It's Good to Be Thick (In Some Places):

Eye pressure is nearly always measured during a comprehensive eye examination. It is an important measurement because high fluid pressure in the eye can cause the gradual death of the optic nerve. This disease process often leads to visual field loss and sometimes central acuity loss, and is known as glaucoma. Other risk factors exist for glaucoma other than high eye pressure. One important risk factor is central corneal thickness.

When eye pressure is measured, the cornea is flattened (applanated) from its normal convex shape to a flat (Plano) state. This flattening is achieved with a puff of air (non-contact tonometry) or with a probe (Goldmann tonometry). The amount of force required to flatten (applanat) the cornea is measured and converted to a fluid pressure in millimeters of mercury. Typical normal eye pressure runs from 8 to 21 millimeters of mercury. When the cornea is flattened, the calculations used to convert the force required to flatten it to fluid pressure, assume that the cornea is “average” thickness. Average is about 550 microns thick centrally.

I was taught in optometry school 30 years ago that almost all corneas are average thickness. We now know that this is not true. Since thin corneas are a contraindication for some refractive surgeries (i.e. LASIK), many practioners have ultrasound–A units that measure central corneal thickness quickly and easily. In my office, use of my ultrasound–A unit (pachymeter) has shown a wide variance among patient’s corneal thickness. It is not unusual to see thick corners (580 microns and higher) or thin corneas (520 microns and lower). Since it is easier to flatten a thin cornea, most experts’ feel that a patients true eye pressure is actually higher than that measured if the patient has a thin cornea. Patients with thick corneas probably have lower pressure than that measured.

Thin corneas are an independent risk factor for glaucoma. Irrespective of a patient’s true pressure, the theory goes that patients with thin corneas probably have other structures in the eye that are thinner than normal, which may be more easily damaged by fluid pressure. I always tell patients it’s good to be thick in some places. The cornea is definitely one of those places.