OPHTHALMOLOGY

Q: I heard that corneal thickness can affect your intraocular pressure (IOP). Does it affect anything else related to the eye? What is normal corneal thickness and why is it important?

A: The cornea is the transparent structure on the front of our eyes shaped like a dome. As the outermost layer of the eye, it helps protect the inner eye segments from injury and harmful matter. The cornea’s primary function is that of a refracting surface and it provides 2/3 of the eye’s focusing power. The cornea bends or refracts incoming light onto the lens of the eye which further focuses the light onto the retina, a layer of light sensing cells at the back of the eye that starts the translation of light into the visual images we see. The cornea is made up of a tough, strong protein called collagen. There are no blood vessels in the cornea. Nourishment for the cornea comes from the tear film on the outside and the aqueous humor which is the clear fluid inside the eye. Corneal thickness is measured with an ultrasound device called a pachymeter. The resulting measurements vary per eye and per individual. Average pachymetry ranges from 540-560 microns, about half a millimeter. Pachymetry is an essential element in both intraocular pressure (IOP) measurements and the diagnosis and treatment of glaucoma. The normal range for IOPs range is 12-21 mm Hg. In those with thicker pachymetry, the IOP can be falsely elevated, or conversely, falsely lower in those with thin corneas. Studies indicate that those with elevated IOPs but who have thicker corneas are at considerably less risk of developing glaucoma than those patients with elevated IOPs and a normal or thin cornea. Thin pachymetry indicates a greater likelihood of potential damage to the optic nerve and suggest that lower IOPs are needed. It is important to note that pachymetry, while important, is not the only factor for assessing, monitoring, and treating elevated IOPs and glaucoma. Other factors that should be taken into account include family history, age, race, other medical conditions, visual field and ocular coherence tomography tests, and results from regular dilated exams. In relation to LASIK, pachymetry is one of the key measurements in the pre-operative evaluation for this surgery. LASIK requires that a certain amount of corneal tissue is used to create a flap that is lifted so that the excimer laser can reshape the cornea optically. If the pachymetry reading is too thin, another procedure such as PRK may be needed. Since both LASIK and PRK thin the cornea, a new pachymetry measurement is established after surgery. This new measurement can thus affect future IOP measurements, and in the event of other refractive surgeries such as cataract surgery, will require additional calculations. Post-LASIK or PRK patients will want to let the treating physician or cataract surgeon know of this previous surgery so that all factors may be taken into account when examining and monitoring the patient and/or when performing additional eye surgery.

Gregory Eippert, MD

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