What is the Cornea?
The cornea is the clear window at the front of the eye. Its main function is to bend incoming light rays and focus them on the retina, while simultaneously serving as an outer protective layer. Just as people differ in height or eye color, there is natural variation in corneal thickness from person to person.
How does corneal thickness relate to glaucoma and intraocular pressure?
Evaluating intraocular pressure (IOP) is largely based on applying gentle physical force to the surface of the cornea, and instruments are calibrated according to average corneal thickness. When the cornea is thinner than average, measurement devices may show a pressure lower than it truly is. Conversely, when the cornea is thicker than average, the device may show an artificially high pressure.
Currently, there is no precise calibration formula to correct pressure measurement based on corneal thickness. However, corneal thickness must be taken into account during every intraocular pressure measurement to more accurately assess the condition of the eye.
Corneal Thickness as an Independent Risk Factor
Beyond its effect on technical measurement, large-scale medical studies have proven that a thin cornea is an independent risk factor for the development and progression of glaucoma, regardless of the measured pressure level. Conversely, a thick cornea serves as a protective factor against the development of glaucoma. Therefore, knowing your corneal thickness is essential for building a complete risk profile.
Is Corneal Thickness Constant or Changing?
In most cases, corneal thickness does not change significantly after adolescence. However, eye surgeries, especially laser vision correction surgeries, can affect the structure of the cornea. These surgeries act by reshaping and thinning the cornea. A patient who underwent laser vision correction in the past will have a thinner cornea, which requires special consideration when evaluating intraocular pressure years later. Various eye diseases can also cause thinning (keratoconus) or thickening (corneal edema), so a thorough history is critically important.
How Do We Measure Corneal Thickness?
Corneal thickness is measured non-invasively and involves no pain or discomfort. There are a variety of instruments used to evaluate corneal thickness. The measurement can be done using several technologies:
- Ultrasound Pachymetry: The use of sound waves for a highly accurate, point-specific evaluation of corneal thickness.
- Optical Pachymetry: The use of light rays for non-contact mapping of corneal thickness at microscopic resolution.