A cornea that is thicker or thinner than average may throw off measurements of intraocular pressure.
Glaucoma, a condition that can lead to blindness by damaging the optic nerve, was once thought to be caused by elevated intraocular pressure (IOP)—the pressure exerted by fluid within the eye. But even people with normal IOP can suffer damage to the optic nerve, and IOP is now considered a risk factor for—rather than a cause of—glaucoma. Recently, researchers also discovered a new risk factor for glaucoma—cornea thickness—that may help explain why many people with normal IOP develop the disease.
Measuring Cornea Thickness
The cornea is a transparent, dome-shaped disk that covers the iris and pupil and allows light to enter the eye. The thickness of the cornea is measured in the center of the eye with an instrument called a pachymeter. The test uses ultrasound and is fast and painless. The average cornea is about 560 microns thick (just more than half a millimeter), but cornea thickness varies greatly from this average in many people.
Researchers have long known that cornea thickness affects the measurement of IOP. Specifically, IOP is underestimated in people with thinner corneas and overestimated in people with thicker corneas. In other words, people with thick corneas may appear to have an elevated IOP when in reality they don’t, and people with thin corneas may appear to have a normal IOP when in fact it is elevated.
Because having a thin cornea gives a falsely low IOP reading, and thus may mask elevated IOP, thin corneas may be a risk factor for glaucoma. This assumption was confirmed in the groundbreaking Ocular Hypertension Treatment Study, which was published in the journal Archives of Ophthalmology.
The study evaluated cornea thickness in 1,398 people with elevated IOP (24 to 32 mm Hg in one eye and 21 to 32 mm Hg in the other) who had no evidence of glaucoma damage. People with average cornea thickness (556 to 588 microns) were 1.7 times as likely to develop glaucoma after five years, and those with the thinnest corneas (555 microns or less) were 3.4 times as likely to develop glaucoma, compared with those with the thickest corneas (589 microns or more).
In addition, regardless of the participants’ IOP level, those with the thinnest corneas were most likely to develop glaucoma. People with the highest IOP levels and the thinnest corneas had the highest risk of glaucoma after five years (36%).
What To Do
Measurement of cornea thickness is not part of a normal eye exam, and many eye care professionals do not perform the test. But the American Academy of Ophthalmology now considers a thin cornea (below 556 microns thick) to be a risk factor for glaucoma and recommends measuring cornea thickness in people with other risk factors for the condition (for example, advanced age, elevated IOP, and black ancestry).
Determining cornea thickness is also useful when people have elevated IOP levels but no glaucoma damage or when they show signs of glaucoma but have apparently normal IOP levels. In these cases, the cornea thickness may reveal that IOP is actually lower or higher, respectively, than indicated by the measurement.
