Central Corneal Thickness, Pachymetry and Glaucoma

If you have glaucoma, or are at risk for glaucoma it is likely that at some point you will have a test called “Pachymetry.” This test is one of the most important tests for anyone who has or may have glaucoma. It tells us a great deal about how likely it is that a person will get glaucoma, and if they have glaucoma, it can help predict how severe it is likely to be.

Glaucoma and Intraocular Pressure are tied up together. The higher the pressure is in a person’s eye, the more likely it is that they will have glaucoma. To treat glaucoma, we lower the pressure. This is the only treatment that is available at this time.

The pressure is measured by resting a weight of one sort or another against the eye. Almost always it is rested against the Cornea, the front clear window of the eye. Our instruments measure how hard it is to dent the cornea, and from this they calculate how high the pressure is.

We have known for a long time that some eyes had corneas that were harder to dent than others. This is a characteristic of the cornea, unrelated to the pressure inside they eye. In other words, two eyes with exactly the same intraocular pressure may give different readings of pressure on the instrument because of differences in the corneas. However, up until recently we thought the differences were small and insignificant.

In 2002, the results of the Ocular Hypertension Treatment Study (OHTS) were reported. This was a large study that tried to determine what characteristics of a person might help to predict who would get glaucoma. One of the characteristics was the thickness of the cornea. When the results were examined, we found that the thickness of the cornea was very good at predicting who would get glaucoma. People with thin corneas have as much as 7 times greater chance of getting glaucoma. More recently, other studies have shown that people who have glaucoma and have thin corneas are more likely to get worse than people with thick corneas.

We don’t know for sure why the thickness of the cornea is so good in making predictions about a person’s glaucoma. It may be that the difference in the thickness of the cornea is just throwing off our measurements; that people with thin corneas have lower readings even when the pressure is high in their eye. It may be that the thickness of the cornea is telling us something about the structure of the eye and how well it can stand up to pressure. We don’t have all the answers yet.

In the meantime, we can say with confidence that people with thin corneas need to be watched more closely and need their pressures lower than people with normal corneas. Conversely, some people with thick corneas may be able to go without treatment, when we might have treated them in the past.

The thickness of the cornea or Central Corneal Thickness (CCT) is measured by an instrument called a Corneal Pachymeter and the test is called Pachymetry. It is done in the office with a device similar to the one that is used to measure the pressure. It requires only a drop to numb the eye. For most people the thickness of the cornea does not change much over time, so most people need it measured only once. Most insurance companies pay for this test when it is performed on people with glaucoma or who are at risk for glaucoma.