The cornea is the clear, dome-shaped tissue at the very front of the eye. This transparent layer is responsible for focusing most of the light that enters the eye, making its health and structure fundamental to clear vision. A specific measurement, known as Central Corneal Thickness (CCT), is a routine diagnostic parameter in modern eye care. CCT refers to the thickness of this tissue at its center, and it is measured in tiny units called microns (µm), where one micron is one-millionth of a meter.
How Corneal Thickness is Measured
Measuring the Central Corneal Thickness is accomplished through a procedure called pachymetry, which is a quick and painless test. Precision is necessary because the cornea is only about half a millimeter thick, requiring specialized instruments. The measurement is typically taken in the center of the cornea, which is generally the thinnest point.
One common technique is ultrasonic pachymetry, where a small, pen-like probe is gently touched to the anesthetized corneal surface. This device uses high-frequency sound waves to calculate the thickness. Another method is optical pachymetry, which uses non-contact devices like Optical Coherence Tomography (OCT) or Scheimpflug imaging. These optical systems use light to create cross-sectional images of the cornea, allowing for very detailed and precise measurements without touching the eye’s surface.
Understanding the Standard Thickness Range
The normal range for Central Corneal Thickness is a statistical range, generally falling between 520 and 560 microns for the average healthy adult. While some studies cite the average CCT as being around 540 to 550 microns, individual variation is common. For example, a cornea measuring 515 microns is thinner than average, while one measuring 580 microns is thicker, but both may still be perfectly healthy.
The measurement must be interpreted alongside a comprehensive eye examination. Differences in CCT can be influenced by factors like age, ethnicity, and underlying medical conditions. What matters is the stability and uniformity of the thickness, which helps determine if further investigation is needed.
Why Thickness Matters for Glaucoma Risk
Central Corneal Thickness is a factor in assessing the risk of glaucoma, primarily because it affects the accuracy of Intraocular Pressure (IOP) readings. The most common method for measuring eye pressure, Goldmann applanation tonometry, works by determining the force needed to flatten a small area of the cornea. This measurement method was standardized assuming a specific average corneal thickness.
A cornea that is thicker than average provides more structural resistance to the measuring device, causing the IOP reading to be artificially inflated. This means a person with a thick cornea might appear to have high eye pressure when the true pressure inside the eye is actually lower. Conversely, a thinner-than-average cornea offers less resistance, which can suppress the IOP reading, making the pressure seem lower than it truly is. An underestimated IOP reading is concerning because it can mask the presence of glaucoma, a condition characterized by damage to the optic nerve. Therefore, eye care professionals use the CCT measurement to help correct the measured IOP, leading to a more accurate estimate of the actual pressure inside the eye.
Structural Integrity and Refractive Surgery
Corneal thickness is a measurement of structural integrity, and deviations from the normal range can indicate underlying conditions or affect eligibility for certain procedures. An extremely thin cornea, often defined as less than 500 microns, can be a sign of a progressive condition like keratoconus. In this disease, the cornea weakens and progressively thins, causing it to bulge outward into a cone shape, which results in significant visual distortion.
CCT is also a factor in the screening process for refractive surgeries, such as LASIK (Laser-Assisted In Situ Keratomileusis), which permanently reshapes the cornea to correct vision. To perform LASIK safely, the surgeon must ensure that enough corneal tissue remains after the procedure to maintain the eye’s structural stability. The procedure involves creating a thin flap, typically 90 to 120 microns thick, and then removing tissue underneath to correct the refractive error. Most surgeons require a minimum residual stromal bed thickness of 250 to 300 microns after the flap is replaced and the tissue is removed. If the cornea is too thin to begin with, the risk of a complication called corneal ectasia, where the cornea weakens and bulges post-surgery, becomes too high, and alternative procedures like PRK are often recommended.