area of precesiely 3.06mm is flattened
The reading on the dial is multiplied by 10 to give the IOP (often shown digitally)
Sources of error: