Q. Recent studies suggest an increased risk for glaucoma in patients with thin central corneas. Are we just underestimating IOP, or is there an inherent problem that puts these patients at greater risk?
A. Both factors may be at work. On the surface, thinner corneas will represent an underestimate of the measured IOP, says optometrist Leo Semes, of the University of Alabama at Birmingham School of Optometry. Similarly, thicker corneas will overestimate measured IOP.
And, since the publication of the Ocular Hypertension Treatment Study (OHTS) last year1, further research has found that thinner central corneal thickness (CCT) significantly increases the risk of conversion from ocular hypertension to glaucoma. The OHTS data clearly shows that as CCT decreases, the risk factors for developing glaucoma increases.
There has even been a suggestion that the risk increases proportionally with thinner CCT, Dr. Semes says.
Even before OHTS, previous studies found myopic laser refractive surgery patients had consistently lower IOP following the procedure, he says.
In a recent study of 58 patients who underwent hyperopic LASIK, the researchers observed a significant difference between the IOP before and after surgery, with lower IOP measured post-op.2 The researchers said that this could not be related to the degree of hyperopia treated or to the treatment zone diameter. They say that the applanation tonometer also underestimates the true IOP after hyperopic LASIK.
However, the decrease in IOP following hyperopic LASIK cannot be explained by a change in central corneal thickness, studies have found. Other possible explanations include the surgical effect on corneal rigidity, corneal curvature change and temporary trabecular meshwork outflow.
This patients pachymetry reading is slightly thicker than normal. Image courtesy of Leo Semes O.D.
Its also possible that an inherent problem puts the patient with thinner CCT at higher risk for glaucoma. The corneal changes may be part of a more global ocular predisposition, says Louis Cantor, M.D., of Indiana University School of Medicine. Or, there may be other ocular factors influencing the risk of glaucoma, such as corneal diameter, curvature, corneal extracellular matrix or a host of other factors that are only now just being explored in more depth.
However, the real answer is not yet known, and we need to consider other factors. What is not clear is: Are the thin corneas a telltale sign of a thin and weak lamina cribrosa, which may not offer much support to the nerve fibers that pass through it, or some other anatomic or physiologic flaw that we have not noticed yet? says Richard Wilson, M.D., of Wills Eye Hospital in Philadelphia. The investigation goes forward but no answers yet.
Even so, researchers at Duke University found that CCT measures may be a powerful clinical predictor of glaucoma progression.3
After reviewing the charts of 429 patients, the researchers determined CCT to be the most powerful clinical predictor of glaucoma severity in patients with primary open-angle or normal-tension glaucoma. By measuring we may be able to identify glaucoma patients at high risk of progression and plan their therapy more aggressively.
On a brighter note, patients with thinner central corneal thickness seem to have a better response to topical agents by showing a greater reduction in IOP than do patients with thicker corneas.
1. Gordon MO, Beiser JA, Brandt JD, et al. The Ocular Hypertension Treatment Study: baseline factors that predict the onset of primary open-angle glaucoma. Arch Ophthalmology 2002 Jun;120(6):714-20.
2. Zadok D, Raifkup F, Landao D, Frucht-Pery J. Intraocular pressure after LASIK for hyperopia. Ophthalmology 2002 Sept;109(9):1659-61.
3. Weizer JS, Stinnett SS, Herndon LW. Central corneal thickness as a risk factor for advanced glaucoma damage. Association for Research in Vision and Ophthalmology (ARVO) Abstract Number 1045, 2003.
