There are many different tests optometrists and ophthalmologists (specialist eye doctors) conduct during the glaucoma diagnosis process. Within these tests, however, there are several factors to take into consideration in order to ensure an accurate diagnosis.

The most common of these factors is your intraocular pressure (IOP) – the pressure of the fluid in your eye. Another is the thickness of your cornea: the eye’s clear, protective outer layer. Here, we break down why corneal thickness can affect glaucoma diagnosis and how it’s detected, in order to help you understand how glaucoma is diagnosed accurately and safely.

How does corneal thickness affect glaucoma?

First of all, IOP is usually measured during a routine eye check-up. This is called a tonometry test. You can learn exactly how this is done in this dedicated article on intraocular pressure in glaucoma.

However, IOP measurement can be affected by the thickness of the cornea. During the tonometry test, a small amount of pressure is applied to the cornea, and any resistance to the pressure helps measure IOP. A thick cornea may not ‘give’ as easily when pressure is applied, giving the impression of a high IOP. Similarly, a thin cornea may ‘give’ more than usual, which can give an impression of an abnormally low IOP.

What is the normal thickness of the cornea?

A normal range of corneal thickness is between 540µm and 560µm - µm represents measurements in micrometres, i.e. one-millionth of a metre.

A thick cornea is 565µm or more, with a very thick cornea being greater than 600µm.

Typical corneal thickness can vary by race, with African-Caribbean typically having a thinner cornea. For anyone with a thinner cornea, the risk is that lower IOP readings will potentially result in an underestimation of the actual level of intraocular pressure (IOP).1, 2, 3

This risk, if unspotted, can lead to misdiagnosis. For example, if a tonometry test comes back with ‘normal’ results and you are showing no signs of high eye pressure, then it can be assumed you’re not ‘glaucoma suspicious’. However, as a thin cornea can cause an artificially low IOP reading, then this diagnosis might not be correct.

Conversely, those who have a thick cornea may show an eye pressure reading that’s higher than it actually is.

This makes it even more critical that the ophthalmologist has an overall picture of your eye health, including your optic disc appearance, your nerve fibre layers, and your visual field test results. Thankfully, technology now is incredibly advanced — meaning glaucoma misdiagnosis is an extremely rare occurrence.

Should you be referred to an ophthalmologist due to being glaucoma suspect (showing signs and symptoms of the condition), the test your eye doctor will perform to measure corneal thickness is called a pachymetry test.

What is a pachymetry test?

The pachymetry test is a painless procedure used to measure central corneal thickness (CCT) and determine if it could be affecting accurate IOP measurements. A pachymeter can use light or ultrasound to assess the thickness of your cornea.

Alternatively, if your ophthalmologist/optometrist has an OCT device, you’ll be asked to simply place your chin on a rest and look at a light while the device takes measurements. This method is quickly becoming popular among eye doctors due to how detailed eye images are thanks to OCT. You can read our article on how optical coherence tomography is used to detect glaucoma here.

Your ophthalmologist may also use a handheld device or diagnostic machine to perform an ultrasound pachymetry test.

Adapting glaucoma diagnosis in line with corneal thickness measurement

Should you have an abnormally thick or thin cornea, your ophthalmologist will reassess your field of vision, as this can pick up signs of nerve damage and can help to define the type of glaucoma you may have. If you have a visual field loss or changes to your optic disc, for example, yet your IOP appears normal, this is called normal-tension glaucoma.

Combining your IOP measurement and visual field test results with your corneal thickness measurement can help the ophthalmologist determine how to interpret your IOP and, as a result, the type of glaucoma you may or may not have.

If you are concerned about anything you have read in this article, you should speak to your optometrist or ophthalmologist. They can examine your eyes and determine whether your corneal thickness is likely to play a role in your glaucoma diagnosis. You can also visit our dedicated glaucoma resource to learn more about glaucoma causesglaucoma diagnosis and treatment of glaucoma.

References
  1. Beutelspacher SC1, Serbecic N, Scheuerle AF. Assessment of central corneal thickness using OCT, ultrasound, optical low coherence reflectometry and Scheimpflug pachymetry. Eur J Ophthalmol. 2011 Mar-Apr;21(2):132-7.
  2. William A.Argus MD Ocular Hypertension and Central Corneal Thickness Ophthalmology Volume 102, Issue 12, December 1995, Pages 1810-1812
  3. James D Brandt MD Julia A Beiser MS Michael A Kass MD Mae O Gordon PhD the Ocular Hypertension Treatment Study (OHTS) Group Central corneal thickness in the ocular hypertension treatment study (OHTS) Ophthalmology Volume 108, Issue 10, October 2001, Pages 1779-1788