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Corneal Thickness: A Risk Factor for Glaucoma?

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Of the myriad risk factors for glaucoma, the most important in determining disease progression is IOP. The Baltimore Eye Survey¹ showed that patients with an IOP of 22 mm Hg developed glaucoma 8.6 times more often than those with the normal IOP of 10 to 21 mm Hg.

Many decades ago, Goldmann and Schmidt² understood that IOP is affected by variables such as scleral rigidity and variations in central corneal thickness (CCT). A thicker CCT could result in overestimation of IOP since more force is needed to flatten the cornea, and conversely, a thinner cornea would result in underestimation.

Since then, studies have shown a correlation between IOP (as measured by Goldmann applanation tonometery [GAT]) and CCT, with a great deal of variation in the general population. Normal CCT is 540 μm. People with ocular hypertension can have measurments 50 μm higher than glaucoma and control patients.^3,4 When measuring GAT in some patients with normal-tension glaucoma (NTG) with thin CCTs, a true diagnosis of primary open-angle glaucoma (POAG) was made after the adjustment. Also, patients with true NTG had thinner CCTs than patients with normal eyes.^5,6 Thus, some patients diagnosed with ocular hypertension and increased CCTs were noted to be normal when their IOPs were adjusted. Today, the ultrasonic pachymetric device is the most common and most reliable method for measuring the CCT.⁷

Population Differences

The landmark Ocular Hypertension Treatment Study (OHTS)⁸ demonstrated that a decreased CCT predicts the development of POAG. People with CCTs ≤555 μm had 3 times the risk of progressing to glaucoma than those with a CCT >588 μm. OHTS also showed significant racial differences in CCT.^5,9-12 Compared with white patients, black patients are 3 to 4 times more likely to develop glaucoma,⁵ have an earlier onset and more aggressive course, and are 8 to 12 times more likely to become blind from the disease. Black patients’ CCTs showed a 20-μm difference, which may account for their increased glaucomatous optic neuropathy at lower IOP levels. Other studies have shown decreased CCT values in Hispanics,¹³ mixed black and white descendents,¹⁰ Mongolians,¹⁴ black patients at a large refractive surgery center seeking surgery¹¹ and black male veterans.³

Other Factors

Although CCT reaches adult thickness around ages 2 to 4 years,¹⁵ one longitudinal study of patients with glaucoma showed thinning of the CCT up to 23 μm over an 8-year period,¹⁶ indicating that CCT is not static and suggesting a need for CCT readings every 5 years for proper IOP management. Older adults and LASIK patients should also have CCT measured every 5 years because the value also decreases with time for this cohort. Other factors that affect CCT measurements are prostaglandins,^17-20 topical dorzolamide,^19-22 topical beta-blockers, estrogen levels during ovulation and pregnancy,^22,23 diabetes²⁴ and optic disc radius.²⁵

The landmark Ocular Hypertension Treatment Study demonstrated that a decreased central corneal thickness predicts the development of POAG.

— Mildred M.G. Olivier, MD

Practical Aspects

The OHTS makes clear that CCT is a potent predictive factor for progression of ocular hypertension to glaucoma and that IOPs need to be regularly measured by GAT and adjusted depending on CCT.² Because of our evolving understanding regarding the significance of variations in CCT, new modalities are being developed so that corneal biomechanics and corneal hysteresis can be factored into the equation, resulting in more accurate measurements of IOP, the single risk factor we can manipulate to prevent disease progression. CCT allows ophthalmologists to decide which patients need more aggressive treatment and which may not need treatment at all.

References

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