The role of central corneal thickness in glaucoma management is not completely understood, but several interesting aspects have come to light. This article reviews key developments in our understanding on the role of CCT in glaucoma diagnosis and management.

Growing Evidence
In the 1970s, Niels Ehlers, MD, and colleagues reported that patients with low-tension glaucoma had thin corneas¹ and individuals with ocular hypertension had thick corneas.² Over time, other factors were associated with thin CCT: black race,³ old age,⁴ myopic refractive errors⁴ and diabetes.⁴
 
Most recently, the Ocular Hypertension Treatment Study found that subjects with CCT of <555 µm had a three-fold greater risk for developing primary open angle glaucomae⁵ Patients with OHT and thin corneas may also demonstrate early visual field defects when tested with short wave automated perimetry.⁶ Similarly, patients with thin corneas may respond to topical therapy differently, as such individuals show greater measured response to therapy.⁷

As shown in Table 1, several studies have demonstrated a wide range of CCT measurements in normal subjects and patients with primary open-angle glaucoma, OHT or normal- tension glaucoma. This has put the standard measurement of intraocular pressure by Goldmann applanation tonometry (GAT) into question because GAT assumes a constant CCT of 520 µm.¹⁵ Therefore, in an individual with a thick cornea, IOP measurement by GAT may show falsely high reading and the opposite may be true if the cornea is thinner than 520 µm. In a manometric study, Dr. Ehlers calculated GAT to be accurate at 520 µm, but a variation of 70 µm in thickness or thinness resulted in IOP change by 5 mmHg correspondingly.²

Similarly, a meta analysis review on CCT studies¹⁶ determined that every 25 µm of change in CCT from a mean of 530 µm resulted in overestimation or underestimation of IOP by 1 mmHg. Another indication of the importance of CCT on IOP measurement is the ever-increasing popularity of corneal refractive surgery. Both photorefractive keratectomy and LASIK result in lower IOP readings after surgery.^17,18 IOP measurement by pneumotonometry (PT) on the other hand is less adversely affected by corneal thickness or curvature, which may explain its relative accuracy after keratorefractive surgery.¹⁹ Several investigators have observed that PT readings tend to be 1 to 2 mmHg higher than GAT.^20,21

Challenging Goldmann
In 2001, a study was undertaken at our institution to determine the effect of race and gender on CCT in patients with POAG and OHT. IOP was measured by GAT and PT. Exclusion criteria were: refractive error of >3D, astigmatism of >1D, prior intraocular procedure, corneal abnormality, and secondary glaucoma. The study enrolled 278 patients, and the results are tabulated in Table 2 and Figure 1. CCT was affected by race: Caucasians had thick corneas, African-Americans had thin corneas and Hispanics had values in between the two.

Table 1. The Chief Studies on Central Corneal Thickness
Author/Year	Country	Race	CCT (um)
			Normal	POAG	NTG	OHT
Barbados Eye Study 2003	Barbados	Black	529.8	520.6+/-37.7	--	533.8+/-34.6
		Mixed	537.8	--	--	--
		White	545.2	--	--	--
S. Shah et al, 1999	UK	N.A.	553.9	550.1	514	--
F.A. LaRosa et al, 2001	USA	Black	533.8+/-33.9	529.5+/-9.6	462	--
		White	555.9+/-33.2
OHTS 2001	USA	Black				555.7+/-40
		White				579.0+/-37
Reykjavik Eye Study 2002	Iceland	White	Male=528+/-41
			Female=526+/-37
A.C. -M.Wong et al, 2002	Hong Kong	Chinese	551.11+/-35.3
L.L. Wu et al, 2002	Japan	Japanese	552.36	550.33	548.33	582.32
R. Thomas et al, 2000	India	Indian	537+/-0.034	534+/-0.03	--	574+/-0.033
B.Y. Emara 1999	Canada	---	556.7+/-35.9	548.2+/-35.0	513.2+/-26.1

Even in this defined and controlled population, there was a wide range of values in individual groups. This finding suggests that there are yet more unknown factors that affect CCT in individuals. GAT measurements were similar in all races, but in comparison, PT values were higher than GAT in each race. This observation challenges the role of GAT as a gold standard in IOP measurement. Perhaps an adjustment for CCT may lead to a more accurate assessment of IOP by GAT in patients with thick or thin corneas. The clinical implication is significant. Patients with thick corneas after adjusted IOP may require fewer medications or less frequent need for surgery; whereas, subjects with thin CCT may be more vigorously treated if they exhibit other risk factors for glaucoma, such as large and deep cup-to-disc ratio.

It is important to stress that apart from CCT differences in races, there are other factors that affect the natural history of glaucoma in various races. More studies are needed to unravel the relationship between CCT, IOP and glaucoma. The association of other structural abnormalities such as high myopia²² and nanophthalmos (high hyperopia and thick sclera),²³ are well-known. There is a need to calculate structural features of all patients with glaucoma or glaucoma suspects and study the effect of structural variations on visual function. For example, it is known that African-Americans have thin corneas, but they also have large optic discs.²⁴ Anatomical specimens have demonstrated that black individuals have larger total lamina cribrosa area and a greater number of laminar pores than whites.²⁵ Individuals with enlarging optic disc area have increasing numbers of nerve fibers.²⁶ But nerve fiber density per disc area decreases with increasing optic disc area.²⁶ Do these race-related structural features have any role in the natural history of glaucoma? CCT has thus provided another piece of the glaucoma puzzle, which is sure to stimulate a new thought process to examine the whole eye structure rather than concentrate on IOP and features of the optic nerve head only. 

This work was supported in part by an unrestricted research grant from Research to Prevent Blindness, Inc. Dr. Kooner is an associate professor of ophthalmology at the University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd. Dallas, Texas 75390-9057. E-mail: kjkooner@ yahoo.com.

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