What Is the Normal Corneal Thickness in Millimeters?

The cornea is the transparent, dome-shaped outer layer at the very front of the eye. It covers the iris, pupil, and anterior chamber, playing a part in vision by refracting, or bending, light as it enters the eye. Because it helps focus light, any change in the cornea’s shape or clarity can impact sight. The thickness of the cornea is a specific attribute that eye care professionals measure to gather data about eye health.

The Average Corneal Thickness Measurement

The central portion of a healthy adult cornea measures between 540 and 560 micrometers, or microns, which is roughly equivalent to half a millimeter. The cornea is not uniform in its dimensions; it is thinnest at its center and gradually becomes thicker towards its outer edge, known as the limbus, where it can be 700 to 900 microns.

Factors such as ethnicity can influence the measurement. For instance, studies have shown that people of African or South Asian descent may have slightly thinner corneas on average compared to Caucasians. This baseline measurement is a useful reference point for eye health assessments.

How Corneal Thickness Is Measured

The clinical test used to determine corneal thickness is called pachymetry, a quick and painless procedure. There are two primary methods: ultrasound and optical pachymetry.

Ultrasound pachymetry uses a small, handheld probe that emits high-frequency sound waves. After applying numbing drops to the eye, a clinician gently touches the probe to the cornea’s surface, and the device calculates thickness based on the time it takes for sound waves to bounce back. Optical pachymetry is a non-contact technique that uses instruments like optical coherence tomography (OCT) to create a cross-sectional map and measure thickness without touching the eye.

Clinical Importance of Corneal Thickness

Measuring corneal thickness is part of a comprehensive eye exam because it has clinical applications in managing glaucoma and assessing candidacy for refractive surgery. For glaucoma, corneal thickness can affect the accuracy of intraocular pressure (IOP) readings. A tonometer measures IOP, the fluid pressure inside the eye, by pressing on the corneal surface. A thinner cornea can lead to an artificially low IOP reading, while a thicker one can produce an artificially high reading.

The Ocular Hypertension Treatment Study (OHTS) revealed that a thin cornea is an independent risk factor for developing glaucoma. This finding suggests that a thinner cornea might have structural differences that make the optic nerve more susceptible to damage. A pachymetry reading allows an ophthalmologist to adjust the IOP measurement for a more accurate assessment of a patient’s glaucoma risk.

Corneal thickness is also a consideration for patients interested in refractive surgery, such as LASIK. During this procedure, a laser reshapes the cornea to correct vision problems. To perform this safely, the surgeon creates a thin flap and removes a specific amount of tissue underneath. A minimum amount of corneal tissue must remain after the surgery to ensure the eye’s long-term structural stability, so an initial thin cornea could compromise its integrity.

The measurement is also used to diagnose and monitor certain corneal conditions. For example, keratoconus is a progressive disease characterized by the thinning and bulging of the cornea into a cone-like shape, which distorts vision. In contrast, conditions like Fuchs’ dystrophy or corneal edema can cause the cornea to swell and thicken, leading to cloudy vision.